EPA/600/R-03/103
September 2002
Considerations in the Design of Treatment
Best Management Practices (BMPs) to
Improve Water Quality
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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Notice
This document has been reviewed in accordance with the U.S. Environmental Protection
Agency's peer and administrative review policies and approved for publiction. Mention of trade
names, commercial products, or design procedures does not constitute endorsement or
recommendation for use.
11
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Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with
protecting the Nation's land, air, and water resources. Under a mandate of national
environmental laws, the Agency strives to formulate and implement actions leading to a
compatible balance between human activities and the ability of natural systems to support and
nurture life. To meet this mandate, EPA's research program is providing data and technical
support for solving environmental problems today and building a science knowledge base
necessary to manage our ecological resources wisely, understand how pollutants affect our
health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's center
for investigation of technological and management approaches for preventing and reducing risks
from pollution that threaten human health and the environment. The focus of the Laboratory's
research program is on methods and their cost-effectiveness for prevention and control of
pollution to air, land, water, and subsurface resources; protection of water quality in public water
systems; remediation of contaminated sites, sediments and ground water; prevention and control
of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both public
and private sector partners to foster technologies that reduce the cost of compliance and to
anticipate emerging problems. NRMRL's research provides solutions to environmental problems
by: developing and promoting technologies that protect and improve the environment; advancing
scientific and engineering information to support regulatory and policy decisions; and providing
the technical support and information transfer to ensure implementation of environmental
regulations and strategies at the national, state, and community levels.
This publication has been produced as part of the Laboratory's strategic long-term
research plan. It is published and made available by EPA's Office of Research and Development
to assist the user community and to link researchers with their clients.
Hugh W. McKinnon, Director
National Risk Management Research Laboratory
in
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Abstract
For the past three decades, municipalities in the United States have successfully
addressed pollution in the watershed by collecting and treating their wastewater. Currently, all
municipalities provide secondary level treatment, and in some cases tertiary treatment, and
industries provide best available/best practicable treatment. This has had great benefits. More
rivers are meeting water quality standards, and the public health is being protected from
waterborne disease. The challenge now facing us is to address pollution associated with storm
water runoff, since this is now the last major threat to water quality.
It is less costly to prevent the generation of polluted runoff than to treat it. Today,
many municipalities are implementing low-cost best management practices (BMPs) that prevent
runoff. The lowest cost BMPs, termed non-structural or source control BMPs, include practices
such as limiting pesticide use in agricultural areas or retaining rainwater on residential lots
(currently termed "low impact development"). There are a set of higher cost BMPs, which
involve building a structure of some kind to store stormwater until it can be discharged into a
nearby receiving water. These can be more costly, especially in areas where land costs are high.
The three most commonly used structural treatment BMPs that will be discussed in the document
are ponds (detention/retention), vegetated biofilters (swales and filter/buffer strips) and
constructed wetlands. Two categories of treatment considered in this document are ponds and
vegetated biofilters. Ponds are probably the most frequently used BMP in the United States.
There are three types of pond BMPs: wet ponds (retention ponds); dry ponds (notably extended
detention ponds); and infiltration basins. Three different types of vegetative biofilter BMP types
are discussed: grass swales, vegetated filter strips, and bioretention cells. Grass swales include
three variations: traditional grass swales, grass swales with media filters and wet swales.
This document presents factors that should be considered in the design of treatment
BMPs to improve water quality. The state-of-the-practice is such that existing design guides
vary and the performance of treatment BMPs shows a wide range of pollutant removal
effectiveness.
IV
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Contents
Notice ii
Foreword iii
Abstract iv
Table of Contents v-vi
List of Figures vii
List of Tables viii
Acronyms and Abbreviations x-xi
Acknowledgments xii
Chapter One
Role of Best Management Practices (BMPs) in Improving Water Quality
1.1 Introduction 1-1
1.2 Impacts of Nonpoint Sources on Receiving Waters 1-2
1.3 Impacts of Urbanization on Receiving Waters - Physical and Chemical .... 1-8
1.4 Impacts of Urbanization on Receiving Waters - Biological Communities . . 1-10
1.5 Pollutant Loadings Associated with Urban Stormwater 1-13
1.6 Stormwater Management - EPA Regulatory Requirements 1-21
1.7 Role of BMPs in Developing an Urban Stormwater Management Plan .... 1-22
1.8 Current Peak Discharge Control Strategies 1-23
1.9 Design of Treatment BMPs to Improve Water Quality 1-25
1.10 Concerns with BMP Performances 1-25
Chapter Two
Watershed Hydrology Pertinent to BMP Design
2.1 Introduction 2-1
2.2 Amount and Distribution of Rainfall Intensity and Volume 2-1
2.3 Hydrologic Concepts for BMP Design 2-4
2.4 Peak Discharge Control Strategies 2-5
2.5 Water Quality Control Strategies 2-13
Chapter Three
Types of BMPs and Factors Affecting their Selection
3.1 Introduction 3-1
3.2 Types of BMPs 3-1
3.3 BMP Selection Criterion - Meeting Stormwater Management Goals 3-3
3.4 BMP Selection Criterion - On-Site vs Regional Controls 3-7
3.5 BMP Selection Criterion - Watershed Factors 3-11
3.6 BMP Selection Criterion - Terrain Factors 3-13
3.7 BMP Selection Criterion - Physical Suitability Factors 3-14
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3.8 BMP Selection Criterion - Community and Environmental Factors 3-16
3.9 BMP Selection Criterion - Location and Permitting Factors 3-17
3.10 Federal Regulations That Impact Stormwater BMP Design 3-19
3.11 State and Municipal Requirements That Impact Stormwater BMP Design .3-25
Chapter Four
BMP Effectiveness in Removing Pollutants
4.1 Introduction 4-1
4.2 Current Flow Control Watershed Management Strategies 4-1
4.3 Pollutant Loading Estimates 4-2
4.4 Effectiveness of Treatment BMPs using Current Design Approaches 4-5
4.5 Importance of Particle Size Distribution 4-8
4.6 Approaches to Implementing BMPs for Improved Water Quality in the Urban
Watershed 4-9
4.7 Removal Processes Occurring in Treatment BMPs 4-12
4.8 Treatment-Train Approach to Improve Water Quality 4-17
Chapter Five
Types of Pond BMPs and Their Ability to Remove Pollutants
5.1 Introduction 5-1
5.2 Design of Wet Ponds to Maximize Sedimentation 5-5
5.3 Design of Extended Detention Basins for Water Quality Improvements ... 5-14
5.4 Maintenance of Pond BMPs 5-17
Chapter Six
Types of Vegetative Biofilters and Their Ability to Remove Pollutants
6.1 Introduction 6-1
6.2 Grass Swale 6-1
6.3 Vegetative Filter Strip 6-5
6.4 Bioretention Cell 6-5
6.5 Role in Water Quality Improvement 6-5
6.6 Design of Grass Swales for Pollutant Removal 6-10
6.7 Design of Vegetative Filter Strips for Pollutant Removal 6-11
6.8 Design of Bioretention Cells for Pollutant Removal 6-15
Glossary G-l
References R-l
VI
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List of Figures
Figure 1-1 Change in Floodplain Elevations 1-5
Figure 1-2 Relationship Between Impervious Cover and the Volumetric Runoff Coefficient
1-6
Figure 2-1 Stormwater Control Points Along the RFS for Maryland 2-2
Figure 2-2 Fifteen Rain Zones of the United States 2-2
Figure 2-3 A Watershed Where the Drainage From a Small Development Site Joins the Flow
From Large Watershed 2-9
Figure 2-4 Alternative Hydrographs From the Watershed 2-10
Figure 4-1 Urban Stormwater Treatment Train Process Flow Diagram 4-19
Figure 5-1 Wet Pond Typical Detail 5-2
Figure 5-2 Typical Dry Pond 5-3
Figure 5-3 Extended Detention Basin, Typical Detail 5-4
Figure 5-4 Infiltration Basin, Typical Detail 5-6
Figure 6-1 Grass Swale 6-3
Figure 6-2 Wet Swale 6-4
Figure 6-3 Typical Vegetative Filter Strip 6-6
Figure 6-4 Typical Bioretention Cell 6-7
Figure 6-5 Pollutant Removal Efficiency Versus Filter Strip Length 6-13
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List of Tables
Table 1-1 Impacts of Urbanization on Receiving Waters 1-9
Table 1-2 Changes Due to Urbanization and Effects on Aquatic Organisms 1-11
Table 1-3 Recent Research Examining the Relationship of Urbanization to Aquatic Habitat
and Organisms 1-12
Table 1-4 National Event Mean and Median Concentrations for Chemical Constituents of
Stormwater 1-14
Table 1-5 Regional Groupings by Annual Rainfall 1-15
Table 1-6 Mean and Median Nutrient and Sediment Stormwater Concentrations for
Residential Land Use Based on Rainfall Regions 1-16
Table 1-7 Percentage of Metal Concentrations Exceeding Water Quality Standards by
Rainfall Region 1-16
Table 1-8 Stormwater Pollutant Event Mean Concentration for Different U.S. Regions
1-17
Table 1-9 Typical Urban Areas and Pollutant Yields 1-18
Table 1-10 Median Stormwater Pollutant Concentration for All Sites by Land Use ... 1-19
Table 1-11 Comparison of Water Quality Parameters in Urban Runoff With Domestic
Wastewater 1-19
Table 1-12 Runoff and Pollutant Characteristics of Snowmelt Stages 1-20
Table 1-13 Impairments Associated with Current Flow Control Strategies 1-24
Table 2-1 Typical Values of Individual Storm Event Statistics for 15 Zones of the United
States 2-3
Table 2-2 Comparison of Model Attributes and Functions 2-6
Table 2-3 Design Storm Frequencies and Assumed Benefits 2-7
Table 2-4 Qualitative Assessment of Peak Discharge Control Strategies with Respect to the
Physical Impact Category 2-8
Table 3-1 ASCE Source Control BMPs 3-2
Table 3-2 Treatment BMPs 3-2
Table 3-3 Summary of Studies on Environmental Impacts for Pond and Wetland BMPs
3-4
Table 3-4 Summary of Studies on Environmental Impacts for Vegetative Biofilter BMPs
3-5
Table 3-5 Summary of Studies on Environmental Impacts for Infiltration BMPs 3-6
Table 3-6 Summary of Studies on Environmental Impacts for Filter BMPs 3-7
Table 3-7 Treatment BMPs vs Watershed Factors 3-12
Table 3-8 BMP Selection - Terrain Factors 3-14
Table 3-9 BMP Selection - Physical Suitability Factors 3-15
Table 3-10 BMP Selection - Community and Environmental Factors 3-17
Table 3-11 Permitting Checklist 3-18
Table 3-12 State or Regional Planning Authority Requirements for Water Quality Protection
Vlll
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3-26
Table 3-13 Municipal or Regional Planning Authority Requirements 3-27
Table 3-14 Minimum Drainage Area Requirements for States 3-28
Table 3-15 Minimum Area Requirements for Local Agencies 3-28
Table 3-16 Peak Discharge Control Criteria for States 3-29
Table 3-17 Peak Discharge Rate Control Requirements, Municipalities 3-30
Table 3-18 Water Quality Regulatory Requirements, States 3-31
Table 3-19 Water Quality Requirements, Municipalities 3-31
Table 4-1 Median Pollutant Removal of Stormwater Treatment Practices 4-5
Table 4-2 Median Effluent Concentration of Stormwater Treatment Practice Groups . . 4 - 6
Table 4-3 Removal Processes Occurring in Treatment BMPs 4-13
Table 4-4 Treatment BMP Expected Performance 4-13
Table 4-5. Pollutant Removal (%) by Mulch from Stormwater Runoff 4-17
Table 4-5 Expected Median Effluent Concentration of Selected Pollutants 4-20
Table 5-1 Hydrologic and Hydraulic Design Criteria for Standard Extended Detention Wet
Pond System 5-7
Table 5-2 Recommended Criteria for Wet Pond Design for Nutrient Removal 5-8
Table 6-1 Estimated Pollutant Removal Capability of Biofilters 6-8
Table 6-2 Land Use and Biofilter Suitability 6-9
Table 6-3 Physical Site Conditions and Biofilter Suitability 6-10
Table 6-4 Pollutant Removal Efficiencies for Grass Swales 6-11
Table 6-5 Pollutant Removal Performance of Bioretention Practices (% Removal) ... 6-16
IX
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Acronyms and Abbreviations
ASCE = American Society of Civil Engineers
BMP = best management practice
BOD = biochemical oxygen demand
CERCLA = Comprehensive Environmental Response, Compensation and Liability Act
CZARA = Coastal Zone Act Reauthorization Amendments
CZMA = Coastal Zone Management Act
COD = chemical oxygen demand
CWA = Clean Water Act
EPA = Environmental Protection Agency
EPT = ephemeroptera (mayflies), plecoptera (stoneflies), and trichoptera (caddisflies)
ESA = Endangered Species Act
EMC = event mean concentration
FIFRA = Federal Insecticide, Fungicide and Rodenticide Act
FWPCA = Federal Water Pollution Control Act
IBI = index of biotic integrity
MDE = Maryland Department of the Environment
MTBE = methyl tertiary butyl ether
NEPA = National Environmental Policy Act
NGPE = native growth protection easement
NMFS = National Marine Fisheries Service
NOAA = National Oceonographic and Atmospheric Administration
NPDES = National Pollution Elimination Discharge Program
NRCS = Natural Resources Conservation Service
NURP = Nati onwi de Urb an Runoff Program
OCZM = Office of Coastal Zone Management
RCRA = Resource Conservation and Recovery Act
RFS = rainfall frequency spectrum
SCS = Soil Conservation Service
SWMM = StormWater Management Model
TMDL = total maximum daily loads
TN = total nitrogen
TP = total phosphorus
TSS = total suspended solids
UDFCD = Urban Denver Flood Control District
USDA = US Department of Agriculture
USFWS = US Fish and Wildlife Service
USGS = US Geological Survey
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USTM = Unitied Stormwater model
WEF = Water Environment Federation
WEPP = Water Erosion Prediction Model
Wqv = water quality volume
WWF = wet weather flow
XI
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Acknowledgments
Many people participated in the creation of this document. Technical direction was
provided by USEPA's National Risk Management Research Laboratory (NRMRL). Technical
writing was carried out in several stages, but culminated into a final product as a cooperative
effort between the NRMRL and the contractors named below. The contractors produced a three-
volume series "BMP Design Guide - Volume 1, General Considerations", "BMP Design Guide -
Volume 2, Vegetative Biofilters" and "BMP Design Guide - Volume 3, Pond BMPs" which
included detailed design guidance. That three-volume series will be published separately by
USEPA. This document overviews factors which should be considered in the design of
treatment best management practices (BMPs) to improve water quality.
Authors of Three-Volume Series
Michael Clar, Ecosite, Inc, Ellicott City, MD
Bill J. Barfield, Oklahoma State University, Stillwater, OK
Shaw Yu, University of Virginia, Charlottesville, VA
USEPA Contributors/Authors
Thomas O'Connor, Water Supply and Water Resources Division, NRMRL, Edison, NJ
Chi-Yuan Fan, Water Supply and Water Resources Division, NRMRL, Edison, NJ
Daniel Sullivan, Water Supply and Water Resources Division, NRMRL, Edison, NJ
Richard Field, Water Supply and Water Resources Division, NRMRL, Edison, NJ
Anthony Tafuri, Water Supply and Water Resources Division, NRMRL, Edison, NJ
Asim Ray, Senior Environmental Employment Program, Edison, NJ
Peer Reviewers of Three-Volume Series
Ben R. Urbonas, Urban Watersheds, LLC, Denver, CO
Eugene D. Driscoll, Oakland, NJ
Jesse Pritts, Office of Water, Washington, DC
Norbert Huang, Office of Water, Washington, DC
King Boynton, Office of Water, Washington, DC
Technical Edit Review
Gelane Gemechisa, Environmental Careers Organization, Edison, NJ
xn
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Chapter 1: Introduction BMP Design Considerations
September 2002
Chapter One
Role of Best Management Practices (BMPs) in Improving Water
Quality
1.1 Introduction
For the past three decades, municipalities in the United States have successfully
addressed pollution in the watershed by collecting and treating their wastewater. Currently, all
municipalities provide secondary level treatment, and in some cases tertiary treatment, and
industries provide best available/best practicable treatment. This has had great benefits. More
rivers are meeting water quality standards, and the public health is being protected from
waterborne disease. The challenge now facing us is to address pollution associated with
stormwater runoff since this is now the last major threat to water quality.
It is less costly to prevent the generation of runoff than to treat it. Today, many
municipalities are looking at low-cost best management practices (BMPs) that prevent runoff.
The lowest cost BMPs, termed nonstructural or source control BMPs, include practices such as
limiting pesticide use in agricultural areas or retaining rainwater on residential lots (currently
termed "low impact development"). There are higher cost BMPs, which involve building a
structure of some kind to store stormwater and enable sedimentation. These can be more costly,
especially in areas where land costs are high. BMPs have been classified a number of different
ways including by stormwater runoff source, pollutant, land use and BMP type. For example,
the Rouge River Restoration Project has seven classifications for BMPs: public information and
participation, urban source control, treatment control, channel restoration/stabilization,
construction erosion and sediment control, and agricultural. The American Society of Civil
Engineers has nine categories (ASCE, 1998) and the State of Texas has three classes.
For the past ten years, the Environmental Protection Agency (EPA) has encouraged that
water pollution controls be approached on a watershed basis. A watershed approach allows
tradeoffs between pollution sources, point source treatment and pollution prevention, and
optimal balances between these. It requires community-level involvement and often includes the
use of both hard (structural) and soft (nonstructural) engineering approaches to protect or restore
watersheds from chemical, physical, or biological stressors. The watershed approach allows
simultaneous pollution, flood, and erosion-sedimentation control by properly siting BMPs within
the watershed to maximize pollutant removals and reduce stormwater-associated stressors.
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Chapter 1: Introduction BMP Design Considerations
September 2002
Historically, BMPs were employed to capture peak flows, assist in local drainage, and
manage the quantity of runoff produced during wet-weather flow (WWF), i.e., flood control.
While these objectives will probably remain a goal of watershed management planners, BMPs
are now also considered for pollutant removal, stream restoration, and groundwater recharge
infiltration.
Source control and pollution prevention are considered "good housekeeping" practices
i.e., practices that keep pollutants out of runoff such as street cleaning, product substitution, and
controlled application of pesticides/herbicides. Runoff source controls are used to reduce runoff
generated at the source of specific activities and are divided into two types: those used on a
temporary basis (e.g., runoff control at construction activities) and those used on a permanent
basis (e.g., hot spot treatment at vehicle repair sites). End-of-pipe or treatment controls are used
to remove pollutants from contaminated runoff.
The three most commonly used treatment BMPs are ponds (retention/detention),
vegetative biofilters (swales, filter/buffer strips, and bioretention cells) and constructed wetlands.
Two other categories of structural treatment BMPs are filters (notably sand filters) and
innovative technology options (catchbasin inserts, filters, etc). This document concentrates on
ponds and vegetative biofilters. BMPs that can be applied to agricultural lands will not be
covered. The key aquatic stressors of concern in the United States are nutrients, suspended
solids and sediments (SSASs), pathogens, flow, and toxic substances. These stressors have
worldwide significance.
1.2 Impacts of Nonpoint Sources on Receiving Waters
WWF discharges are the leading cause of water quality impairment in the United States
and pose significant risk to both human health and the downstream ecosystems. These
discharges include stormwater and, in many urban areas, sewer overflows (from combined
sewers and sanitary sewers). WWFs have the potential for widespread, short-term high
exposures to infectious agents which result in gastrointestinal illness and even death. In
addition, there is an increase in chronic long-term contamination of sediments and the aquatic
food chain through the release of persistent, bioaccumulative toxic agents. The Office of Water,
in its "National Water Program Agenda - 2001-2002" identifies the management of WWF
dischargers as one of the key priority areas remaining to assure clean water and safe drinking
water. Furthermore, this agenda states:
• Almost 40% of rivers, lakes, and coastal waters monitored by States do not meet
water quality goals.
• Wet weather results in stormwater discharges and runoff from diffuse, nonpoint
sources of pollution (e.g., agricultural operations, city streets, and construction)
and causes significant water pollution problems throughout the country.
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Chapter 1: Introduction BMP Design Considerations
September 2002
• Pollution from diffuse or non-point sources during and after rainfalls is now the
single largest cause of water pollution.
WWF discharges cause significant negative impacts on the downstream ecosystems and
create human health concerns since these downstream waters may be used for drinking water
sources and recreational purposes. There may be significant pathogen microorganism counts
(salmonella, straphylococcus aureus, pseudomonas aeruginosa) and viruses in stormwater. A
recent epidemiology study in Santa Monica Bay, California, documented an increased risk of
illness associated with swimming near storm drains. A special concern focuses on the public
health aspects of beach closures and shellfish bed closures and minimizing the impacts of WWFs
associated with body contact in swimming water during recreational activities. Exposure to
these pathogens is of particular concern after major rainfall events which cause discharges from
both point sources (e.g., sanitary sewer overflows (SSOs), combined sewer overflows (CSOs)
and stormwater) and non-point sources (e.g., non-sewered urban runoff, animal feedlots,
malfunctioning septic tanks, and other wild and domestic animal wastes). According to the
Natural Resources Defense Council's eighth annual survey on beach water safety, at least 4,153
beach closings and advisories were caused by pollution in 1997 alone - "and adequate
monitoring and notification procedures are still lacking at many of the nation's most popular
beaches." This number of advisories may be an underestimate of incidents of contamination
because many States and localities do not conduct, nor are they required to have, regular
recreational water quality monitoring programs.
The 1992 National Water Quality Inventory cites numerous public health adversities
associated with WWF: (1) toxic pollution from urban storm runoff into the Southern San
Francisco Bay has caused heavy metal increases (copper, lead, nickel and zinc) and impairments
to water quality for this salmon and herring fishery and recreational resource; (2) metals (copper,
lead, zinc, mercury and cadmium) and organic toxicants (notably PCBs) have degraded water
quality and contaminated sediments in the Duwamish River, Washington; (3) urban storm runoff
in the New York metropolitan area has been implicated in increasing coliforms and reduced DO
in the western end of the Long Island Sound, causing closure of beaches and commercial
shellfish beds due to high fecal coliform concentrations; and (4) high concentrations of coliform
bacteria observed after rainfall events in the Westport River, Massachusetts, have caused
violations of primary contact recreation water quality criteria and forced the closure of shellfish
beds.
WWFs have caused a decrease in flora/fauna species diversity, species types, and tissue
bioassay impacts in many streams, as well as dissolved oxygen depletions. Urban storm runoff
is a major cause of eutrophication, especially along the eastern coastal estuaries. In Lake Eola,
Florida, urban runoff was found to be the sole cause of lake degradation associated with
phosphorus increases and algal growth. The Village Creek in Birmingham, Alabama turned
dark green with a putrid odor and contained considerable oil and grease due to upstream
pollution; the creek was anaerobic with no fish or other biological life. Urbanization also creates
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Chapter 1: Introduction BMP Design Considerations
September 2002
higher stream flows causing bank and bottom erosion and deposition which has a high and
significant national impact not only in terms of physical upsets but ecological as well.
There are localized economic losses associated with WWFs, including lost work hours
due to illness; medical expenses; increased drinking water treatment costs (turbidity, metals,
pathogens); lost tourist trade due to beach closings, fishing advisories; lost supply of shellfish
and other fish (commercial shell fishing operations have been wiped out and economic losses
also extend to recreational diggers); response, investigation, medical care, and insurance costs;
and property value losses (e.g., to lake and river properties affected by floatables, siltation or
eutrophi cation).
Flooding Impacts Flow events that exceed the capacity of the stream channel spill out into
adjacent flood plains. These are termed "overbank" floods and can damage property and
downstream drainage structures. While some overbank flooding is inevitable and even desirable,
the historical goal of drainage design in many jurisdictions has been to maintain pre-
development peak discharge rates for both the two and ten-year frequency storms after
development, in an attempt to maintain the level of overbank flooding the same over time. This
prevents costly damage or maintenance for culverts, drainage structures, and swales.
Overbank floods are ranked in terms of their statistical return frequency. For example, a
flood that has a 50% statistical probability of occurring in any given year is termed a "two-year"
flood. The two-year storm is also often used as a surrogate for the "bankfull flood", as
researchers have demonstrated that most natural stream channels have just enough capacity to
handle a runoff event with a return frequency of 1 to 2 years, before spilling into the floodplain
(Wolman, 1960; Leopold, 1964, 1968).
Similarly, a flood that has a 10% probability of occurring in any given year is termed a
"ten-year flood." Under traditional engineering practice, most channels and storm drains in many
jurisdictions are designed with enough capacity to safely pass the peak discharge from the
ten-year design storm.
The level areas bordering streams and rivers are known as flood plains. Operationally,
the floodplain is usually defined as the land area within the limits of the 100-year storm flow
water elevation. The 100-year storm has a 1 % statistical probability of occurring in any given
year and typically serves as the basis for controlling development in many states and establishing
insurance rates by the Federal Emergency Management Agency. These floods can be very
destructive and can pose a threat to property and human life. Flood plains are natural flood
storage areas and help to attenuate downstream flooding.
Flood plains are very important habitat areas, encompassing riparian forests, wetlands,
and wildlife corridors. Consequently, many local jurisdictions restrict or even prohibit new
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Chapter 1: Introduction
September 2002
BMP Design Considerations
development within the 100-year floodplain to prevent flood hazards and conserve habitats.
Nevertheless, prior development that has occurred in the floodplain remains subject to periodic
flooding during these storms.
As with overbank floods, development sharply increases the peak discharge rate
associated with the 100-year design storm. As a consequence, the elevation of a stream's 100
year floodplain becomes higher and the boundaries of its floodplain expands (Figure 1-1). In
some instances, property and structures that had not previously been subject to flooding are now
at risk. Additionally, such a shift in a floodplain's hydrology can degrade wetlands and forest
habitats.
C. RESPONSE OF STREAM GEOMETRY
Figure 1-1 Change in Floodplain Elevations (MDE, 2000)
Hydrologic Regime Alterations Associated with Imperviousness Development increases the
amount of impervious area in a watershed and thus can have a profound influence on the quality
of receiving waters. Land use changes caused by agriculture, construction and urban
development can dramatically alter the local hydrologic regime, primarily due to the increase in
runoff due to impervious surfaces. The hydrology of an area changes during the initial clearing
and grading that occur during construction. Trees, meadow grasses, and agricultural crops that
had previously intercepted and absorbed rainfall are removed and natural depressions that had
temporarily ponded water are graded to a uniform slope. Cleared and graded sites easily erode,
are often severely compacted, and can no longer prevent rainfall from being rapidly converted
into stormwater runoff. Very large errors in soil infiltration rates can be made if published soil
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Chapter 1: Introduction
September 2002
BMP Design Considerations
maps and most available models are used for typical disturbed urban soils, as these tools ignore
compaction (Pitt etal, 2000). Any disturbance of a soil profile can significantly change its
infiltration characteristics and with urbanization, native soil profiles may be mixed or removed
or fill material from other areas may be introduced (USDA, 1986). Some local agencies have
attempted to address this issue by requiring that the pre-development hydrologic soil group
(HSG) type be downgraded for post development hydrologic analysis. For example pre-
development HSG types A, B, and C would be downgraded respectively to a B, C, and D.
The situation worsens after construction. Rooftops, roads, parking lots, driveways and
other impervious surfaces no longer allow rainfall to soak into the ground. Consequently, most
rainfall is converted directly to stormwater runoff. This phenomenon is illustrated in Figure 1-2,
which shows the increase in the volumetric runoff coefficient (Rv) as a function of area
imperviousness. The runoff coefficient expresses the fraction of rainfall volume that is
converted to runoff. As can be seen, the volume of stormwater runoff increases sharply with
impervious cover. For example, a one acre parking lot can produce 16 times more stormwater
runoff than a one acre meadow each year (MDE, 2000).
Runoff Coefficient (Rv}
1O
2O 30 40 SO 60 70 SO
Watershed Imperviousness (%)
too
Figure 1-2 Relationship between Impervious Cover and the Volumetric Runoff
Coefficient (Schueler, 1987)
Groundwater Recharge Impacts The slow infiltration of rainfall through the soil layer is
essential for replenishing groundwater. The amount of rainfall that recharges groundwater
varies, depending on the slope, soil, and vegetation.
Groundwater is a critical water resource in many areas of the U.S. Not only do many
people depend on groundwater for their drinking water, but the health of many aquatic systems is
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Chapter 1: Introduction BMP Design Considerations
September 2002
also dependent on its steady discharge. For example, during periods of dry weather,
groundwater sustains flows in streams and helps to maintain the hydrology of non-tidal wetlands.
Because development creates impervious surfaces that prevent natural recharge, a net decrease in
groundwater recharge rates may result in urban watersheds. As previously mentioned, many
construction and development practices disturb the natural soil processes, through clearing of
vegetation, grading and compaction, limiting infiltration in the post development landscape.
Thus, during prolonged periods of dry weather, stream flow sharply diminishes. In smaller
headwater streams, the decline in stream flow can cause a perennial stream to become seasonally
dry.
Urban land uses and activities can also degrade groundwater quality if stormwater runoff
is directed into the soil without adequate treatment. Certain land uses and activities are known to
produce higher loads of metals and toxic chemicals and are designated as stormwater hot spots.
Typical urban hot spots include industrial facilities, gasoline stations, parking lots, bus depots,
golf courses and nurseries. The following land uses and activities are not normally considered
hot spots: residential streets and rural highways; residential development; institutional
development; commercial and office developments; non-industrial rooftops; pervious areas,
except golf courses and nurseries.
Impacts on Stream Channel Stability Stormwater runoff is a powerful force that influences
the geometry of streams. After development, both the frequency and magnitude of storm flows
increase dramatically. Consequently, urban stream channels experience more frequent out of
bank flows as well as intermediate flows that have sufficient energy to erode and destabilize the
stream channel than they had prior to development.
As a result, the stream bed and banks are exposed to highly erosive flows more
frequently and for longer periods. Streams typically respond to this change by increasing
cross-sectional area to handle the more frequent and erosive flows either by channel widening or
down cutting, or both. This results in a highly unstable phase where the stream experiences
severe bank erosion and habitat degradation. In this phase, the stream often experiences some of
the following changes as it adjusts to a new flow regime: rapid stream widening, increased
streambank and channel erosion, change in sinuosity, decrease in slope, decline in stream
substrate quality (through sediment deposition and embedding of the substrate), loss of
pool/riffle structure in the stream channel, and degradation of stream habitat structure.
The decline in the physical habitat of the stream, coupled with lower base flows and
higher stormwater pollutant loads, has a severe impact on the aquatic community including
decline in aquatic insects, freshwater mussels, and fish diversity, and degradation of aquatic
habitat. Traditionally, some local agencies have attempted to provide some measure of channel
protection by imposing the two-year storm peak discharge control requirement, which requires
that the discharge from the two-year post development peak rates be reduced to pre-development
levels. However, hydrologic analysis (McCuen, 1987) and recent field experience indicate that
1 -7
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Chapter 1: Introduction BMP Design Considerations
September 2002
the two-year peak discharge criterion is not capable of protecting downstream channels from
erosion. For some receiving waters, controlling the two-year storm may actually accelerate
streambank erosion because it exposes the channel to a longer duration of erosive flows than it
would have otherwise received.
Thermal Impacts In some urbanized regions of the country, summer in-stream temperatures
have been shown to increase significantly (5 to 12 F°) in streams due to direct solar radiation,
lack of riparian buffer, runoff from heat absorbing pavement and discharges from stormwater
ponds. Increased water temperatures can preclude temperature sensitive species from being able
to persist in urban streams.
Galli (1991) reported that stream temperatures throughout the summer are increased in
urban watersheds, and the degree of warming appears to be directly related to the
imperviousness of the contributing watershed. He monitored five headwater streams in the
Maryland Piedmont over a six-month period, with each of the streams having differing levels of
impervious cover. Each of the urban streams had mean temperatures that were consistently
warmer than a forested reference stream, and the size of the increase appeared to be a direct
function of watershed imperviousness. Other factors, such as lack of riparian cover and ponds,
were also demonstrated to amplify stream warming, but the primary contributing factor appeared
to be watershed impervious cover.
1.3 Impacts of Urbanization on Receiving Waters - Physical and Chemical
General impacts of pollutants on different receiving waters are reported in Table 1-1.
Impervious surfaces accumulate pollutants deposited from the atmosphere, leaked from vehicles,
or washed off/windblown from adjacent areas. During storm events, these pollutants quickly
wash off and are rapidly delivered to downstream waters. Pervious areas are major contributors
of erosion products (sediment), nutrients, and pesticide/herbicides.
Urban runoff has elevated concentrations of both phosphorus and nitrogen, which can
enrich streams, lakes, reservoirs and estuaries. In particular, excess nutrients have been
documented to be a major factor in the decline of major estuarine areas such as the Chesapeake
Bay and western Long Island Sound. Excess nutrients promote algal growth that blocks sunlight
1 -
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Chapter 1: Introduction
September 2002
BMP Design Considerations
from reaching underwater grasses and depletes oxygen in bottom waters. Urban runoff has been
identified as a key and controllable source.
Table 1-1 Impacts of Urbanization on Receiving Waters
Lakes
Reservoirs
Aquifers
Wetlands
Streams
Shellfish
Beaches
Estuaries
Sea grasses
Sediment
• •
• •
• •
• •
• •
• •
• •
• •
• •
Pathogens
• •
• •
• •
• •
• •
• •
• •
• •
• •
Metal and
Hydrocarbon
Toxicity
• •
• •
• •
• •
• •
• •
• •
• •
• •
Nutrients/
Eutrophication
• •
• •
• •
• •
• •
• •
• •
• •
• •
Pesticide /
Herbicide
• •
• •
• •
• •
• •
• •
• •
• •
?
Chloride
• •
• •
• •
• •
• •
• •
• •
• •
• •
MTBE
• •
• •
• •
1
• •
1
• •
• •
?
• • Standard violation concerns / significant concern / loss of beneficial use
• • Occasional Standard violation / site specific concerns
• "Rarely affects receiving area
? Insufficient information
Sources of sediment include washoff of particles that are deposited on impervious
surfaces and erosion of streambanks and construction sites. Both suspended and deposited
sediments can have adverse effects on aquatic life in streams, lakes and estuaries. Sediments
also transport other attached pollutants.
Organic matter, washed from impervious surfaces during storms, can present a problem
in slower moving downstream waters. As organic matter decomposes, it can deplete dissolved
oxygen in lakes and tidal waters. A modest number of currently used and recently banned
insecticides and herbicides have been detected in urban streamflow at concentrations that
approach or exceed toxicity thresholds for aquatic life uses.
Bacteria levels in stormwater runoff routinely exceed public health standards for water
contact recreation. Stormwater runoff can also lead to the closure of adjacent shellfish beds and
1 -9
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Chapter 1: Introduction BMP Design Considerations
September 2002
swimming beaches and may increase the cost of treating drinking water at water supply
reservoirs. Viruses and protozoa (e.g., cryptosporidium and giardia) cause additional difficulties.
Motor vehicles leak oil and grease that contain a wide array of hydrocarbon compounds,
some of which can be toxic at low concentrations to aquatic life. Cadmium, copper, lead and
zinc are routinely found in stormwater runoff. These heavy metals can be toxic to aquatic life at
certain concentrations and can also accumulate in the sediments of streams, lakes and estuaries.
Deicing salts that are applied to roads and parking lots in the winter months appear in stormwater
runoff and meltwater at much higher concentrations than many freshwater organisms can tolerate
and have been known to cause closures of well water supplies.
Impervious surfaces may increase temperature in receiving waters, adversely impacting
aquatic life that requires cold and cool water conditions (e.g., trout). Considerable quantities of
trash and debris are washed through storm drain networks. The trash and debris accumulate in
streams and lakes and detract from their natural beauty and decrease property value.
1.4 Impacts of Urbanization on Receiving Waters - Biological Communities
The physical and chemical impacts identified above cause a decline in both the quantity
of the aquatic biota and the quality of their habitat. This section examines some of the impacts
that urbanization exerts on the aquatic community, focusing specifically on macro-invertebrates,
fish, amphibians and freshwater mussels. The fundamental change in hydrology, as well as the
chemical composition of runoff in urban and urbanizing streams causes both a decrease in
biological diversity and a shift from more pollutant sensitive to less sensitive aquatic organisms.
Urbanization can significantly alter the land surface, soil, vegetation, water quality and
stream hydrology and create adverse impacts for aquatic organisms through habitat loss or
modification. Table 1-2 summarizes some of the changes to aquatic ecosystems as a result of
urbanization and the effects on the biological community.
The effects of urbanization on aquatic community structure has been the subject of
several recent studies as summarized in Table 1-3. A number of the studies have examined the
link between urbanization and its impact on aquatic organisms and habitat. These studies reveal
that the onset of urbanization almost always has a negative affect on the aquatic biota of
receiving waters. The degradation in the biological diversity of aquatic environments is the
result of the variety of influences that added impervious cover exerts on aquatic systems. Table
1-3 presents some of the key findings of prior research involving aquatic organisms and the
problems associated with increases in impervious cover.
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Table 1-2 Changes Due to Urbanization and Effects on Aquatic Organisms
Impact | Effect on ecosystem
Effects on organisms
Chemical Impacts
Heavy Metals
Chemical
Pollutants
Sediment
Nutrients
Reduction in Water
Quality
Increase in Turbidity
Algae Blooms
Reduced survival of eggs and alevins, toxicity to juveniles
and adults, increased physiological stress, reduced
biodiversity.
Reduced survival of eggs, reduced plant productivity,
physiological stress on aquatic organisms.
Oxygen depletion due to algal blooms, increased
eutrophication rate of standing waters, possibly toxicity to
eggs and juveniles from certain nutrients.
Physical Impacts
Hydrologic
Geo-morphology
Thermal
Channel
Modification
Increased Flow
Volumes/ Channel
Forming Storms
Decreased Base
Flows
Increase in Sediment
Transport
Loss of Pools and
Riffles
Changes in Substrate
Composition
Loss of Large Wood
Debris
Increase in
Temperature
Loss of First Order
Streams
Creation of Fish
Blockages
Loss of Vegetative
Rooting Systems
Straightening or
Hardening of
Channel
Alterations in habitat complexity, changes in availability of
food organisms related to timing of emergence and recovery
after disturbance, reduced prey diversity, scour-related
mortality, long-term depletion of large woody debris,
accelerated erosion of streambanks.
Crowding and increased competition for foraging sites,
increased vulnerability to predation, increased fine sediment
deposition.
Reduced survival of eggs and alevins, loss of habitat due to
deposition, siltation of pool areas, reduced macro-invertebrate
production.
Shift in the balance of species due to habitat change, loss of
deep water cover and feeding areas.
Reduced survival of eggs, loss of inter-gravel spaces used for
refuge by fry, reduced macroinvertebrate production, reduced
biodiversity.
Loss of cover from predators and high flows, reduced
sediment and organic matter storage, reduced pool formation,
reduced organic substrate for macro-invertebrates
Changes in migration patterns, increased metabolic activity,
increased disease and parasite susceptibility, higher mortality
of sensitive species, reduced biodiversity in stream
community.
Loss of valuable habitat especially for more sensitive species
Loss of spawning habitat for adults; inability to reach
overwintering sites, loss of summer rearing habitat, increased
vulnerability to predation.
Creates problems with decreased channel stability, increased
streambank erosion, reduced streambank integrity .
Increased stream flows, loss of habitat complexity
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Table 1-3 Recent Research Examining the Relationship of Urbanization to
Aquatic Habitat and Organisms
Indicator
Aquatic habitat
Aquatic insects
and fish
Insects, fish,
habitat water
quality, riparian
zone
Aquatic insects
and fish
Fish, Aquatic
insects
Insects, fish,
habitat, water
quality, riparian
zone
Aquatic insects
and fish
Aquatic insects
and fish
Wetland plants,
amphibians
Aquatic insects
and fish
Aquatic insects
and fish
Key Finding
There is a decrease in the quantity of large woody debris (LWD)
found in urban streams at around 10% impervious cover.
In a comparison of three stream types, urban streams had lower EPT
{Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera
(caddisflies)}, (22% vs 5% as number of all taxa, 65% vs 10% as
percent abundance) and poor index of biotic integrity (IBI) scores.
Steepest decline of biological functioning after 6% imperviousness.
There was a steady decline, with approx 50% of initial biotic
integrity at 45% impervious area.
Macroinvertebrate and fish diversity decline significantly beyond
10-12% impervious area.
A study of five urban streams found that as land use shifted from
rural to urban, fish and macroinvertebrate diversity decreased.
Physical and biological stream indicators declined most rapidly
during the initial phase of the urbanization process as the
percentage of total impervious area exceeded the 5-10% range.
There was significant decline in the diversity of aquatic insects and
fish at 10% impervious cover.
Evaluation of runoff effects in urban and non-urban areas found that
native species dominated the non-urban portion of the watershed,
but accounted for only 7% of the number of species found at the
monitoring stations located in urban areas. Benthic taxa were more
abundant in non-urbanized portions of the watershed.
Mean annual water fluctuation inversely correlated to plant &
amphibian density in urban wetlands. Declines noted beyond 10%
impervious area.
Residential urban land use in Columbus watersheds caused a
significant decrease in fish attainment scores at around 33%. For
Cuyahoga watersheds, a significant drop in IBI scores occurred at
around 8%, primarily due to certain stressors which functioned to
lower the non-attainment threshold. When watersheds smaller than
lOOmi2 were analyzed separately, the level of urban land use for a
significant drop in IBI scores occurred at around 15%.
All 40 urban sites sampled had fair to very poor IBI scores,
compared to undeveloped reference sites.
Reference
Booth et al
1991
Crawford &
Lenatl989
Horner et al.
1996
Kleinl979
Masterson &
Bannerman
1994
May et al.
1997
MWCOG
1992
Pitt 1982
Taylor 1993
Yoder et. al
1999
Yoder 1991
Location
Washington
North
Carolina
Puget
Sound,
Washington
Maryland
Wisconsin
Washington
Washington,
DC
California
Seattle
Ohio
Ohio
1 - 12
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Chapter 1: Introduction BMP Design Considerations
September 2002
Increases in imperviousness appear to cause detrimental effects in the integrity of the
biological community, beginning at fairly low levels of impervious cover. Studies suggest that
above 10% watershed imperviousness levels, significant signs of degradation are easily found.
These signs include loss of species diversity, reductions in overall species abundance, and
reproductive failure and juvenile mortality. Additional research is required to firmly establish
the exact level of imperviousness at which the biological community of a receiving water begins
to face significant impacts to its health, to identify regional variations in the impervious cover
levels at which aquatic diversity is affected and to assess the effects of disconnected impervious
area vs directly connected pervious areas.
1.5 Pollutant Loadings Associated with Urban Stormwater
Water quality impacts of urbanization encompass a broad range of parameters.
Essentially, any pollutant deposited or derived from an activity on the land surface will likely
end up in stormwater runoff in some concentration. However, there are certain pollutants and
activities that are consistently more likely to result in degradation of a stream or receiving water.
These more frequently occurring pollutants can be grouped into several broad categories
including: nutrients, sediment, metals, hydrocarbons, gasoline additives, pathogens, deicers,
herbicides and pesticides.
The direct effects of these pollutants on receiving waters is often a function of the size of
the receiving water and the sensitivity of the inhabiting organisms. Toxins tend to accumulate in
lakes, ponds, estuaries, or other fixed receiving water bodies and concentrations in streams tend
to rapidly rebound to background conditions. Toxic pollutants from stormwater tend to be a
short term problem for fast moving waters. A small stream receiving a large load of
hydrocarbons or metals from a well used parking lot is more likely to experience toxic effects
than would a large river. Sensitive species such as trout and stoneflies may be more susceptible
to a range of pollutants than more pollution tolerant organisms such as the black-nosed dace or
certain leeches.
The beneficial use of the receiving water is an important consideration when evaluating
the concentrations of pollutants in urban stormwater. Certain pollutants even at low levels are of
greater concern when receiving waters have specific beneficial uses such as swimming or
fishing. Drinking water reservoirs, especially ones without filtration, may be more sensitive to
lower levels of pollutants because the water is being managed for human consumption.
Data in Table 1-4 represent typical concentrations of chemical constituents discussed in
this section. Concentrations for most pollutants are derived from Smullen and Cave (1998).
This study represents a compilation of NURP data, combined with later data from the USGS, as
well as NPDES Phase I stormwater monitoring.
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Table 1-4 National Event Mean and Median Concentrations for Chemical
Constituents of Stormwater
Constituent (Units)
Suspended Solids (mg/1)
Total Phosphorus (mg/1)
Soluble Phosphorus (mg/1)
Total Nitrogen (mg/1)
Total Kjeldhal Nitrogen (mg/1)
Nitrite and Nitrate (mg/1)
Copper (• g/1)
Lead («g/l)
Zinc (-g/1)
BOD (mg/1)
COD (mg/1)
Organic Carbon (mg/1)
Cadmium (• g/1)
Chromium («g/l)
Oil and Grease (mg/1)
Fecal Conform (col/ 100 ml)
Fecal Strep (col/100 ml)
Cryptosporidium (organisms)
Giardia (organisms)
MTBE (• g/1)
Chloride (snowmelt) (mg/1)
Diazonon (• g/1)
Chlorpyrifos (• g/1)
Atrazine (• g/1)
Prometon (• g/1)
Simazine (• g/1)
Source of Data (% detection)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Pooled NUPJVUSGS(l)
Nationwide - Stormwater Inflow (4)
NURP(3)
Dallas-FW NPDES (2)
NURP(3)
Nationwide Stormwater inflow (4)
Nationwide Stormwater inflow (4)
NY (5)
NY (5)
National Study 16 cities (6)
Minnesota (7)
Baseflow (75%)
Stormflow (2) (92% - residential only)
Nationwide baseflow (41%)
Nationwide baseflow (86%)
Nationwide baseflow (84%)
Nationwide baseflow (88%)
Concentration
Mean
78.4
0.315
0.129
2.39
1.73
0.658
13.35
67.5
162
14.1
52.8
0.7
4
3
15,038
35,351
37.2
41.0
Median
54.5
0.259
0.103
2.00
1.47
0.533
11.1
50.7
129
11.5
44.7
11.9
3.9
6.4
1.6
116
0.025
0.55
0.023
0.031
0.039
Number
of Events
3047
3094
1091
2016
2693
2016
1657
2713
2234
1035
2639
19
150
32
NA
34
17
78
78
592
49
326
76
327
327
327
327
(1) Smullenand Cave, 1998, (2)
1996, (6) Delzer 1996, (7) Oberts
Brush et al. 1995, (3) Crunkilton et al. 1996, (4) Schueler 2000, (5) Stern et al.
1989.
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Chapter 1: Introduction
September 2002
BMP Design Considerations
National Data for Major Pollutants The amount of rainfall, temperature differences, and the
period between rain events are important factors causing stormwater quality differences. Arid
and semi-arid regions generally experience longer dry periods where pollutants build up from
different sources and subsequently runoff in higher concentrations during storm events. In cold
climates, snow accumulation in winter coincides with pollutant build up; therefore, greater
concentrations of pollutants are found during runoff events.
The USGS National Stormwater Database of 1123 storms for 98 stations in 20
metropolitan cities is a primary data source. This regional analysis of stormwater data was
chosen based on the lack of standard techniques across other data sources including NPDES,
NURP, and USGS. Tasker and Driver (1988) performed regression analyses to determine which
factors had the greatest influence on stormwater concentrations. Annual rainfall had the greatest
influence on the majority of the parameters. The water quality data was then grouped based on
the amount of yearly average rainfall. Table 1-5 shows the rainfall groupings and the cities
represented.
Table 1-5 Regional Groupings by Annual Rainfall (Driver and Tasker, 1990)
Region
Region I
Region II
Region III
Annual Rainfall
Less than 20 inches
20 to 40 inches
More than 40
inches
Cities monitored
Anchorage, AK;
Fresno, CA; Denver,
CO; Albuquerque, NM;
Salt Lake City, UT
HA, IL, MI, MN, MI,
NY, Austin,TX, OR,
OH, WA, WI
FL, MD, Boston,MA;
NC, Durham, NH, Long
Island, NY; Houston
TX, Knoxville, TN and
Little Rock, AR
Concentration Data
Highest mean and
median values for TN,
IP, TSS, COD, total
ammonia + organic
nitrogen
Higher mean and
median values than
Region III for TSS,
dissolved phosphorus
and cadmium
Lower values for many
parameters likely due to
the frequency of storms
and the lack of build up
in pollutants
Region I, the region with the lowest annual rainfall (less than 20 inches), typically had
higher concentrations of a number of pollutants. Mean and median concentrations of total
nitrogen, total phosphorus, dissolved phosphorus, total suspended solids and total ammonia +
organic nitrogen were all much higher in Region I. Additionally, a large proportion of stream
1 - 15
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Chapter 1: Introduction
September 2002
BMP Design Considerations
flow in arid or semi-arid regions comes from turbid urban sources such as municipal wastewater
effluent, return flow from irrigation, and urban storm flow (Caraco, 2000). This is probably due
to the greater amount of sediment eroded from pervious surfaces in arid or semi-arid regions, due
to the sparsity of protective vegetative cover. In Tables 1-6 and 1-7, the higher concentrations of
TSS, TP and TN from the regions with less rainfall are shown, as well as the tendency to exceed
chronic toxicity standards for metals (Driver, 1988).
Table 1-6 Mean and Median Nutrient and Sediment Stormwater
Concentrations for Residential Land Use Based on Rainfall Regions
(adapted from Tasker and Driver, 1988)
Region I (under 20 in)
Region II (20 to 40 in)
Region III (over 40 in)
TN (median) mg/1
4
2.3
2.3
TP (median) mg/1
0.45
0.31
0.31
TSS (mean) mg/1
320
250
120
Table 1-7 Percentage of Metal Concentrations Exceeding Water Quality
Standards by Rainfall Region (Driver and Tasker, 1990)
Rainfall
Region
I
II
III
Rainfall
<20 inches
20-40 inches
> 40 inches
Water Quality Standard Freshwater and Chronic Toxicity
10' g/1
Cadmium
1.5%
0
0
12 • g/1
Copper
89%
78%
75%
32 • g/1
Lead
97%
89%
91%
47 • g/1
Zinc
97%
85%
84%
Stormwater data gathered from different regions of this country, using disparate
stormwater data sources such as NPDES, USGS, and local Stormwater data, generally confirm
the trend determined by Driver (1988). That values for nutrients, suspended sediment and metals
tend to be higher in arid and semi-arid regions and tend to decrease as the amount of rainfall
increases. Arid regions do not experience build up of pollutants such as PAHs because they are
degraded rather rapidly by photo-degradation. Table 1-8 shows the distribution of rainfall and
pollutant concentrations from various monitoring sources for a number of American cities.
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Table 1-8 Stormwater Pollutant Event Mean Concentration for Different U.S.
Regions (adapted from Caraco, 2000)
National
Phoenix, AZ
San Diego, CA
Boise, ID
Denver, CO
Dalles, TX
Marquette, MI
Austin. TX
MD NPDES
Louisville, KY
GA NPDES
FL NPDES
MN Snowmelt
Annual
Rainfall
(in.)
--
7.1
10
11
15
28
32
32
41
43
51
52
NA
Events
2000-
3000
40
36
15
35
32
12
--
107
21
81
--
49
SS
(mg/1)
78.4
227
330
116
242
663
159
190
67
98
258
43
112
BOD
(mg/1)
14.1
109
21
89
--
112
15.4
14
14.4
88
14
11
--
COD
(mg/1)
52.8
239
105
261
227
106
66
98
--
38
73
64
112
Total N
(mg/1)
2.39
3.26
4.55
4.13
4.06
2.7
1.87
2.35
1.94*
2.37
2.52
1.74
4.3
Total P
(mg/1)
0.32
0.41
0.7
0.75
0.65
0.78
0.29
0.32
0.33
0.32
0.33
0.38
0.70
Soluble P
(mg/1)
0.13
0.17
0.4
0.47
--
--
0.04
0.24
--
0.21
0.14
0.23
0.18
Copper
(•g/1)
14
47
25
34
60
40
22
16
18
15
32
1.4
--
Lead
(•g/1)
68
72
44
46
250
330
49
38
12.5
60
28
8.5
100
Zinc
(•g/1)
162
204
180
342
350
540
111
190
143
190
148
55
--
TKN - total Kjeldahl nitrogen.
NA - not applicable
Land development generates pollutants from traditional point sources, such as
wastewater, and from more diffuse sources, such as stormwater runoff. The Clean Water Act has
had stringent controls in force for decades to control point source discharges through the NPDES
program. The diffuse sources are controlled in part by NPDES stormwater programs, which
involve less rigorous controls. Table 1-9 (Burton and Pitt, 2002) presents typical urban areas
and pollutant yields on an annual basis, while Table 1-10 provides median EMC values. Some
of these pollutants are released at concentrations in excess of the woodland conditions that
existed at some time prior to construction. These pollutants include nutrients, bacteria, and
metals. Other pollutants are new to the receiving waters, such as forms of volatile synthetic
materials. Various petroleum products and additives are also new to many receiving waters.
Additional pollutants can also include trash, sediment loads, temperature, and even non-native
and invasive biological species.
Table 1-9 Typical Urban Areas and Pollutant Yields (Burton & Pitt, 2002)
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Pollutant
Total Solids
SS
Cl
TP
TKN
NH3
NO3 + NO2
BOD5
COD
Pb
Zn
Cr
Cd
As
Land Use (lb/acre/year)a
Com-
mercial
2100
1000
420
1.5
6.7
1.9
3.1
62
420
2.7
2.1
0.15
0.03
0.02
Parking
Lot
1300
400
300
0.7
5.1
2
2.9
47
270
0.8
0.8
NA
0.01
NA
Residential - Density
High
670
420
54
1
4.2
0.8
2
27
170
0.8
0.7
NA
0
NA
Medium
450
250
30
0.3
2.5
0.5
1.4
13
50
0.1
0.1
0
0
0
Low"
65
10
9
0
0.3
0
0.1
1
7
0
0
0
0
0
High-
ways
1700
880
470
0.9
7.9
1.5
4.2
NA
NA
4.5
2.1
0.09
0.02
0.02
Industry
670
500
25
1.3
3.4
0.2
1.3
NA
200
0.2
0.4
0.6
0
0
Parks
NAC
3
NA
0.03
NA
NA
NA
NA
NA
0
NA
NA
NA
NA
Shopping
Center
720
440
36
0.5
3.1
0.5
0.5
NA
NA
1.1
0.6
0.04
0.01
0.02
a The difference between Ib/acre/year and kg/ha/yr is less than 15%, and the accuracy of the values shown in this
table cannot differentiate between such close values
b The monitored low-density residential areas were drained by grass swales
c NA = Not available
Table 1-11 indicates that the concentration of pollutants in stormwater runoff can be
comparable to treated domestic wastewater. Exceptions include nutrients and solids; a higher
percentage of stormwater solids are inorganic from the local geology, which has implications for
treatment. When the concentration is multiplied by the large quantity of water in runoff, the total
loading from urban areas can be greater than that of treated domestic wastewater. Thus, when
untreated urban runoff is discharged directly into receiving waters, the pollutant loads can be
much greater than those from treated domestic sewage and are rightfully a matter of concern.
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Table 1-10 Median Stormwater Pollutant Concentration for All Sites by Land
Use (EPA, 1983)
Constituents
BOD5, mg/1
COD, mg/1
TSS, mg/1
Total Pb, ug/1
Total Cu, ug/1
Total Zn, ug/1
TKN, ug/1
NO2+NO3(as N), ug/1
TP, ug/1
Soluble P, ug/1
Land Uses
Residential
Median
10
73
101
144
33
135
1900
736
383
143
cova
0.41
0.55
0.96
0.75
0.99
0.84
0.73
0.83
0.69
0.46
Mixed Land Use
Median
7.8
65
67
114
27
154
1289
558
263
56
cov
0.52
0.58
1.14
1.35
1.32
0.78
0.5
0.67
0.75
0.75
Commercial
Median
9.3
57
69
104
29
226
1179
572
201
80
COV
0.3
0.4
0.9
0.7
0.8
1.1
0.4
0.5
0.7
0.7
Open/
Non-urban
Median
-
40
70
30
-
195
965
543
121
26
COV
-
0.78
2.92
1.52
-
0.66
1
0.91
1.66
2.11
"COV: coefficient of variation = standard deviation/mean
Table 1-11 Comparison of Water Quality Parameters in Urban Runoff With
Domestic Wastewater (EPA, 1986)
Constituent
COD (mg/1)
TSS (mg/1)
Total P (mg/1)
Total N (mg/1)
Lead (mg/1)
Copper (mg/1)
Zinc (mg/1)
Fecal Coliform
per 100ml
Urban Runoff
Separate Sewers
Range
10-275
20-2,890
0.02-4.30
0.4-20.0
0.01-1.20
0.01-0.40
0.01-2.90
400-50,000
Typical
75
150
0.36
2
0.18
0.05
0.02
Domestic Wastewater
Before Treatment
Range
250-1,000
100-350
36630
20-85
0.02-0.94
0.03-1.19
0.02-7.68
106-108
Typical
500
200
8
40
0.1
0.22
0.28
After Secondary
Treatment
Typical
80
20
2
30
0.05
0.03
0.08
200
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Chapter 1: Introduction
September 2002
BMP Design Considerations
Cold Region Snowmelt Data In cold regions, greater than 50% of the annual load for sediment,
nutrients, PAHs, and some metals can come from snowmelt runoff during late winter and early
spring (Oberts, 1989). In areas where there is infrequent melting, buildup of pollutants takes
place in the snowpack, contributing to high concentrations of the pollutants during snowmelt
runoff. Oberts (1994) describes four types of snowmelt runoff events and the resulting pollutants
(Table 1-12).
Source areas for pollutants associated with snowmelt include snow dumps and roadside
areas. Concentrations of pollutants in snow dumps can be more than five times greater than
typical stormwater pollutant concentrations. These areas can build up a tremendous amount of
pollutants over the winter months and many of these pollutants can be lost in just one rain or
snow event in the early spring. Metals, PAHs, chloride, sediment and nutrients are all
parameters which build up in the snowpack.
The only significant regional differences for PAHs and oil and grease were reported for
snowmelt events. These pollutants can build up in snow in urban areas and be released during
significant snowmelt events. Oberts (1994) and others have reported that 90% of the load can be
released during the last 10% of the runoff event. The regional concentration data based on
rainfall and the snowmelt process has implications for stormwater managers. Stormwater cannot
be managed or regulated in the same manner across regional boundaries. Northern climates must
use different strategies to manage runoff from snowmelt conditions and utilize stormwater
practices which can treat a larger amount of runoff.
Table 1-12 Runoff and Pollutant Characteristics of Snowmelt Stages (Oberts,
1994)
Snowmelt
Stage
Pavement Melt
Roadside Melt
Pervious Area
Melt
Rain-on- Snow
Melt
Duration /
Frequency
Short, but many
times in winter
Moderate
Gradual, often
most at end of
season
Short
Runoff
Volume
Low
Moderate
High
Extreme
Pollutant Characteristics
Acidic, high concentrations of soluble pollutants,
Cl, nitrate, lead. Total load is minimal.
Moderate concentrations of both soluble and
particulate pollutants.
Dilute concentrations of soluble pollutants,
moderate to high concentrations of particulate
pollutants, depending on flow.
High concentrations of particulate pollutants,
moderate to high concentrations of soluble
pollutants. High total load.
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Chapter 1: Introduction BMP Design Considerations
September 2002
Pollutant Concentrations and Loadings When developing a design estimate of pollutant
concentrations and loadings, two general situations, or a combination of the two, may be
encountered. The first case occurs when planning a new facility on previously undeveloped
land, and an estimate of anticipated post-development pollutant loads is needed. This situation
will require estimates based on data from similar land uses and the site specific geology for
particle size and density characteristics.
The second situation occurs when the BMP design consists of a water quality retrofit for
an existing developed area. In this case, actual data for the existing land uses and their runoff
can be collected. Because water quality monitoring is very expensive and time consuming,
however, the designer may choose to develop estimates based on available data associated with
similar land uses. The designer can also use a combination of the two approaches using limited
storm monitoring and sampling to verify and calibrated modeling estimates.
There are two well documented approaches for developing pollutant loading estimates
from existing data. These include the nationwide regression equation method developed by the
USGS (Tasker & Driver, 1988), and the simple method developed by the Metropolitan
Washington Council of Governments (Schueler, 1987).
1.6 Stormwater Management - EPA Regulatory Requirements
In response to the 1987 Amendments to the Clean Water Act (CWA), EPA developed
Phase I of the National Pollutant Discharge Elimination System (NPDES) Stormwater program
in 1990. The Phase I program addressed sources of Stormwater runoff that had the greatest
potential to negatively impact water quality at that time. Under Phase I, EPA required NPDES
permit coverage for Stormwater discharges from:
• "Medium" and "large" municipal separate storm sewer systems (MS4s) located in
incorporated places or counties with populations of 100,000 or more; and
Eleven categories of industrial activity, one of which is construction activity that
disturbs five or more acres of land.
Operators of the facilities, systems, and construction sites regulated under the Phase I NPDES
program can obtain permit coverage under an individually-tailored NPDES permit (developed
for MS4s and some industrial facilities) or a general NPDES permit (used by most operators of
industrial facilities and construction sites).
The Phase II Final Rule, published in the Federal Register on December 8, 1999, requires
NPDES permit coverage for Stormwater discharges from:
• certain regulated small municipal separate storm sewer systems (MS4s); and
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Chapter 1: Introduction BMP Design Considerations
September 2002
• construction activity disturbing between 1 and 5 acres of land (i.e., small
construction activities).
In addition to expanding the NPDES Storm Water Program, the Phase II Final Rule revises the
"no exposure" exclusion and the temporary exemption for certain industrial facilities under
Phase I of the NPDES Storm Water Program.
The number of watersheds implementing BMPs is expected to increase dramatically with
the implementation of Phase II of the NPDES stormwater permitting regulations. The
cornerstone of this regulation is to ensure that BMPs to prevent and minimize water quality
impacts from runoff are implemented and maintained. Phase II requires NPDES permits for
smaller systems (populations of 10,000 or more), primarily all those in urbanized areas and
smaller construction sites (one to five acres in size). There are six minimum control measures
outlined under the Phase II Rule: public education/outreach, public involvement and
participation, illicit discharge detection and elimination, erosion control for construction sites
from 1 to 5 acres, post-construction BMPs for control in new and redeveloped urban areas, and
pollution prevention and good housekeeping. Many more industrial, commercial and
institutional sites will be included under Phase II.
1.7 Role of BMPs in Developing an Urban Stormwater Management Plan
The primary method to control stormwater discharges in urban areas is through the use of
BMPs (EPA website http://cfpub.epa.gov/npdes/stormwater/swphasel.cfm). These could be a
combination of practices for source control and for treatment. The overall goal would be to get
to a watershed condition that existed prior to development. Runoff from development has
significant impacts on local streams, especially when areas are paved and the amount of
impervious surface in the watershed increases.
Source control is a important component of a sound watershed management plan. Both
the volume of runoff and its quality should be addressed. The volume can be reduced by
allowing infiltration of the rainwater into ground surfaces. Use of low impact development
approaches and retrofit of paved areas with bioretention cells demonstrates this approach. The
quality of runoff can be improved by product substitution (e.g., replacing "hazardous" building
materials) and treating hazardous "hot spot" sources on site where they occur. Reduction in the
amount of impervious surfaces and minimizing soil compaction impacts during construction has
major beneficial results.
There are a number of sources that describe BMPs and their role in urban watershed
management:(http://www.bmpdatabase.org/, http://www.ce.utexas.edu/centers/crwr/index.htm,
http ://cfpub .epa.gov/npdes/stormwater/menuofbmps/menu. cfm,
http://www.tnrcc.state.tx.us/admin/topdoc/rg/348/index.html, http://www.txnpsbook.org/
http://www.lowimpactdevel opment.org/mainhome.html,http://www. stormwatercenter.net/).
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Chapter 1: Introduction BMP Design Considerations
September 2002
1.8 Current Peak Discharge Control Strategies
Urban drainage systems have been designed to remove stormwater runoff as rapidly as
possible. For flood control, storm sewers were commonly designed for a 10-yr storm with a
range from 2-4 in./hr of rainfall intensity in urban catchments, and the design frequency range
from 25 to 100 years for culverts and small bridges in highway drainage systems. As a result,
the increased magnitude and frequency of these flow peaks tend to aggravate stream channel
erosion and increase downstream flooding. To restrain the peak flow affects, many
municipalities now have ordinances requiring any storm that is greater than 0.2 in./hr with a
greater than 2-yr return interval be controlled so the post-development peak flow for a given
return interval storm (e.g., 2-yr; 10-yr; or 25-yr) does not exceed the pre-development peak flow
for the same storm. This can be achieved by using local detention storage to shave the post-
development peak flow. Although the controlled basin outflow may reduce downstream
flooding, the effect on stream erosion still remains due to the prolonged period of discharging
erosive flows.
Peak discharge control strategies represent a basic approach to control or mitigate
impacts from urban runoff and are the oldest and most widely used strategy in urban watershed
management. It is relatively straightforward and consists of a general policy that post-
development runoff rates cannot substantially exceed existing pre-development runoff rates.
Runoff (both the total volume and the peak discharge values) after development is usually much
greater due to the establishment of impervious surfaces (roadways, rooftops, etc). The flow
control approach generally requires that facilities be provided to temporarily store the additional
runoff, which is then discharged after the storm at an allowable release rate (which is usually
based on the discharge from a predisturbed design storm with the same return period).
This level of control is currently being provided by many states and municipalities under
the NPDES stormwater regulatory approach. It provides two performance criteria that are
closely related: (1) flood control and (2) peak discharge control. Peak discharge control is not
necessarily flood control. Peak discharge control strategies are often used for a range of storm
frequencies including the 1, 2, 5, 10, 25, 50 and 100 year storms. Typically the smaller
frequencies 1 and 2 year are used to prevent channel erosion rather than flood protection. By
definition flooding does not occur until a stream overtops its banks and spreads out into its
floodplain. The limits of the flood prone area is often arbitrarily set as the limit of the 100 year
storm, although the Corps of Engineers also looks at the Standard Project Flood which is the 500
year flood. Some practitioners have concluded that on a watershed-wide scale, uniform
detention strategies are a failure because they do not maintain base flows, do not necessarily
improve water quality, and in some case fail to control floods (Ferguson, 1998).
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Chapter 1: Introduction
September 2002
BMP Design Considerations
This peak discharge or flood control approach has some major drawbacks as summarized
in Table 1-13.
Table 1-13 Impairments Associated with Current Flow Control Strategies
(Collins etal., 2001)
Category
Physical
Habitat
Biological
Chemical
(Water
Quality)
Impact Type / Metric
Hydrologic
regime
Geomorphic
Runoff volume
Peak discharge
Flow duration &
frequency
Groundwater
recharge, water table
elevation & baseflows
Channel geometry
Sediment transport
Flooding
Thermal
Attachment Sites
Embeddedness
Fish Shelter
Channel alteration
Sediment deposition
Stream velocity and depth
Channel flow status
Bank vegetation protection
Bank condition score
Riparian vegetation zone
Total taxa
Ephemeroptera, Plecoptera, Tricoptera
(EPT) taxa
% taxa
% EPT
Family Biotic Index (FBI)
Sediment
Nutrients
Metals
Oil and Grease
Pathogens
Organic Carbon
MTBE
Herbicides and Pesticides
Deicers
Impairment or Change to Beneficial Use
Flooding, Groundwater recharge, hydrologic
balance, etc.
Flooding, channel erosion, habitat loss
Channel erosion, habitat loss
Water table, local wells, baseflows, habitat loss
Channel erosion, sediment deposition, habitat
loss
Aggradation, Degradation, Channel capacity
Loss of property
Habitat impairment
Impairment or loss of habitat structure
results in reduction or losses in biologic
conditions and communities.
Biologic conditions and communities can be
reduced or eliminated as a result of impairment or
loss of habitat structure caused by physical
impacts resulting from construction and
development activities.
Water quality degradation or impairment can
have many negative consequences: drinking
water violations, increased water treatment costs,
beach closures, shellfish bed closures, loss of
boating use, fishery loss, reduction of reservoir
and lake volumes due to sediment volume.
1.9 Design of Treatment BMPs to Improve Water Quality
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Chapter 1: Introduction BMP Design Considerations
September 2002
There are many publications that address BMP design for flood control based on
hydrological procedures; however most do not provide satisfactory guidance for water quality
control. The most basic level of design is based upon flow to minimize flooding. Many
stormwater controls were initially employed for flood control, i.e., to capture peak flows, assist
in local drainage, and manage the quantity of runoff produced during WWF. In this regard,
many states and municipalities only require the control of peak stormwater discharges.
In response to the provisions of the Clean Water Act, a number of activities (such as the
nationwide urban runoff program (NURP)) were initiated to characterize and quantify the water
quality impacts of WWF, and municipalities started to adapt BMPs for pollutant removal. In
recent years, watershed approaches have considered that BMPs can result in water quality
improvements, and procedures have been established to assure removal of pollutants. Coastal
zone states, for example, must remove 80 percent of total suspended solids (TSS) from new
construction areas under the Coastal Zone Management Act of 1972. Other approaches look
toward controlling the first flush of pollutants associated with a storm, mandating the capture of
the first l/2 to 1 inch of runoff (typically generated in the first hours of the one year storm).
More recently in response to a growing national awareness and understanding of the wide
range of environmental impacts associated with land use changes, particularly urbanization,
BMPs have begun to be designed for stream channel protection and restoration, groundwater
infiltration, and protection of riparian habitat and biota. Collected runoff has also been used for
irrigation, toilet flushing and other non-potable purposes, including ponds and wetlands that also
enhance urban aesthetics. This approach involves control of larger storms to achieve additional
ecological benefits, such as preventing erosion of stream banks, recharging groundwaters, or
minimizing thermal impacts.
1.10 Concerns with BMP Performances
The overriding concern with treatment BMPs is that it is often difficult to link their
installation to water quality improvements. In fact, receiving water quality at times seems
unchanged before and after the construction of BMP. Two other major concerns are the degree
to which pollutant removal associated with a particular BMP can be predicted and whether
identical designs, not considering suspended (particulate) and dissolved solids characteristics,
can produce the same performance levels at different locations. Finally, there is a lack of
methodologies/models to tell water quality managers where to place the BMP in the watershed to
get optimal water quality results. Concerns about BMP performance leads to a research program
that addresses how the BMPs work, how to design for water quality control what they cost, how
effective they are, and where to best place them in the watershed.
Improved Understanding BMPs as Unit Processes Research is needed to improve the
understanding of the key mechanisms working within the BMP to reduce the effluent load.
Useful starting points have been established, however, not to the extent of a clear understanding
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Chapter 1: Introduction BMP Design Considerations
September 2002
of several independent mechanisms such as infiltration and settling. Residence time within the
BMP likely will be the most important controlling factor. It is affected not only by design
considerations, but also by rain event factors (duration, runoff volume, inter-event timing) or, in
general, the combination of watershed hydrology with BMP hydraulics. An improved
understanding of BMP unit processes would lead to better design of the commonly used BMPs.
Proper design must include:
• Better definitions of influent mass loadings (flowrate, pollutant concentrations,
suspended solids size/settling velocity, dissolved solids, partitioning of pollutants
to solids). Current design is often based on needing to capture a large (2-year)
infrequent storm or based on typical/default stormwater characteristics, however
much of the pollutant loading occurs during frequent small storms (typically up to
80% of annual pollutant load). This approach should be modified to better
characterize the influent specific to that watershed. Design should be based on
continuous (wet and dry weather) long-term rainfall-runoff-channel flow (BMP
influent) simulation emanating from the BMP drainage area using appropriate
urban hydrology.
Better application of engineering principles. Traditional BMPs have been
designed for flood protection rather than removal of pollutants. Typically longer
detention time is required for treatment to be effective. Only recently have some
States emphasized the water quality aspects of BMP design. Engineering
principles should be applied more fully. For example, improved designs should
be based on discharge rates that allow particulate settling, consideration of
velocity/size distributions, allowance for removal of the dissolved solids fraction,
and improved soil infiltration practices. As noted above, site-specific
characterization of stormwater is imperative.
• Need to consider all pollutants. Examples of this are toxic and oxygen-
demanding substances and thermal changes that could result in significant
receiving-water upsets when they are overlooked. Avoidance of sediment
resuspension is also important.
Design is an "inexact science" and that variations in performance should be expected. Current
variations in sewage treatment plant sediment removals (established practice for over 100 years)
range from 40 to 60%.
No Universally-Accepted Definition of Effectiveness A generally-accepted definition for the
"effectiveness" of BMPs has not yet been obtained. Without this common metric, there will be
no way to establish whether the BMP meets specified performance criteria, surpasses minimum
needs, or fails to suffice. The EPA definition and the engineering definition must be precisely
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Chapter 1: Introduction BMP Design Considerations
September 2002
worded to convey the fully intended meaning. While some users have an ecological inference
when discussing "effectiveness," the definition may need to be revised to have an engineering
foundation.
For example, "the fractional pollutant mass removed by the mature BMP in a climatically
average calendar year," according to one feasible and defensible definition, will not provide the
same measure of efficiency as "the fractional concentration reduction across the BMP in a 1-year
storm event," or "the fractional mass reduction by a BMP over its useful lifetime". None
considers the real issue that highly seasonal considerations linked to the life-cycle needs of the
indigenous fauna may exist. This becomes considerably messier when looking at non-chemical
stressors (e.g., flow, temperature). Also BMP effluent performance typically approaches a
limiting concentration regardless of influent concentration. Does this mean that they are less
efficient for lower influent concentrations than higher concentrations? In all cases long-term
pollutant mass loading removal must be the emphasis.
Further, the correct pollutants need to be managed. For example, concerns with
removing total phosphorus, may be misguided as it is the bioavailable phosphorus that is most
important to prevent water quality impacts. This suggests that the wrong stressor is being
managed.
Quantifying the efficiency of BMPs has often centered on examinations and comparisons
of "percent removal" defined in a variety of ways. BMPs do not typically function with a
uniform percent removal across a wide range of influent water quality concentrations. For
example, a BMP that demonstrates a large percent removal under heavily polluted influent
conditions may demonstrate poor percent removal where low influent concentrations exist. The
decreased efficiency of BMPs receiving low concentration influent has been demonstrated and it
has been shown that in some cases there is a minimum concentration achievable through
implementation of BMPs for many constituents (Schueler, 1996). Percent removal alone, even
where the results are statistically significant, often does not provide a useful assessment of BMP
performance.
High Degree of Uncertainty Associated with Load and Performance One of the major
criticisms by the National Research Council of the TMDL Model process is the large amount of
uncertainty associated with applying the models. In fact, many current models result in
inaccurate predictions so when the controls are applied they do not work. There are large
amounts of uncertainty in simple tasks such as flow measurement, leading to propagated error.
Uncertainty in measurement and modeling leads to probable errors linked with the associated
loads, the reductions within a BMP, or natural attenuation.
Inability to Link "Cause to Effect" - Post-BMP Complications It is difficult to link BMP
performance with in-stream response. There are many "causes" (pollutant sources) in the
watershed giving rise to receiving-water "effects", and these represent complex interactions.
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Chapter 1: Introduction BMP Design Considerations
September 2002
Receiving-water effects are further obscured by background (upstream) flow, ground-water
(baseflow) entry, direct air pollutant deposition, bottom benthos/sediment legacy and
resuspension, multi-stressor synergisms/antagonisms, pollutant fate and effect routing through to
ecological "food-network," flow energy biota impacts, thermal impacts, and the inability to
relate human disease risk to microbial indicators of pathogens
The in-receiving water mechanisms can be at least partially modeled, but a sufficient
understanding of some relevant issues is yet to be achieved. Recovery time is an obvious
example. Similarly, there is no mechanism to incorporate intermittent point sources with
variable loadings which BMPs represent, within the permitted loading to the receiving waters.
Inadequate Basis for Placement of BMPs in an Urban Watershed There is no current basis
for identifying the optimal location of BMPs in an urban watershed. All pollutant sources and
surface runoff needs to be accounted for. The overall watershed drainage routing must include
the interception and capture of the required amount of surface or pollution source-generated
runoff causing the receiving-water problem.
Inability to Determine Changes in BMP Effectiveness Over Time Proper
monitoring/evaluation techniques are hardly ever conducted. Proper monitoring requires
continuous (wet- and dry-weather) pollutant quality sampling of the influent and effluent points
synchronized with flowrate measurement of these points. This will allow determination of
pollutant mass loading reduction. Sampling devices must be capable of handling both heavy and
light particles at short time intervals in order to represent the pollutant loadigraph properly. The
flow meters must be capable of measuring highly variable flowrates going from very low liquid
levels to surcharge flow conditions.
This also involves administering proper long-term maintenance and monitoring for
determination of BMP unit process effectiveness.
Responsibilities for Implementing BMPs At some point in the last few decades, municipalities
shifted the burden of storm drainage to the developers and the future owners of the development.
There are many possible problems with this approach, i.e., limited understanding of downstream
flooding potential from multiple discharges, the lack of centralized treatment, and the tendency
to discharge runoff to the nearest receiving water without consideration of water quality impacts.
This "delegation" in managing stormwater drainage was promulgated by most
municipalities as a cost savings measure. However costs may actually be higher as monies are
needed to restore eroded stream channels. More and more municipalities are turning to
stormwater utilities or impervious area taxes to raise money to rectify damage to receiving
waters. The current problem may have been averted if municipalities had adopted a more
comprehensive shift in drainage control. Many municipalities had retained building codes for
regional control, i.e. curb and gutter and drainage pipes for centralized stormwater collection
1 -28
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Chapter 1: Introduction BMP Design Considerations
September 2002
system and discharge, when the municipalities were responsible for the drainage, but have in fact
shifted to a local control practice for drainage control, where curb and gutter approaches are
accentuating problems to receiving waters.
Discharges to small streams (e.g., non - navigable streams, headwaters) require multiple
levels of control to slow the flows coming off impervious areas - dry detention ponds for floods
is not enough. If regional controls are not used and discharges are continued to be made to the
nearest receiving water body then a series of treatment BMPs will most likely be required.
No BMP design will be adequate unless the receiving water body is capable of
assimilating the flow. In addition to pollution effects, high flows which cause stream channel
erosion must be considered. A receiving water channel that is being eroded is often more likely
to be a problem than "pollution" associated with runoff. Once the channel is stabilized by
reducing the flows, than the pollutant constituents to the stormwater can be addressed.
Discharging stormwater runoff to the nearest waterway is intended to keep water in the
channels. However, this approach is misguided as the flows from impervious areas have been
documented to be too flashy in nature and do not maintain baseflows. Base flows can only be
maintained by allowing for pervious areas and possibly increasing infiltration through infiltration
capable BMPs in developments with impervious areas.
1 -29
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
Chapter Two
Watershed Hydrology Pertinent to BMP Design
2.1 Introduction
Hydrology is the science dealing with the properties, distribution, and circulation of
water on the land surface (including surface waters), and subsurface (including groundwater). In
the context of watershed management, the focus is on the quantity of runoff produced by various
storms and how it moves (or is routed) through the watershed. Hydrology depends highly on
rainfall, topography, soil and drainage characteristics and will vary between regions of the
country and land uses.
The hydrologic concepts of interest with respect to the design of BMPs are closely
related to the design objectives of the BMP. Design of BMPs can be focused on flow control
(normally control of peak discharges), runoff volume control, pollutant removal for water quality
improvements, a host of ecological sustainability goals (e.g., groundwater recharge, stream
channel protection, prevention of thermal impacts) or a combination of two or more of these
objectives. Each objective has somewhat different hydrologic parameter requirements. The
hydrologic data which must be understood in order to design effective BMPs and evaluate water
quality impacts in urban watersheds include (1) the amount and distribution of rainfall intensity
and volume; and (2) the amount of rainfall contributing to runoff volume, i.e., rainfall minus
abstractions. These abstractions include interception, evapotranspiration, soil infiltration, and
depression or pocket storage.
2.2 Amount and Distribution of Rainfall Intensity and Volume
A rainfall frequency spectrum (RFS), defined as the distribution of all rainfall events in
an area, is a useful tool to place in perspective many of the relevant hydrologic parameters.
Represented in this distribution is the rainfall volume from all storm events ranging from the
smallest, most frequent events to the largest, most extreme events, such as the 100-year storm.
An RFS example is shown in Figure 2-1.
2- 1
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Chapter 2: Watershed Hydrology Pertinent to BMP Design
September 2002
BMP Design Considerations
The distribution and magnitude of the RFS varies across the country. Driscoll et al.
(1989) subdivided the U.S. into 15 distinct rainfall regions, as shown in Figure 2-2 and
summarized in Table 2-1.
8
"t/5" I
o»
~S f~~s V^3
»_ anc
Channel
erosson and
flood contra!
channel
a, erosion
£ pre
1 4
J§
c
2
0
Ground water
recharge and
water quality
^^^
^~-~~~^~~~~~"^
0.01 0.1
ent>on
x^
^
X"
/
X
/
/
flood >/
control ,/
/
,X
j^ 1
1
1
1 10 100
Rainfall Recurrence Interval (years)
Figure 2-1 Stormwater Control Points Along the RFS for Maryland (CRC, 1996)
Figure 2-2 Fifteen rain zones of the United States (Driscoll et al, 1989)
2-2
-------
Table 2-1 Typical Values of Individual Storm Event Statistics for 15 Zones of the United States (Driscoll, etal.
1989)
Rain Zone
Northeast
Northeast, coastal
Mid-Atlantic
Central
North Central
Southeast
East Gulf
East Texas
West Texas
Southwest
West, inland
Pacific Southwest
Northwest, inland
Pacific Central
Pacific Northwest
Annual
No. of Storms
Avg.
70
63
62
68
55
65
68
41
30
20
14
19
31
32
71
CV
0.13
0.12
0.13
0.14
0.16
0.15
0.17
0.22
0.27
0.30
0.38
0.36
0.23
0.25
0.15
Duration
(hr)
Avg.
11.2
11.7
10.1
9.2
9.5
8.7
6.4
8
7.4
7.8
9.40
11.6
10.4
13.7
15.9
CV
0.81
0.77
0.84
0.85
0.83
0.92
1.05
0.97
0.98
0.88
0.75
0.78
0.82
0.80
0.80
Intensity
(in./hr)
Avg.
0.067
0.071
0.092
0.097
0.087
0.122
0.178
0.137
0.121
0.079
0.055
0.054
0.057
0.048
0.035
CV
1.23
1.05
1.20
1.09
1.20
1.09
1.03
1.08
1.13
1.16
1.06
0.76
1.20
0.85
0.73
Volume
(in.)
Avg.
0.50
0.66
0.64
0.62
0.55
0.75
0.80
0.76
0.57
0.37
0.36
0.54
0.37
0.58
0.50
CV
0.95
1.03
1.01
1.00
1.01
1.10
1.19
1.18
1.07
0.88
0.87
0.98
0.93
1.05
1.09
Storm Separation
(hr)
Avg.
126
140
143
133
167
136
130
213
302
473
786
476
304
265
123
CV
0.94
0.87
0.97
0.99
1.17
1.03
1.25
1.28
1.53
1.46
1.54
2.09
1.43
2.00
1.50
Notes: CV = coefficient of variation of the logarithm of the observations (CV=S/M); S = standard deviation of the logarithms of the observations, (S=[
2-3
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
M = natural logarithm of the mean value of the EMC observations; x = natural logarithm of an individual EMC
observations; N = number of observations
2-4
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
In the absence of site-specific information, the RFS can be used to establish reasonable
design volumes for various BMPs. Runoff intensity and volume are the most important
hydrologic variables for water quality protection and design; they are related to capture and
treatment of the mass load of pollutants. Peak runoff rate is the most commonly used hydrologic
variable for drainage system and flooding analysis used in current design practices. Stormwater
BMPs designed to remove pollutants are built to treat a specified volume of runoff for the full
duration of a storm event, as opposed to accommodating only an instantaneous peak at the most
severe portion of a storm event.
A more accurate method to determine runoff specific to a particular watershed is to
measure it using rain gauges, flow meters, and other monitoring equipment. These data can then
be put into various rainfall/runoff models (there are a number of them) which can be used to
predict runoff levels. The models can also predict pollutant loadings, but need specific data on
the concentrations of contaminants in that region's runoff. The models run continuous
evaluations of rainfall/runoff relationships: rainfall is typically modeled on a daily basis and
runoff and loading predicted in response to the rainfall on a daily basis as well. Return period
information is determined by conducting many years of simulation, typically 25 to 100 years,
and doing a return period analysis on the predicted values.
2.3 Hydrologic Concepts for BMP Design
Most frequently recurrent rainfall events are small (less than 1 inch of daily rainfall). For
example, 90% of the annual rainfall comes in storms smaller than 0.9 in./day in Cincinnati, OH
(Roesner et a/., 2001). These often wash down the land surface, generating a relatively high
"first flush" concentration of pollutants. The capture and treatment of these small storms would
lead to improved water quality since the total pollutant load to receiving streams would be
minimized.
Current design, however, focuses on capturing large storms to minimize flooding. These
rainfall events typically range from 2 inches to 10 inches of daily rainfall and occur much less
frequently (every 2 years to 100 years). Although these storms may contain significant pollutant
loads (Chang et a/., 1990), their contribution to the annual average pollutant load is really quite
small due to the infrequency of their occurrence.
The computational procedures for large storm hydrology refer to procedures to estimate
or model runoff hydrographs from the larger storm events typically ranging from the 2-yr to the
100-yr storm. The procedures for conducting these analysis are well documented at both the
national and regional level.
At the national level a variety of models are available and well documented to simulate
the rainfall-runoff processes for watersheds and the design of BMPs. The selection of the
appropriate modeling technique will often depend on the level of detail and rigor required for the
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
application and the amount of data available for setup and testing of the model results. However,
in many instances the local regulatory agencies may specify which models are acceptable for
design and review purposes. For example in the state of Maryland, the state regulatory authority,
the Maryland Department of the Environment requires that BMP design be performed using the
NRCS TR-55 and TR-20 models. Table 2-2 summarizes a number of national and regional level
models that are frequently used for BMP large storm design.
A number of large storm models have also been developed by local and regional
government. Some of these models include:
• The Penn State Runoff Model (PSRM) which is used widely in Pennsylvania and
Virginia
• The Illinois Urban Area Simulator (ILLUDAS) which was developed by the
Illinois State Water Survey in concert with USEPA, based on the RRL (Roads
Research Laboratory) research and is widely used in Illinois and neighboring
mid-western states.
The Urban Drainage and Flood Control District (UDFCD) model developed by
the Denver Urban Drainage Flood Control District (UDFCD, 1999). This model is
used widely in Colorado and adjoining states.
• The Santa Barbara Urban Runoff Hydrograph developed for the City of Santa
Barbara, California. This model is widely used in California and other Pacific
coast states (Oregon and Washington).
2.4 Peak Discharge Control Strategies
Peak discharge control is the oldest and most widely used strategy for controlling the
drainage and flood impacts of urban runoff. The strategy is relatively straightforward and
consists of a general policy that post-development discharge rates cannot substantially exceed
existing or pre-development discharge rates. Post-construction runoff conditions (both total
volume and the peak discharge values) are usually much greater than pre-development
conditions. Therefore, the peak discharge approach generally requires that storage facilities be
provided to temporarily store the additional runoff volume, which is then discharged at the
allowable release rate, based on the "design storm".
2-6
-------
Table 2-2 Comparison of Model Attributes and Functions
ATTRIBUTE
Sponsoring Agency
Simulation Type
Water Quality Analysis
Rainfall/Runoff Analysis
Sewer System Flow Routing
Dynamic Flow Routing Equations
Regulators, Overflow Structures
Storage Analysis
Treatment Analysis
Data and Personnel Req.
Overall Model Complexity
MODEL
National
HSPF
USGS
Continuous
Yes
Yes
None
None
None
Yes
Yes
High
High
SWMM
USEPA
Continuous
Yes
Yes
Yes
Yes
Yes
Yes
Yes
High
High
TR-55/
TR-20
NRCS1
Single
Event
None
Yes
None
None
None
Yes
None
Medium
Low
HEC-
HMS
CORPS2
Single
Event
None
Yes
None
None
None
Yes
None
Medium
High
Rational
Method
Single
Event
None
Yes
None
None
None
Yes
None
Low
Low
Regional
PSRM
PSU3
Single
Event
Yes
Yes
Yes
Yes
None
Yes
Yes
Medium
Low
ILLUDAS
ISWS4
Single
Event
None
Yes
Yes
None
None
Yes
None
Medium
Low
UDFCD
UDFCD5
Single
Event
None
Yes
Yes
None
None
Yes
None
Low
Low
Santa
Barbara
Single
Event
None
Yes
None
None
None
Yes
None
Medium
Low
1 NRCS = National Resources Conservation Service
2 CORPS = US Army Corps of Engineers
3 PSU = The Pennsylvania State University
4 ISWS = Illinois State Water Survey
5 UDFCD = Urban Denver Flood Control District
6 USGS = US Geologic Survey
2-7
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Chapter 2: Watershed Hydrology Pertinent to BMP Design
September 2002
BMP Design Considerations
Design Storms The design storm is the particular storm event which generates runoff rates and
volumes which the BMP is designed to handle (the term design storm has other definitions as
when used for sewer design capacity and in various models). The peak discharge control
strategy is closely tied to the use of design storms. The selection of a specific design storm
generally incorporates a number of implicit assumptions relating to the stormwater runoff
impacts being controlled, and thus provides a good starting point for a scientific assessment
relating to actual versus perceived benefits of this strategy.
As Table 2-3 documents, a number of the assumptions implicit in the selection of a
design storm do not hold up under scientific scrutiny and have never been validated by field
monitoring. As the table indicates, the implicit assumption that peak discharge control of the 2-
year storm as a strategy for channel protection is not supported by geomorphic science or field
monitoring data. On the contrary, the geomorphic data predicts that the strategy is flawed, and
the prediction is being confirmed by limited field monitoring data.
Table 2-3 Design Storm Frequencies and Assumed Benefits
Design
Storm
!/2 - < 1
inch
rainfall
1-inch
rainfall
1-year
2 -year
10-year
100-year
Assumed Benefits
70-80 percent control of annual
runoff volume used for water quality
volume control
90 percent control of annual runoff
volume used for water quality
volume control
Water quality management and
stream channel protection
Used by most municipalities to
provide protection from accelerated
channel erosion and for habitat
protection
Used to provide flood protection
from intermediate storm events
Used for flood control protection
from major storms; also used to
maintain 100-year floodplain limits
Comments
Used by many municipalities on the
east coast
Replacing !/2 inch as basis for water
quality control (predominantly east
coast)
Used by some municipalities for water
quality management. Maryland is now
using for channel protection.
Geomorphic science does not support
this assumption. Very limited field
monitoring indicates that the strategy is
flawed.
Use of this storm frequency is mostly a
carryover from storm drainage design
practices. Flood control benefits are
very limited. In some cases increases
potential for downstream flooding due
to super-positioning of hydrograph
peaks. There is no geomorphic basis for
the use of this storm.
Flood control benefits are very limited.
In some cases increases the potential
for downstream flooding due to super-
positioning of hydrograph peaks..
References
MDE 2000
MDE 2000
Leopold, 1964;
McCuen et al.,
1987, MacRae,
1996; Jones,
1997; Maxted
and Shaver, 1997
Skupien, 2000 ,
Ferguson, 1998,
Debo & Reese,
1995
Skupien, 2000;
Ferguson, 1998;
Debo & Reese,
1995
2-8
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Chapter 2: Watershed Hydrology Pertinent to BMP Design
September 2002
BMP Design Considerations
Peak Discharge Strategies and Control of Physical Impacts Table 1-13 provided a summary
of the major impact categories (physical, habitat, biological, and chemical), the impact types or
metrics, and the impairment or change to the use of the receiving waters. With respect to the
physical impact category, the major areas of impairment are:
Increased flooding
Channel instability and erosion
• Reduction in groundwater recharge and related issues
• Increased sediment transport
• Thermal impacts
Table 2-4 provides a qualitative assessment of the benefits provided by peak discharge control
strategies with respect to the physical impacts category.
Table 2-4 Qualitative Assessment of Peak Discharge Control Strategies with
Respect to the Physical Impact Category
Physical Impact
Category
Increased flooding
Channel instability
and erosion
Reduction in
groundwater recharge
and related issues
Increased sediment
transport
Thermal impacts
Control Strategy
Peak discharge control of
10- and 100-year storms
Peak discharge control of
2-year storm
Not addressed by peak
discharge control
Peak discharge of 2-year
storm
Not addressed by peak
discharge control
Assessment
Peak discharge control of 10- and 100-
year storms Peak discharge strategy
provides limited downstream control.
In some cases, it aggravates
downstream flooding condition.
Requires coordinated permitting at
watershed scale. (Skupien, 2000;
Ferguson, 1998; Debo & Reese, 1995)
Both geomorphic science and limited
field monitoring indicate that this
strategy does not work. (McCuen et al.,
1987; MacRae, 1996)
N/A
Both geomorphic science and limited
field monitoring indicate that this
strategy does not work. (McCuen et al.,
1987; MacRae, 1996)
N/A
2-9
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Chapter 2: Watershed Hydrology Pertinent to BMP Design
September 2002
BMP Design Considerations
Control of Increased Flooding The ability of land use changes, and in particular land
development activities, to increase runoff quantity and cause downstream flooding and erosion
has been recognized for many decades. This has led many states, counties and municipalities,
and other agencies to require onsite detention of increased project area runoff with peak site
outflows set equal to the pre-developed conditions. This requirement has become popular, since
it can be applied during development design and reviewed on a case-by-case basis without large-
scale watershed analysis. This popularity has led to the frequent use of onsite detention and
retention basins, which have become standard features on many land development projects.
However, the limitations of peak discharge control strategies documented by Leopold
and Maddock (Leopold, 1954) have been largely ignored. At the exact spot where a detention
basin discharges through its outlet, it reduces the peak rate of storm flow. We know this
conclusively from the laws of physics and applied hydraulics. While there is no argument on this
point, farther downstream, a basin's effect on peak rate depends partly on how its discharge
combines with the flow from other tributaries. In practice, on any given site, detention should be
applied with caution and should be based on an appropriate watershed-wide and downstream
analysis.
Combined flow
Figure 2-3 A Watershed Where the Drainage From a Small Development Site
Joins the Flow From Large Watershed (Ferguson, 1998)
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Chapter 2: Watershed Hydrology Pertinent to BMP Design
September 2002
BMP Design Considerations
cr
o
£
"3
Combined flow
without detention
Combined flow
with
Flow from
vwalersh
Figure 2-4
Time, hours
Alternative Hydrographs From the Watershed Shown in Figure 2-3
(Ferguson, 1998)
Ferguson (1998) has provided a good example of this condition as illustrated in Figures
2-3 and 2-4. Figure 2-3 shows a small development site discharging into the main stem of a
larger watershed. As shown in Figure 2-4, the storm hydrograph from the development site is
short and fast compared with that from the main watershed. Because the development site's
flow drains out before the main watershed's peak arrives, it does not contribute to the magnitude
of a flood downstream. But if detention is added to the developed site, outflow will be delayed,
so that it overlaps onto the peak flow in the main stream and contributes to a new, higher
combined peak flow.
One can imagine two detention basins on different sites in the same watershed,
constructed by different developers at about the same time. When hydrographs from the two
basins combine downstream, their delayed flows combine in a way that has never existed before
development, and a larger flood may be created. In spite of this knowledge, numerous local
governments are requiring every developer to reduce the peak rate during a design storm to its
pre-development level. The effect of this approach has been a random proliferation of small
detention basins over urbanizing watersheds, none of which is designed with regard to its
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
specific location in the drainage network. The potential conflict between a basin and its
watershed, first identified by Leopold and Maddock (1953) has been confirmed by a number of
more recent studies. Independent modeling studies throughout the U.S., including the studies
listed below have all confirmed that randomly sited basins have failed to provide downstream
flood and channel protection:
• McCuen (1979) for a Maryland watershed;
• Ferguson (1991, 1995), Hess and Inman(1994) for watersheds in Colorado,
Georgia and Virginia;
Debo and Reese (1992) for watersheds in North and South Carolina; and
• Skupien (2000) for a watershed in New Jersey
Enough studies have been conducted and reported that the following generalizations can
be drawn from them:
Some watershed-wide systems of detention basins help, in the sense that they
keep downstream peak discharges during a given storm lower than it would be
without them.
• Other individual basins do the opposite of help; they actually increase
downstream peak discharges as a result of the overlapping of their detained
volumes with mainstream peaks.
• No watershed-wide system of uniform basins works to the extent for which they
were designed. If they were designed to reduce peak discharges during a given
storm to pre-development levels, then their aggregate effect, although it may
result in a reduction in peak discharge, is usually not a reduction to the designed
degree, because of the accumulation of runoff volumes downstream.
Detention basins can reduce flood peaks only when they are selectively located in their
watersheds as explained by Leopold and Maddock (1953). Selective planning of publicly
financed reservoirs led to the effective flood control for the Miami River in Ohio, when the
Miami Conservancy District (Morgan, 1951) identified specific flood hazards in Dayton and
other cities, then located a combination of multiple-purpose reservoirs, levees, and channels to
work in concert to reduce flood damage at those points.
Downstream Analysis The issue of downstream analysis is often not addressed by local
stormwater management ordinances. Debo and Reese (1992) conducted studies for the City and
County of Greenville, South Carolina and Raleigh, North Carolina to demonstrate how such a
policy could be developed. This study used a hydrologic-hydraulic computer model to analyze
the downstream effects of storm runoff from developments of different size, shape, physical
characteristics, and location within larger drainage basins. The study also examined different
size flood events and different types of downstream drainage systems. The results of this study
revealed that the effects of the development process stabilizes at the point where the proposed
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
development represents 5 to 10 percent of the total drainage area, depending on the size of the
development and the amount of increased impervious area. This analysis was used as the basis
for the formulation of the following policy concerning downstream impacts (Debo and Reese,
1992):
"In determining downstream effects from stormwater management structures and the
development, hydrologic-hydraulic engineering studies shall extend downstream to a
point where the proposed development represents less than ten (10) percent of the total
watershed draining to that point. "
Channel Instability, Bank Erosion, and Sediment Transport A related issue associated with
the peak discharge control strategy is the well-documented problem of increases in the frequency
and duration of stormwater discharges. Peak discharge control strategies using detention ponds
do not eliminate runoff, they simply delay it. The volume discharging from a detention basin is
the same as the inflow. When the post development volumes from different tributaries join
downstream, there is nothing to prevent them from combining to produce inadvertently high
peak rates. In the fortunate cases in which flood peaks are consistently reduced, the receiving
streams may still erode and become unstable, because in accommodating the increased volume
of runoff, relatively high erosive flows still pass through for longer periods (McCuen, et al,
1987). As demonstrated by McCuen, et al. (1987), the practice of detaining the extra volume of
stormwater runoff and discharging it at pre-construction peak discharge rates until the extra
volume is fully dissipated has the result of creating more in-stream erosion than if no stormwater
control were present. This occurs when the selected design storm focuses predominately on
downstream flood control and not on in-stream erosion (channel protection) and the protection of
aquatic habitat and biology.
Reduction in Groundwater Recharge and Related Issues Peak discharge control strategies
are often referred to as end-of-pipe control strategies, because they typically make use of small
BMP ponds placed at the low topographic point on development sites. This approach does not
usually address groundwater recharge and related issues, such as lowering of groundwater levels
and reduction or loss of base flows in small streams. There are two exceptions to this general
case. One is where infiltration ponds are used as the BMP. The other exception to this condition
consists of recent initiatives in the state of Florida, where stormwater management ponds are
being used as sources of gray water for lawn watering. This initiative is in part a response to the
alarming lowering of water tables in many areas of Florida.
Thermal Impacts A negative consequence of the peak discharge control strategy and the
associated use of pond BMPs is the associated increase in thermal warming of runoff waters.
The problem is particularly acute in regions of the country that support cold-water habitat,
particularly trout and salmon fisheries.
Summary of Peak Discharge Strategies Peak discharge strategies represent an approach to
control or mitigation of impacts from urban runoff. This level of control is currently being
2- 13
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
provided by many states and municipalities under the NPDES stormwater regulatory approach.
It provides two performance criteria that are closely related: (1) flood control and (2) peak
discharge control. Some practitioners have concluded that on a water shed-wide scale, uniform
detention strategies are a failure because they do not maintain base flows, do not necessarily
provide water quality improvement, and in some case fail to fulfill their single explicit purpose
of controlling floods (Ferguson, 1998).
A recent technology assessment for the major impact categories as summarized in Table
1-13 concluded that approaches based solely on peak discharge control are not adequate to
address the range of impacts associated with urban runoff issues (Collins et al., 2001). The
following is a summary of findings:
• While this approach does provide some degree of flood control in the upstream
areas it can in some instances actually transfer or aggravate flooding conditions
downstream.
This approach not only fails to provide protection for stream channel stability, but
may actually aggravate and accelerate stream channel degradation and impacts.
The approach does not address groundwater recharge issues including lowering of
water tables and maintenance of stream base flows.
• The approach does not address, but can actually aggravate, thermal impacts on
receiving waters.
• This approach does not address or guarantee water quality management and
pollutant removal, although both can be achieved if the BMPs are properly
designed.
This approach does not provide control for the degradation and loss of riparian
habitat.
• This approach does not provide control for the degradation and loss of biological
communities.
Peak discharge control strategies, in and of themselves, appear unable to meet the objectives of
the Clean Water Act and other legislation. Their effectiveness is limited primarily to some flood
control in the upstream areas.
2.5 Water Quality Control Strategies
Water quality control of urban runoff is still a relatively new and developing technology.
The addition of water quality considerations in the design of BMPs has introduced a new
dimension to the traditional hydrologic considerations for BMP design. Prior to the introduction
of water quality considerations hydrologic design methods were focused on flood event
hydrology with focus on storms typically ranging from the 2-yr (bankfull); to the 10-year (storm
drainage conveyance storm) to the 100-yr (floodplain storm). Water quality considerations
created a shift from flood events to a continuous long-term rainfall-runoff BMP design volume
2- 14
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
approach and the pollutant loads associated with these volumes. This new focus has given rise
to concepts such as the rainfall frequency spectrum (Figure 2-1) and small storm hydrology.
Need to Address Small Storms Early efforts in stormwater management focused on flood
events ranging from the 2-year to the 100-year design storm. Increasingly stormwater
professionals have come to realize that small storms (e.g., < 1 inch rainfall) dominate watershed
hydrologic parameters typically associated with water quality management issues and BMP
design.
Large storms occur infrequently and are of primary concern for overbank flows and
flooding of structures located in the floodplains of stream channels and in urban areas. Most
rainfall events are much smaller than design or large storms used for urban drainage models. In
any given area, most frequently recurrent rainfall events are small (less than 1 inch of daily
rainfall). For example, 90% of the annual rainfall comes in storms smaller that 0.9 in./day in
Cincinnati, OH (Roesner et al., 2001). For small rains, impervious areas contributed most of the
runoff flows and pollutants (Pitt, 1987). The capture and treatment of these small storms would
lead to improved water quality since the total pollutant load to receiving streams would be
minimized. These small storms are responsible for most annual urban runoff and groundwater
recharge.
Urban runoff models play an important role in evaluations for stormwater BMPs.
Unfortunately, many commonly used models incorrectly estimate runoff flows and the washoff
of particles from impervious surfaces during small rains. Typical washoff prediction procedures
used in urban runoff models greatly over-predict particulate residue washoff from impervious
surfaces, especially for large particles (Pitt, 1987).
Current design, however, typically still focuses on capturing large storms to minimize
flooding and control drainage. These rainfall events typically range from 2 to 10 inches of daily
rainfall and occur over a much longer return period range from 2 to 100-years. Although these
storms may contain significant pollutant loads (Chang et al., 1990), their contribution to the
annual average pollutant load is really quite small due to the infrequency of their occurrence. In
addition, longer periods of recovery are available to receiving waters between larger storm
events. These periods allow systems to flush themselves and allow the aquatic environment to
recover.
Storms with a return frequency of six months to 2 years are the dominant storms that
determine the size and shape of the receiving streams. These storms will remain critical for
design of BMPs to protect stream channels from accelerated erosion and degradation. However,
the use of small storm (those with a return frequency under six months) approaches should
dominate design of BMPs for pollutant removals.
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Chapter 2: Watershed Hydrology Pertinent to BMP Design BMP Design Considerations
September 2002
Small Storm Hydrology Two different approaches to small storm hydrology computations are
pertinent. The first approach is based on the probabilistic approach developed by the US EPA
(1986) in the publication, "Methodology for Analysis of Detention Basins for Control of Urban
Runoff Quality". This approach is well suited to the design of water quality control BMPs for
larger drainage areas. The following are related to this computational procedure:
Long-term rainfall characteristics
• Capture of stormwater runoff
• An approach for estimating stormwater quality capture volume
• Example of a water quality capture volume estimate
A second approach is based largely on the work of Pitt (1994) that is tailored for very
small urban sites and is closely linked to the presence of impervious surfaces. This approach has
been adopted by the State of Maryland (MDE, 2000) and may provide a simpler computational
tool that is better suited for use by Phase II communities. The following are related to this
approach:
• Small site hydrology approach
• The 90% rule regarding cumulative rainfall volume for water quality treatment
Short-cut method for estimating the water quality volume for BMP design using
small storms.
Estimating peak discharges for the water quality storm.
To treat the bulk of the pollutant loads from stormwater runoff, many states and
municipalities specify a treatment volume that is designed to capture the initial component of the
stormwater runoff. In practice this is achieved by specifying a rainfall amount (such as the first
%-inch, 1-inch, or other rainfall depth over impervious areas) or the capture of a stormwater
runoff volume that correlates to a design storm (such as the 6-month, 1-year, or 2-year frequency
storm).
Design Storm vs Continuous Flow Simulation Design storms, primarily derived from IDF
(e.g., intensity, duration, frequency) or NRCS type curves, have been the primary tools used to
predict runoff rates. These are used with a wide variety of single storm models including
HEC-HMS (US Army Corps of Engineers, 2001), SWMM (US EPA, 1971), Sedimot II (Wilson
et al., 1982) and Sedimot II (Barfield, et al., 1996). The assumption made in the single storm
models is that the return period of the peak discharge is the same as the return period of the
design rainfall event and that watershed parameters are invariant with return period rainfall.
Studies have shown that constant watershed parameters are not a good assumption. For
example, Haan and Edwards (1988) evaluated predictions of peak discharge on six watersheds in
Ohio, Nebraska, Arizona, and Oklahoma using the NRCS curve number approach. For each
storm on the watershed, they calculated the parameter »S, the maximum potential abstraction from
rainfall, for each storm event. Their results showed that:
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September 2002
The value of S, varied widely for each storm event on each watershed due to
changing soil moisture and vegetative characteristics.
• When considering the joint variability of both S and rainfall, the return period
discharge was always greater than that predicted assuming a constant S and
varying rainfall. This is because the probability distributions for both
precipitation and S are skewed.
• In general, considering the variability of S improved predictions for the rare
events but increased the error for the lower return period events (probability less
than 80 percent).
One advantage of using continuous simulation models is that they could capture some of
the variability occurring with input parameters. Another advantage is that they could, assuming
accurate algorithms and input data, give a good representation of lower frequency, less than one
year, events.
The problem of matching single storm predictions based on rainfall with return period
flow rates is easy to evaluate when considering runoff. The problem is amplified when
considering pollutants such as sediment, toxics, nutrients, and pathogens. The standard
assumption is that the pollutant loading in runoff from a design storm, such as the NRCS Type
storms, will match observed return period pollutant loadings.
An advantage of using continuous simulation models with pollutant loadings, and
particularity with BMPs, is that the inter-arrival time between storms can have a significant
impact on trapping performance of the BMP. Driscoll et al. (1986) addressed this issue in a
model of sedimentation in reservoirs and developed procedures for estimating performance
under these conditions. This model has potential to be used to estimate dynamic and quiescent
condition settling in reservoirs used as BMPs. The WEPP model (Lane et al., 1989) also
contains a reservoir model known as WEPPSIE (Lindley et al., 1998a and b).
The advantages of using a continuous simulation model must be weighed against the
problems with such an approach. Specifically, these include:
Greatly increased data set requirement for the models. The models must not only
predict hydrologic and water quality responses, they must also predict changes in
vegetative cover resulting from annual growth and death cycles. In addition, the
models must have good climatic simulators to simulate rainfall and other climatic
variables. Since algorithms within models are only as good as their inputs,
assuring that the models have good predictors of watershed and climatic variables
is critical.
• Greatly increased complexity in setting up and executing the models, thus
increasing the knowledge base requirement of the user. The validity of a model
prediction is as much dependent on the skill of the user as it is on the reliability of
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September 2002
the model algorithms. If the complexity of the model is such that an advanced
degree in hydrology and water quality is required for its proper execution, the
average user is not likely to generate good BMP designs from its use. Likewise,
reviewers are not as likely to be competent in interpreting permit applications.
Therefore, the modeling technique must be selected with the skill of the average
user, both in the design community as well as in the regulatory organizations.
The continuous simulation models are most appropriate for larger regional watersheds
and may be necessary for predicting the effect of discharges from many BMPS on a watershed
scale; therefore, their development should be encouraged. Continuous simulation models are
better at predicting accumulation and washoff of pollutants during the inter-arrival time between
storms, and can thus have a significant impact on the trapping performance of the BMP.
Continuous simulation is needed for watershed based approaches to solve habitat and water
quality issues in urban streams (Strecker, 2002). Continuous simulation offers possibilities for
design and management that do not currently exist. However, the widespread adoption in
design and permit review depends in part on the models becoming sufficiently user friendly and
the input guidelines developed that the user community can execute the models with confidence
and competence.
Created Use of Extended Detention The extended detention concept was introduced to
overcome the limitations of early detention pond strategies and provide more and better control
of the smaller and more frequent storm events. Basically, extended detention refers to designing
the outlet so that the smaller storms which pass through ponds are now detained for longer
periods than they would otherwise be held. Thus, trapping of those particles could be enhanced.
The extended detention approach can be designed to provide extended detention of 6 to 48 hours
which provides longer holding times, increased removal for lower settling velocity particles and
thus higher pollutant removal performance.
BMPs that encompass both peak discharge hydrology and small storm hydrology would
optimally use a system that incorporates on-site treatment and storage of stormwater for the
smaller storms while protecting downstream areas from floods. Regardless of the specific
method used for modeling the peak discharge design volume, the ultimate pond design will
typically be greater than necessary for the water quantity volume alone. However, the outlet
control structures have typically been designed more for the flood control volume than the
discharge of the more frequent storms. Redesigning the discharge from the outlet control
structures may be the most critical design aspect to prevent future deleterious downstream water
quality effects and this may be the most cost effective measure to retrofit in existing detention
ponds and improve water quality improvement performance of BMPs on a watershed basis.
Roesner et al. (2001) believe the problem with peak discharge BMPs is not the BMPs
themselves but the design guidance for BMP outlet flow control which does not take into
account the geomorphologic character of the receiving stream. The uncontrolled section of the
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September 2002
flow frequency curve causes stream reaches downstream from BMPs to continue to exhibit
habitat degradation and reduced biological indices. Because the recommended design storm for
sizing most BMPs are small, it is often possible to retrofit existing regional flood control
detention basins with small, low-level outlets thus providing extended detention basins for
treatment of these small storms (Roesner etal. 2001). Newman etal. (2000) found that
optimized designs (based on SWMM) of extended detention ponds provided superior pollutant
removals compared to the original designs. The optimized designs used smaller outlet orifices to
maximize detention times of the smaller storms.
In addition to extended detention, the limitations of peak discharge strategies can be
supplemented with volume control techniques using control measures that include vegetated
swales, infiltration trenches, and bioretention cells in a treatment train approach to achieve the
goals of legal mandates. Retrofitting dry basins with permanent wet pools has been proposed.
By including these supplemental measures using either distributed and/or centralized controls,
the peak discharge control strategies can be upgraded to address water quality control.
Recognizing the Dominant Role of Sedimentation and Filtration Mechanisms promoting the
removal of stormwater pollutants involve physical, chemical, and biological processes. Owing
to the intermittent nature of stormwater inflow, physical processes associated with detention for
sedimentation and filtration (either through vegetated systems or through an infiltration medium)
are the principle mechanisms by which stormwater contaminants are first intercepted.
Subsequent chemical and biological processes can influence the transformation of these
contaminants. Wong et al. (2001) state that various stormwater treatment components by which
the contaminants are first intercepted and detained can be described using a unified model.
Grass swales, wetlands, ponds, and infiltration systems are considered to be a single continuum
of treatment based around flow attenuation and detention, and particle sedimentation and
filtration. Hydraulic loading, vegetation density and aerial coverage, hydraulic efficiency and
the characteristics of the target pollutants (e.g. particle size distribution and contaminant
speciation) largely influence their performance. In this context, the infiltration systems are
simply vertical filtration systems compared to the horizontal filtration systems of grass swales
and wetlands, reliant on enhanced sedimentation and surface adhesion (promoted by biofilm
growth) for removal of fine particles.
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September 2002
The validity of this unified conceptual approach to simulating the operation of
stormwater treatment measures is demonstrated by empirical analysis of observed water quality
(primarily TSS) improvements in swales, wetlands, ponds and infiltration basins and also by
fitting observed water quality data from these treatment systems to a unified stormwater model
(USTM) developed by Wong et al. The USTM provides a mechanism by which the urban
catchment and waterway managers can predict and assess the performance of stormwater
treatment measures.
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Chapter 3: BMP Types and Selection BMP Design Considerations
September 2002
Chapter Three
Types of BMPs and Factors Affecting their Selection
3.1 Introduction
This chapter provides a brief review and summary of the major BMP types and the
factors that govern the selection of the appropriate BMP for a specific site. The most important
criterion governing selection for water quality improvement is the effectiveness of the BMP to
remove pollutants. Guidance is also provided on the other important criteria, including
stormwater management goals, on-site vs regional considerations, watershed and terrain factors,
physical suitability factors, community and environmental factors, and location and permitting
factors.
3.2 Types of BMPs
BMPs for control of urban runoff can be generally grouped into two major categories that
include: 1) source control BMPs, and 2) treatment BMPs (ASCE, 1998). Source control BMPs
are practices that prevent pollution by reducing potential pollutants at their sources before they
come into contact with stormwater, while treatment BMPs are methods to treat or remove
pollutants from stormwater. Many treatment BMPs are considered to be structural in that they
involve some sort of earthen or concrete structure. Table 3-1 provides a list of typical source
control BMPs and Table 3-2 is a list of treatment BMPs.
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Chapter 3: BMP Types and Selection
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BMP Design Considerations
Table 3-1 ASCE Source Control BMPs (ASCE 1998)
Major Categories
A. Public Education
B. Planning and
Management
C. Materials Handling
D. Street / Storm Drain
Maintenance
E. Spill Prevention &
Cleanup
F. Illegal Dumping
Controls
G. Illicit Connection
Control
H. Stormwater Reuse
Source Control
Practice
Al- Public Education and Outreach
Bl- Better Site Planning
B2 - Vegetative Controls
B3 - Reduce Impervious Areas
Cl - Alternative Product Substitution
Dl - Street Cleaning
D2 - Catch Basin Cleaning
D3 - Storm Drain Flushing
D4 - Road & Bridge Maintenance
El - Above Ground Tank Spill Control
Fl - Storm Drain Stenciling
F2 -Household Hazwaste Collection
Gl - Illicit Connection Prevention
G2 - Illicit Connection - Detection & Removal
HI - Landscape watering
B4- Disconnect Impervious Areas
B5 - Green roofs
C2 - Housekeeping Practices
D5 - BMP maintenance
D6 - Storm Channel & creek
Maintenance
E2 - Vehicle spill Control
F3 - Used Oil recycling
F4 - Illegal Dumping Controls
G3 - Leaking Sanitary Sewer
Control
H2 - Toilet Flushing
Table 3-2 Treatment BMPs (adapted from ASCE 1998).
Major Categories
Ponds
Vegetative Biofilters
Constructed Wetlands
Filters
Technology Options and
Others
Treatment BMPs
Wet (Retention) Pond Infiltration Pond
Dry Detention / Extended Detention
Basin
Grass Swales (Wet, Dry)
Filter Strip / Buffer
Bioretention Cells
Constructed Wetlands
Sand Filter
Perimeter Filter
Inlet Filters
Multi-Chambered Treatment
Media Filter
Underground Filter
CDS
Train Chemical Treatment
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Chapter 3: BMP Types and Selection BMP Design Considerations
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As managers seek to reduce pollutant loadings in their watersheds to meet total maximum
daily load (TMDL) reduction requirements, a combination of BMPs will likely apply.
Depending on the stormwater management goals and objectives identified for a specific site or
area, a combination of source controls, as well as the use or one or more treatment BMPs may
need to be used to meet the design objectives, in what is often referred to as a treatment train
approach. The distinction between source controls and treatment controls is very clear in some
cases, but less so in others. Street sweeping for pollutant removal is one BMP that could be
considered either source control or treatment control. The use of vegetation to treat disconnected
impervious surfaces such as rooftops, driveways, parking lots and streets is another example of a
BMP that could be considered source control or a treatment BMP. Some of the newer concepts
for urban runoff management such as the better site planning techniques (CWP, 1998) and low
impact development (LID) technology (EPA, 2000a,b) focus on the use of planning techniques
and micro scale integrated landscape based practices to prevent or reduce the impacts of urban
runoff at the very point where this impacts would be generated. These approaches tend to have
very close overlap between preventive source control approaches and small scale treatment
approaches that blur the distinction between these two types of BMPs.
This document is focused primarily at selected treatment BMPs. Two major groups are
presented, ponds and vegetative biofilters, with an aim to give design criteria that will improve
their pollutant removal capacity and therefore improve water quality.
Historically stormwater management technology has focused more on the treatment
BMPs, particularly pond BMPs. However the current trend in BMP technology, spurred by our
growing awareness of the range and complexity of issues associated with our overall goals of
maintaining the ecological integrity of our receiving waters, as mandated by the CWA, is
towards the use of integrated stormwater management approaches that include one or more
source controls, as well as one or more treatment (i.e., treatment train) controls.
3.3 BMP Selection Criterion - Meeting Stormwater Management Goals
Different regions of the United States (and localities within these regions) have differing
needs and issues that lead them to adopt stormwater management programs. This document does
not attempt to define what an appropriate level of stormwater management is for any given area,
or what design goals and objectives should be used. Rather it recognizes that different goals
exist and provides guidance on how to address and select the BMPs that are appropriate for a
given design objective. A series of tables, Tables 3-3 thru 3-6, summarize the available
qualitative or quantitative data concerning the chemical, physical and biological impacts
associated with each major BMP group (Clar, et al., 2001). The percentage removals are
presented without hydraulic parameters or pollution loadings and therefore cannot be used in
comparison or as guidelines for expected removals.
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
Table 3-3 Summary of Studies on Environmental Impacts for Pond and
Wetland BMPs
BMP
Chemical Impacts
Physical Impacts
Habitat and
Biological Impacts
Wet
(Retention)
Pond
Over 33 studies reporting on the
effectiveness of wet ponds at
reducing / removing TSS, TP, TN,
OP, NO3, Metals, Bacteria (ASCE,
1999* ;CWP, 2000)
Dry / Extended
Detention
Pond
Over 24 studies reporting on the
effectiveness of dry / extended
detention basins at reducing /
removing TSS, TP, TN, OP, NO3,
Metals, Bacteria (ASCE, 1999*;
CWP, 2000)
Implementation of BMPs
has been largely
ineffective in controlling
the physical impacts on
the stream channel
resulting from
urbanization. Ponds
usually do not provide
groundwater recharge.
Ponds can provide peak
discharge control, but
sometimes increase
downstream flooding.
Structural storm water
practices have either
little or no ability to
mitigate the adverse
impacts of urban storm
water runoff on the
macro invertebrate
community. Ponds
pose a risk to cold
water systems because
of their potential for
stream warming.
Pond-Wetland
System /
Extended
Detention
Wetland /
Shallow Marsh
Over 15 studies reporting on the
effectiveness of pond/wetland
system / extended detention
wetland and shallow marsh at
reducing/removing TSS, TP, TN,
NO3, Metals, Bacteria (ASCE,
1999*; CWP, 2000)
Submerged
Gravel
Wetland
1 study reporting on the
effectiveness of pond/wetland
system at reducing / removing
TSS, TP, TN, NO3, Metals,
Bacteria (ASCE, 1999*; CWP,
2000)
Wetlands can be designed
for flood control by
providing flood storage
above the level of
permanent pool, but are
subject to the same
limitations as ponds.
Wetlands are ineffective
at protecting channels.
Wetlands usually do not
provide groundwater
recharge.
Wetlands pose a risk to
cold water systems
because of their
potential for stream
warming.
*www .bmpdatabase.org
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
Table 3-4 Summary of Studies on Environmental Impacts for Vegetative Biofilter BMPs
BMP
Chemical Impacts
Physical Impacts
Habitat &
Biological
Bioretention
Davis (1997) reported on the effectiveness of
bioretention at removing TP (81%), TN (43%),
NH4 (79%), Metals (93-99%).
Yu (1999) reported the following performance
parameters; TSS (86%), TP (90%), COD (97%),
Oil & Grease (67%)
Bioretention practices
are being designed to
provide water quality,
flood control, channel
protection, and ground
water recharge (Clar,
2000). There is
emerging evidence mat
bioretention can help
make post development
runoff equivalent to pre-
development runoff.
Field data
information
not available
Grassed
Swales
3 studies have reported on the effectiveness of
grassed channels at removing TSS, TP, TN,
NO3, Metals, and Bacteria
4 studies have reported on the effectiveness of
dry swales at removing TSS, TP, TN, NO3, and
Metals
2 studies have reported on the effectiveness of
wet swales at removing TSS, TP, TN, NO3, and
Metals
7 studies have reported on the effectiveness of
drainage channels at removing TSS, TP, TN,
NO3, and Metals (ASCE, 1999; CWP, 2000)
Grassed swales can be
used to reduce peak
discharges for small
storm events, and
provide groundwater
recharge (MDE, 2000).
Field data
information
not available
Grassed
Filter Strips
1 study has reported on the effectiveness of 75 ft
and 150 ft grassed filters strips at removing TSS
(54%, 84%), Nitrate, Nitrite (-27%, 20%), TP
(-25%, 40%), Lead (-16%, 50%), and Zinc
(47%, 55%)
(ASCE, 1999; CWP, 2000)
Grassed filter strips do
not have the capacity to
detain large storm
events, but can be
designed with a bypass
system that routes these
flows around the toe of
the slope. Grassed filter
strips can provide a
small amount of
groundwater recharge.
Field data
information
not available
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Chapter 3: BMP Types and Selection
September 2002
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Table 3-5 Summary of Studies on Environmental Impacts for Infiltration BMPs
BMP*
Chemical Impacts
Physical Impacts
Habitat &
Biological
Infiltration
Basin
Very little information available,
one study reported that infiltration
basin sized to treat runoff form
1-inch storm is effective at
removing TSS (75%), P (60 to
70%), N (55 to 60%), Metals (85 to
90%), Bacteria (90%) (Schueler
1987; ASCE, 1999; CWP, 2000)
Full infiltration basins will
control post-development peak
discharge rates at or below
pre-development levels (given
that the basin has sufficient
infiltration capacity). Basins are
effective at recharging
groundwater. Infiltration basins
effectively reduce the increase in
post-development runoff volume
produced from small and
moderate sized storms.
No
information
available
Infiltration
Trench
Infiltration trench sized to treat
runoff form 1-inch storm is
effective at removing TSS (75%), P
(60 to 70%), N (55 to 60%), Metals
(85 to 90%), and Bacteria (90%)
(Schueler; 1987; ASCE, 1999;
CWP. 2000)
Effective at recharging
groundwater
No
information
available
Pervious
and
Modular
Pavement
A study in Prince William VA
(Schueler, 1987) recorded pollutant
removal effectiveness for TSS
(82%), TP (65%), TN (80%)
A study in Rockville, MD
(Schueler, 1987) recorded pollutant
removal effectiveness for TSS
(95%), TP (65%), TN (85%), COD
(82%), Metals (98 to 99%) (ASCE,
1999; CWP, 2000)
Effective at recharging
groundwater (approximately 70
to 80% annual rainfall) (Gburek
and Urban 1980)
No
information
available
* Under certain circumstances (e.g. near-surface water tables) there may be concerns about groundwater
pollution.
Table 3-6 Summary of Studies on Environmental Impacts for Filter BMPs
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Chapter 3: BMP Types and Selection
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BMP Design Considerations
BMP
Sand Filters
Peat/Sand
Filters
Compost Filter
System
Multi-
chambered
Treatment
Train
Perimeter Sand
Filter
Surface Sand
Filter
Vertical Sand
Filter
Chemical Impacts
1 study reporting on the effectiveness of sand
filters at removing TSS (87%), TN (44%), NO,
(-13%), Metals (34-80%), Bacteria (55%)
1 study reporting on the effectiveness of peat
sand filters at removing TSS (66%), TN (47%),
N03 (22%), Metals (26-75%)
2 studies reporting on the effectiveness of
compost filter systems at removing TSS,
Nitrate, and Metals
3 studies reporting on the effectiveness of
multi-chamber treatment trains at removing
TSS, and Metals
3 studies reporting on the effectiveness of
perimeter sand filter at removing TSS, TP, TN
NO3, and Metals
6 studies reporting on the effectiveness of
surface sand filter at removing TSS, TP, TN
NO3, Bacteria, and Metals
2 studies reporting on the effectiveness of
vertical sand filter at removing TSS, TP, TN
NO3, and Metals
Physical Impacts
Some groundwater
recharge is possible
with the exciter design,
however, other sand
filter designs cannot
provide recharge.
These systems are not
expected to have
significant role in
preventing channel
degradation or
providing peak
discharge control.
Habitat & Biological
No field data
information available.
Some systems may help
prevent thermal
impacts.
These systems are not
expected to have
significant role in
preventing habitat and
biological impairment
resulting from channel
degradation.
3.4 BMP Selection Criterion - On-Site vs Regional Controls
The decision of whether to use an on-site or a regional approach can have a strong
influence on the selection of the BMP type. Some treatment BMPs , such as ponds and
wetlands, can be used either as stand-alone, on-site treatment controls, or as part of regional
controls for stormwater management. Others, including swales, filters strips, infiltration and
percolation, media filters, oil and water separators, are designed only for on-site use. Within the
on-site use group there is a new subset of emerging practices referred to as micro-scale multi-
functional management practices, known as LID, that are intended to be integrated into a site's
landscape. Many of the onsite practices such as the swale, and filter strips fall within this group,
as well as some new biofilters practices such as the bioretention cell.
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Chapter 3: BMP Types and Selection BMP Design Considerations
September 2002
On-Site Controls Three schools of thought have emerged in stormwater management
technology, each of which reflects one of the three application identified above. The most
widespread approach being used nationwide is the use of on-site controls where structural
treatment practices on individual sites are designed to provide peak discharge control. While
this approach has many flaws, it is often selected because of the ease of application and
implementation. For many jurisdictions, the use of on-site controls is perceived to be the only
practical institutional and political alternative. Every site that meets the minimum area
requirements is required to provide on-site controls. Concerns expressed by public works
practitioners include (ASCE, 1998):
• Because large numbers of on-site controls, sometimes exceeding several hundred
or even thousand, can eventually be installed within an urban watershed, it
becomes difficult to reliably quantify their cumulative effects on receiving waters.
• Large numbers of on-site controls complicate the quality assurance during design
and construction because they are typically designed by a variety of individuals
and are constructed by a variety of different contractors under varying degrees of
quality control.
Onsite BMPs may be maintained and operated in a variety of ways impossible to
anticipate or control.
• Unless these on-site controls are coordinated at a watershed scale, which
typically, they are not, these controls not only fail to provide downstream
protection for peak discharge, but in many instances will accelerate the rate of
channel degradation.
Regional Controls The second school of thought on stormwater management takes the position
that using regional controls serving 80 to 600 ac offers a more rational approach over the use of
on-site controls (ASCE, 1998). The proponents of the regional approach site the following
advantages:
• Regional controls eliminate the uncertainty of large numbers of on-site controls
• Regional controls can use multistage outlets to "throttle " and release small runoff
events in 12 to 24 hours and empty the total water quality capture volume in 24 to
48 hours.
Regional controls are perceived to be more cost effective because fewer controls
are less expensive to build and maintain than a large number of on-site controls
(Wiegand, etal., 1986).
By serving larger drainage areas the outlet works are larger and easier to design,
build, operate, and maintain.
• Regional controls are generally under the control of a public agency and therefore
more likely to receive ongoing maintenance.
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Chapter 3: BMP Types and Selection BMP Design Considerations
September 2002
• Regional controls can provide treatment for existing and new developments and
typically will capture all runoff from public streets, which is often not addressed
by on-site controls.
Because regional controls cover large land areas, this allows other compatible
uses such as recreation, wildlife habitat, or aesthetic open space to occur within
their boundaries.
The regional approach to stormwater management is currently being successfully utilized by a
number of metropolitan areas such as the Denver Metropolitan area. Some other areas of the
U.S. such as Prince George's County (PGC) in Maryland, however, experimented with regional
controls and found them to be unacceptable. PGC was requested by the permitting agencies to
conduct a cumulative impact assessment of its regional facilities program, as a condition for
continued issuance of permits. During the course of the cumulative impact assessment, PGC
identified so many fatal flaws associated with its regional facilities program that it decided to
abandon the regional approach and identify viable alternatives. Some of the fatal flaws
associated with the regional approach identified by PGC included:
The regional controls, which are typically a peak discharge control strategy, failed
to provide downstream protection of stream channels
The regional facilities typically failed to provide significant flood relief for
downstream properties, and where such relief was provided, the downstream
control were very limited. (PGC ultimately adopted a floodplain management
program that includes early flood warning, flood insurance, flood proofing, and
the purchase and removal of flood prone structures)
• Maryland receives over 40 inches of annual rainfall. Regional facilities, did not
solve the targeted problems but also introduced additional environmental
problems that are identified below.
Regional facilities created fish passage blockages that were unacceptable to the
permitting agencies
• Regional facilities tend to be located in perennial streams and their construction
tends to create wetland impacts which were unacceptable to the permitting
agencies.
• The regional facilities resulted in increased stream temperatures which were
unacceptable in cold fisheries streams.
By relying on the regional facilities the feeder stream to these facilities were left
unprotected, and often experienced severe accelerated erosion that delivered large
volumes of sediment to the regional facilities, which greatly accelerated the
maintenance program.
• Disposal of pond and lake sediments in urban settings became extremely
expensive.
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Chapter 3: BMP Types and Selection BMP Design Considerations
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Other problems with the implementation of regional approaches have been identified (ASCE,
1998):
The regional facility approach requires advanced planning and up-front financing
Lack of financing early in the watershed's land development process, before
sufficient developer contributions are available, can preclude their timely
installation.
Low Impact Development (LID) Technology The third school of thought relating to
stormwater management technology, unlike the two approaches above that have been in use for
over thirty years, is still in its early development and largely unknown to most local jurisdictions.
This approach which is more commonly known as LID technology, was pioneered by Prince
George's County, Maryland, after having applied both on-site and regional approaches. The
proponents of this approach cite the following benefits of the LID approach (P.G. Co., 1997;
EPA 2000 a,b; Coffman, etal, 1998; Clar 2000):
Use of these techniques helps to reduce off-site runoff and ensure adequate
groundwater recharge.
• Since every aspect of site development affects the hydrologic response of the site,
LID control techniques focus mainly on site hydrology. Hydrologic functions
such as infiltration, frequency and volume of discharges, and groundwater
recharge can be maintained with the use of reduced impervious surfaces,
functional grading, open channel sections, disconnection of hydrologic flowpaths,
and the use of bioretention/filtration landscape areas.
LID also incorporates multi-functional site design elements into the stormwater
management plan. Such alternative stormwater management practices as on-lot
micro-storage, functional landscaping, open drainage swales, reduced
imperviousness, flatter grades, increased runoff travel time, and depression
storage can be integrated into a multi functional site design.
• Specific LID controls called Integrated Management Practices (EVIPs) can reduce
runoff by integrating stormwater controls throughout the site in many small,
discrete units.
IMPs are distributed in a small portion of each lot, near the source of impacts,
virtually eliminating the need for a centralized BMP facility such as a stormwater
management pond.
• LID designs can also significantly reduce development costs through smart site
design by:
• • Reducing impervious surfaces (roadways), curb, and gutters
• • Decreasing the use of storm drain piping, inlet structures, and
• • Eliminating or decreasing the size of large stormwater ponds.
In some instances, greater lot yield can be obtained using LID techniques,
increasing returns to developers (Clar, 2000)
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Chapter 3: BMP Types and Selection BMP Design Considerations
September 2002
• Reducing site development infrastructure can also reduce associated project
bonding and maintenance costs
LID techniques such as bioretention cells can be used as a water quality control
technique for infill development (Clar, 2000)
LID techniques can also be used as a water quality retrofit for existing urban areas
(Clar, 2000)
• The LID approach can be used as a volume control method to provide
downstream peak discharge protection for major storm events (Clar, 2000).
• The LID approach can be used as an improved approach to protect water supply
reservoirs, as demonstrated in the High Point, NC case study (Tetra Tech, 2000;
Clar, 2001).
The LID approach can be used to address total impervious area (TIA) limitations
(Clar, 2000).
Some practitioners have found LIDs site oriented micro-scale control approach to be
controversial, as it sometimes conflicts with building codes, challenges conventional stormwater
management paradigms and is perceived by some to accommodate urban sprawl. A recent
critique of the LID approach questioned the use of the term "low impact", and also critiqued the
adequacy of the hydrological design procedures utilized to substantiate the effectiveness of the
techniques (Strecker, 2001).
Integration of Approaches Clearly the discussion above reveals that there is no clear
consensus on which school of thought is the right approach. It appears that perhaps no single
approach is adequate for all cases, and that the one size fits all approach is not the way to
proceed. The appropriate approach for a semi-arid mountain region such as Colorado or Utah,
may be considerably different from the approach selected in a humid climates such as are found
in the Mid Atlantic or Pacific Northwest. In addition, within a specific state or region, the
appropriate approach for an existing degraded urban area may be considerably different from the
approach selected to protect a high quality rural area. Ultimately each region or municipality
will need to identify its watershed and water resources protection goals and objectives and select
the approach or combination of approaches that are appropriate to meet these goals.
3.5 BMP Selection Criterion - Watershed Factors
The design of urban BMPs can be strongly influenced by the nature of the downstream
water body that will be receiving the stormwater discharge. Consequently, designers should
determine the Use Designation of the watershed in which their project is located prior to design.
In some cases, higher pollutant removal or environmental performance may be needed to fully
protect aquatic resources and/or human health and safety within a particular watershed or
receiving water. Therefore, a shorter list of BMPs may need to be considered for selection
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BMP Design Considerations
within these watersheds or zones. The areas of concern are summarized in Table 3-7 and
include: cold-water streams, sensitive streams, aquifer protection, reservoir protection,
shellfishing, and recreational contact.
Table 3-7 Treatment BMPs vs Watershed Factors (modified from MDE, 2000)
BMPs
Ponds and
Wetlands
Infiltration
Vegetative
Biofilters
Filters (Sand,
Perimeter,
Underground)
Watershed Factors
Cold Water
Restricted due to
thermal impacts
Offline design
recommended
Yes, if site has
suitable soils
OK
OK for small
volumes
Sensitive
Stream
May be limited
or require
additional
volume for
channel
erosion
impacts
Yes, if site has
suitable soils
OK, if channel
protection
volume is met
OK for WQ,
no channel
protection
Aquifer
Protection
May require liner
if A soils are
present and water
table high
Pretreat hot spots
Requires safe
distance from
wells & water
table
Pretreat hot spots
OK
OK for WQ, no
recharge
Reservoir
Protection
May be limited
due to channel
erosion
May require
additional
volume control
Requires safe
distance from
bedrock &
water table
OK
OKforWQ
Recreational
Contact
May require use
of permanent
pools to increase
bacteria removal
Yes, but needs
safe distance to
water table
OK, but wet
swale has poor
bacteria removal
OK, moderate to
high bacteria
removal
Coldwater Streams Cold and cool water streams have habitat qualities capable of supporting
trout and other sensitive aquatic organisms. Therefore, the design objective for these streams is
to maintain habitat quality by preventing stream warming, maintaining natural recharge,
preventing bank and channel erosion, and preserving the natural riparian corridor. Techniques
for accomplishing these objectives may include:
• Minimizing the creation of impervious surfaces,
• Minimizing surface areas of permanent pools,
• Preserving existing forested areas
Bypassing existing baseflow and/or springflow, or
Providing shade-producing landscaping
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Chapter 3: BMP Types and Selection BMP Design Considerations
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Some BMPs, especially those with permanent pools with large surface areas can have adverse
downstream impacts on cold water streams and their use is highly restricted.
Sensitive Streams Sensitive streams are defined as streams with a watershed impervious cover
of less than 15%. These streams may also possess high quality cool water or warm water aquatic
resources. The design objectives are to maintain habitat quality through the same techniques
used for cold water streams, with the exception that stream warming is not as severe of a design
constraint. These streams may also be specially designated by local authorities.
Aquifer Protection Areas that recharge existing public water supply wells present a unique
management challenge. The key design constraint is to prevent possible groundwater
contamination by preventing infiltration of hotspot runoff. At the same time, recharge of
unpolluted stormwater is needed to maintain flow in streams and wells during dry weather.
These issues are particularly important in areas with Karst geology.
Reservoir Protection Watersheds that deliver surface runoff to a public water supply reservoir
or impoundment are of special concern. Depending on the treatment available at the water
intake, it may be necessary to achieve a greater level of pollutant removal for the pollutants of
concern such as bacteria pathogens, nutrients, sediment or metals. One particular management
concern for reservoirs is ensuring that stormwater hot spots are adequately treated so that they do
not contaminate drinking water.
Shellfish/Beach Protection Watersheds that drain to specific shellfish harvesting areas or
public swimming beaches require a higher level of BMP treatment to prevent closings caused by
bacterial contamination from stormwater runoff. In these watersheds, BMPs are explicitly
designed to maximize bacteria removal.
Other Criteria Designers should consult with the appropriate review authority to determine if
their development project is subject to additional stormwater BMP criteria as a result of an
adopted local watershed plan or protection zone. Table 3-7 provides a summary assessment of
the suitability of the treatment practices with respect to the watershed factors discussed above.
3.6 BMP Selection Criterion - Terrain Factors
Three key terrain factors to consider are low-relief, karst and mountainous terrain.
Special geotechnical testing requirements may be needed in karst areas. Table 3-8 summarizes
the key issues that need to be considered for each BMP type with respect to the three terrain
factors.
Table 3-8 BMP Selection - Terrain Factors (Modified from MDE, 2000)
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September 2002
BMP Design Considerations
BMPs
Ponds
Wetlands
Infiltration
Vegetative
Biofilter
Filter
Terrain Factor
Low Relief
May be limited by depth
to water table
OK
Minimum distance to
water table of 2 feet
OK
Some designs limited by
head requirements
Karst
Geotechnical testing reqd
May require liner
Ponding depth may be limited
May be prohibited by local
authority
OK
Require liner
Mountainous
Embankment heights
restricted
Max slope 15%
Swales may be limited
by steep slopes
OK
The type of structure used can be impacted by terrain factors. For example, in very flat areas, it
is difficult to construct a basin with a dam as would be possible in steeper sloped watersheds. In
the case of the flatter areas, it may be necessary to construct the basin by excavation. Also, the
type of outlet can be controlled by the terrain with drop inlets being useful in steeper slopes but
weir and open channel outlets favored for flat terrain.
3.7 BMP Selection Criterion - Physical Suitability Factors
The watershed and terrain factors should enable the BMP designer to narrow the BMP
list to a manageable number and proceed to the consideration of the physical suitability factors
that characterize the physical conditions at a site. Table 3-9 cross-references testing protocols
needed to confirm physical conditions at the site. The six primary physical suitability factors
include: soils, water table, drainage area, slope, head, and urban conditions.
Soils The key evaluation factors are based on an initial investigation of the USD A (1986)
hydrologic soils groups (HSGs) at the site. The HSG is defined by 4 groups: A - sand, loamy
sand, or sandy loam; B - silt loam or loam; C - sandy clay loam; and, D - clay loam, silty clay
loam, sandy clay, silty clay or clay. More detailed geotechnical tests are usually required for
infiltration feasibility and during design to confirm permeability and other factors.
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BMP Design Considerations
Table 3-9 BMP Selection - Physical Suitability Factors (modified from MDE,
2000)
BMP
Ponds
-Wet
-Dry
Wetlands
Infiltration
- Trench
- Basin
Biofilters
- Bioretention
- Swales
- Filter strip
Filters
-Sand
- Perimeter
- Underground
Soils
"A" soils may
require liner
"B" soils may
require testing
"A" soils may
require liner
0.52 in./hr
minimum
Uses made soil
OK
Water
Table
• 4 ft2 if
Hotspot or
Aquifer
• 4 ft2 if
Hotspot or
Aquifer
• 4 ft (• 2 ft
for flatter
areas)
•2ft
•2ft.
Drainage
Area (acre)
25 minimum3 for
wet pond
25 minimum3 for
wet pond
5 maximum
10 maximum
2 maximum
5 maximum
N/A
10 maximum
2 maximum
2 maximum
Slope
None
None
15%
maximum
None
•4%
• iO%
None
Head
(ft)
6-8
3-5
•i
•3
•5
•4
None
•5
2-3
5-7
Urban
Not practical;
Requires too
much area to
be functional
Yes
Not practical
OK
Not practical
Not practical
OK
Notes: OK = not restricted
1 = Should be based on the erosion resistance of soils; some circumstance may require structural reinforcement.
2 = Four feet separation distance to the seasonally high water table elevation
3 = Unless adequate water balance and anti-clogging device installed
Water Table This column indicates the minimum depth to the seasonally high water table from
the bottom or floor of a BMP.
Drainage Area This column indicates the recommended minimum or maximum drainage area
that is considered suitable for the practice. If the drainage area present at a site is slightly greater
than the maximum allowable drainage area for a practice, some leeway is permitted or more than
one practice can be installed. The minimum drainage areas indicated for ponds and wetlands to
maintain a permanent pool are flexible depending on water availability (baseflow or
groundwater).
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Slope Restriction This column evaluates the effect of slope on the practice. Specifically, the
slope restrictions refer to how flat the area where the practice may be.
Head This column provides an estimate of the elevation difference needed at a site (from the
inflow to the outflow) to allow for gravity operation within the practice.
Urban Sites This column identifies BMPs that work well in the downtown urban environment,
where space is limited and original soils have been disturbed. These BMPs are frequently used
at redevelopment sites.
3.8 BMP Selection Criterion - Community and Environmental Factors
Another group of factors that should be considered by the BMP designer includes the
community and environmental factors. This group of factors includes the following four factors:
ease of maintenance, community acceptance, construction costs, and habitat quality. Table 3-10
employs a comparative index approach indicating whether the BMP has a high or low benefit.
Maintenance Requirements This column assesses the relative maintenance effort needed for a
BMP in terms of three criteria: frequency of scheduled maintenance, chronic maintenance
problems (such as clogging) and reported failure rates. All BMPs require routine inspection and
maintenance.
Community Acceptance This column assesses community acceptance as measured by three
factors: market and preference surveys, reported nuisance problems, and visual aesthetics. A
low rank can often be improved by a better landscaping plan.
Construction Cost The BMPs are ranked according to their relative construction cost per
impervious acre treated as determined from cost surveys and local experience.
Habitat Quality BMPs are evaluated on their ability to provide wildlife or wetland habitat,
assuming that an effort is made to landscape them appropriately. Objective criteria include size,
water features, wetland features and vegetative cover of the BMP and its buffer.
Other Factors This column indicates other considerations in BMP selection.
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BMP Design Considerations
Table 3-10 BMP Selection - Community and Environmental Factors (modified
from MDE, 2000)
BMP
Ponds
-Dry
-Wet
Wetlands
Infiltration
- Trench
- Basin
Biofilters
Filters
Maintenance
Requirements
Easy
Medium
Medium
High
Medium
Varies
High
Community
Acceptance
Medium
High
Medium
High
Low
High
High
Cost
Low
High
Medium
Medium
Medium
Medium
High
Habitat
Quality*
Low
High
High
Low
Low
Medium
Low
Other Factors
Trash and debris can
be a problem
Limited depth
Avoid large stone
Frequent pooling
Landscaping
Out of sight
Traffic bearing
Filter media
* Habitat quality refers to ability to provide habitat quality in the BMP facility
3.9 BMP Selection Criterion - Location and Permitting Factors
The checklist in Table 3-11 provides a condensed summary of current BMP restrictions
as they relate to common site features that may be regulated under local, State or federal law.
These restrictions fall into one of three general categories:
Locating a BMP within an area that is expressly prohibited by law.
• Locating a BMP within an area that is strongly discouraged and is only allowed
on
a case by case basis. Local, State and/or federal permits shall be obtained and the
applicant will need to supply additional documentation to justify locating the
BMP within the regulated area.
• BMPs must be setback a fixed distance from the site feature.
This checklist is only intended as a general guide to location and permitting requirements as they
relate to siting stormwater BMPs. Consultation with the appropriate regulatory agency is the
best strategy.
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BMP Design Considerations
Table 3-11 Permitting Checklist (Modified from MDE, 2000)
Feature
Jurisdictional
Wetland
Stream
Channels
100 Year
Floodplains
Stream Buffer
Forest
Conservation
Critical Areas
Utilities
Roads
Structures
Septic Drain
Fields
Water Wells
Sinkholes
Location and Permitting Guidance
• Wetlands should be delineated prior to siting stormwater BMPs
• Use of wetlands for stormwater treatment strictly discouraged and requires federal permit
• BMPs require 25 ft setback from wetlands
• Buffers can be used as nonstructural filter strip
• Stormwater must be treated prior to discharge into a wetland
• Stream channels should be delineated prior to design
• Instream ponds may require review and permit.
• Instream ponds may be restricted or prohibited in cold water streams
• May need to implement measures that reduce downstream warming.
• Grading and fill for BMP construction is strongly discouraged within the ultimate 100 year
floodplain, as delineated by FEMA flood insurance rate, FEMA flood boundary and
floodway, or local floodplain maps.
• Floodplain fill cannot raise floodplain water surface elevation more than a tenth of a foot.
• Consult local authority for stormwater policy.
• BMPs are strongly discouraged in the stream-side zone (within 25 feet of streambank).
• Consider how outfall channel will cross buffer to reach stream.
• BMPs can be located within the outer portion of a buffer.
• Check with local regulatory agency for applicable forest conservation requirements
• BMPs are strongly discouraged within Priority 1 Forest Retention Areas.
• BMPs must be setback at least 25 feet from the critical root zone of specimen trees
• Designers should consider the effect of more frequent inundation on existing forest stands.
• BMP buffers are acceptable as reforestation sites if protected by conservation agreement
• Check with local regulatory agency for applicable critical areas requirements
• BMPs w/in the Critical Area shoreline buffer may be prohibited unless a variance is
obtained from the local review authority.
• BMPs are acceptable within mapped buffer exemption areas.
• Note the location of proposed utilities to serve development.
• BMPs are discouraged within utility easements or rights of way (public or private).
• Consult local DOT or DPW for any setback requirement from local roads.
• Obtain approval for any discharges to local or State-owned conveyance channel.
• Consult local review authority for BMP setbacks from structures.
• Consult local health authority.
• Recommended setback is a minimum of 50 feet from drain field edge.
• 100 foot setback for stormwater infiltration.
• 50 foot setback for all other BMPs.
• Water appropriation permit needed if well water used for water supply to a BMP.
• Infiltration or pooling of stormwater near sinkholes is prohibited.
• Geotechnical testing may be required within karst areas
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3.10 Federal Regulations That Impact Stormwater BMP Design
The design of stormwater management BMPs is mandated and regulated by regulatory
requirements at several levels including; federal, state, regional and/or local. This section
provides a brief review of the regulatory requirements that drive the design of these BMPs.
At the federal level, the requirements of the following agencies are summarized:
• U.S. Environmental Protection Agency (EPA)
• The National Oceanographic and Atmospheric Administration (NOAA) of the
Department of Commerce
The U.S. Fish and Wildlife Service (USFWS)
In addition a recent compilation of the stormwater management requirements of state, regional
and local government agencies is summarized.
Clean Water Act Originally, this act was entitled the Federal Water Pollution Control Act of
1948 (FWPCA) which prescribed a regulatory system consisting mainly of State-developed
ambient water quality standards applicable to interstate or navigable waters. In 1972, FWPCA
amendments established a system of standards, permits and enforcement aimed at "goals" of
"fishable and swimmable waters by 1983" and "total elimination of pollutant discharges into
navigable waters by 1985." (33 U.S.C. § 1251 (a) (2)). Further amendments were passed in
1977, when the Act was officially denominated The Clean Water Act (CWA). Today, the CWA
is the nation's primary mechanism for protecting and improving water quality. The broad
purpose of the Act is "to restore and maintain the chemical, physical, and biological integrity of
the Nation's waters," (33 U.S.C. § 1251 (a)), and its emphasis is to declare unlawful the
unregulated discharge of pollutants into all waters of the United States.
The strength of the CWA lies in its comprehensive, nationwide approach to water quality
protection which requires Federal, State, and local governments to act cooperatively for the
achievement of common goals. The Act makes the States and the EPA jointly responsible for
identifying and regulating both point and nonpoint sources (NPS) of pollution. Point sources are
controlled by a permit-based system, while nonpoint sources are approached with a management
strategy. The Act's framework thus allows for both environmental quality (water quality
standards) and technology-based (treatment processes and Best Management Practices)
approaches to water pollution control. Each State is required to develop and adopt water quality
standards which enumerate the designated uses of each water body as well as specific criteria
deemed necessary to protect or achieve those designated uses. The CWA requires States to
develop and implement water quality standards in accordance with EPA regulations and
guidance.
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Under current EPA regulations, water quality management planning is focused on
priority water quality issues and geographic areas. This process requires the development of
Total Maximum Daily Loads (TMDLs), which set the amount of pollution that may be
discharged while still complying with water quality standards (WQS). These allocations are
implemented through the issuance of permits for point sources and the use of BMPs for nonpoint
sources (NFS). In addition, State water quality programs are required to integrate three
components (1) a designation of uses for all State waters, (2) criteria to meet those uses, and (3)
an antidegradation policy for waters that meet or exceed criteria for existing uses (40 CFR §
131.10- 131.12). State water quality management plans are also required to identify priority
point and nonpoint problems, consider alternative solutions, and recommend control measures.
In order to comply with the CW A, State water quality standards must, theoretically, include
indicators of the health of ecological habitats and the level of biological diversity, and ambient
water quality standards were to be supplemented by discharge standards in the form of effluent
limitations applicable to all point sources.
The Act also specifically provides that State water quality criteria must include both
numeric standards for quantifiable chemical properties and "narrative criteria or criteria based
upon biomonitoring." (33 U.S.C. §1313(c)(2)(a)). As defined in the Act, the term "biological
monitoring" means: the determination of the effects on aquatic life, including accumulation of
pollutants in tissue, in receiving waters due to the discharge of pollutants by techniques and
procedures, including sampling of organisms representative of appropriate levels of the food
chain appropriate to the volume and the physical, chemical, and biological characteristics of the
effluent, and at appropriate frequencies and locations (33 U.S.C. § 1362).
CWA amendments, EPA regulations, and State water quality programs addressing point
and nonpoint sources have continued to evolve over the years as increased knowledge is
accumulated on the impacts of urban development. Stormwater runoff from increased
impervious surfaces in urban areas has emerged as a significant threat to water quality. Several
sections of the CWA apply to urban runoff, both as a point and nonpoint source of pollution, as
well as impacts of any activities which may result in the disturbance of natural wetlands,
regulated by section 404 of the Act. The following paragraphs describe these sections, with
emphasis on their relevance to Stormwater runoff and land development activities, both during
the construction phase and the post construction phase..
Section 304(m) of the CWA, added by the Water Quality Act of 1987, requires EPA to
establish schedules for (i) reviewing and revising existing effluent limitations guidelines and
standards and (ii) promulgating new effluent guidelines. On January 2, 1990, EPA published an
Effluent Guidelines Plan (55 FR 80), in which schedules were established for developing new
and revised effluent guidelines for several industry categories. Natural Resources Defense
Council, Inc., challenged the Effluent Guidelines Plan in a suit filed in the U.S. District Court of
the District of Columbia (NRDC et al. v. Browner, Civ. No. 89-2980). The Court entered a
consent decree (the "304(m) Decree"), which established schedules for, among other things,
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EPA's proposal and promulgation of effluent guidelines for a number of point source categories.
The Effluent Guidelines Plan was published in the Federal Register on September 4, 1998 (63
FR 47285).
The most recent update to this plan occurred in April 1999, when EPA announced that it
was preparing to develop effluent limitations guidelines for construction and development. EPA
is proposing regulatory options to address storm water discharges from construction sites under
the authority of Sections 301, 304, 306, 307, 308, and 501 of the Clean Water Act (CWA) (the
Federal Water Pollution Control Act), 33 United States Code (U.S.C.) 1311, 1314, 1316, 1317,
1318, and 1361. The public may submit comments on the proposal through October 22, 2002.
Effluent limitations guidelines may be finalized sometime thereafter; refer to
http://www.epa.gov/OST/guide/construction for updates.
NPDES Phase I and Phase II Stormwater Rules The National Pollutant Discharge
Elimination System (NPDES) is a permit system established under the CWA to enforce effluent
limitations. Operators of construction activities, including clearing, grading and excavation are
required to apply for permit coverage under the NPDES Phase I and II storm water rules. Under
the Phase I rule (promulgated in 1990), construction sites of 5 or more acres must be covered by
either a general or an individual permit. General permits covering the Phase I sites have been
issued by EPA regional offices and state water quality agencies. Permittees are required to
develop storm water pollution prevention plans that include descriptions of BMPs employed,
although actual BMP selection and design are at the discretion of permittees (in conformance
with applicable state or local requirements). There exists considerable variability throughout the
states and localities with respect to these requirements which are summarized below.
Construction sites between 1 and 5 acres in size are subject to the NPDES Phase II storm
water rule (promulgated in 1999). The construction activities covered under Phase II are termed
small construction activities and exclude routine maintenance that is performed to maintain the
original line and grade, hydraulic capacity, or original purpose of the facility. Under the Phase II
program, NPDES permit requirements for construction activities are similar to the Phase I
requirements because they are covered under similar general permits.
Water Quality Certifications (Section 401) The purpose of section 401 of the CWA is to
ensure that federally permitted activities comply with the Act, State water quality laws and any
other appropriate State laws. This is accomplished through a State certification process. Any
applicant for a Federal permit for any activity that could result in a discharge of a pollutant to a
State's waters is required to obtain a certification from the State in which the activity is to occur
(EPA, Region 2, 1993). In essence, the State certifies that the materials or pollutants discharged
comply with the effluent limitation, water quality standards, and any other applicable conditions
of State law. Examples of Federal permits and licenses requiring State certification include:
NPDES permits, section 404 permits, permits for activities regulated by the Rivers and Harbors
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Act, and hydroelectric discharge-related activities (Doppelt, et al., 1993). If the State denies the
certification, the Federal permitting agency must deny the permit application. If the State
imposes conditions on a certification, the conditions become part of the Federal permit (EPA,
Region 2, 1993). A certification obtained for construction activities must also pertain to the
subsequent operation of the structure (EPA, Region 2, 1993).
Certification processes differ from state to state, with some states participating early
enough in a project's development to have an impact on determining alternatives and mitigation
processes (Doppelt, etal., 1993). Typically, the process begins when the State receives the
permit information from the Federal agency receiving the request from the applicant. The State
regulatory agency designated with certification authority notifies the Federal permitting
authority of its decisions concerning certification for the proposed activity. States must act to
grant or deny certification within a reasonable time (not to exceed one year) after a request is
received, or certification authority will be deemed to have been waived (Doppelt, etal., 1993).
Pollution Prevention Act of 1990 The Pollution Prevention Act of 1990 (PPA) (42 U.S.C.
13101 et seq., P. L. 101-508, November 5, 1990) "declares it to be national policy of the United
States that pollution should be prevented or reduced whenever feasible; pollution that cannot be
prevented should be recycled in an environmentally safe manner whenever feasible; pollution
that cannot be recycled should be treated in an environmentally safe manner whenever feasible;
and disposal or release into the environment should be employed only as a last resort and should
be conducted in an environmentally safe manner" (Section 6602; 42 U.S.C. 13101 (b)). In short,
preventing pollution before it is created is preferable to trying to manage, treat, or dispose of it
after it is created. The Pollution Prevention Act directs EPA to, among other things, "review
regulations of the Agency prior and subsequent to their proposal to determine their effect on
source reduction" (Section 6604; 42 U.S.C. 13103(b)(2)).
This regulation has not yet been widely or systematically applied to stormwater
management technology. Situations where it has been applied include the use of source control
measures to reduce or prevent the generation of pollutants. A recent innovation in stormwater
management technology, the low impact development (LID)approach does address this
regulation through its stated goal of mimicking the pre-development hydrology of sites in order
to preclude or reduce the environmental impacts traditionally associated with these hydrologic
changes and the use of end-of-pipe approaches for stormwater management control.
Coastal Zone Management Act (CZMA) The Coastal Zone Management Act of 1972
(CZMA) was passed by Congress in order to "preserve, protect, develop, and where possible, to
restore or enhance, the resources of the Nation's coastal zone for this and succeeding
generations." (16 U.S.C. §1452) The Act established a program to encourage States and
territories to develop comprehensive programs to protect and manage coastal resources,
including the Great Lakes (Terrene Institute, 1994). Much of the Act is geared toward managing
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and steering development of coastal energy resources. To encourage States to develop coastal
zone management programs, Congress incorporated several major incentives in the CZMA. For
example, the Act provides Federal grants to States for the development and administration of
coastal management programs. The Act also provides a mechanism by which a State can
allocate some of its funds to a local government or interstate agency, thus encouraging the
coordination of coastal management on a regional level.
The CZMA is overseen by the Secretary of Commerce, acting through the National
Oceanic and Atmospheric Administration (NOAA). However, the Act focuses on the States as
being key players in the management of coastal zone areas. The legislation emphasized the State
leadership in the program, and allowed States to participate in the Federal program by submitting
their own coastal zone management proposals to the Office of Coastal Zone Management
(OCZM) at NOAA for approval. To receive Federal approval and implementation funding,
States and territories had to demonstrate programs and enforceable policies sufficiently
comprehensive and specific to regulate land and water uses and coastal development, and to
resolve conflicts between competing uses (Terrene Institute, 1994). Once the OCZM has
approved a State program., Federal agency activities within a coastal zone must be consistent "to
the maximum extent practicable." with the program.
Areas subjected to CZMA planning include wetlands, floodplains, estuaries, beaches,
dunes, barrier islands, coral reefs, and fish and wildlife and their habitat. Management plans
developed by States must include an inventory and designation of coastal resources, designate
those of national significance and establish standards to protect those so designated. The State
plans should also include a process for assessing and controlling shoreline erosion, and a
description of the organizational structure proposed to implement the program with specific
references to the inter-relationships and responsibilities between various jurisdictions. States are
also encouraged to prepare special area management plans addressing such issues as natural
resources, coastal dependent economic growth, and protection of life and property in hazardous
areas. These resource management and protection plans are accomplished through State laws,
regulations, permits, and local plans and zoning ordinances. Section 307(c) of the CZMA
requires any applicant seeking a Federal permit to furnish a certification that the proposed
activity will comply with the State's coastal zone management program. No Federal permit will
be issued until the State has concurred with the applicant's certification of consistency (U.S.
Environmental Protection Agency, Region 2, 1993).
The Coastal Zone Act Reauthorization Amendments of 1990 (CZARA) These specifically
charged State coastal programs and State nonpoint source programs to address nonpoint source
pollution issues affecting coastal water quality. Under CZARA, coastal States must develop
appropriate management programs in order to continue to receive funding and participate in the
CZMA. EPA has developed technical guidance to help States develop the CZARA mandated
control programs. The guidance specifies management measures for sources of nonpoint
pollution in coastal waters, including coastal stormwater control. Management measures are
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Chapter 3: BMP Types and Selection BMP Design Considerations
September 2002
defined as "economically achievable measures to control the addition of pollutant to coastal
waters; that is, they reflect the greatest degree of pollutant reduction available through the
application of the best available nonpoint pollution control practices, technologies, processes,
site criteria, operating methods or other alternatives" (Terrene Institute, 1994). Coastal
stormwater control programs are not intended to supplant existing coastal zone management
programs or nonpoint source management programs (Camp, Dresser, and McKee, 1993a).
Rather they serve to update and expand existing programs and are to be coordinated closely with
other nonpoint source management plans (U.S. Environmental Protection Agency, 1991).
Many States have an approved coastal zone management plan which may apply to
activities in specific local regions, jurisdictions, or areas within the State. In these designated
areas, projects affecting coastal waters, ecology, or land use may require additional permitting
and/or compliance with State laws or local zoning regulations and ordinances.
Endangered Species Act (ESA) This Act seeks to conserve endangered and threatened species
through requiring Federal agencies, in consultation with the Secretaries of the Interior and
Commerce, to ensure that their actions "do not jeopardize the continued existence of endangered
or threatened species or result in the destruction or adverse modifications of the critical habitat of
such species" (16 U.S.C. § 1536). An endangered species is "any species which is in danger of
extinction throughout all or a significant portion of its range" (16 U.S.C. § 1532). A species is
threatened if it is "likely to become an endangered species within the foreseeable future through
all or a significant portion of its range" (16 U.S.C. § 1532). The Fish and Wildlife Service
(FWS) takes jurisdiction over listings for terrestrial and native freshwater species, and the
National Marine Fisheries Service (NMFS) is responsible for listings of marine species or
anadromous species (Doppelt, et a/., 1993). Under the Act, the FWS and NMFS determined
critical habitat for the maintenance and recovery of endangered species, and requires that the
impacts of human activities on species and habitat be assessed. While States can compile their
own lists of species and the degrees of protection required, species on the Federal list are under
the jurisdiction and protection of the Federal Government, and a violation of the act carries
Federal penalties (Corbitt, 1990). Another important provision of the Act is the establishment of
an Endangered Species Committee to grant exemptions from the Act.
When a species is listed under the ESA, the lead Federal agency is required to issue a
biological assessment whenever an action in which the Federal Government is involved (as in
the issuance of Department of Army permits) "may affect" a listed or threatened species (16
U.S.C. § 1536). The agency must consult with the Fish and Wildlife Service if the results of the
biological assessment show a listed species may be affected by the project. If an action will
jeopardize a listed species or its habitat, the lead agency must provide mitigation measures for,
or alternatives to, the proposed activity (Corbitt, 1990).
Projects that impact such areas may be subject to ESA regulation even if a "water right"
exists through Federal or State compact in compliance with State water laws or the Clean Water
3-24
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Chapter 3: BMP Types and Selection BMP Design Considerations
September 2002
Act. As a matter of law, the ESA supersedes most other Federal laws and policies. Given this, it
is still unclear whether State water law and water rights are immune to ESA regulation.
However, the case law indicates that the ESA does authorize a reduction in the power of existing
water rights through regulation (Doppelt, et al., 1993).
The ESA applies to activities directly affecting water resources designated as "critical
habitat" areas, and may include receiving waters from highway or urban runoff. For example,
stream quality in the Pacific Northwest has become an important issue in regards to protection of
the salmon population. Highway construction, runoff quality, mitigation activities, and
maintenance may be subject to review under the ESA due to the identification of certain
receiving waters as "critical habitat" for salmon runs. In many cases, the NEPA process required
for all significant Federal activities uncovers the existence of a listed species, and the subsequent
EIS must deal with potential adverse impacts, project modifications or the project site relocation.
3.11 State and Municipal Requirements That Impact Stormwater BMP Design
States and municipalities have been regulating discharges of runoff from construction and
land development industry to varying degrees for some time. A recent compilation of state and
selected municipal regulatory approaches was prepared in support of EPA's ongoing
development of effluent limitations guidelines for the construction and land development
industries (Tetra Tech, 2000) to help establish the baseline for national and regional levels of
control. Data were collected by reviewing state and municipal web sites, summary references,
state and municipal regulations and storm water guidance manuals. All states (and the selected
municipalities) were contacted to confirm the data collected and to fill in data gaps; however,
only 87 percent of the states and a much smaller percentage of municipalities responded. The
state and municipal regulatory information is presented only to demonstrate that there is a
considerable amount of variance in state and local regulatory requirements related to stormwater
management. Many states and local agencies are currently in the process of revising and
updating their requirements and consequently the data provided in the tables is subject to
constant updates and revisions.
The compilation of state and municipal regulations was prepared to determine national
and regional approaches towards controlling storm water. The data were collected by reviewing
state and municipal web sites, summary references, and state and municipal regulations and
storm water guidance manuals. States and municipalities were contacted to confirm the data
collected and to fill in data not available by these methods. Many months were allocated to
collecting the regulatory data and repeated attempts to obtain and confirm regulatory data ceased
at the end of August 2000.
A summary of criteria and standards that are implemented by states and municipalities as
of August 2000 are presented in Tables 3-12 and 3-13, respectively. State requirements are
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
generally equal to or less stringent than municipalities that are covered under the federal Clean
Water Act NPDES Storm Water Program because state requirements apply to all development
within their boundaries including single site development and low to high density developments.
NPDES Storm Water Program designated municipalities generally have a population of 100,000
or more and can collect and fund the resources necessary to design, implement, and monitor
separate and potentially more stringent storm water management programs. Table 3-12 contains
responses from 47 of the 54 state controlling agencies. The total is greater than 50 because
Florida has 5 regional authorities that are self-regulating. Some state data were uncertain and
repeated contacts to the responsible state agencies to confirm the data were not returned. For the
same reason, some of the data sought from municipal agencies also are not available for this
report.
The data collected reflect a cross section of the U.S. geography but are representative
primarily of municipalities that have a population of 100,000 or greater and only a few
municipalities of smaller population. Thirty-one municipalities are included in the summary
tables, which is a small data set compared to the approximately 240 municipalities with NPDES
programs and nearly 3000 municipalities nationwide. Therefore, the relative use of control
measures that are presented for the states on Table 3-12 is considered to be fairly accurate while
the relative use
Table 3-12 State or Regional Planning Authority Requirements for Water Quality
Protection
Generic Standard
Solids or sediment percent
reduction
Numeric effluent limits for
TSS, settleable solids, or
turbidity
Minimum design depth or
volume for water quality
treatment
Habitat/biological measures
Physical in-stream
condition controls
Chemical monitoring
control
States with
Requirement (%)
24
11
53
7
17
6
States without
Requirement (%)
61
76
28
80
70
83
No Data (%)
15
13
19
13
13
11
for the municipalities presented on Table 3-13 is not considered to be accurate but does reflect
the diversity of control measures used at the municipal level.
Table 3-13 Municipal or Regional Planning Authority Requirements
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
Generic Standard
Design storm for peak
discharge control
Solids or sediment percent
reduction
Numeric design depth,
storm, or volume for water
quality treatment
Design storm for flood
control
Habitat/biological
measures
Physical in-stream
condition controls
Existing Requirement (%)
39
7
-
39
3
10
No Requirement (%)
45
77
-
16
65
58
Unknown (%)
16
16
~
23
32
32
Tables 3-12 and 3-13 show that the following key control measures employed by states and
municipal/regional authorities generally meet the intent of the federal, state, and municipal
regulations that address features of the CWA NPDES Stormwater Program:
storms designed for peak discharge control
storms designed for water quality control
The state and local regulations at the state and local level can be grouped into 3 major categories;
maximum drainage areas that can be disturbed prior to requiring a NPDES permit; requirements
for flood control and peak discharge; and requirements for water quality management.
Drainage Area The compilation of state regulations revealed that the minimum drainage area
requirement among states that triggered a requirement for a NPDES permit ranged from 5000
square feet to 5 acres. The results of the compilation are summarized in Table 3-14.
The compilation for regional and local governments found a wider breakdown for
drainage area limits for local governments especially for the smaller drainage area limits. The
drainage area requirements ranged from 500 square feet to 5 acres. The results of the
compilation are summarized in Table 3-15.
Table 3-14 Minimum Drainage Area Requirements for States (Tetra Tech, 2000)
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
Drainage Area
5 acres
3 acres
1 acre
5000 ft2
No area
requirement
Comments
The majority of States (34 of 48) have adopted the NPDES Phase 1 requirement of 5 acres.
Most of these States will increase this requirement to one acre as the Phase II NPDES
requirements go into effect.
The State of West Virginia uses a 3 acre limit.
Currently 2 States (Geogia and Washington) are already using a one acre limit.
4 States (DE, MD, NJ, and PA) use a 5,000 ft2 limit
2 States have no maximum statewide area limit that requires an NPDES permit. Only
areas in these States comply with NPDES Phase I requirements
MS4
Table 3-15 Minimum Area Requirements for Local Agencies (Tetra Tech, 2000)
Drainage Area
5 acre
2 acres
1 acre
10,000 ft2
5,000 ft2
< 5,000 ft2
Comments
Of the 35 municipalities that were sampled, 17 use the NPDES Phase I
requirement of 5 acres. These municipalities will change to a one acre
requirement when Phase II is implemented.
2 municipalities use a 2 acre.
5 municipalities are currently using a one acre limit
1 municipality reported using a 10,000 ft2 limit
3 municipalities reported using a 5,000 ft2 limit
the following size limits were reported by one or more communities
4,000 ft2; 2,500 ft2; 1,350 ft2; and 500 ft2
Peak Discharge Rate Requirements for Flood Control The second major grouping of
regulatory requirements consisted of agency requirements to control peak discharges to a pre-
development level in order to control increased flooding, channel protection or water quality.
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
The peak discharge requirements were usually expressed as a design storm event. Design storm
frequencies found in these regulations ranged from the 1A -year or six month storm to the 100
year storm. The results of the compilation are summarized in Table 3-16.
Table 3-16 Peak Discharge Control Criteria for States (Tetra Tech, 2000)
Peak Discharge
Control Criteria
No statewide control
requirements
2 yr, 24 h storm
5 yr., 24 h storm
2& lOyr., 24 h storms
lOyr., 24 h storms
1, 10, 1 00 yr., 24 h duration
2, 10 & lOOyr., 24 h storm
2,25, 1 00 yr., 24 h storms
25 yr
Comments
The majority of the states (30) do not currently have any
statewide requirements for peak discharge control
3 States (CA, ME, VT) require peak discharge of the 2 yr.,
24 h duration
Pennsylvania requires peak discharge control of the 5 yr.,
24 h duration storm
Virginia requires peak discharge control of the 2 & 10 yr.,
24 h duration storms
North Carolina requires control of the 10 yr storm
Maryland requires control of 3 storms
Massachusetts requires control of these 3 storms
Rhode Island also requires control of 3 storm frequencies
Florida requires peak discharge control of the 25 yr. storm.
The southern district uses the 3 day duration storm; while the
SW and St. John's River districts use the 24 h duration storm.
The compilation for regional and local governments found similar peak discharge requirements
usually expressed as a design storm event. Design storm frequencies found in these regulations
closely followed the range of storms addressed by the state regulations did not reveal as much
range as the state requirements, but instead appear to focus on the 2, 10 and 100-yr storms. The
results of the compilation are summarized in Table 3-17.
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
Table 3-17 Peak Discharge Rate Control Requirements, Municipalities (Tetra
Tech, 2000)
Peak Discharge Rate
Control
No Requirement
2& 10 yr., 24 h
2, 10 & lOOyr, 24 h
1 yr, 24 h
O.Syr (6 mo), 24 h
lOyr., 24 h duration
10&25yr., 24 h
10 & lOOyr., 24 h
25 & lOOyr., 24 h
50 & lOOyr., 24 h
lOOyr, 24 h
NA
Comments
17 of the 35 municipalities in the sample do not have peak
discharge rate control requirements
4 municipalities use this requirement
4 municipalities use this requirement
These requirements are each used by one of the municipalities in
the sample.
Requirements were not identifiable for 4 municipalities
Water Quality Control Requirements The compilation of state regulations revealed that the
states typically used one of two criteria for water quality control; 1) a specified runoff depth,
and/or 2) a percent removal rate. Table 3-18 summarizes the results of the compilation. The
runoff depth required was either 1A inch or 1 inch. With respect to the percent removal
requirement the most frequently used requirement is 80 percent removal of TSS. The
compilation revealed a similar trend at the regional and municipal level. The results are shown
are summarized in Table 3-19.
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Chapter 3: BMP Types and Selection
September 2002
BMP Design Considerations
Table 3-18 Water Quality Regulatory Requirements, States (Tetra Tech, 2000)
Water Quality
Requirements
Comments
None
38 of the 48 States in the sample currently have no requirements for water quality
control in stormwater management
Runoff Depth
None
0.5in
1.0 in
44 of the 48 States in the sample reported no specific volume requirement for water
quality control
2 States (DE, FLA) require management of the first 1A inch of runoff
2 States (MA, MD) require management of the first inch of runoff
% Removal
None
80% TSS
Other
37 of the States sample do not have specific pollutant removal requirements
10 States reported this requirement which is based on CZARA
1 State (IN) requires 70% removal of TSS
The St. John's River District of FLA requires 80% removal of all pollutants
The Chesapeake District of VA requires 10% removal of TP
Table 3-19 Water Quality Requirements, Municipalities (Tetra Tech, 2000)
Water Quality
Requirements
Comments
None
28 of the 35 municipalities in the sample reported no water quality requirements for
stormwater
Runoff Depth
None
0.5 in
0.75 in
1.0 in
25 municipalities reported no specific volume requirements
5 municipalities require control of the first 0.5 inch of runoff
2 municipalities require control of the first 0.75 inch of runoff
4 municipalities require control of the first 1A inch of runoff
% Removal
None
80% TSS
Other
28 municipalities reported no specific pollutant removal requirements
2 municipalities reported this requirement which is based on CZARA
20% reduction in annual copper loadings by 2001 (Alameda, Co., CA)
65% TP (Washington Co., OR)
0.5 mg/L - TN, 0.1 mg/L - TP, 0.5 mg/L - Iron, 20NTU - Turbidity, 50 mg/L - TSS, 2
mg/L - grease and oil (Lahontan RWQCB Lake Tahoe)
50% TP (Prince William Co., VA)
100% all pollutants (Montgomery Co, MD)
80% TSS - all site; 50%TP - discharge to sensitive lake; 50% ZN - discharge to
stream resource area; <10 mg/L Alkalinity, 50% TP, 40% nitrates + nitrites -
discharge to sphagnum bogs (King Co., Washington)
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Chapter 4: BMP Effectiveness in Removing Pollutants BMP Design Considerations
September 2002
Chapter Four
BMP Effectiveness in Removing Pollutants
4.1 Introduction
As indicated in Chapter 1, the most basic level of design is based upon controlling peak
discharges to minimize flooding. Many stormwater controls were initially employed for flood
control, i.e., to capture peak flows, assist in local drainage, and manage the quantity of runoff
produced during wet weather flow (WWF). In this regard, many States and municipalities
require the control of peak stormwater discharges. As pollution removals for point source
discharges are nearing the point of diminishing returns, increasingly states and municipalities are
relying on the water quality/pollutant removal potential of BMPs as a factor in watershed
management to improve receiving water quality.
Land use changes associated with development invariably increase runoff quantity and
cause downstream flooding and erosion. This has led many states, counties and municipalities,
and other agencies to require onsite detention of this increased runoff with the objective of peak
outflows from detention basins being equal to the pre-developed conditions. This requirement
has become popular, since it can be applied during the development design and review process
on a case-by-case basis without large-scale watershed analysis. This popularity has led to the
frequent use of onsite detention and retention basins, which have become standard features on
many land development projects.
4.2 Current Flow Control Watershed Management Strategies
Pond and wetland BMPs can be designed to provide effective pollutant removal. Water
quality control designs are focused more on the annual volume of runoff rather than peak storm
events. Effective water quality control requires management of the smaller storm events, such as
the 1-inch rainfall events and smaller storms, that typically account for approximately 90 percent
of the annual rainfall and runoff volumes. Many of the older detention facilities used for peak
discharge control include low flow pilot channels that allow these frequent storm events to flow
through the facilities with little or no management.
The most widely used approach to water quality design currently in use throughout the
U.S. consists of a volume-based approach to BMP design. This typically uses a predetermined
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Chapter 4: BMP Effectiveness in Removing Pollutants BMP Design Considerations
September 2002
control volume (i.e., water quality volume, WQv, the volume needed to treat or capture 90% of
average annual stormwater runoff) such as the first 0.5-in. or first 1-in. of runoff, in conjunction
with a diverse set of other design criteria. The State and local jurisdictions assume that if these
volumes and criteria are properly applied then typical pollutant removal percentages will be
achieved. Where possible it is best to incorporate water quality control and peak discharge
control in the same BMP for economic reasons, though this is not always a necessary or
desirable approach. Water quality control can be improved in older BMPs designed under the
older peak discharge principles by retrofitting.
4.3 Pollutant Loading Estimates
There are many methods to estimate the concentration and loading of pollutants to
surface waters. Physically based models attempt to mimic the accumulation and removal of
pollutants as well as the chemical reactions within the receiving streams. More empirical models
rely on general data and information on pollution concentrations in surface runoff and then
predict pollution through an estimation of surface runoff volumes. Regression equations use
significant variables to predict loadings of various constituents based on data sets. This type of
model can be used with little or no data but are very rough in their estimates. They are less
effective for "what if analysis which may extend the situation beyond the limits of data bases,
nor are they very accurate in prediction of acute or shock loadings. Physically based models
require substantial data for calibration over the range of expected conditions but can be very
effective when data exist and can simulate the most important physical, biological and/or
chemical aspects of the problem.
EPA (1983) determined that, based on the sampling done during the Nationwide Urban
Runoff Program (NURP), there are certain pollutants that may be typically found in urban storm
water. Some of the conventional pollutants show up in significant concentrations in most
samples, notably the metals, but most others were present in measurable quantities in less than
15 percent of the samples. Many of these constituents are related to automotive traffic or
industrial activities, while others are characteristic of fertilizing and insect control practices.
Automotive sources and street locations are generally the two key factors, other than illicit
connections and dumping pollutant sources. Common pollutants addressed in studies include:
coliform bacteria; total suspended sediment (TSS), total phosphorus (TP), total nitrogen, (TN),
5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total copper
(TCu), total lead (TPb), total zinc (TZn), and oil and grease.
Event Mean Concentration Method NURP was designed and executed under the auspices of
EPA in the late 1970's and early 1980's. Its main goal was to provide reliable data and
information characterizing runoff from urban sites (USEPA, 1983). Twenty-eight sites were
monitored from across the United States. While there were some differences in the objectives
and procedures of the sites, a common base of information emerged. Later sampling data from
municipalities with NPDES permits confirm NURP's findings. Because of the variability of
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Chapter 4: BMP Effectiveness in Removing Pollutants BMP Design Considerations
September 2002
measurements within storms, among different storms at one site and among sites it was desirable
to use a measure which tended to reduce this variability somewhat.
The measure of the magnitude of urban runoff pollution chosen is termed the event mean
concentration (EMC). EMC is defined as the total constituent mass discharge divided by the
total runoff volume for a given storm event. With few exceptions EMCs were not found to vary
significantly for similar land uses from site to site for the same constituent and were found to be
distributed log-normally. Therefore measures of central tendency (median and mean) and scatter
(standard deviation, coefficient of variation), as well as expected values at any frequency of
occurrence, could be calculated by using the logarithmic transformation of the raw data.
Standard statistical tests and sampling theory can also be used on the log-normally distributed
data.
In selecting a method for estimation of potential washoff loads for a particular site, it is
often decided to use methods that estimate washoff loads by land use type. Total loadings are
then determined based on EMCs of pollutants and runoff volumes. Table 1-4 presented typical
EMC for various land uses and percent imperviousness based on the NURP data and other
sources. This information should be compared to local information, when available. Initial data
from a number of municipalities throughout the East and Midwest indicate that, other than lead,
most constituents did not vary significantly from the NURP information. The reduction in lead is
thought to be based on the use of lead-free gasoline since the NURP data were collected.
Nationwide Regression Equations Method Reconnaissance studies of urban storm-runoff
loads commonly require preliminary estimates of mean seasonal or mean annual loads of
chemical constituents at sites where little or no storm runoff or concentration data are available.
To make preliminary estimates, a regional regression analysis can be used to relate observed
mean seasonal or mean annual loads at sites where data are available for physical, land use, or
climate characteristics. As discussed in Section 1.5, a major study by the U.S. Geological
Survey and the U.S. Environmental Protection Agency resulted in the development of regression
equations that can be used to estimate mean loads for COD, SS, dissolved solids, total nitrogen,
total ammonia nitrogen, total phosphorous, dissolved phosphorous, total copper, total lead, and
total zinc (Tasker and Driver, 1988).
USGS has developed equations for determining pollutant loading rates based on
regression analyses of data from sites throughout the country (Driver and Tasker, 1990). This
method consists of three sets of equations for analysis of runoff pollutant load. The first set of
equations allows for calculation of storm pollutant constituent loads and storm runoff volumes.
The second set of equations is used to calculate the storm runoff mean concentrations. The third
set of equations is used to calculate mean seasonal and annual pollutant loads.
The country is divided into three regions based on mean annual rainfall to increase the
precision of the regression equations. Region I consists of States with a mean annual rainfall of
less than 51 mm (20 in.) and includes the Western States, excluding Hawaii, Oregon, and
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Chapter 4: BMP Effectiveness in Removing Pollutants BMP Design Considerations
September 2002
Washington. Region II consists of States with a mean annual rainfall between 508 mm (20 in.)
and 1,020 mm (40 in.) and includes the Midwestern and Great Lakes States, the Pacific
Northwest, and Hawaii. Region III consists of States with a mean annual rainfall of more than
1,020 mm and includes the Southern States and the coastal Northeastern States. All of the
constituents are modeled for regions I and II; dissolved solids and cadmium are not modeled for
Region III due to a lack of data.
The Simple Method The Simple Method, as its name implies, is an easy-to-use empirical
equation for estimating pollutant loadings of an urban watershed (Schueler, 1987). The method
is applicable to watersheds less than 1 square mile in area, and can be used for analysis of
smaller watersheds or for site planning. The method was developed using the database
generated during a NURP study in the Washington, D.C., area and the national NURP data
analysis. The equations, however, may be applied anywhere in the country. Some precision is
lost as a result of the effort to make the equation general and simple.
Data and Measurement Needs While use of literature values is helpful in a first cut analysis or
preliminary design work, it is important to characterize SS on a site-specific basis; the transport
of settleable solids is a function of local conditions which include topography, geology, and
antecedent dry period. Topography influences slope or gradient, with milder slopes causing
greater solid amounts to be deposited; subsequently, these solids are resuspended during
intensive storm flows. The surrounding geology, or more specifically the soil, affect the SS and
settleable solids concentration and particle-settling-velocity distribution. Seasonal effects may
also be considered.
The monitoring and analyses needed prior to installation for proper assessment, design,
and application of BMPs may be expensive and complex in the short term; however, reliable
data collection may save even more expensive construction costs and may help designs improve
water quality. Sampling devices must be able to capture the heavier SS or settleable solids and
not manifest biased results due to stratification of the heavier solids.
Site-specific solids characterization is necessary for the satisfactory design of physical
treatment, e.g., sedimentation. Sedimentation in BMPs is dependent upon the (1) fraction of
settleable solids and SS, (2) SS-settling-velocity distribution, and (3) hydraulic loading
(gal/ft2/min). Common sieve analysis or more advanced light scattering techniques can be used
for particle-size-distribution analyses. These analyses will enable a site-specific estimate of the
percent of SS and their associated pollutants that the intended CSO control facility may be
capable of removing. The settling characteristic analyses (settleable solids) should be the
gravimetric type with data presented in mg/L to determine the fraction of settleable solids in the
storm flow. Indicator organism and pathogenic analyses may also require some special
procedures before analysis.
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Chapter 4: BMP Effectiveness in Removing Pollutants
September 2002
BMP Design Considerations
Technological advances and improvements in real-time monitors can also allow
continuous measurements of certain parameters, e.g., pH and turbidity; however, even modern
probe-type-monitoring devices must remain wet (submerged) which becomes a problem.
4.4 Effectiveness of Treatment BMPs using Current Design Approaches
The existing data bases on pollutant removal by BMPs may or may not identify the
design method used. Many of the BMPs monitored will have been designed using water quality
measures such as control of first flush, extended detention or retention; however some of the data
are representative of peak discharge control strategies. The levels reported in databases such as
the EPA-ASCE National BMP Database and others as illustrated in Table 4-1 and Table 4-2:
Table 4-1 Median Pollutant Removal of Stormwater Treatment Practices
(Brown and Schueler, 1997)
Treatment BMP
Stormwater Detention
Ponds
Stormwater Retention
Ponds
Stormwater Wetlands
Filtering Practices (2)
Infiltration Practices
Water Quality Swales (3)
Median Pollutant Removal Efficiency (%)
TSS
47
80 (67)
76 (78)
86 (87)
95(1)
81 (81)
TP
19
51(48)
49(51)
59 (51)
70
34 (29)
SolP
-6.0
66 (52)
35 (39)
3 (-31)
85(1)
38 (34)
TN
25
33(31)
30(21)
38 (44)
51
8(41)
NOx
4
43 (24)
67 (67)
-14 (-13)
82(1)
31
Cu
26«
57 (57)
40 (39)
49 (39)
N/A
51(51)
Zn
26
66 (51)
44 (54)
88 (80)
99 (i)
71 (71)
1. Data based on fewer than five data points
2. Excludes vertical sand filters and filter strips
3. Refers to open channel practices designed for water quality
Notes:
• Data in parentheses represent values from the First Edition;
• N/A = data are not available, TSS = Total Suspended Solids; TP = Total Phosphorus; Sol P = Soluble
Phosphorus; TN = Total Nitrogen; NOx = Nitrate and Nitrite Nitrogen; Cu = Copper; Zn = Zinc
Table 4-2 Median Effluent Concentration of Stormwater Treatment Practice
Groups (Brown and Schueler, 1997)
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Chapter 4: BMP Effectiveness in Removing Pollutants
September 2002
BMP Design Considerations
Treatment BMP
Stormwater Detention Ponds
Stormwater Retention Ponds
Stormwater Wetlands
Filtering Practices (2)
Infiltration Practices
Water Quality Swales (4)
Median Effluent Concentration (mg/L)
TSS
28®
17
22
11
17®
14
TP
0.18(2)
0.11
0.2
0.1
0.05 ®
0.19
OP
0.13(2)
0.03
0.09
0.08
0.003 (2)
0.08
TN
0.86 (2)
1.3
1.7
1.1®
3.8 (2)
1.12
NOx
N/A(3)
0.26
0.36
0.55 (2)
0.09 (2)
0.35
Cu(1)
9.0 (2)
5
7
10
4.8 ®
10
Zn(1)
98 (2)
30
31
21
39(2)
53
1. Units for Zn and Cu are micrograms per liter.
2. Data based on fewer than five data points
3. Excludes vertical sand filters and filter strips.
4. Refers to open channel practices designed for water quality
Notes:
N/A = data are not available, TSS = Total Suspended Solids; TP = Total Phosphorus; OP = Ortho-Phosphorus; TN = Total
Nitrogen; NOx = Nitrate and Nitrite Nitrogen; Cu = Copper; Zn = Zinc
These databases and their associated summary tables at the very best should be used only to very
roughly provide information on BMP effectiveness. There is no single value for pollutant
removal which is based on influent loadings and characteristics. A treatment train approach
(discussed later) and source controls could increase the pollutant removals, which is a benefit for
the receiving stream and is more important than achieving targeted removals. These tables are
also presented without stating the "margin of safety" or uncertainty.
Percent Removal of Pollutant is a Poor Measure of BMP Performance The quantification of
efficiency of BMPs has often centered on examinations and comparisons of "percent removal"
defined in a variety of ways. BMPs do not typically function with a uniform percent removal
across a wide range of influent water quality concentrations. For example, a BMP that
demonstrates a good percent removal under heavily polluted influent conditions may
demonstrate poor percent removal when low influent concentrations exist. The decreased
efficiency of BMPs receiving influent with low contaminant concentration has been
demonstrated. For many constituents, there is a minimum concentration necessary to achieve
any reduction. Percent removal alone, even where the results are statistically significant, often
does not provide a useful assessment of BMP performance.
The goal in watershed management is to reduce the pollutant load either through source
control (the most effective way to do it) or through multi-stage treatment (treatment trains).
Although individual BMPs may be less effective on a percent basis, if they cumulatively still
result in a lower effluent concentration (or load), they benefit the watershed. BMPs should
therefore not be designed for percent removal but for pollutant removal to achieve a given
effluent level.
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Other recommended parameters for measuring BMP efficiency include measurements of
how performance varies from pollutant to pollutant, with large or small storm events, with
rainfall intensity, and whether the BMP reduces toxicity and whether it can cause an
improvement in downstream biotic communities (Strecker etal, July 2000).
Monitoring and Current Evaluations of Treatment BMP Effectiveness BMP monitoring is
difficult, complex, and costly to conduct properly. It is almost impossible to locate a BMP in the
field that has been evaluated and monitored properly for its effectiveness, including the
aforementioned and other databases. Proper BMP evaluation must include:
• Mass-balance (synchronized flowrate measurement with sampling times)
monitoring of all influent and effluent points (vectors whether point or diffuse).
This becomes extremely complex when monitoring wetlands, filter strips, and
swales having multiple, changing or diffuse influent/effluent locations and
variable groundwater infiltration/exfiltration.
Representative and satisfactory monitoring equipment, i.e., sampling devices
(capable of withdrawing stratified, heavy particles and surface films), flowmeters
(capable of accurate/precise measurement under the adverse stormflow conditions
of unsteady, surcharged, low, and non-uniform flow), and non-fouling sensors.
• Continuous and representative monitoring of the influent and effluent during both
wet- and dry-weather flow conditions.
• Long-term (at least one year) monitoring covering all seasons.
• Monitoring of soil-infiltration capacity representative of the entire BMP bottom
and sides/embankments.
Satisfactory and uniform quality parameters and standard analytical methods
(with an approved QA/QC program) to enable reliable site-to-site comparisons
and achieve proper evaluation. Details and results of the QA/QC program should
be reported in monitoring study reports and summarized in applicable papers.
• Pollutant removal as it relates to BMP size (volume, plan area) and BMP influent
flowrate and volume. For example, 100% pollutant removal is achievable in an
oversized BMP retention infiltration pond having a very high bottom/side soil-
infiltration rate that is capable of capturing its total long-term inflow volume and
allows quick draw-down between storms.
An adequate sample population of each specific BMP type evaluated.
If a BMP relies heavily on particle settling for its effectiveness, then without particle-settling-
velocity laboratory analyses, effectiveness evaluation will fall short. This can be explained by
using the case of a BMP that relies on settling that is situated in an area that contains unsettleable
surface particles (e.g., fine clay). In this case treatment by settling will be minimal and without
knowing particle-settling velocity distributions an improper BMP effectiveness assessment could
be made. Also, if BMP treatment relies on filtration, then it is important to use particle-size
distribution as an evaluation parameter.
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At best, the available databases today have only limited sample populations per BMP
type. Some of these were designed under the peak discharge control approach to stormwater
management which happen to include water quality design approaches currently in use and,
therefore, can be listed under both categories.
4.5 Importance of Particle Size Distribution
Treatment of contaminated stormwater runoff relies almost exclusively on sedimentation
with the added assumption that a majority of the pollutants are present as adsorbed species on the
finer fraction of the suspended solids /sediments. Particle-size-distribution of solids directly
affects the SS settling characteristics. Therefore, determination of the particle-size-distribution
of solids in the stormwater and analysis of the various fractions of the sediment are necessary for
selecting and designing an efficient BMP treatment-train system.
Miiller (2001) and his colleagues studied the distribution of heavy metals in different
fractions of river sediment and recommended that 'preference should be given to the rapid,
simple, and economic separation by sieving (20* m); this fraction corresponds fairly closely to
the former suspended load of a riverine transport.' Studies show that sorption of pollutants by
particulate matter is bimodal in nature. For example, Charlesworth and Lees (1999) studied
speciation of heavy metals vs particle size (<63 • m and 2 • m) in urban sediments and found that
both size fractions contained similar amounts of heavy metals. Furthermore, the majority of the
metals, irrespective of size fraction, are associated with organic matter of that fraction.
Distributions of heavy metals and hydrocarbons in urban stormwater are associated with their
particulate fractions and the relative size of SS. Particles finer than 250 • m contain more heavy
metals and total petroleum hydrocarbons (TPHs) than particles larger than 250 • m, and about
70% of the heavy metals are attached to particles finer than 100 • m (Ellis and Revitt 1982).
Gao (1977) found that distribution of pesticides (atrazine and bifenox) in sediments is
also bimodal, governed both by the particle size and their organic matter content. Wang (2001)
reported that the majority of the PAHs occurred in the coarser fractions (>250 • m) of the
sediment. Their study also showed that particulate organic matter like charcoal and plant
detritus in the sediment appear to absorb PAHs more strongly than organic matter associated
with clay. Walling and his colleagues (Walling, 2001) studied sediment-associated nutrient
transport during a storm event and found that:
particle size has limited influence on the nutrient content of the suspended solids,
• sediment properties, like iron and manganese content, control the phosphorous
content of the sediment,
• TOC content of the sediment appears to be related to nitrogen content of the
sediment,
• Total nitrogen (TN) content of sediment is predominantly (94-98%) in organic
form.
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4.6 Approaches to Implementing BMPs for Improved Water Quality in the Urban
Watershed
There are several strategies being used to improve water quality in the urban watershed,
including setting standard pollutant reduction levels, establishing maximum pollutant levels for
new development, using annual flow volumes rather than flood events, basing design on first
flush principals, and developing designs using treatment train BMPs. These are discussed below
and in subsequent sections of this document.
Setting Standard Pollutant Reduction Levels A strategy for controlling the mass of pollutants
released into receiving waters is to require that a specified amount of pollutant be removed from
the stormwater runoff before it is discharged. The reduction has been commonly specified as a
percent decrease of the pollutant. Pollution reduction standards can be applied to impervious
areas or to the entire developed area (including open space and pervious areas). The pollution
reduction strategy may require a specific reduction in the average mass of pollutant or may
require that the average mass of pollutants after development be reduced to preconstruction
levels.
An example is the federal guideline issued pursuant to the CZARA that specifies that
urban runoff from a new and stabilized development site have 80 percent of the TSS removed
before it is discharged from a construction site. This is a voluntary program and applies only to
new land development in municipalities not covered by the NPDES stormwater program in
coastal states. When calculating the average mass of total suspended solids, the CZARA
considers only discharges generated by the 2-year, 24-hour frequency storm or smaller storms.
Implementing the pollution reduction strategy requires knowledge of the preconstruction
and post development average mass of pollutant. This is usually derived by using pollutant
loading factors from a developed site or by using event mean concentrations (EMCs) from sites
that are comparable to a proposed development site. It is possible to conduct long-term
monitoring to determine the mass of preconstruction pollutants, but the post development masses
need to be estimated so that stormwater management controls can be designed and permitted.
Actual loadings may also be used when there is sufficient data available. This is the most
accurate approach and is proposed for use in watershed TMDL modeling. Post development
monitoring has not been normally required or implemented as part of the permit approval
process in the past, but some municipalities are beginning to require it. The stormwater
management controls that are proposed for a site development are designed and approved by
permitting agencies based on the best available knowledge. Once the design is approved and
constructed as designed, developers are not usually expected to retrofit stormwater controls even
if monitoring indicates that they do not achieve the expected pollutant reduction goal.
The pollution reduction strategy is an effective means of reducing the mass of new and
additional pollutants arising from land development activities. It also specifies a goal to be
achieved without mandating the specific controls that to are be used. The strategy is generally
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considered to be effective if the regulating municipality selects the appropriate pollutant
reduction percentage and ensures that the stormwater controls are properly selected, designed,
constructed, operated, and maintained. There are several limits to the effectiveness of this
approach, including:
• The strategy permits an increase in total pollutant mass released into receiving
waters since the allowable percentage discharged may actually exceed the pre-
disturbed loadings.
• The strategy is designed to control pollutants discharged from a development site.
It does not explicitly require protections at the receiving waters, so discharges
from numerous development sites could combine to exceed desired pollutant
masses in receiving waters.
The reduction goal needs to be generic to accommodate the variety of site
conditions in a municipality. Pre construction effluent characteristics and
receiving water requirements will vary across a municipality as will post
development characteristics. Criteria and standards developed to control water
quality pollution from the broad range of environmental conditions present could
be too lenient in some cases and too strict in others.
• The pollutant removal efficiencies of stormwater technologies have not been well
defined in the past. Existing guidance on the design of stormwater controls have
typically used a broad range of pollutant removal efficiencies based on existing
monitoring methods (which are poor in many cases). This range in reported
effectiveness leads to uncertainties in the selection and design of the treatment
processes used to meet the pollutant reduction goals.
Some of these concerns are being addressed by ongoing investigations and innovative
approaches that are being developed and tested by some municipalities.
With regards to monitoring, evaluating compliance with the pollutant reduction strategy
may entail a subjective judgment because monitoring standards and guidance generally are not
well documented and implemented. A continuing study jointly funded by the ASCE and USEPA
seeks to provide tools that describe stormwater control monitoring and expand a database that
can be used to estimate stormwater control effectiveness. This project has resulted in the
development of the ASCE/EPA BMP database web site (http/www.bmpdatabase.com). Several
municipalities and professional organizations are also studying the impacts of pollutant loads on
receiving water quality and aquatic biology. These studies are expected to refine the relationship
between development activities, stormwater controls, and receiving water responses
Establishing Maximum Pollutant Levels for New Development Another strategy designed to
prevent short- and long-term harm to humans and the environment is to specify that pollutants of
concern in stormwater discharged at the MS4s from developed sites cannot exceed specified
concentrations. A number of states and municipalities have established maximum permissible
concentration criteria and standards for pollutants such as TSS or turbidity, and some have also
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developed criteria and standards for nutrients, oil and grease, metals, and other pollutants. While
these concentrations are typically specified at the MS4 discharge locations from developments,
States or municipalities may require that the development activity not cause impacts to the
receiving waters that exceed minimum concentrations of some pollutants such as dissolved
oxygen.
By requiring that pollutants in stormwater effluent not exceed predetermined
concentrations, municipalities can control worst-case conditions. As commonly implemented,
however, such a requirement does not prevent the average pollutant load released from a
development site from exceeding pre-construction conditions. The design of structures that
achieve these controls are subject to the same degree of uncertainty as described above for the
percentage reduction strategy, but the not-to-exceed concentration strategy gives the governing
municipality a ready means (i.e., effluent monitoring) of ensuring that its goal is met and puts the
responsibility on the developer to properly design, and retrofit if necessary, the stormwater
controls needed to achieve the effluent concentration requirements. Another drawback to the
strategy is that the establishment of concentration limits is based on the existing understanding of
how water quality and aquatic biology respond to changes in pollutant loads. The current
understanding is an estimate of both the ability of the receiving water to accommodate changes
in pollutant loads and the impacts that aquatic biology can withstand in the short- and long-
terms.
Using Annual Flow Volumes for Design The addition of water quality considerations in the
design of BMPs has introduced a new dimension to the traditional hydrologic considerations for
BMP design. Prior to the introduction of water quality considerations hydrologic design
methods were focused on flood event hydrology with focus on storms typically ranging from the
2-yr (bankfull); to the 10-year (storm drainage conveyance storm) to the 100-yr (floodplain
storm). Water quality considerations created a shift from flood events to annual rainfall volumes
and the pollutant loads associated with these volumes. This new focus has given rise to concepts
such as the rainfall frequency spectrum and small storm hydrology.
Basing Design on First Flush Principles The tendency for solids and associated constituents to
be washed off of paved areas during the initial portion of the storm event is referred to as the
first flush. In general, the potential for first flush is determined by the storm characteristics, the
size of the subwatershed, and the partitioning characteristics of the pollutants of concern.
To treat the bulk of the pollutant loads from stormwater runoff, many states and
municipalities specify a treatment volume that is designed to capture the first flush component of
the stormwater runoff. In practice this is achieved by specifying a rainfall amount (such as the
first !/2-inch, 1-inch, or other rainfall depth) over impervious areas or the capture of a stormwater
runoff volume that correlates to a design storm (such as the 6-month, 1-year, or 2-year frequency
storm).
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Working with a very small (300-square-meter) highway segment, Sansalone, et al. (1994)
found a pronounced first flush for solids, dissolved zinc and dissolved copper, but not dissolved
lead. The first flush for the particulate-bound fractions of these metals was not well defined.
While the first flush is commonly treated using settling technologies, filtering and cation
exchange technologies may also be warranted depending upon the subwatershed characteristics
and the pollutants of concern.
In some cases, such as Austin and San Antonio, TX, the first flush of water is diverted to
a separate treatment system for settling followed by sand filtration. The remainder of the storm
is directed into a stormwater detention basin that may be a retention or detention pond.
Designing Using Treatment Train BMPs As discussed in subsequent sections, targeted
pollutant removals can usually be achieved using a series of BMPs in a treatment train. This can
apply to new designs as well as to retrofit existing BMP facilities. Simply put a treatment train
is comprised of several BMPs, e.g., filter strips draining to swales that convey the stormwater to
a retention pond designed with a forebay.
A treatment train BMP process should be capable of achieving the targeted pollutant(s)
removal or degradation in the designed treatment system. Effectiveness may be assessed in
terms of specific stressor of concern (e.g., flow, nutrients, pathogens, sediment, or toxics) or
groups of pollutants. If there are no existing pollutant removal or water quality control measures
currently being implemented and the planned BMP provided certain degrees of treatment, then
the BMP system may be considered effective by default. Furthermore, the designed BMP
treatment train (or multi-tiered approach) should achieve pollutant reduction sufficient to
produce effluent water quality parameters that comply with the regulatory requirements.
Otherwise the recommended BMP treatment system should not be considered effective.
4.7 Removal Processes Occurring in Treatment BMPs
The processes occurring in treatment BMPs (Table 4-3) include: settling, sorption,
filtration, infiltration, biodegradation/bioassimilation, nitrification/denitrification, volatilization
and phytoremediation. One or more of these treatment processes may occur in the treatment
BMP systems to remove pollutants. Table 4-4 summarizes expected performance values of each
treatment BMP.
Table 4-3 Removal Processes Occurring in Treatment BMPs
Pollutant
Constituents
Treatment BMP Type and Process Mechanism
Pond
Wetland
Infiltration
Biofilter
Sand Filter
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BMP Design Considerations
Heavy Metals
Toxic
Organics
Nutrients
Solids
Oil & Grease
BOD5
Pathogens
Sorption
Settling
Sorption
Biodegradation
Settling
Phytovolatilization
Bioassimilation
Settling
Sorption
Settling
Biodegradation
Settling
UV (sunlight)
Sorption
Settling
Phytoremediation
Sorption
Biodegradation
Settling
Phytovolatilization
Bioassimilation
Phytoremediation
Sorption
Settling
Sorption
Settling
Biodegradation
UV (sunlight)
Predation
Sorption
Filtration
Adsorption
Filtration
Sorption
Sorption
Filtration
Sorption
Biodegradation
Filtration
Sorption
Filtration
Phytoremediation
Settling
Sorption
Filtration
Settling
Phytovolatilization
Sorption
Bioassimilation
Phytoremediation
Sorption
Filtration
Settling
Sorption
Settling
Biodegradation
Filtration
Settling
Sorption
Filtration
Sorption
Filtration
Sorption
Filtration
Sorption
Biodegradation
Filtration
Predation
Table 4-4 Treatment BMP Expected Performance (ASCE, 2001)
BMP Type
Detention Pond
Retention Pond
Alum System
Sand Filters
Swales
Buffer Strips
Infiltration Trenches
Expected Pollutant Removal Efficiency (%)
Suspended
Solids
70
85
90
70-90
60-80
20-80
70-90
Total Nitrogen
10
40
50
30-40
0-20
20-60
40-70
Total Phosphorus
20
50
90
50-60
30-40
20-60
50-70
Total Heavy Metals
30-70
25-70
80-90
20-80
30
20-80
70-90
Settling Also known as sedimentation, settling occurs when particles have a greater density than
the surrounding liquid. The settling process in stormwater management is determined by the
particle size and settling velocity, turbulence or short-circuiting, peak flow-through rate, and
volume of water (Stahre and Urbonas, 1990). Soil particles and TSS are removed primarily
through settling. In addition because many of the other pollutants including nitrogen,
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phosphorus, metals, and bacteria are attached to the soil particles they are also removed from the
water column.
Particle size directly affects the pollutant settling ability: the smaller the particle size, the
longer it takes to settle. Conversely, the larger the particle, the faster its settling velocity.
Particle size, however, is not the only factor in settling ability. This relationship also depends on
the difference between the density of the fluid suspending the particle and the density of the
particle. Large, dense particles, such as sand, will fall through fluid at a faster rate than smaller,
less dense particles, such as clay. The volume of particles suspended within the fluid also factors
into this process. Stahre and Urbonas (1990) indicates that the more particles suspended within
the fluid, the faster the rate of settling but at some point, the rate of settling will bottom out.
Turbulence, eddies, multilayered flows, circulation currents, and diffusion at inlets and
outlets affect the settling ability of particles. Each of these factors can resuspend particles into
the water column. Kuo (1976) found that sedimentation would improve as flow-through rate and
surface loading decreases. The difference was most significant for larger particles; however, this
study did not go beyond the laboratory. Actual field conditions must take into account the
particle settling velocity and surface loading rates during runoff conditions. Sediment removal
under these conditions varies with storm intensity. The size of the body of water relative to
stormwater runoff will also determine the settling ability of sediment. The larger the stormwater
loading rate, the lower the removal of sediment by settling. Settling also occurs after stormwater
is trapped and ponded between storms. Because the intervals between storm events are a random
process, understanding the effective ratio of storage volume to mean runoff rate and the ratio of
sediment volume removed to mean runoff rate is essential to predicting long-term averages.
The most widely used stormwater management practices that employ the sedimentation
process are retention and detention structures such as ponds and constructed wetlands. These
can be designed to effectively remove sediment from stormwater. Several factors are considered
during the design processes: retention or detention feature, detention time, and period storms.
Stormwater management basins with a permanent pool of water have a removal
percentage of total suspended solids of about 50 to 90 percent (Wotzka and Oberts, 1988; Yousef
et al., 1986; Cullum, 1985; Driscoll, 1983, 1986; MWCOG, 1983; OWML, 1983; Holler, 1989;
Martin, 1988; Dorman et al., 1989; City of Austin, 1990). Extended detention ponds have a
similar percentage of removal (MWCOG, 1983; City of Austin, 1990; OWML, 1987). Some
researchers have found, however, that detention ponds will have a lower sediment removal
efficiency over the long term than retention ponds. This is because an opportunity exists for new
storm flows to resuspend sediments deposited on the detention pond bed from previous storm
events.
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The typical detention time for detention basins in the United States is from 12 to 48
hours. The longer the detention time, the more time particles have to settle before the
stormwater is discharged to the receiving water. The detention time must be long enough for the
desired particulates to settle from the storm water, yet the full volume of storage should also be
available for the next storm event. Thus a 2-day period for the temporary storage and treatment
of stormwater is the typical maximum period since this seems to balance the pollutant removal
goals with the between-storm interval during the rainy season in many locations.
As mentioned earlier, the settling process can remove particulate materials and those
dissolved materials which may adsorb to settleable particles. However, the removal rate by
settling of pollutants other than sediment particles is inconclusive. Part of the confusion is
related to which removal process in a stormwater management structure is responsible for
removing a pollutant. In retention ponds, for example, several processes occur simultaneously:
settling, biological uptake, volatilization, infiltration to groundwater, and adsorption. While
nitrogen, phosphorus, and bacteria may be removed to some extent by absorption to larger
particulates, this is not expected to be a primary mechanism for their treatment. Metals,
however, are present in particulate and dissolved form and some metals species can be removed
by coagulation and sedimentation.
With respect to speciation, recent runoff data from a heavily traveled highway site in
Cincinnati, OH, indicate that, in general, cadmium, copper, and zinc can be found substantially
in the dissolved form, depending on the storm event (Sansalone etal., 1994). For a series of five
storm events, the event mean dissolved fraction ranged from 0.535 to 0.955 mg/L for zinc, from
0.446 to 0.964 mg/L for cadmium, and from 0.310 to 0.713 mg/L for copper. In contrast, lead
tends to be in the particulate form; the dissolved fraction ranged from 0.179 to 0.451 mg/L.
Factors cited by Sansalone et al. (1994) that affect the event-to-event variation in dissolved
fraction include rainfall pH and the average residence time of the runoff. Similar results have
been found for runoff from parking lots on the west coast (Woodward-Clyde, 1996).
With respect to particle size fractions, a number of researchers have found that the
smaller particles tend to be more mobilized during storm events and the concentration of metals
was found to increase with decreasing particle size (Sartor et al., 1974). Recent highway runoff
particle fraction data show that the surface area per unit of mass within different size fractions
increases as particle size decreases (Sansalone etal. 1994), and thus metal concentrations would
similarly increase with the smaller sized particles. On the basis of 13 monitored events from the
highway runoff site in Cincinnati, the median particle diameter was about 570 jim (Sansalone et
a/., 1994).
Filtration The filtration process can remove sediment and other pollutants as stormwater passes
through a filtering system. Typically, stormwater filters remove particulates and adsorbed
pollutants, such as sediment, organic carbon, phosphorus, and many trace metals. Particulate
pollutants are trapped by cation/anion exchange (discussed in the following paragraph) or are
prevented from moving beyond the filter. In some cases, the filtration process can increase the
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pollutant level of storm water. Filters that inadvertently become anaerobic and nitrify organic
nitrogen can release ammonia and nitrate into storm water. Existing media filtration practices
commonly use trenches filled with sand or peat. Once the treatment volume is achieved during a
given storm the excess runoff bypasses the filter and is untreated.
Sorption The clay and organic particles in soil hold a negative charge. The ability of soil and
organic matter to hold cations, such as phosphorus and aluminum, represent the soil's cation
exchange capacity. This process is most readily used to filter pollutants from storm water.
Organic matter, such as peat or leaf matter, in the filter media will use its cation exchange
capacity to bind pollutants to the filter. The treatment of all runoff through filter media (Stewart,
1992), and biofilters, such as the bioretention cell (Clar, 1993) are other examples of cation
exchange processes. A shallow basin collects the runoff and gradually discharges through a
filter media filled with planting soil, peat or composted leaf media. The media traps particulates
(through filtration), adsorbs organic chemicals, and removes up to 90 percent of solids, 85
percent of oil and grease, and 82 to 98 percent of heavy metals (through cation exchange from
leaf decomposition).
The extent to which a given metal is adsorbed is affected by a number of factors,
including competitive effects of other ionic metals, the presence of iron and manganese oxides,
the presence of organic carbon, and especially pH (Maidment, 1993). Treatment trains that
include adsorptive media may provide effective treatment for dissolved metals. Such media
include compost, granulated activated carbon, or diatomaceous earth, all of which work on a
cation exchange principle. Pilot laboratory testing of different filter media conducted by Robert
Pitt at the University of Alabama/Birmingham show the following removal efficiencies (Pitt,
1986):
• Sand filter - 45 percent (zinc)
• Composted leaves - 88 percent (zinc), 67 percent (copper)
• Peat moss - 80 percent (trace metals in general)
An in-house research by EPA (Wojtenko et.al, 2002) demonstrated the capability of
common tree mulches used for landscaping to remove pollutants commonly found in urban
storm water runoff. In an experiment 2L of storm water spiked with a mixture of heavy metals
(Cu, Cd, Cr, Pb, and Zn (each at 5,000 • g/L) and priority organic pollutants benzo(a)pyrene,
naphtahalene, fluoranthene, 1.3-dichloro benzene, and butyl-benzyl phthalate (each at 1,000
• g/L) were mixed with known weights of mulch. Pollutant removals are shown in Table 4-5.
Table 4-5. Pollutant Removal (%) by Mulch from Stormwater Runoff (Wojentko
et. a/, 2002)
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BMP Design Considerations
Pollutant
Cd
Cu
Cr
Pb
Zn
Benzo(a)pyrene
Naphthalene
Floranthene
1,3 Dichloro benzene
Butylbenzyl phthalate
Initial Concentrations of Pollutants (2L solution)
Metals each 5,000 • g/L; Organics each 1,000 • g/L
Mass of Mulch
100 g
94
50
100
100
75
100
100
100
100
100
500 g
99
100
100
100
99
100
99
100
100
100
Phytoremediation Plants are able to degrade (break down) organic pollutants through their
metabolic processes. Aquatic plants have been used to treat wastewater, such as wetlands have
been used to treat farming effluent and mining runoff. Phytoremediation refers to the use of
plants to degrade, sequester, and stabilize organic and metal pollutants in stormwater. Plants are
able to volatilize contaminants (volatile organic compounds, i.e., solvents, etc.) from soil or
water (i.e., phytovolatilization). More recently, the bacterial activity associated with the roots of
grasses and other plants has been explored for its organic degradation potential. The efficiency
of phytoremediation may vary depending on the depth of soil and the type and species of
pollutants in water that are most available for plant uptake.
4.8 Treatment-Train Approach to Improve Water Quality
Several treatment processes are applicable to treat stormwater runoff. The following unit
processes can be selected to compose a treatment-train for a site-specific application:
• settling
• infiltration
filtration
sorption
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• biodegradation
• nitrification/denitrification
bioassimilation
phytoremediation
One or more of these treatment processes may be used to achieve the desired effluent
quality of stormwater released from urban watershed. Depending on the stormwater
management goals and objectives identified for a specific site or area, a combination of one or
more treatment BMPs may need to be used to meet the design objectives, in what is often
referred to as a treatment train approach. No single BMP is as effective as a "train" (that is,
series) of practices and controls (ASCE & WEF 1998).
Pretreatment is recommended where the site has sufficient space, to aid in reducing
incoming velocities as well as capturing coarser sediment particles to extend the design life and
reduce replacement maintenance of the primary BMP downstream. The pretreatment method
may include a vegetative filter strip, swale or incorporate other techniques to aid in extending the
design life of the primary BMP. Historically, the primary purpose of a vegetated filter strips has
been to enhance the quality of stormwater runoff on small sites in a treatment system approach,
or as a pretreatment device for another BMP. The dense vegetative cover facilitates
conventional pollutant removal through detention, filtration by vegetation, sediment deposition,
infiltration and adsorption to the soil (Yu and Kaighn, 1992). Vegetated filter strips may be used
as a pretreatment BMP in conjunction with a primary BMP. Retention and detention basins
should be designed to promote sediment deposition near the point of inflow. A forebay with a
volume equal to approximately 10% of the total design volume can help with the maintenance of
the basin, and extend the service life of the remainder of the basin. This reduces the sediment
and particulate pollutant load that would reach the primary BMP, which, in turn, would reduce
the BMP's maintenance costs and enhance its pollutant removal capabilities.
Because detention ponds operating alone have been documented to be ineffective, it is
not possible to recommend them as a viable water quality control measure (Moffa et al. 2000).
However, they can be very effective when used in conjunction with other stormwater control
practices. At a minimum, a two-stage basin is preferable for extended detention ponds. The
lower stage has a micropool that fills frequently. This reduces the periods of standing water and
sediment deposition in the remainder of the basin. These recommendations does not necessarily
apply to large, regional extended detention basin and the impact of these considerations varies
with climate and soil types.
Integrated Treatment-Train Systems The basic treatment BMPs indicated in the previous
section are essentially singular processes; however, system optimization should be considered to
enhance overall treatment BMP effectiveness to meet water quality goal. An integrated
treatment train to produce four different levels of effluent water quality. The various unit BMPs
form the following control or treatment trains as presented in Figure 4-1:
4- 18
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Chapter 4: BMP Effectiveness in Removing Pollutants
September 2002
BMP Design Considerations
• Control/pretreatment - source control and natural drainage system followed by
storage basins constructed with spillway for by-passing high-intensity and long-
duration storm generated flows to regional ponds.
Primary treatment - detention and retention ponds with polishing constructed
wetland [Class D].
• Secondary treatment - primary treatment with chemical addition and disinfection
[Class C].
• Tertiary treatment - secondary treatment plus filtration and disinfection [Class B].
• Advanced treatment - tertiary treatment plus activated-carbon adsorption with
disinfection [Class A].
Each treatment train is expected to produce a different degree of effluent water quality to meet
the receiving water quality criteria as illustrated in Table 4-5. The effluent presented are based
solely on expected unit process removals. Only effluent D is indicative of the potential removals
using the current "passive" management of stormwater based solely on gravity flow, as opposed
to a more "active" management typified by wastewater treatment plants.
Low-Impact
Development/
Source Control
Ponds/High-Rate
Filtration, GAC, and
Disinfection
Ponds/High-Rate
Filtration
and Disinfection
Ponds with
Chemical Additior
and Disinfection
Ponds/Infiltration
Basins and Constructed
Wetland
Regional Ponds
D
D
Figure 4-1 Urban Stormwater Treatment Train Process Flow Diagram
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Chapter 4: BMP Effectiveness in Removing Pollutants
September 2002
BMP Design Considerations
Table 4-5 Expected Median Effluent Concentration of Selected Pollutants
Water Quality
Constituent
Suspended solids (mg/L)
BOD, (mg/L)
Total nitrogen (mg/L)
Total phosphorus (mg/L)
Total coliform (MPN per 100 ml)
Oil and grease (mg/L)
Potential Effluent Water Quality Concentration
A
5
5
<0.1
<0.1
<100
1
B
10
10
0.5
<0.1
< 100
5
c
10
10
< 1
<0.1
100
10
D
30
30
1.5
1
--
15
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
Chapter Five
Types of Pond BMPs and Their Ability to Remove Pollutants
5.1 Introduction
Ponds are probably the most frequently used stormwater BMP in the United States. For
the purposes of this document, pond BMPs are grouped into three types: wet ponds or retention
ponds; dry basins or ponds including detention basins (ponds) and extended detention basins
(ponds); and infiltration basins.
Group 1 - Wet Ponds/Retention Ponds A wet pond is a small artificial lake often with
emergent wetland vegetation around the perimeter and littoral zone, designed to capture runoff
and remove pollutants from stormwater. This BMP is sometimes referred to as a "retention
pond" or a "wet retention basin". In this document, it is referred to as a wet pond to distinguish
it from the extended detention basin (a type of dry pond) described below. Removal rates of
solids by wet ponds typically outperform detention basins. The larger permanent pool of wet
ponds allows water to reside in the interval between storms while further treatment occurs. A wet
pond can be sized to remove nutrients (10-40%) and dissolved constituents; however settling is
the primary mechanism of treatment. Permanent pools that may be associated with an extended
detention basin are smaller and are provided for aesthetics, as discussed under the extended
detention discussion above. Figure 5-1 illustrates the elements of a wet retention pond.
Group 2 - Dry Ponds/Detention Ponds/Dry Detention Basins and Extended Detention
Ponds/Basins Detention of urban stormwater runoff began appearing as an urban stormwater
management practice in the 1960s in North America to control runoff peaks from new land
development sites. Figure 5-2 shows a typical dry pond. While many jurisdictions initially
applied this approach to control the 10-, 25-, 50-, or 100-year flow rates, a small number of
jurisdictions also mandated detention (i.e., storage of stormwater in the dry pond for a short
period of time) to control the 2-year peak flow rate for stream bank erosion control purposes.
Unfortunately, this policy has not been able to achieve its objective of stream channel protection.
5- 1
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Chapter 5: Pond BMPs
September 2002
BMP Design Considerations
POND BUFFER
(25 FEET MINIMUM)
OVERFLOW
SP1LLW8AY
EMERGENCY
SPILLWAY
HARDENED
PAD
MAINTENANCE —'
ACCESS ROAD
NATIVE LANDSCAPING AROUND POOL
SAFETY BENCH
RISER IN
EMBANKMENT
PLAN VIEW
EMBANKMENT-
RISER-
EMERGENCY
SPILLWAY
OrtfcJTi rtDAUU /
FORESAY
PROFILE
Figure 5-1 Wet Pond Typical Detail (MDE, 2000)
5-2
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Chapter 5: Pond BMPs
September 2002
BMP Design Considerations
Figure 5-2 Typical Dry Pond
In the early 1980s, watershed managers began to restrict the flow out of the dry ponds
even further so that a pool of stormwater would remain (be detained) in the pond for much
longer periods of time. This approach to dry pond design was called extended detention ponds
(see Figure 5-3). . Extended detention for stormwater quality began to be used for new
installations of extended detention ponds or as retrofits of old dry ponds. By the late 1980s,
sufficient empirical data were available to design extended detention basins for water quality
purposes with reasonable confidence in their performance. Extended detention refers to a basin
designed to extend detention beyond that required for stormwater control to provide some water
quality affect. Extended detention basins are best at removing settleable constituents. They are
as effective in removing soluble solids as other BMPs that incorporate other treatment
mechanisms.
The amount of pollutant reduction achievable in ponds depends on a wide variety of
factors, including: continuous long term-inflow, surface area and effective volume of the basin,
peak outflow rate, size distribution of the particles, specific gravity of particles, fraction of the
sediment that is active clay, type of associated pollutant concentrations, and fraction of influent
solids that are colloidal, dissolved and or unsettleable.
5-3
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Chapter 5: Pond BMPs
September 2002
BMP Design Considerations
Side Slopes No Steeper than 4:1
Top Stage with
2% Slope f k»r Drainage
Embankment Side Slope
No Steeper than 3*1
Embankment
Access to Outlet
\ Outlet w/Trash Rack
3535?"
PLAN
NOT TO SCALE
""flow
Presedementation
, Forebay
Frequent
Runoff Poo!
10% to 25% of WQCV
Secondary tern
Water Quality Capture
volume level {including
20% additional volume
for sediment storage)
Flow
Dispersing
Inlet
Solid Driving
Surface
Top of Low
Flow Channel
t
Emergency Spillway Rood
Level
@ Spillway Crest
te g 100~vr( SPF, PMF. etc.;
Spillway Crest
Cutoff Collar
Size Outlet &
Drain Forebay
Volume in 45
Minutes
Invert of
Low Flow
Channel
S=0.0%±
OuiJet Works
(see detail)
_ SECTION,
NOT TO SCALE
Figure 5-3 Extended Detention Basin, Typical Detail (UDFCD, 1999)
Extended detention basins sometimes have a small permanent pool below the invert of
the low flow outlet. This is normally so small that it does not materially impact trapping of
suspended solids and chemicals and is typically included for aesthetics or to cover deposited
sediments.
Regional facilities often offer economies of scale and greater reliability in capturing
stormwater when they are used, while on-site facilities offer institutional and fiscal advantages of
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
implementation as the land is urbanized. The advantages and disadvantages of regional and on-
site facilities are described in Chapter Three.
Group 3 - Infiltration Basins Infiltration basins are dry ponds constructed to allow infiltration
to occur simultaneously with other treatment processes. Figure 5-4 provides a typical detail for
an infiltration basin. The operating characteristics of infiltration basins is essentially the same as
for dry detention ponds, with a few significant exceptions:
1. Infiltration basins also remove dissolved and colloidal solids in the volume of
infiltrated water, whereas extended detention ponds can only remove the fraction
of colloidal solids sorbed to settleable solids
2. The settling velocities of sediment particles and particulate (settleable) chemicals
are increased by a value equal to the infiltration rate in the basin. The impact
would, of course be more important for the clay (colloidal) sized particles than for
silt, sand, and small or large aggregates.
3. Infiltration practices differ from typical dry basins because they contribute to
groundwater recharge, therefore providing an additional element of performance.
4. Because they can provide volume control, infiltration basins can effectively
address the issues of increased frequency and duration of peak flows that are
important in providing downstream channel protection.
5. Because they operate by infiltration of runoff into the subsurface soils, infiltration
basins are able to preclude the thermal impacts issues associated with detention ,
extended detention and wet ponds.
The use of infiltration practices depends on careful site investigation. Table 3-7 and 3-8
previously addresses some of the concerns with infiltration, which primarily focus on possible
contamination of groundwater. If allowed by local conditions, i.e. allowed by local regulation
and provided that the infiltrating soil has sufficient infiltration capacity, infiltration basins are an
excellent watershed management tool to enhance water quality.
5.2 Design of Wet Ponds to Maximize Sedimentation
The primary removal mechanism for pollutants in wet ponds is by settling of the solid
materials. Thus, wet ponds should be designed to maximize sedimentation within the permanent
pool. The permanent pool of water is equal to some fraction or multiple of the runoff volume.
The runoff displaces a portion of the pool volume and is treated during the dry period and in turn
is displaced by the next storm. A schematic of this wet pond design is illustrated in Figure 5-1.
Schueler and Helfrich (1989) summarized some typical design criteria for this approach in Table
5-1.
5-5
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Chapter 5: Pond BMPs
September 2002
BMP Design Considerations
STJUJNG
BASIN
EMERGENCY
SPILLWAY
RIS01/
BARREL
PLAN VIEW
INFLOW
^STfLLWGBASW
100 YEAR LEVEL
\J 10 YEAR LEVEL
=" ^Cp, Of 2 YEAR LEVEL
EMBANKMENT
RISER
IX
INFILTRATION STi
CLEANOUTS-_ VALVE-,
|1 ^|^=h»
=nail=j
BARREL
BACKUP IWD6RDRAIN PIPE W CASE OF .
STANDING WATB^ PROBLEMS
FITER DIAPHRAGM
EMERGENCY
SPILLWAY
PROFILE
Figure 5-4 Infiltration Basin, Typical Detail (MDE, 2000)
5-6
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Chapter 5: Pond BMPs
September 2002
BMP Design Considerations
Table 5-1 Hydrologic and Hydraulic Design Criteria for Standard Extended
Detention Wet Pond System (Schueler and Helfrich, 1989)
Permanent Pool Storage
Design criteria
Storage volume
Water surface elevation
Pipe Sizing (Pool drain)
Treat first flush of runoff
For first inch of rainfall = Rv * drainage area (sq ft)/ 12, where
Rv = 0.05 + (0.009 * percent imperviousness) - see footnote
Established by invert of ED Pipe
Drain pool volume within 24 hours
Extended Detention Storage
Design criteria
Storage volume
Water surface elevation
Pipe sizing - Allowable release
rate (Qr)
Provide minimum 24 hours of detention for next one-half inch
watershed runoff
One-half inch * watershed area
Upper limit set at beginning of 2 year storm water storage
(Qr) = [(0.5 acre - in.)(43560 cf / acre) (ft / 12 in)] / [2(24hrs)]
Two Year Storm Event Peak Discharge Control Storage
Design criteria
Storage volume
Water surface elevation
(W.S.E.)
Maintain Pre-Development Peak Discharge for the Two Year
Design Storm Event
Obtained from TR-55, short cut method , TR-20, HEC-HMS or
other methods which produce similar results
Upper limit: bottom of 100-year storage.
Lower limit: top of extended detention storage.
Safety Storm / Emergency Spillway
Design criteria
Storage volume
Safety storm (SS): Design event depends on hazard class
Emergency Spillway: Must pass safety storm
SS: Obtained from TR-20 (NRCS, 1982)
ES: Obtained from NRCS Spillway charts (NRCS, 1982)
Footnote: Using this formula, to control the first inch of rainfall at a 5 acre development with 25% imperviousness,
you would need: storage volume = ((0.05 + (0.009 * 25)) * 217,800/12 = 4,990 cu ft. 25% is 25 in this equation, not
0.25. Also, one-drainage area inch * drainage area is the volume of rain falling on the drainage area in inch-square
feet.
A second approach treats the wet pond as a lake with controlled levels of eutrophication
to account for the biological and physical/chemical processes that are principal mechanisms for
nutrient removal (Hartigan, 1989 and Walker, 1987). General criteria for this approach are
summarized in Table 5-2. Both approaches relate the pollutant removal efficiencies to hydraulic
residence time.
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Chapter 5: Pond BMPs
September 2002
BMP Design Considerations
Table 5-2 Recommended Criteria for Wet Pond Design for Nutrient Removal*
(Hartigan et al, 1989)
Design Parameter
1 . Storage Volume
(Permanent Pool)
2. Mean Depth (Permanent
Pool)
3 . Surface Area (Permanent
Pool)
4. Drainage area
5. Side slopes
6. Length/width ratio
7. Soils at site
Recommended Criteria
On-Site Wet Pond
a. T = 2 weeks or more
b. VB/VR>4ormore
3 to 6 feet
> 0.25 acres
Minimum of 20 - 25 acres
5:1 to 10:1 (H:V)
2:1 or greater
Hydrologic Soil Groups B,C, and
D (Compaction may be required
on A and B soils)
Regional
Same as onsite
Same as onsite
3 to 5 acres or more
100-300 acres depending on
impervious cover
T = average hydraulic residence time
* Projected Nutrient removal (P=65%, Solids 85-90%)
The design approach should be selected based upon the target of the control efforts as
well as site and economic constraints. The controlled eutrophication approach requires longer
residence times and larger storage volumes comparable to those of the solids settling approach.
However, where the chief concern is to control nutrient levels in waters such as lakes and
reservoirs, it is then advantageous to use the controlled eutrophi cation. If the major goal is the
removal of a broad spectrum of pollutants, especially those adsorbed onto suspended matter, it
may be preferable to base the design criteria on the sedimentation models. Presently, most pond
water quality practice designs for runoff pollution control rely heavily on the sedimentation
process.
Volume of the Permanent Pool This volume, in relation to the drainage area or runoff volume,
is the most critical parameter in the sizing of the wet pond and its ability to remove pollutants.
Various design criteria or rules of thumb are expressed in terms of the VB/VR ratio where VB is
the volume of the permanent pool and VR is the volume of runoff for an average storm. A
starting point for selecting a design would be to size the pool for a hydraulic detention time,
which is a simple calculation to make. An estimate of the detention time T(in years) is given by
dividing the permanent pool volume VB by the product of the total number of runoff events per
year, n, namely:
5-
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
T- VB
nVR
Field studies indicate that an optimum nutrient removal of approximately 50% occurs at T values
of 2 to 3 weeks for pools with mean depths of 3 to 6 ft (Hartigan, 1989). In the eastern U.S., this
optimum range for T values corresponds to VB/VR ratios of 4 to 6. Ponds with values of T
greater than 2 to 3 weeks have a greater risk of thermal stratification and anaerobic bottom
waters, resulting in an increased risk of significant export of nutrients from bottom sediments
and possible odor problems if the pond becomes anaerobic.
State and regional stormwater management regulations and guidelines often address
design criteria for the permanent pool storage volume in terms of either average hydraulic
retention time, T, the ratio VB/VR, or minimum total suspended sediment removal rate. For
example, the State of Florida requires an average hydraulic retention time of 14 days, equivalent
to VB/VR of 4; the Urban Drainage and Flood Control District's BMP criteria manual in the
Denver, Colorado, area (UDFCD, 1992) specifies that the permanent pool storage volume should
be 1.0 to 1.5 times the "water quality capture volume," which is equivalent to VB/VR on the
order of 1.5 to 2.5. A municipal BMP handbook published by the California State Water
Resources Control Board (Camp Dresser & McKee et al., 1993) recommends that retention pond
permanent pools be sized for a VB/VR of 3.
Some State or local regulations require detention of a specified runoff volume as
surcharge above the permanent pool. Storage in the surcharge zone is released during a specified
period through an outlet structure. This surcharge detention requirement is intended to reduce
short circuiting and enhance settling of total suspended sediments. Settling-solids analysis
shows that retention ponds sized for nutrient removal with a minimum detention time, T, of 2
weeks and a minimum VB/VR of 4 achieve total suspended sediment removal rates of 80 to
90%. North Carolina's stormwater disposal regulations for coastal areas and water supply
watersheds specify that the permanent pool should be sized to achieve a total suspended
sediment removal rate of 85%, which is equivalent to a VB/VR in the range of 3 to 4 when no
surcharge extended detention is provided. With surcharge extended detention, 85% removal of
total suspended sediments has been achieved with a VB/VR of 2 or less.
Addition of an extended detention zone above the permanent pool is unlikely to produce
measurable increases in the removal of total suspended sediments. Still, a surcharge extended
detention volume is recommended whenever the VB/VR, is less than 2.5. Whenever one is used
or required, it is suggested that the maximized event-based volume with a 12-hour drain time be
used. In cases where relatively permeable soils (HSG A and B) are encountered and infiltration
basins are not an option, the risk of drawdown may be minimized by installing a six inch clay
liner at the bottom of the pond (HSG A), or simply by compacting the pond soils (HSG B).
5-9
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
Pool Depth The depth of the permanent pool is an important design parameter since it affects
solids settling. Mean depth of the pool is obtained by dividing the storage volume by the pool
surface area. The pool should be shallow enough to ensure aerobic conditions and avoid thermal
stratification, yet be deep enough to minimize algal blooms and resuspension of previously
deposited materials by major storms and wind generated disturbances. Prevention of thermal
stratification will minimize short-circuiting and maintain aerobic bottom waters, thus
maximizing pollutant uptake and minimizing the potential release of nutrients to the overlying
waters. An average depth of 3 to 6 ft is sufficient to maintain the environment within the pool. A
ten-foot wide and one-foot deep bench is needed around the perimeter of the pool to promote
native aquatic vegetation and to reduce a potential safety hazard to the public. Shallow depth
near the inlet structure is desirable to concentrate sediment deposition in a smaller and easily
accessible area. The riser should be located in a deeper area to facilitate withdrawal of cold
bottom water for the mitigation of downstream thermal impacts, if any.
Mean depth of the permanent pool is calculated by dividing the storage volume by the
surface area. The minimum depth of the open water area should be greater than the depth of
sunlight penetration to prevent emergent plant growth in this area, namely, on the order of 6 to 8
ft. A mean depth of approximately 3 to 10 ft should produce a pond with sufficient surface area
to promote algae photosynthesis and should maintain an acceptable environment within the
permanent pool for the average hydraulic retention times recommended above, although separate
analyses should be performed for each locale. If the pond has more than 2 ac of water surface,
mean depths of 6.5 ft will protect it against wind generated resuspension of sediments. The
mean depths of the more effective retention ponds monitored by the NURP study typically fall
within this range. A water depth of approximately 6 ft over the major portion of the pond will
also increase winter survival offish (Schueler, 1987).
A maximum depth of 10 to 13 ft should be used to reduce the risk of thermal
stratification. However, in the State of Florida, pools up to 30 ft deep have been successful when
excavated in high groundwater areas. This is probably because of improved circulation at the
bottom of the pond as a result of groundwater moving through it.
Readily visible stormwater management facilities receive more and better maintenance
than those in less visible, more remote locations. Readily visible facilities can also be inspected
faster and more easily by maintenance and mosquito control personnel. If maintained at the
recommended 3 to 6 ft depth, the permanent pool can serve as aquatic habitat.
Minimum Surface Area of Permanent Pool Minimum surface area will be contingent upon
local topography, minimum depth and solids settling guidelines. For on-site wet pond water
quality basins, the typical minimum pool surface area is 0.25 acres.
Minimum Drainage Area and Pond Volume The minimum drainage area for an on-site wet
pond water quality structure should be large enough to sustain the wet pond during the summer
periods. The drainage area should permit sufficient base flow to prevent excessive retention
5- 10
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
times or severe drawdown of the permanent pool during dry seasons. Unless regional experience
is available for determining the minimum drainage area required in a particular location, it is
recommended that a water balance calculation be performed using local runoff,
evapotranspiration, exfiltration, and base flow data to ensure that the base flow is adequate to
keep the pond full during the dry season. Baseflow will, of course, vary considerable from
watershed to watershed in a region. However, a regional analysis would be helpful. This
information is typically available from the USGS offices in a state or possibly the local NRCS
office.
The maximum tributary catchment area should be set to reduce the exposure of upstream
channels to erosive stormwater flows, reduce effects on perennial streams and wetlands, and
reduce public safety hazards associated with dam height. Again, regional experience will be
useful in providing guidelines. For example, in the southeastern U.S., some stormwater master
plans have restricted the maximum tributary catchments to 100 to 300 acres, depending on the
amount of imperviousness in the watershed, with highly impervious catchments restricted to the
lower end of this range and vice versa. On the other hand, experience in semiarid areas has
shown that even a small area of new land development can cause downstream erosion and that
drainage way stabilization is needed between the new development and the pond for relatively
small catchments.
As a rule of thumb, a minimum drainage area of 20 acres is required to sustain the
desired dry weather inflow. In general, 4 acres of contributing drainage area are needed for each
acre-foot of storage. As indicated earlier, however, a local analysis is needed.
Side Slopes Side slopes along the shoreline of the retention pond should be 4H: IV or flatter to
facilitate maintenance (such as mowing) and reduce public risk of slipping and falling into the
water. In addition, a littoral zone should be established around the perimeter of the permanent
pool to promote the growth of emergent vegetation along the shoreline and deter individuals
from wading. The emergent vegetation around the perimeter serves several other functions: it
reduces erosion, enhances the removal of dissolved nutrients in urban stormwater discharges,
may reduce the formation of floating algal mats, and provides habitat for aquatic life and wetland
wildlife. This bench for emergent wetland vegetation should be at least 10 ft wide with a water
depth of 0.5 to 1.5 ft. The total area of the aquatic bench should be 25 to 50% of the permanent
pool's water surface area. Local agricultural agencies or commercial nurseries should be
consulted about guidelines for using wetland vegetation within shallow sections of the
permanent pool.
Pond Configuration Length-to-width ratio of the pond should be as large as possible to
simulate conditions found in plug flow reaction kinetics. Under the ideal plug flow conditions, a
"plug" or "pulse" of runoff enters the basin and moves as a plug through the pond without
mixing. Relatively large length-to-width ratios can help reduce short circuiting, enhance
sedimentation, and help prevent vertical stratification within the permanent pool (Griffin et al.,
1988) showed that the dead storage for length to width ratios less than 2:1 was in the range of 27
5- 11
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
percent and for length to width ratios greater than 2:1 was in the range of 17 percent. A
minimum length-to-width ratio of 2:1 is therefore recommended for the permanent pool. The
permanent pool should expand gradually from the basin inlet and contract gradually toward the
outlet, maximizing the travel time from the inlet to the outlet. Baffles or islands within the pool
can increase the flow path length and reduce short circuiting. Hartigan et al. (1989)
recommendations of a minimum 3:1 ratio for optimal sedimentation (Table 5-1) did not consider
the use of baffles.
To reduce the frequency of major clean out activities within the pool area, a sediment
forebay with a hardened bottom should be constructed near the inlet to trap coarse sediment
particles. A frequently used value for the forebay storage capacity is approximately 10% of the
permanent pool storage. Access for mechanized equipment should be provided to facilitate
removal of sediment. The forebay can be separated from the remainder of the permanent pool by
one of several means: a lateral sill with wetland vegetation, two ponds in series, differential pool
depth, rock-filled gabions, a retaining wall, or a horizontal rock filter placed laterally across the
permanent pool.
Outlets An outlet for a retention pond typically consists of a riser with a hood or trash rack to
prevent clogging and an adequate antivortex device for basins serving large drainage areas.
Antiseep collars should be installed along outlet conduits passing through or under the dam
embankment. If the pond is a part of a larger peak-shaving detention basin, the outlet should be
designed for the desired flood control performance. Typically, the riser structure should be sized
to drain the permanent pool within 40 hours so that sediments may be removed mechanically
when necessary. The drain pipe should be controlled by a lockable gate valve at the outlet. Flat
areas may require the use of weirs instead of risers.
An emergency spillway must be provided and designed using accepted engineering
practices to protect the basins embankment. The return period of the design storm for the
emergency spillway depends on the hazard classification, which can vary from region to region.
The designer should make certain that the pond embankment and spillway are designed in
accordance with federal, state, and local dam safety criteria.
Documentation of the classification of dams is normally required for plan approval by the
local regulatory agency. Such documentation typically includes, but is not limited to, location
and description of dam, configuration of the valley, description of existing development (houses,
utilities, highways, railroads, farm or commercial buildings, and other pertinent improvements),
potential for future development, and recommended classification. The classification of a dam is
normally the responsibility of the designer, and subject to review and concurrence by the
approving authority. The classification of a dam is normally determined only by the potential
hazard from failure. Classification factors can be obtained in the NRCS National Engineering
Manual.
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
The inlet design should dissipate flow energy and diffuse the inflow plume where it
enters the forebay or permanent pool. Examples of inlet designs include drop manholes, energy
dissipaters at the bottom of paved rundown, a lateral bench with wetland vegetation, and the
placement of large rock deflectors.
Thermal Effects Thermal effects of the wet pond must be considered since the pool acts as a
heat sink during the summer period, between the storm events. When the water is displaced
from the pool, it may be as much as 10 degrees Fahrenheit warmer than naturally occurring
baseflow. Large impervious surfaces can also significantly raise the temperature of runoff in the
summer months. The net result of elevated pool temperatures may have an adverse impact on
downstream coldwater uses such as trout production. Most streams in mature urban areas do not
fall into this category. However, in newly urbanizing areas, the pond designer should pay
special attention to the potential of thermal effects on downstream water bodies supporting cold
water fisheries. Thermal impacts in such areas may be eliminated or mitigated by: (a) prohibiting
wet ponds altogether, (b) diverting most of the baseflow and bypassing the wet pond entirely, (c)
utilizing a design with a drastically undersized permanent pool, (d) using a design with a deep
pool and positioning the inlet of the outlet pipe to withdraw cooler water from near the bottom,
(e) planting shade trees on the periphery of the pool (other than the dam) to reduce warming in
the summer, (f) directing baseflow through the wetland while channeling storm flow to a fringe
pool area and (g) employing a series of pools in sequence rather than a single one.
Other Considerations A wet pond basin contains a permanent pool in addition to the flood
control storage. To maintain water quality (oxygen levels), control mosquito breeding and
prevent stagnation, a sufficient inflow of water (either surface or ground water) is necessary on a
regular basis. A fountain or solar powered aerator may be used for oxygenation of water. The
potential effects of sediment loading on the permanent pool should be considered when
determining if a site is suitable for a wet pond basin. The use of existing lakes and ponds as wet
ponds for treatment of stormwater is sometimes prohibited.
A well designed pond will accumulate considerable quantities of sediment. A typical
clean out cycle for a wet pond in a stabilized watershed is approximately 10 years, with sediment
removal at each cycle costing as much as 20 - 40% of the initial construction cost.
5.3 Design of Extended Detention Basins for Water Quality Improvements
Design Considerations - Sizing the Basin Extended detention basins are normally sized to
store the peak storm flow and then discharge it after the rainfall subsides. This means that the
peak flow after urbanization matches the pre-development peak flow. Procedures for making
this design are straightforward and equations range from simple relationships to those that
require the use of computer models such as HEC-HMS (U.S. Army Corps of Engineers, 2001)
and the NRCS TR20 (USDA, 1986) program. The design can be for an average storm or for 2,
10, 25, 50, and 100 year storms, depending on the regulatory authority.
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
Basin Configuration Extended detention basins should be made an integral part of the
community as much as possible. Consideration should be given to multiple uses, aesthetics,
safety, and the way the facility will fit into the urban landscape. Also, maintenance is an
important consideration, and the design layout must provide access for maintenance equipment.
Although these basins provide passive treatment with no operational attention, continued
successful performance will depend on good maintenance.
Figure 5-3 shows an idealized layout for an extended detention basin. The individuality
of each on-site or regional facility and its place within the urban community make it incumbent
on the designer to seek out local input, identify site constraints, identify the community's
concerns, and consider a wide array of possibilities during design.
Storage Volume Storage volume, sometimes called capture volume, is needed to detain the
flow long enough to capture the desired pollutants and keep the peak discharge less than the pre-
developed peak. If significant sedimentation is occurring, an additional volume should be added
to account for the deposited solids. For critical areas, a complete sediment yield analysis over a
period of years (e.g., 20 years) would need to be made to determine the probable build-up of
deposited sediment. For less critical areas, an addition of 20% to this detention volume to
provide for sediment accumulation is a reasonable assumption. Randall et al. (1982) and
Whipple and Hunter (1981) suggest that such detention basins be designed to promote
sedimentation of small particles, namely smaller than 60 microns in size, which account for
approximately 80% of the suspended sediment mass found in stormwater (Urbonas and Stahre,
1993).
Selection of pond volumes and design of outlets should allow time for most of the
stormwater particles to settle. This includes particles in the first part of the storm, so
consideration should be given to providing an outlet to empty less than 50% of the design
volume in the first one-third of the design emptying period (that is, 12 to 16 hours). This ensures
that small runoff events will be detained long enough to remove small suspended solids. Also,
the discharge from the pond will be slower immediately after a storm than hours later, while
releasing small rain events more slowly than larger ones. A long emptying time-thus the term
extended detention-permits smaller particles to attach to the bottom of the basin and become
trapped.
Flood Control Storage Whenever feasible the functions of the extended detention basin should
be incorporated within a larger flood control facility. The designer may want to consider
combining water quality and flood control functions in a single detention basin.
Basin Geometry The basin should gradually expand from the inlet and contract toward the
outlet to reduce short circuiting. Griffin et al. (1985) found that an aspect ratio (length to width
ratio) of 2:1 or greater reduces short circuiting within the pond.
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
Two-Stage Design A two-stage basin is preferable. The lower stage has a micropool that fills
frequently. This reduces the periods of standing water and sediment deposition in the remainder
of the basin. The upper stage should be 2 to 6 ft deep, its bottom sloping at approximately 2%
toward a low-flow channel. The bottom pool should be 1.5 to 3 ft deeper and should be able to
store 15 to 25% of the capture volume. These recommendations do not necessarily apply to
large, regional extended detention basins. The impact of these considerations varies with climate
and soil types.
Basin Side Slopes Basin side slopes must remain stable under saturated soil conditions. They
also need to be sufficiently gentle to limit rill erosion, facilitate maintenance, and address the
safety issue of individuals falling in when the basin is full of water. Side slopes of 4:1 H:V and
flatter provide well for these concerns.
Forebay The basin should be designed to encourage sediment deposition to occur near the
point of inflow. A forebay with a volume equal to approximately 10% of the total design
volume can help with the maintenance of the basin, and the service life of the remainder of the
basin can be extended. A stabilized access and a concrete or soil cement lined bottom should be
used to prevent mechanical equipment from sinking into the bottom. This should also facilitate
sediment removal, since the procedure of scraping material from a concrete bottom will not
necessitate reforming the bottom or resetting/repairing liners.
Basin Inlet Most erosion and sediment deposition occurs near the inlet. An ideal inflow
structure will convey stormwater to the basin while preventing erosion of the basin's bottom and
banks, reducing resuspension of previously deposited sediment and facilitating deposition of the
heaviest sediment near the inlet. These design goals are achievable in most cases, allowing for
minor compromises. Inflow structures can be drop manholes, rundown chutes with an energy
dissipater near the bottom, a baffle chute, a pipe with an impact basin, or one of the many other
types of diffusing devices.
Low-Flow Channel A low-flow channel may be required by local regulation to convey trickle
flows and the last of the captured volume to the outlet. This device prevents water logging and
enhances the growth of vegetation. It also accelerates flows from small storms and is not
recommended from a receiving water quality standpoint.
Outlet Type and Protection An outlet capable of slowly releasing the design capture volume
over the design emptying time should be used. An arrangement of an outlet was suggested by
Schueler et al. (1992), wherein a hooded and perforated riser is located in a small permanent
pool, such that a micro pool is formed. Additionally, a number of alternative details for outlet
structures are available.
Because extended detention basins are designed to encourage sediment deposition and
urban stormwater has substantial quantities of settleable and floatable solids, basin outlets are
prone to being clogged. This can make the design of reliable outlet structures for extended
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
detention basins difficult. A clogged outlet will invalidate the hydraulic function of even the
best design.
The diameter of the low flow orifice is a key element of outlet design. ASCE (1985),
ASCE (1992), DeGroot (1982), Roesner et al. (1989), Schueler (1987), Schueler et al. (1992),
Urbonas and Roesner (Eds.) (1986), and Urbonas and Stahre (1993) reported many reasons for
outlet problems, which include clogging by trash and debris, burial by silt, vandalism, animals
blocking an outlet (i.e., rodent nests) and other factors that modify its discharge characteristics.
Each outlet has to be designed with clogging, vandalism, maintenance, aesthetics, and safety in
mind. An orifice that is too large may result in high discharge rates for smaller storms. The
smaller storms which contain the bulk of the annual pollution load would have short residence
times in the BMP and this would result in limited water quality benefit. Smaller outlet orifices
are necessary to maximize detention times of smaller storms (Newman et al., 2000).
If the outlet is not protected by a gravel pack, some form of a trash rack should be
provided. Wrapping a perforated outlet in a geotextile filter cloth, which will clog quickly, is
not a recommended practice.
Dam Embankment The dam embankment should be designed and built so that it will not fail
during storms larger than the water quality design storm. An emergency spillway should be
provided or the embankment designed to withstand overtopping commensurate with the size of
the embankment, the volume of water that can be stored behind it, and the potential of
downstream damages or loss of life if the embankment fails. Emergency spillway designs vary
widely with local regulations. Embankments for small on-site basins should be protected from at
least the 100-year flood, while the larger facilities should be evaluated for the probable
maximum flood. Consulting the state's dam regulatory agency is always a good idea..
Embankment slopes should typically be no steeper than 3:1, preferably 4:1 or flatter.
They also need to be planted with turf-forming grasses. Embankment soils should be compacted
to 95% of their maximum density at optimum moisture.
Vegetation A basin's vegetation provides erosion control and enhances sediment entrapment.
The basin can be planted with native grasses or with irrigated turf, depending on the local
setting, basin design, and its intended other uses (such as recreation). Sediment deposition,
along with frequent and prolonged periods of inundation, make it difficult to maintain healthy
grass cover on the basin's bottom. Options for an alternative bottom liner include a marshy
wetland bottom, bog, layer of gravel, riparian shrub, bare soil, low weed species, or other type
that can survive the conditions found on the bottom of the basin.
Maintenance Access Vehicular maintenance access to the forebay should be available along
with the outlet areas with grades that do not exceed 8 to 10% and have a stable surface of gravel-
stabilized turf, a layer of rock, or concrete pavement.
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Chapter 5: Pond BMPs BMP Design Considerations
September 2002
5.4 Maintenance of Pond BMPs
Regular inspection and maintenance of BMPs are necessary if these facilities are to
consistently perform up to expectations. Stormwater management systems are expected to
perform quality and quantity control functions as long as the land use they serve exists. Failure
to maintain these systems can create the following adverse impacts:
• Increased discharge of pollutants downstream.
• Increased risk of flooding downstream.
Increased downstream channel instability, which increases sediment loadings and
reduces habitat for aquatic organisms.
Potential loss of life and property, resulting from catastrophic failure of the
facility.
• Aesthetic or nuisance problems, such as mosquitoes or reduced property value,
due to a degraded facility appearance.
Most of these impacts can be avoided through proper and timely inspection and maintenance. A
major concern associated with these impacts is the general public's expectations relating to the
quality of life provided, in part, by construction of these systems. Inadequate maintenance means
the general public may have a false sense of security. The most common cause of stormwater
system failure is the lack of adequate and proper operation, inspection, maintenance, and
management. If stormwater management systems are not going to be adequately maintained, the
facilities should not be constructed in the first place.
Good design and construction can reduce subsequent maintenance needs and costs but
they cannot eliminate the need for maintenance altogether. Maintenance requires a long term
commitment of time, money, personnel, and equipment. Monitoring the overall performance of
the stormwater management system is a major aspect of any maintenance program. Wet
detention and wetland systems are especially complex environments which require a healthy
aquatic ecosystem to provide maximum benefits and to minimize needed maintenance.
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Chapter 6: Vegetative Biofilters BMP Design Considerations
September 2002
Chapter Six
Types of Vegetative Biofilters and Their Ability to Remove
Pollutants
6.1 Introduction
Historically vegetative biofilters, such as grass swales, were used primarily for
stormwater conveyance (Ree, 1949; Chow, 1959; Temple, 1987). However with the passage of
the Clean Water Act, and the focus on water quality management of urban runoff, the potential
for the application of these techniques has begun to be reconsidered and many additional benefits
have been identified. Today biofilters are being applied to address design objectives of urban
stormwater management. These include: reduction of urban runoff impacts, groundwater
recharge, water quality control, stream channel protection, and peak discharge control for both
small storms (6-month and 1-year frequency storms), and large storms (2, 10 and 100-year
storms). The most common application of the biofilters, however, is typically their use as the
first stage of the treatment train approach and their purpose is to partially address groundwater
recharge and water quality control for small headwater areas.
Three different types of vegetative biofilter BMP types have been identified and are
described in this document. These are: grass swales, vegetated filter strips, and bioretention
cells. Grass swales include three variations: traditional grass swales, grass swales with media
filters and wet swales.
6.2 Grass Swale
Grass swales have traditionally been used as a low cost stormwater conveyance practice
in low-to-medium density residential developments (e.g., 1A- acre lots). Most public works
agencies throughout the U.S. have a typical rural road sectional that allows the use of vegetated
swales within the public right of way. During the early years of stormwater management
technology the focus was on peak discharge control and grass swales were not given much
consideration. As the focus of stormwater management programs expanded to include water
quality considerations and pollutant reduction, the grassed swale has been perceived to represent
6- 1
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Chapter 6: Vegetative Biofilters BMP Design Considerations
September 2002
a potentially important element of the treatment train approach to total stormwater management
(Yousef, et al, 1986; Yu, 1992; Yu, 1993).
Grass swales have a number of desirable attributes with respect to total stormwater
management (MDE, 2000; ASCE, 1998; CRC, 1996; Yu, 1993;) including:
• slower flow velocities than pipe systems that result in longer times of
concentration and corresponding reduction of peak discharges;
ability to disconnect directly connected impervious surfaces, such as driveways
and roadways thus reducing discharge;
filtering of pollutants by grass media;
infiltration of runoff into the soil profile thus reducing discharges, providing
additional pollutant removal, and increasing groundwater recharge; and
• uptake of pollutants by plant roots (phytoremediation)
A typical grass swale is shown in Figure 6-1. The section shows that the water quality volume
(WQv) is a fraction of the typical 2 and 10 year design storms.
Grass Swale with Media Filters Also known as a dry swale, this grass swale consists of an
open channel that has been modified to enhance its water quality treatment capability by adding
a filtering medium consisting of a soil bed with an underdrain system (CRC, 1996). It is
designed to temporarily store the design water quality volume (WQv) and allow it to percolate
through the treatment medium. The system is designed to drain down between storm events
within approximately one day. The water quality treatment mechanisms are similar to
bioretention cells except that the pollutant uptake is likely to be more limited since only a grass
cover crop is available for nutrient uptake.
Wet Swale The wet swale also consists of a broad open channel capable of temporarily routing
and storing the water quality volume (WQv) but does not have an underlying filtering bed (CRC,
1996). It is constructed directly within existing soils and may intercept the water table. Like the
dry swale, the WQv within the wet swale should be stored for approximately 24 hours. The wet
swale has water quality treatment mechanisms similar to stormwater wetlands, both of which
rely primarily on settling of suspended solids, adsorption, and uptake of pollutants by vegetative
root systems. Figure 6-2 illustrates the design components of the wet swale (MDE, 2000).
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Chapter 6: Vegetative Biofilters
September 2002
BMP Design Considerations
CHANNEL LENGTH IS DIRECTLY PROPORTIONAL TO ROADWAY LENGTH
- OPTIONAL CHECK DAM
- ROADWAY —»
PLAN VIEW
VELOCITY LESS THAN 1 -0 tos
FOR r RAINFALL
r to r WIDTH
(tor WQV ONLY)
SECTION
Figure 6-1 Grass Swale (MDE, 2000)
6-
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Chapter 6: Vegetative Biofilters
September 2002
BMP Design Considerations
RIPRAP
,— GRAVEL INLET TRENCH
i r-1/2 ROUND PIPE-WEIR
PRETRiATMENT
/ (FOREBAr)
. ROADWAY -
PLAN VIEW
SMOULDER
ROADWAY
SLOf^ OR FLATTER
^_2
FILTER FABRIC
PERFORATED PIPE
SECTION
Figure 6-2 Wet Swale (MDE 2000)
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Chapter 6: Vegetative Biofilters BMP Design Considerations
September 2002
6.3 Vegetative Filter Strip
Vegetative filter strips (VFSs) and buffers are areas of land with vegetative cover that are
designed to accept runoff as overland sheet flow from upstream development. They can either
be constructed or existing. Dense vegetative cover facilitates sediment attenuation and pollutant
removal for the design storms. Unlike grass swales, vegetated filter strips are primarily designed
for overland sheet flow. Grading and level spreaders can be used to create a uniformly sloping
area that distributes the runoff evenly across the filter strip. For small storms that do not
discharge, infiltration becomes the primary removal mechanism.
Filter strips have been used to treat runoff from roads and highways, roof downspouts,
very small parking lots, and pervious surfaces. They can also be used as the "outer zone" of a
stream buffer but are usually most effective as pretreatment to another treatment BMPs such as
infiltration basins or trenches. Figure 6-3 illustrates the primary design components of the filter
strip (CRC, 1996).
6.4 Bioretention Cell
The bioretention concept was originally developed in the early 1990's as an alternative to
traditional BMP structures (Clar, etal., 1993, 1994). Bioretention is a practice to manage and
treat stormwater runoff using a conditioned planting soil bed and planting materials to filter
runoff stored within a shallow depression. The method combines physical filtering and
adsorption with biological processes and usually takes place in a bioretention cell. The system
consists of a flow regulation structure, a pretreatment filter strip or grass channel, a sand bed, a
pea gravel overflow curtain drain, a shallow ponding area, a surface organic layer of mulch, a
planting soil bed, plant material, a gravel underdrain system, and an overflow system. Figure 6-
4 illustrates these primary design components of the bioretention cell (MDE, 2000).
6.5 Role in Water Quality Improvement
Table 6-1 summarizes the pollutant removal capability reported as percent removal of
biofilter BMPs for the following constituents: TSS, total phosphorus (TP), total nitrogen (TN),
Nitrates, (NO3), and metals. Biofilters have some similarities with respect to performance, but
their flow reduction and pollution removal capabilities are basically a function of their size
relative to the inflow drainage volume (or long-term infiltration capacity volume) ratio
(volume/area). For example, all of these facilities typically report relatively high removal rates
of suspended sediment, ranging from 68% for the grass channel to 90% or more for the dry
swale and the bioretention cell. The bioretention cell is typically much smaller than the other
units; therefore, the total loading would be smaller.
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Chapter 6: Vegetative Biofilters
September 2002
BMP Design Considerations
RIPRAP
AOOmOKAi STORAGE
[i****************
************* *JF**
************* ****
^*************
i^** *****************
;******
3F3F3F3FW-*
***** PLANTINGS
*********************
-SHOULDER—^
^ y y
y y
«_ ROADWAY—,
PLAN VIEW
WATER TABLE (VARIABLE)
V-NOTCH WEIR
PROFILE
Figure 6-3 Typical Vegetative Filter Strip (CRC, 1996)
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Chapter 6: Vegetative Biofilters
September 2002
BMP Design Considerations
iou wax.
Flow Length
V
Lawn
* Flow Length
\
*
s
y
; — i
Park
ing
Planted With Grass Tolerant
to Frequent Inundation
\ /
Filter Strip
25" Min.
Length
Curb
Stops
Pea Gravel
Diaphragm
Maximum Pervious
Ponding Limit Material
^- Berm
rz:
Overflow Spillways
Forest Buffer
-— Outlet
Pipes, Spaced
25' Centers
Curb
Stop
Parking V
Lot *
Grass Filter Strip Length (25' Min.)
Shallow Ponding Limit
Pervious Berm
(Sand/Gravel Mix)
12"x24"
Pea Gravel
Diaphragm
Water Quality
Treatment Volume
Outlet Pipes Buffer
12" Max.
PROFILE
Figure 6-4 Typical Bioretention Cell (MDE, 2000)
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Chapter 6: Vegetative Biofilters
September 2002
BMP Design Considerations
Table 6-1 Estimated Pollutant Removal Capability of Biofilters (Winer, 2000; Yu
and Kaighn, 1992, Davis et al., 1998)
Biofilter
Grass Swale
Dry Swale
Wet Swale
Filter Strip
Bioretention
TSS*
68
93
74
70
95
TP
29
83
28
25
83
TN
N/A
92
40
NA
43
NO3
-25
90
31
10
23
Other / Comments
Metals: Cu (42); Zn (45)
Hydrocarbons: 65%
Bacteria: Negative
Metals: Cu (70); Zn (86)
Metals: Cu(ll);Zn (33)
Metals: 40-50%
Metals: 93-99%
* Removals shown as percentages
Some differences have been observed in the comparative ability to remove total
phosphorus. The best performers were the dry swale and bioretention cells with removal rates of
83% and 70% respectively. Grass channels, wet swales and filter strips were less reliable, at 10
to 29% average removal. Vegetative biofilters display a wide range of total nitrogen removal.
The dry swale exhibited a very high removal rate of 92%.
While all biofilter designs showed at least moderate capacity to remove trace metals such
as copper, lead, and zinc, most of the removed metals were already attached to particles.
Designs that showed promise in removing dissolved metals include the dry swale and
bioretention cell.
Pollutant removal and mechanisms rely on processes in a generally aerobic environment,
as opposed to anaerobic environment. Filters which go anaerobic tend to release previously
captured phosphorous as iron phosphates break down.
Compatibility with Land Use Type As a group, vegetative biofilters can be applied to a
diverse range of land use types. However, individual designs are limited to a much narrower
range. These common land use situations include ultra-urban sites, parking lots, road and streets,
small residential subdivisions and backyard/rooftop drainage. Table 6-2 is a matrix that
illustrates the most economical and feasible biofilter designs for each of these five broad
categories of development, as well as those that are not applicable. As previously discussed,
devices that rely on infiltration should take into consideration the fate of possible pollutants in
the groundwater.
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Chapter 6: Vegetative Biofilters BMP Design Considerations
September 2002
Table 6-2 Land Use and Biofilter Suitability
Urban Retrofit
Parking Lots
Roads & Highways
Residential
Rooftops
Bioretention cell has proven very versatile for use in retrofit conditions.
Swales are usually not well suited.
Bioretention cell is well suited for use in parking lots.
Swales may be suitable under certain conditions (space, soils, water table).
Filter strips can be effective (Figure 6-1)
City streets generally do not provide enough space for any biofilter Suburban
areas, specially large to medium lot subdivision can accommodate all of the
biofilters.
Highways may accommodate biofilters if sufficient space is available in
median or side slopes.
Low density residential affords opportunities for all biofilter uses.
High density residential may offer limited opportunity based on space
availability.
Roof drain disconnections to filter strips or bioretention areas are
recommended where feasible.
For example, in ultra-urban or retrofit settings where space is at a premium, the
bioretention cell is one of the most versatile biofilters. In most cases, the space requirements of
swales and filter strips are so great that they can be eliminated from consideration in downtown
urban areas, but bioretention cells may be considered as a retrofit to partially treat urban runoff.
Compatibility with Site Conditions Table 6-3 compares how each biofilter design compares
with respect to a number of site conditions, including soils, water table, drainage area, slope head
and space consumed.
6.6 Design of Grass Swales for Pollutant Removal
Pollutants are removed in swales by settling, deposition in low velocity areas, or by
infiltration into the subsoil. The primary pollutant removal mechanism is through sedimentation
of suspended materials for larger particles and infiltration for colloidal particles and dissolved
solids. Therefore, suspended solids and adsorbed metals are most effectively removed through
the traditional grass swale (rather than the swale with filter media or wet swale). Removal
efficiencies reported in the literature vary, but generally fall into the low-to-medium range, with
some swale systems recording no water quality effects at all. Schueler (1992), reported that of
10 swales monitored, 50 percent registered moderate pollutant removal, while the remainder
showed negligible or negative removal.
The amount of pollutant removed will depend on the length of the swale. Table 6-4
presents the pollutant removal efficiencies for 200 ft and 100 ft swale lengths. Although
research results varied, these data clearly indicate increased pollutant removal efficiencies with
longer swales.
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Chapter 6: Vegetative Biofilters
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BMP Design Considerations
Table 6-3 Physical Site Conditions and Biofilter Suitability
(modified from MDE, 2000)
Biofilter
1) Grass Swale
Dry Swale
Wet Swale
2) Filter strip
3) Bio-
retention Cell
Soils
OK
Filter
Media
OK
OK
Filter
Media
Water Table
(depth)
2 feet
2 feet
Below WT
2 feet
2 feet
Drainage Area
(acres)
5 max
5 max
5 max
N/A
2 max
Slope Limits
6% max.
6% max
6% max
15% max
None
Head
2 feet
3 to 6 feet
Ifoot
N/A
5 feet
Area
Required
6.5%
10-20%
10-20%
100%
5.0%
Notes: Soils - the key evaluation factors are based on an initial investigation of the USDA HSG at the site. More
detailed geotechnical tests are usually required for infiltration feasibility and during design to confirm permeability
and other factors
Water table - the minimum depth to the seasonally high water table from the bottom or floor of a BMP.
Drainage Area - the recommended minimum or maximum drainage area that is considered suitable for the practice.
If
the drainage area present at a site is slightly greater than the maximum allowable drainage area for a practice, some
leeway is permitted or more than one practice can be installed.
Slope Restriction - the effect of slope on the practice. Specifically, the slope restrictions refer to how flat the area
where the practice may be.
Head - an estimate of the elevation difference needed at a site (from the inflow to the outflow) to allow for gravity
operation within the practice.
Area Required - indicates percentage of total drainage area requirement for BMP.
Table 6-4 Pollutant removal efficiencies for grass swales (Barret, et al., 1993;
Schueler, 1991; Yu,1993; Yousef, et al,, 1985; Horner, 1996)
Design
200-ft grass swale
100-ft grass swale
Pollutant Removal Efficiencies (%)
Solids
TSS
83
60
Nutrients
TN
25*
0
TP
29
45
Metals
Zn
63
16
Pb
67
15
Cu
46
2
Other
Oil&
Grease
75
49
COD**
25
25
*Some swales, particularly 100-ft systems, showed negligible or negative removal for TN.
**Data is very limited.
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Chapter 6: Vegetative Biofilters BMP Design Considerations
September 2002
In general, the current literature reports that a well-designed, well-maintained swale
system can be expected to remove 70% of total suspended solids (TSS), 30 percent for total
phosphorus (TP), 25 percent for total nitrogen (TN), and 50 to 90% for trace metals (Barret, et
al., 1993; GKY, 1991). The TN removals may be fairly optimistic, given that studies conducted
by Yousef et al. (1985) and others produced negative nitrogen removal in many cases, possibly
due to the remobilization of nitrogen from grass clippings and other organic materials.
Seasonal differences in swale performance can be important. In temperate climates, fall
and winter temperatures force vegetation into dormancy, thereby reducing uptake of runoff
pollutants, and removing an important mechanism for flow reduction. Decomposition in the fall,
and the absence of grass cover in the winter can often produce an remobilization of nutrients,
and may expose the swale to erosion during high flows, increasing sediment loads downstream.
Pollutant removal efficiencies for many constituents can be markedly different during the
growing and dormant periods (Driscoll and Mangarella, 1990).
6.7 Design of Vegetative Filter Strips for Pollutant Removal
Pollutants are removed in filter strips mainly by settling for larger particles and by soil
infiltration for colloidal particles. Under low-to-moderate velocity, filter strips effectively
reduce particulate pollutant levels by removing sediments and organic materials and trace metals
(Schueler, 1992). Research has shown removal of 70% for TSS, 40% to 50% for phosphorus
(particulate) and zinc, 25% for lead, and 10% for nitrate/nitrite (Florida Department of
Transportation, 1994). Settling of aggregate containing clay particles removes sorbed nutrients
and other pollutants. Removal of free soluble pollutants in filter strips is accomplished when
pollutants infiltrate into the soil, some of which are subsequently taken up by rooted vegetation.
Therefore, removal of solubles depends on the infiltration rates. The mechanism for infiltration
is minor in most filter strips during design storms or larger storms since only a modest portion of
the incoming runoff is infiltrated and most discharges, resulting in low removal rates for
solubles, but is the dominant mechanism for small storms that totally infiltrate.
Pollutant removal in filter strips is a function of length, slope, soil permeability, size of
contributing runoff area and its long-term contributing inflow volume, particle size and settling
velocity, and runoff velocity (Schueler, 1987 and Hayes et al., 1984). A wide range of values for
minimum length in the flow direction have been reported in the literature. Frequently cited
values range from 20 ft to lengths of 100 to 300 ft for adequate removal of the smaller particles.
The design guidance that follows provides analytical procedures for computing these values.
Regardless of vegetation type, the length of the filter strip is shown to have significant
influence on pollutant removal. Figure 6-5 provides one example of percent pollutant removal
efficiency versus length (Yu and Kaighn, 1992). In Figure 6-5, the relative value of adding
additional length to a filter strip for pollutant removal levels off significantly after 59 ft, with the
most significant rise in removal occurring between 19 to 59 ft. However, strip length alone does
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Chapter 6: Vegetative Biofilters
September 2002
BMP Design Considerations
not entirely define pollutant removal. The existing longitudinal slope and soil infiltration
capacity will also influence the ultimate length of the system. These factors may dictate a strip
longer than would be necessary if pollutant removal alone was the only consideration.
80
70
60
50
30 ..
I 20
10
10
20 30
Length in Meters
40
-TotalSuspendcd Solids Zinc
Lead Nitrite-Nitrate
50
Total Phosphorus
Figure 6-5 Pollutant Removal Efficiency Versus Filter Strip Length
(Yu and Kaighn, 1992)
In design, the variables that can be effectively manipulated include length and slope of
the strip, soil characteristics and vegetative cover. According to Yu and Kaighn (1992),
optimum lengths were between 20 to 30 m for a given sheetflow over the filter strip and inflow
to outflow pollutant removals. Higher pollutant removal rates for longer lengths were feasible;
however, further improvements in pollutant removal are relatively minor. The design length
would be expected to vary widely with slope, settleable particle size, soil type, infiltration
capacity and vegetation type. Avoiding the potential for concentrated flows and "gullies" will
effectively "short-circuit" the filter strip and significantly reduce removal rates. Width can also
influence pollutant removal but is often constrained by the area available.
VFS Enhancements - Level Spreader A level spreader should be provided at the upper edge
of a vegetated filter strip when the width of the contributing drainage area is greater than that of
the filter. Runoff may be directed to the level spreader as sheet flow or concentrated flow.
However, the design must ensure that runoff fills the spreader evenly and flows over the level lip
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September 2002
as uniformly as possible. The level spreader should extend across the width of the filter, leaving
only 10 feet open on each end.
There are many alternative spreader devices, with the main consideration being that the
overland flow spreader be distributed equally across the strip. Level spreader options include
porous pavement strips, stabilized turf strips, slotted curbing, rock-filled trenches, concrete sills,
or plastic-lined trenches that act as a small detention pond (Yu and Kaighn, 1992). The outflow
and filter side lip of the spreader should have a zero slope to ensure even runoff distribution (Yu
and Kaighn, 1992). Once in the filter strip, most runoff from high storm flow events will not be
infiltrated and will require a collection and conveyance system. Grass-lined swales are often
used for this purpose and can provide another BMP level. A filter strip can also drain to a storm
sewer or street gutter (Urbonas, 1992).
VFS Enhancements - Pervious Berm A pervious berm may be installed at the foot of the
strip to force ponding in a VFS. It should be constructed using a moderately permeable soil such
as ASTM ML, SM, or SC. Soils meeting USDA sandy loam or loamy sand texture, with a
minimum of 10 to 25% clay, may also be used. Additional loam should be used on the berm (±
25%) to help support vegetation. An armored overflow should be provided to allow larger
storms to pass without overtopping the berm. Maximum ponding depth behind a pervious berm
should be 1 foot.
VFS Enhancements - Types of Vegetation to Use A VFS should be densely vegetated with a
mix of erosion resistant plant species that effectively bind the soil. Certain plant types are more
suitable than others for urban stormwater control. The selection of plants should be based on
their compatibility with climate conditions, soils, and topography and the their ability to tolerate
urban stresses from pollutants, variable soil moisture conditions and ponding fluctuations.
A filter strip should have at least two of the following vegetation types: deep-rooted
grasses and ground covers; or deciduous and evergreen shrubs; or under- and over-story trees.
Native plant species should always be specified . This will facilitate establishment and long term
survival. Non-native plants may require more care to adapt to local hydrology, climate,
exposure, soil and other conditions. Also, some non-native plants may become invasive,
ultimately choking out the native plant population. This is especially true for non-native plants
used for stabilization.
Newly constructed stormwater BMPs will be fully exposed for several years before the
buffer vegetation becomes adequately established. Therefore, plants which require full shade,
are susceptible to winter kill or are prone to wind damage should be avoided. Plant materials
should conform to the American Standard for Nursery Stock, current issue, as published by the
American Association of Nurserymen. The botanical (scientific) name of the plant species
should be according to the landscape industry standard nomenclature. All plant material
specified should be suited for USDA Plant Hardiness Zones.
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Grassed filter strips should be constructed of dense, soil-binding deep-rooted water-
resistant plants. Dense turf is needed to promote sedimentation and entrapment, and to protect
against erosion (Yu and Kaighn, 1992). Turf grass should be maintained to a blade height of 50
to 60 mm (2 to 4 in). Most engineered, sheet-flow systems are seeded with specific grasses.
Common grasses established for filter strip systems are rye, fescue, reed canary, and Bermuda
(Horner, 1996). Tall fescue and orchard grasses grow well on slopes and under low nutrient
conditions (Horner, 1996). The grass species chosen should be appropriate for the climatic
conditions and maintenance criteria for each project.
Retaining existing trees and woody vegetation have been shown to increase infiltration
and improve performance of filter strips. Trees and shrubs provide many stormwater
management benefits by intercepting some rainfall before it reaches the ground, and improving
infiltration and retention through the presence of a spongy, organic layer of materials that
accumulates underneath the plants (Schueler, 1987). As discussed previously in this section,
wooded strips have shown significant increases in pollutant removal over grass strips.
Maintenance for wooded strips is lower than grassed strips, another argument for using trees and
shrubs. However, there are drawbacks to using woody plants. Since the density of the
vegetation is not as great as a turf grass cover, wooded filter strips need additional length to
accommodate more vegetation. In addition, shrub and tree trunks can cause uneven distribution
of sheet flow, and increase the possibility for development of gullies and channels.
Consequently, wooded strips require flatter slopes than a typical grass cover strip to ensure that
the presence of heavier plant stems will not facilitate channelization.
Filter strips managed to allow "natural succession" of vegetation from grasses to shrubs
and trees provides excellent urban wildlife habitat. Judicious planting of selected native shrub
and trees can be used to enhance the quality of food and cover for a variety of animal species
(Schueler, 1987). Compaction of soils during construction may not be appropriate for planting
of shrubs and trees as growth of a healthy root structure may be inhibited. To facilitate this
approach, a landscaping plan should be included in the project specifications.
Construction Guidelines Overall, widely accepted construction standards and specifications,
such as those developed by the USD A Natural Resources Conservation Service or the U.S. Army
Corps of Engineers, should be followed where applicable to construct a vegetated filter strip.
The specifications should also satisfy all requirements of the local government.
Sequence of Construction Vegetated filter strip construction should be coordinated with the
overall project construction schedule. Rough grading of the filter strip should not be initiated
until adequate erosion controls are in place.
6.8 Design of Bioretention Cells for Pollutant Removal
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BMP Design Considerations
Since this is a relatively new BMP, the available data on the pollutant removal
performance of bioretention cells is scarce. The preliminary reports from field monitoring
activities (Table 6-5) are verifying that this BMP not only met local water quality control
criteria, but actually ranked as one of the most effective pollutant removal BMPs available.
Percent removals will depend on filter media used, influent pollution concentrations, hydraulic
loadings and other factors.
Table 6-5 Pollutant Removal Performance of Bioretention Practices (%
Removal) (Davis et al, 1997)
Upper
Zone
Middle
Zone
Lower
Zone
Cu
90
93
93
Pb
93
99
99
Zn
87
98
99
P
0
73
81
TKN
37
60
68
NH,
54
86
79
NO,
-97
-194
23
TN
-29
0
43
The University of Virginia, Charlottesville, Virginia has initiated a long term study of the
performance of a bioretention cell. This study differs from the two bioretention studies
conducted in Maryland that monitored a single storm event (3 inches of rainfall). The UVA
study is providing performance data based on an annual hydrologic budget. Initial, first year
results indicate that the performance of the bioretention cells will exceed all expectations. First
year removal results are as follows: 86% for TSS, 90% for TP, 97% for COD and 67% for oil
and grease (Yu, et al. 1999).
Unlike the other vegetative biofilters that have a dual function of stormwater transport or
detention and pollutant removal, bioretention cells primary function is pollutant removal. For
this reason, bioretention cells would perform best as part of a treatment train. Bioretention can
also be an effective retrofit BMP for existing urban areas that already have stormwater drainage
systems.
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Glossary
Acute: A stimulus severe enough to rapidly induce an effect; in aquatic toxicity tests, an effect
observed in 96 hours or less is typically considered acute. When referring to aquatic toxicology
or human health, an acute effect is not always measured in terms of lethality.
Adjacent Steep Slope: A slope with a gradient of 15 percent or steeper within 500 feet of the
site.
Adsorption: The adhesion of a substance to the surface of a solid or liquid; often used to
extract pollutants by causing them to be attached to such adsorbents as activated carbon or silica
gel. Hydrophobic, or water repulsing adsorbents, are used to extract oil from waterways when oil
spills occur. Heavy metals such as zinc and lead often adsorb onto sediment particles.
Antidegradation: Policies which ensure protection of water quality for a particular water body
where the water quality exceeds levels necessary to protect fish and wildlife propagation and
recreation on and in the water. This also includes special protection of waters designated as
outstanding natural resource waters. Antidegradation plans are adopted by each state to minimize
adverse effects on water.
Anti-seep Collar: A device constructed around a pipe or other conduit and placed through a
dam, levee, or dike for the purpose of reducing seepage losses and piping failures.
Anti-vortex Device: A device designed and placed on the top of a riser or the entrance of a
pipe to prevent the formation of a vortex in the water at the entrance.
Aquatic Bench: A bench which is located around the inside perimeter of a permanent pool and
is normally vegetated with aquatic plants; the goal is to provide pollutant removal and enhance
safety in areas using stormwater pond BMP's.
Aquifer: A porous water bearing geologic formation generally restricted to materials capable of
yielding an appreciable supply of water
"As-Built": Drawing or certification of conditions as they were actually constructed.
Baffles: Guides, grids, grating or similar devices placed in a pond to deflect or regulate flow
and create a longer flow path.
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Bankfull Discharge: A flow condition where streamflow completely fills the stream channel
up to the top of the bank. In undisturbed watersheds, the discharge conditions occurs on average
every 1.5 to 2 years and controls the shape and form of natural channels.
Barrel: The closed conduit used to convey water under or through an embankment; part of the
principal spillway.
Baseflow: The stream discharge from groundwater.
Berm: A shelf that breaks the continuity of a slope; a linear embankment or dike.
Best Available Technology Economically Achievable (BAT): Technology-based standard
established by the Clean Water Act (CWA) as the most appropriate means available on a
national basis for controlling the direct discharge of toxic and nonconventional pollutants to
navigable waters. BAT effluent limitations guidelines, in general, represent the best existing
performance of treatment technologies that are economically achievable within an industrial
point source category or subcategory.
Best Conventional Pollutant Control Technology (BCT): Technology-based standard for the
discharge from existing industrial point sources of conventional pollutants including BOD, TSS,
fecal coliform, pH, oil and grease. The BCT is established in light of a two-part "cost
reasonableness" test which compares the cost for an industry to reduce its pollutant discharge
with the cost to a POTW for similar levels of reduction of a pollutant loading. The second test
examines the cost-effectiveness of additional industrial treatment beyond BPT. EPA must find
limits which are reasonable under both tests before establishing them as BCT.
Best Management Practice (BMP): Permit condition used in place of or in conjunction with
effluent limitations to prevent or control the discharge of pollutants. May include schedule of
activities, prohibition of practices, maintenance procedure, or other management practice. BMPs
may include, but are not limited to, treatment requirements, operating procedures, or practices to
control plant site runoff, spillage, leaks, sludge or waste disposal, or drainage from raw material
storage.
Physical, structural, and/or managerial practices that, when used singly or in combination, reduce
the downstream quality and quantity impacts of stormwater.
Best Practicable Control Technology Currently Available (BPT): The first level of
technology-based standards established by the CWA to control pollutants discharged to waters of
the U.S. BPT effluent limitations guidelines are generally based on the average of the best
existing performance by plants within an industrial category or subcategory.
Bioassay: A test used to evaluate the relative potency of a chemical or a mixture of chemicals
by comparing its effect on a living organism with the effect of a standard preparation on the
same type of organism.
Biochemical Oxygen Demand (BOD): A measurement of the amount of oxygen utilized by
the decomposition of organic material, over a specified time period (usually 5 days) in a
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wastewater sample; it is used as a measurement of the readily decomposable organic content of a
wastewater.
Biofiltration: The simultaneous process of filtration, infiltration, adsorption, and biological
uptake of pollutants in stormwater that takes place when runoff flows over and through vegetated
areas.
Biofiltration Swale: A sloped, vegetated channel or ditch that provides both conveyance and
water quality treatment to stormwater runoff. It does not provide stormwater quantity control but
can convey runoff to BMPs designed for that purpose.
Biological Control: A method of controlling pest organisms by means of introduced or
naturally occurring predatory organisms, sterilization, the use of inhibiting hormones, or other
means, rather than by mechanical or chemical means.
Bioretention: A stormwater management practice that utilizes shallow storage, landscaping
and soils to control and treat urban stormwater runoff by collecting it in shallow depressions
before filtering through a fabricated planting soil media.
Buffer: The zone contiguous with a sensitive area that is required for the continued
maintenance, function, and structural stability of the sensitive area. The critical functions of a
riparian buffer (those associated with an aquatic system) include shading, input of organic debris
and coarse sediments, uptake of nutrients, stabilization of banks, interception of fine sediments,
overflow during high water events, protection from disturbance by humans and domestic
animals, maintenance of wildlife habitat, and roomfor variation of aquatic system boundaries
over time due to hydrologic or climatic effects. The critical functions of terrestrial buffers
include protection of slope stability, attenuation of surface water flows from stormwater runoff
and precipitation, and erosion control.
Catchbasin: A chamber or well, usually built at the curb line of a street, for the admission of
surface water to a sewer or subdrain, having at its base a sediment sump designed to retain grit
and detritus below the point of overflow.
Catchment: Surface drainage area.
Channel: A natural stream that conveys water; a ditch or channel excavated for the flow of
water and is open to the air.
Channelization: Alteration of a stream channel by widening, deepening,straightening,
cleaning, or paving certain areas to change flow characteristics.
Channel Stabilization: Erosion prevention and stabilization of velocity distribution in a
channel using jetties, drops, revetments, structural linings, vegetation and other measures.
Check Dam: A small dam constructed in a gully or other small watercourse to decrease flow
velocity (by reducing the channel gradient), minimize scour, and promote deposition of
sediment.
Chemical Oxygen Demand (COD): A measure of the oxygen-consuming capacity of
inorganic and organic matter present in wastewater. COD is expressed as the amount of oxygen
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consumed in mg/1. Results do not necessarily correlate to the biochemical oxygen demand
(BOD) because the chemical oxidant may react with substances that bacteria do not stabilize.
Chronic: A stimulus that lingers or continues for a relatively long period of time, often one-
tenth of the life span or more. Chronic should be considered a relative term depending on the life
span of an organism. The measurement of a chronic effect can be reduced growth, reduced
reproduction, etc., in addition to lethality.
Chute: A high velocity, open channel for conveying water to a lower level without erosion.
Clay Lens: A naturally occurring, localized area of clay which acts as an impermeable layer to
runoff infiltration.
Clay (Soils): 1. A mineral soil separate consisting of particles less than 0.002 millimeter in
equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine grained soil (more than 50
percent passing the No. 200 sieve) that has a high plasticity index in relation to the liquid limit.
(Unified Soil Classification System)
Clean Water Act (CWA): The Clean Water Act is an act passed by the U.S. Congress to
control water pollution. It was formerly referred to as the Federal Water Pollution Control Act of
1972 or Federal Water Pollution Control Act Amendments of 1972 (Public Law 92-500), 33
U.S.C. 1251 et. seq., as amended by: Public Law 96-483; Public Law 97-117; Public Laws 95-
217, 97-117, 97-440, and 100-04.
Closed Depression: An area which is low-lying and either has no, or such a limited, surface
water outlet that during storm events the area acts as a retention basin.
Coconut Rolls: Also known as coir rolls, these are rolls of natural coconut fiber designed to be
used for streambank stabilization.
Cohesion: The capacity of a soil to resist shearing stress, exclusive of functional resistance.
Combined Sewer Overflow (CSO): A discharge of untreated wastewater from a combined
sewer system at a point prior to the headworks of a publicly owned treatment works. CSOs
generally occur during wet weather (rainfall or snowmelt). During periods of wet weather, these
systems become overloaded, bypass treatment works, and discharge directly to receiving waters.
Combined Sewer System (CSS): A wastewater collection system which conveys sanitary
wastewaters (domestic, commercial and industrial wastewaters) and storm water through a single
pipe to a publicly owned treatment works for treatment prior to discharge to surface waters.
Compaction (Soils): Any process by which the soil grains are rearranged to decrease void
space and bring them in closer contact with one another, thereby increasing the weight of solid
material per unit of volume, increasing the shear and bearing strength and reducing permeability.
Composite Sample: Sample composed of two or more discrete samples. The aggregate sample
will reflect the average water quality covering the compositing or sample period.
Conduit: Any channel intended for the conveyance of water, whether open or closed.
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Constructed Wetland: A wetland that is created on a site that previously was not a wetland.
This wetland is designed specifically to remove pollutants from stormwater runoff.
Contour: 1. An imaginary line on the surface of the earth connecting points of the same
elevation. 2. A line drawn on a map connecting points of the same elevation.
Core Trench: A trench, filled with relatively impervious material intended to reduce seepage
of water through porous strata.
Conventional Pollutants: Pollutants typical of municipal sewage, and for which municipal
secondary treatment plants are typically designed; defined by Federal Regulation [40 CFR
401.16] as BOD, TSS, fecal coliform bacteria, oil and grease, and pH.
Conveyance: A mechanism for transporting water from one point to another, including pipes,
ditches, and channels.
Conveyance System: The drainage facilities, both natural and manmade, which collect,
contain, and provide for the flow of surface and stormwater from the highest points on the land
down to a receiving water. The natural elements of the conveyance system include swales and
small drainage courses, streams, rivers, lakes, and wetlands. The human-made elements of the
conveyance system include gutters, ditches, pipes, channels, and most retention/detention
facilities.
Cradle: A structure usually of concrete shaped to fit around the bottom and sides of a conduit
to support the conduit, increase its strength and, in dams, to fill all voids between the underside
of the conduit and the soil.
Created Wetland: A wetland that is created on a site that previously was not a wetland. This
wetland is created to replace wetlands that wereunavoidably destroyed during design and
construction of a project. This wetland cannot be used for treatment of stormwater runoff.
Crest: 1. The top of a dam, dike, spillway or weir, frequently restricted to the overflow portion.
2. The summit of a wave or peak of a flood, volume.
Criteria: The numeric values and the narrative standards that represent contaminant
concentrations that are not to be exceeded in the receiving environmental media (surface water,
ground water, sediment) to protect beneficial uses.
Curve Number (CN): A numerical representation of a given area's hydrologic soil group,plant
cover, impervious cover, interception and surface storage derived in accordance with Natural
Resources Conservation Service methods. This number is used to convert rainfall depth into
runoff
Cut: Portion of land surface or area from which earth has been removed or will be removed by
excavation; the depth below original ground surface to excavated surface.
Cut-and-Fill: Process of earth moving by excavating part of an area and using the excavated
material for adjacent embankments or fill areas.
Cutoff: A wall or other structure, such as a trench, filled with relatively impervious material
intended to reduce seepage of water through porous strata.
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CZARA: Acronym used for the Coastal Zone Act Reauthorization Amendments of 1990.
These amendments sought to address the nonpoint source pollution issue by requiring states to
develop coastal nonpoint pollution control programs in order to receive federal funds.
Dam: A barrier to confine or raise water for storage or diversion, to create a hydraulic head, to
prevent gully erosion, or for retention of soil, sediment or other debris.
Dead Storage: The permanent pool volume located below the out structure of a storage device.
Dead storage provides water quality treatment but does not provide water quantity treatment.
Depression Storage: The amount of precipitation that is trapped in depressions on the surface
of the ground.
Design Storm: A prescribed hyetograph and total precipitation amount (for a specific duration
recurrence frequency) used to estimate runoff for a hypothetical storm of interest or concern for
the purposes of analyzing existing drainage, designing new drainage facilities or assessing other
impacts of a proposed project on the flow of surface water.
Detention: The temporary storage of storm water runoff in a BMP with the goals of controlling
peak discharge rates and providing gravity settling of pollutants.
Detention Facility / Structure: An above or below ground facility, such as a pond or tank, that
temporarily stores stormwater runoff and subsequently releases it at a slower rate than it is
collected by the drainage facility system. There is little or no infiltration of stored stormwater,
and the facility is designed to not create a permanent pool of water.
Detention Time: The theoretical time required to displace the contents of a stormwater
treatment facility at a given rate of discharge (volume divided by rate of discharge).
Dike: An embankment to confine or control water, for example, one built along the banks of a
river to prevent overflow to lowlands; a levee.
Discharge: Outflow; the flow of a stream, canal, or aquifer. One may also speak of the
discharge of a canal or stream into a lake, river, or ocean. (Hydraulics) Rate of flow, specifically
fluid flow; a volume of fluid passing a point per unit of time, commonly expressed as cubic feet
per second, cubic meters per second, gallons per minute, gallons per day, or millions of gallons
per day.
Disturbed Area: An area in which the natural vegetative soil cover has been removed or
altered and, therefore, is susceptible to erosion.
Diversion: A channel with a supporting ridge on the lower side constructed across the slope to
divert water to areas where it can be used or disposed of safely. Diversions differ from terraces
in that they are individually designed.
Drainage: Refers to the collection, conveyance, containment, and/or discharge of surface and
storm water runoff.
Drainage Area (Watershed): That area contributing runoff to a single point measured in a
horizontal plane, which is enclosed by a ridge line.
Drainage Basin: A geographic and hydrologic sub-unit of a watershed.
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Drainage Channel: A drainage pathway with a well-defined bed and banks indicating frequent
conveyance of surface and stormwater
Drainage Course: A pathway for watershed drainage characterized by wet soil vegetation;
often intermittent in flow.
Drainage Divide: The boundary between one drainage basin and another.
Drain: A buried pipe or other conduit (closed drain). A ditch (open drain) for carrying off
surplus surface water or ground water.
Drainage Easement: A legal encumbrance that is placed against a property's title to reserve
specified privileges for the users and beneficiaries of the drainage facilities contained within the
boundaries of the easement.
Drainage, Soil: The removal of water from a soil.
Drop Structure: A structure for dropping water to a lower level and dissipating surplus
energy; a fall.
Dry Pond: A facility that provides stormwater quantity control by containing excess runoff in a
detention basin, then releasing the runoff at allowable levels.
Dry Swale: An open drainage channel explicitly designed to detain and promote the filtration
of stormwater runoff through an underlying fabricated soil media.
Dry Vault/Tank: A facility that treats stormwater for water quantity control by detaining
runoff in underground storage units and then releases reduced flows at established standards.
Effluent Limitation: Any restriction imposed by the Director on quantities, discharge rates,
and concentrations of pollutants which are discharged from point sources into waters of the
United States, the waters of the contiguous zone, or the ocean.
Effluent Limitations Guidelines (ELG): A regulation published by the Administrator under
Section 304(b) of CWA that establishes national technology-based effluent requirements for a
specific industrial category.
Emergency Spillway: A dam spillway, constructed in natural ground, that is to discharge low
in excess of the principal spillway design discharge.
Energy Dissipator: Any means by which the total energy of flowing water is reduced. In
stormwater design, they are usually mechanisms that reduce velocity prior to, or at, discharge
from an outfall in order to prevent erosion. They include rock splash pads, drop manholes,
concrete stilling basins or baffles, and check dams.
Enhancement: To raise ecological value, desirability, or attractiveness of an environment
associated with surface water.
Erosive Velocities: Velocities of water that are high enough to wear away the land surface.
Exposed soil will generally erode faster than stabilized soils. Erosive velocities will vary
according to the soil type, slope, structural, or vegetative stabilization used to protect the soil.
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Erosion: 1. The process by which the land surface is worn away by the action of water, wind,
ice, or gravity. 2. Detachment and movement of soil or rock fragments by water, wind, ice or
gravity. The following terms are used to describe different types of water erosion:
Accelerated erosion - Erosion much more rapid than normal, natural or geologic
erosion,primarily as a result of the influence of the activities of man or, in some cases, of other
animals or natural catastrophes that expose base surfaces.
Gully erosion - The erosion process whereby water accumulates in narrow channels and removes
the soil from this narrow area to considerable depths ranging from 1 or 2 feet to as much as 75 to
100 feet.
Rill erosion - An erosion process in which numerous small channels only several inches deep are
formed. See rill.
Sheet erosion - The spattering of small soil particles caused by the impact of raindrops on wet
soils. The loosened and spattered particles may or may not subsequently be removed by surface
runoff.
Erosion and Sediment Control: Any temporary or permanent measures taken to reduce
erosion, control siltation and sedimentation, and ensure that sediment-laden water does not leave
a site.
Erosion and Sediment Control Facility: A type of drainage facility designed to hold water for
a period of time to allow sediment contained in the surface and stormwater runoff directed to the
facility to settle out so as to improve the quality of the runoff.
Exfiltration: The downward movement of water through the soil; the downward flow of runoff
from the bottom of an infiltration BMP into the soil.
Existing Site Conditions: The conditions (ground cover, slope, drainage patterns) of a site as
they existed on the first day that the project entered the design phase. Projects which drain into a
sensitive area designated by a federal, state, or local agency may be required to use undisturbed
forest conditions for the purposes of calculating runoff characteristics instead of using existing
site conditions
Extended Detention: A stormwater design feature that provides for the gradual release of a
volume of water in order to increase settling of pollutants and protect downstream channels from
frequent storm events.
Filter Bed: The section of a constructed filtration device that houses the filter media and the
outflow pipe.
Filter Fence: A geotextile fabric designed to trap sediment and filter runoff.
Filter Media: The sand, soil, or other organic material in a filtration device used to provide a
permeable surface for pollutant and sediment removal.
Filter Strip: A strip of permanent vegetation above ponds, diversions and other structures to
retard the flow of runoff, causing deposition of transported material, thereby reducing
sedimentation.
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Fines (Soil): Generally refers to the silt and clay size particles in soil.
Floodplain: Areas adjacent to a stream or river that are subject to flooding or inundation
during a storm event that occurs, on average, once every 100 years (or has a likelihood of
occurrence of 1/100 in any given year).
Flood Frequency: The frequency with which the flood of interest may be expected to occur at
a site in any average interval of years. Frequency analysis defines the "n-year flood" as being the
flood that will, over a long period of time, be equaled or exceeded on the average once every
"n"years.
Flood Fringe: That portion of the floodplain outside of the floodway which is covered by
floodwaters during the base flood. It is generally associated with standing water rather than
rapidly flowing water.
Flood Peak: The highest value of the stage or discharge attained by a flood; thus, peak stage or
peak discharge.
Flood Routing: An analytical technique used to compute the effects of system storage
dynamics on the shape and movement of flow represented by a hydrograph.
Flood Stage: The stage at which overflow of the natural banks of a stream begins.
Floodway: The channel of the river or stream and those portions of the adjoining flood plains
which are reasonably required to carry and discharge the base flood flow. The portions of the
adjoining flood plains which are considered to be "reasonably required" is defined by flood
hazard regulations.
Flow Splitter: An engineered, hydraulic structure designed to divert a percentage of storm flow
to a BMP located out of the primary channel, or to direct stormwater to a parallel pipe system or
to bypass a portion of baseflow around a BMP.
Forebay: An easily maintained, extra storage area provided near an inlet of a BMP to trap
incoming sediments before they accumulate in a pond or wetland BMP.
Freeboard (Hydraulics): The distance between the maximum water surface elevation
anticipated in design and the top of retaining banks or structures. Freeboard is provided to
prevent overtopping due to unforeseen conditions.
French Drain: A type of drain consisting of an excavated trench filled with pervious material,
such as coarse sand, gravel or crushed stone; water percolates through the voids in this material
and flows to an outlet.
Frost-Heave: The upward movement of soil surface due to the expansion of water stored
between particles in the first few feet of the soil profile as it freezes. May cause surface
fracturing of asphalt or concrete.
Frequency of Storm (Design Storm Frequency): The anticipated period in years that will
elapse, based on average probability of storms in the design region, before a storm of a given
intensity and/or total volume will recur; thus a 10-year storm can be expected to occur on the
average once every 10 years. Sewers designed to handle flows which occur under such storm
conditions would be expected to be surcharged by any storms of greater amount or intensity.
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Functions (wetlands): The ecological (physical, chemical, and biological) processes or
attributes of a wetland without regard for their importance to society (see also Values). Wetland
functions include food chain support, provision of ecosystem diversity and fish and wildlife
habitat, flood flow alteration, ground water recharge and discharge, water quality improvement,
and soil stabilization.
Gabion: A rectangular or cylindrical wire mesh cage filled with rock and used as a protecting
agent, revetment, etc., against erosion. Soft gabions, often used in stream bank stabilization, are
made of geotextiles filled with dirt, in between which cuttings are placed.
Gabion Mattress: A thin gabion, usually six or nine inches thick, used to line channels for
erosion control.
Gage: Device for registering precipitation, water level, discharge, velocity, pressure,
temperature, etc.
Gaging Station: A selected section of a stream channel equipped with a gage, recorder, or
other facilities for determining stream discharge.
Gauge: A measure of the thickness of metal; e.g., diameter of wire, wall thickness of steel pipe.
Grab Sample: A sample which is taken from a wastestream on a one-time basis without
consideration of the flow rate of the wastestream and without consideration of time.
Grade: 1. The slope or finished surface of a road, channel, canal bed, roadbed, top of
embankment, bottom of excavation, or natural ground; any surface prepared for the support of
construction, like paving or laying a conduit. 2. To finish the surface of a canal bed, roadbed, top
of embankment or bottom of excavation.
Grass Channel: An open vegetated channel used to convey runoff and to provide treatment by
filtering pollutants and sediments.
Gravel: 1. Aggregate consisting of mixed sizes of 1/4 inch to 3 inches which normally occur in
or near old streambeds and have been worn smooth by the action of water. 2. A soil having
particle sizes, according to the Unified Soil Classification System, ranging from the No. 4 sieve
size, angular in shape, as produced by mechanical crushing.
Gravel Diaphragm: A stone trench filled with small, river-run gravel used as pretreatment and
inflow regulation in stormwater filtering systems.
Gravel Filter: Washed and graded sand and gravel aggregate placed around a drain or well
screen to prevent the movement of fine materials from the aquifer into the drain or well.
Gravel Trench: A shallow excavated channel backfilled with gravel and designed to provide
temporary storage and permit percolation of runoff into the soil substrate.
Ground Water Table: The free surface of the ground water, that surface subject to
atmospheric pressure under the ground, generally rising and falling with the season, the rate of
withdrawal, the rate of restoration, and other conditions. It is seldom static.
Gully: A channel or miniature valley cut by concentrated runoff through which water
commonly flows during and immediately after heavy rains or snow melt. The distinction
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between gully and rill is one of depth. A gully is sufficiently deep such that it would not be
obliterated by normal tillage operations, whereas a rill is of lesser depth and would be smoothed
by ordinary farm tillage or grading activities.
Harmful Pollutant: A substance that has adverse effects to an organism including immediate
death, chronic poisoning, impaired reproduction, cancer or other effects.
Heavy Metals: Metals of high specific gravity, present in municipal and industrial wastes, that
pose long-term environmental hazards. Such metals include cadmium, chromium, cobalt, copper,
lead, mercury, nickel, and zinc.
Head (Hydraulics): 1. The height of water above any plane of reference. 2. The energy, either
kinetic or potential, possessed by each unit weight of a liquid expressed as the vertical height
through which a unit weight would have to fall to release the average energy possessed. Used in
various terms such as pressure head, velocity head, and head loss.
High Marsh: A pondscaping zone within a stormwater wetland that exists from the surface of
the normal pool to a six inch depth and typically contains the greatest density and diversity of
emergent wetland plants.
Hotspot: Area where land use or activities generate highly contaminated runoff, with
concentrations of pollutants in excess of those typically found in stormwater.
Hydraulic Gradient: The slope of the hydraulic grade line. That includes static and potential
head.
Hydrodynamic Structure: An engineered structure to separate sediments and oils from
stormwater runoff using gravitational separation and/or hydraulic flow.
Hydrograph: A graph of runoff rate, inflow rate or discharge rate, past a specific point over
time.
Hydrologic Soil Groups (HSG): A soil characteristic classification system defined by the U.S.
Soil Conservation Service in which a soil may be categorized into one of four soil groups (A, B,
C, or D) based upon infiltration rate and other properties.
Hydrology: The science of the behavior of water in the atmosphere, on the surface of the earth,
and underground.
Hydroperiod: A seasonal occurrence of flooding and/or soil saturation; it encompasses depth,
frequency, duration, and seasonal pattern of inundation.
Hydroseed: An application of seed or other material applied with forced water in order to
revegetate.
Hyetograph: A graph of precipitation versus time.
Impervious Surface / Cover (I): A hard surface area which either prevents or retards the entry
of water into the soil. Common impervious surfaces include rooftops, walkways, patios,
driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen
materials, and oiled surfaces.
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Industrial Stormwater Permit: An NPDES permit issued to an identified land use that
regulates the pollutant levels associated with industrial stormwater discharges or specifies onsite
pollution control strategies.
Infiltration: The downward movement of water from the surface to the subsoil.
Infiltration Facility (or system): A drainage facility designed to use the hydrologic process of
surface and stormwater runoff soaking into the ground, commonly referred to as a percolation, to
dispose of surface and stormwater runoff.
Infiltration Pond: A facility that provides stormwater quantity control by containing excess
runoff in a detention facility, then percolating that runoff into the surrounding soil.
Infiltration Rate (/): The rate at which stormwater percolates into the subsoil measured in
inches per hour.
Inflow Protection: A water handling device used to protect the transition area between any
water conveyance (dike, swale, or swale dike) and a sediment trapping device.
Inlet: A form of connection between surface of the ground and a drain or sewer for the
admission of surface and stormwater runoff.
Invert: The lowest point on the inside of a sewer or other conduit.
Invert Elevation: The vertical elevation of a pipe or orifice in a pond which defines the water
level.
Isopluvial Map: A map with lines representing constant depth of total precipitation for a given
return frequency.
Karst Geology: Regions that are characterized by formations underlain by carbonate rock and
typified by the presence of limestone caverns and sinkholes.
Lag Time: The interval between the center of mass of the storm precipitation and the peak flow
of the resultant runoff.
Land Disturbing Activity: Any activity that results in a change in the existing soil cover (both
vegetative and nonvegetative) and/or the existing soil topography. Land disturbing activities
include, but are not limited to demolition, construction, clearing, grading, filling and excavation.
Leachate : Liquid that has percolated through soil and contains substances in solution or
suspension.
Leaching: Removal of the more soluble materials from the soil by percolating waters.
Level Spreader: A temporary BMP used to spread stormwater runoff uniformly over the
ground surface as sheet flow. The purpose of level spreaders are to prevent concentrated, erosive
flows from occurring. Level spreaders will commonly be used at the upsteam end of wider
biofilters to ensure sheet flow into the biofilter.
Low Flow Channel: An incised or paved channel from inlet to outlet in a dry basin which is
designed to carry low runoff flows and/or baseflow, directly to the outlet without detention.
Major Storm: A precipitation event that is larger than the typically largest rainfall for a year.
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Mass Wasting: The movement of large volumes of earth material downslope.
Mean Depth: Average depth; cross-sectional area of a stream or channel divided by its surface
or top width.
Mean Velocity: The average velocity of a stream flowing in a channel or conduit at a given
cross-section or in a given reach. It is equal to the discharge divided by the cross-sectional area
of the reach.
Metals: Elements, such as mercury, lead, nickel, zinc and cadmium, that are of environmental
concern because they do not degrade over time. Although many are necessary nutrients, they are
sometimes magnified in the food chain, and they can be toxic to life in high enough
concentrations. They are also referred to as heavy metals.
Micropool: A smaller permanent pool which is incorporated into the design of larger
stormwater ponds to avoid resuspension of particles and minimize impacts to adjacent natural
features.
Million Gallons per Day (mgd): A unit of flow commonly used for wastewater discharges.
One mgd is equivalent to 1.547 cubic feet per second.
Mitigation: means, in the following order of preference:
1. Avoiding the impact altogether by not taking a certain action or part of an action;
2. Minimizing impacts by limiting the degree or magnitude of the action and its implementation,
by using appropriate technology, or by taking affirmative steps to avoid or reduce impacts;
3. Rectifying the impact by repairing, rehabilitating or restoring the affected environment;
4. Reducing or eliminating the impact over time by preservation and maintenance operations
during the life of the action; and
5. Compensation for the impact by replacing, enhancing, or providing substitute resources or
environments.
Monitor: To systematically and repeatedly measure something in order to track changes.
Monitoring: The collection of data by various methods for the purposes of understanding
natural systems and features, evaluating the impacts of development proposals on such systems,
and assessing the performance of mitigation measures imposed as conditions of development.
Municipal Stormwater Permit: An NPDES permit issued to municipalities to regulate
discharges from municipal separate storm sewers for compliance with EPA regulations.
Municipal Separate Storm Sewer System (MS4): A conveyance or system of conveyances
(including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches,
manmade channels, or storm drains) owned by a state, city, town or other public body, that is
designed or used for collecting or conveying storm water, which is not a combined sewer, and
which is not part of a publicly owned treatment works. Commonly referred to as an "MS4" [40
CFR 122.26(b)(8)].
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National Pollutant Discharge Elimination System (NPDES): The national program for issuing,
modifying, revoking and reissuing, terminating, monitoring and enforcing permits, and imposing
and enforcing pretreatment requirements, under Sections 307, 318, 402, and 405 of CWA.
NGVD: National Geodetic Vertical Datum
Native Growth Protection Easement (NGPE): An easement granted for the protection of
native vegetation within a sensitive area or its associated buffer. The NGPE shall be recorded on
the appropriate documents of title and filed with the County Records Division.
New Development: Includes the following activities: land disturbing activities, structural
development, including construction, installation or expansion of a building or other structure;
creation of impervious surfaces; Class IV — general forest practices that are conversions from
timber land to other uses; and subdivision and short subdivision of land as defined in RCW
58.17.020. All other forest practices and commercial agriculture are not considered new
development.
New Impervious Area: The impervious area that is being created by the project.
Nonconventional Pollutants: All pollutants that are not included in the list of conventional or
toxic pollutants in 40 CFR Part 401. Includes pollutants such as chemical oxygen demand
(COD), total organic carbon (TOC), nitrogen, and phosphorus.
Nonpoint Source Pollution: Pollution that enters a water body from diffuse origins on the
watershed and does not result from discernible, confined, or discrete conveyances.
Non-Structural BMPs: Stormwater runoff treatment techniques which use natural measures to
reduce pollution levels, do not require extensive construction efforts and/or promote pollutant
reduction by eliminating the pollutant source.
Normal Depth: The depth of uniform flow. This is a unique depth of flow for any combination
of channel characteristics and flow conditions. Normal depth is calculated using Manning's
Equation.
Nutrients: Essential chemicals needed by plants or animals for growth. Excessive amounts of
nutrients can lead to degradation of water quality and algal blooms. Some nutrients can be toxic
at high concentrations.
Off-Line: A management system designed to control a storm event by diverting a percentage of
Stormwater events from a stream or storm drainage system.
Off-site: Any area lying upstream of the site that drains onto the site and any area lying
downstream of the site to which the site drains.
One Year Storm: A Stormwater event which occurs on average once every year or statistically
has a 100% chance on average of occurring in a given year.
One Hundred Year Storm: An extreme flood event which occurs on average once every 100
years or statistically has a 1% chance on average of occurring in a given year.
On-Line: A management system designed to control Stormwater in its original stream or
drainage channel.
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Orifice: An opening with closed perimeter, usually sharp-edged, and of regular form in a plate,
wall, or partition through which water may flow, generally used for the purpose of measurement
or control of flow.
Outlet: Point of water disposal from a stream, river, lake, tidewater, or artificial drain.
Outlet Channel: A waterway constructed or altered primarily to carry water from man-made
structures, such as terraces, tile lines, and diversions. Also known as swale, grass channel, and
biofilter. This system is used for the conveyance, retention, infiltration and filtration of
stormwater runoff.
Overflow: A pipeline or conduit device, together with an outlet pipe, that provides for the
discharge of portions of combined sewer flows into receiving waters or other points of disposal,
after a regular device has allowed the portion of the flow which can be handled by interceptor
sewer lines and pumping and treatment facilities to be carried by and to such water pollution
control structures.
pH: A measure of the hydrogen ion concentration of water or wastewater; expressed as the
negative log of the hydrogen ion concentration in mg/1. A pH of 7 is neutral. A pH less than 7 is
acidic, and a pH greater than 7 is basic.
Peak Discharge Rate: The maximum instantaneous rate of flow during a storm, usually in
reference to a specific design storm event.
Permanent Seeding: The establishment of perennial vegetation which may remain for many
years.
Permeability Rate: The rate at which water will move through a saturated soil.
Permeable Soils: Soil materials with a sufficiently rapid infiltration rate so as to greatly reduce
or eliminate surface and stormwater runoff. These soils are generally classified as SCS
hydrologic soil types A and B.
Permeable Cover: Those surfaces in the landscape consisting of open space, forested areas,
meadows, etc. that infiltrate rainfall.
Permissible Velocity (Hydraulics): The highest average velocity at which water may be
carried safely in a channel or other conduit. The highest velocity that can exist through a
substantial length of a conduit and not cause scour of the channel. A safe, non-eroding or
allowable velocity
Perviousness: Related to the size and continuity of void spaces in soils; related to a soil's
infiltration rate.
Pesticide: A general term used to describe any substance, usually chemical, used to destroy or
control organisms; includes herbicides, insecticides, algicides, fungicides, and others. Many of
these substances are manufactured and are not naturally found in the environment. Others, such
as pyrethrum, are natural toxins which are extracted from plants and animals.
Piping: Removal of soil material through subsurface flow channels.
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Point Source: Any discernible, confined, and discrete conveyance, including but not limited to
any pipe, ditch, channel, tunnel, conduit, well, discrete fixture, container, rolling stock,
concentrated animal feeding operation, landfill leachate collection system, vessel, or other
floating craft from which pollutants are or may be discharged.
Pollutant: Dredged spoil, solid waste, incinerator residue, filter backwash, sewage, garbage,
sewage sludge, munitions, chemical wastes, biological materials, radioactive materials (except
those regulated under the Atomic Energy Act of 1954, as amended (42 U.S.C. 2011 et seq.)),
heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal, and
agricultural waste discharged into water.
Practicable: Available and capable of being done after taking into consideration cost, existing
technology, and logistics in light of overall project purposes.
Pretreatment: The removal of material such as gross solids, grit, grease,and scum from flows
prior to physical, biological, or physical treatment processes to improve treatability. Pretreatment
may include screening, grit removal, stormwater, and oil separators.
Pond Buffer: The area immediately surrounding a pond which acts as a filter to remove
pollutants and provide infiltration of stormwater prior to reaching the pond. Provides a
separation barrier to adjacent development.
Pond Drain: A pipe or other structure used to drain a permanent pool within a specified time
period.
Pondscaping: Landscaping around stormwater ponds which emphasizes using native
vegetative species to meet specific design intentions. Species are selected for up to six zones in
the pond and its surrounding buffer based on their ability to tolerate inundation and/ or soil
saturation.
Porosity («): Ratio of pore volume to total volume.
Pretreatment: Techniques employed in stormwater BMPs to provide storage or filtering to
help trap coarse materials and other pollutants before they enter the system.
Principal Spillway: The primary pipe or weir which carries baseflow and storm flow through a
dam embankment.
Rare, Threatened, or Endangered Species: Plant or animal species that are regionally
relatively uncommon, are nearing endangered status, or whose existence is in immediate
jeopardy and is usually restricted to highly specific habitats. Threatened and endangered species
are officially listed by federal and state authorities, whereas rare species are unofficial species of
concern that fit the above definitions.
Rational Method: A means of computing storm drainage flow rates (Q) by use of the formula
Q = CIA, where C is a coefficient describing the physical drainage area, I is the rainfall intensity
and A is the area.
Reach: A length of channel with uniform characteristics.
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Receiving Waters: Bodies of water or surface water systems receiving water from upstream
manmade (or natural) streams.
Recharge: The flow to ground water from the infiltration of surface and stormwater runoff.
Recharge Rate: Annual amount of rainfall which contributes to groundwater as a function of
hydrologic soil group.
Recharge Volume: The portion of the water quality volume (WQv) used to maintain
groundwater recharge rates at development sites.
Redevelopment: Any construction, alteration, or improvement exceeding five thousand square
feet of land disturbance performed on sites where existing land use is commercial, industrial,
institutional, or multifamily residential.
Regional: An action (here, for stormwater management purposes) that involves more than one
discrete property.
Regional Detention Facility: A stormwater quantity control structure designed to correct
existing excess surface water runoff problems of a basin or subbasin. The area downstream has
been previously identified as having existing or predicted significant and regional flooding
and/or erosion problems. This term is also used when a detention facility is used to detain
stormwater runoff from a number of different businesses, developments or areas within a
catchment.
Release Rate: The computed peak rate of surface and stormwater runoff for a particular design
storm event and drainage area conditions.
Restoration: Actions performed to reestablish wetland functional characteristics and processes
that have been lost by alterations, activities, or catastrophic events in an area that no longer
meets the definition of a wetland.
Retention: The process of collecting and holding surface and stormwater runoff with no
surface outflow. The amount of precipitation on a drainage area that does not escape as runoff. It
is the difference between total precipitation and total runoff.
Retention/Detention Facility (R/D): A type of drainage facility designed either to hold water
for a considerable length of time and then release it by evaporation, plant transpiration, and/or
infiltration into the ground; or to hold surface and stormwater runoff for a short period of time
and then release it to the surface and stormwater management system.
Retrofitting: The renovation of an existing structure or facility to meet changed conditions or
to improve performance.
Return Interval: A statistical term for the average time of expected interval that an event of
some kind will equal or exceed given conditions (e.g., a stormwater flow that occurs every 2
years).
Reverse-Slope Pipe: A pipe which draws from below a permanent pool extending in a reverse
angle up to the riser and determines the water elevation of the permanent pool.
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Right-of-Way: Right of passage, as over another's property. A route that is lawful to use. A
strip of land acquired for transport, conveyance or utility construction.
Rill: A small intermittent watercourse with steep sides, usually only a few inches deep. Often
rills are caused by an increase in surface water flow when soil is cleared of vegetation.
Riprap: A facing layer or protective mound of stones placed to prevent erosion or sloughing of
a structure or embankment due to flow of surface and storm water runoff.
Riparian: Pertaining to the banks of streams, wetlands, lakes or tidewater.
Riser: A vertical pipe extending from the bottom of a pond BMP that is used to control the
discharge rate from a BMP for a specified design storm.
Roughness Coefficient (Hydraulics): A factor in velocity and discharge formulas representing
the effect of channel roughness on energy losses in flowing water. Manning's "n" is a commonly
used roughness coefficient.
Runoff: That portion of the precipitation on a drainage area that is discharged from the area in
the stream channels. Types include surface runoff, groundwater runoff or seepage.
Safety Bench: A relatively flat area above the permanent pool and surrounding a stormwater
pond designed to provide a separation to adjacent slopes.
Sanitary Sewer: A pipe or conduit (sewer) intended to carry wastewater or water-borne wastes
from homes, businesses, and industries to the POTW.
Sanitary Sewer Overflow (SSO): Untreated or partially treated sewage overflow from a
sanitary sewer collection system.
SBUH: Santa Barbara Urban Hydrograph Method. An event-based hydrographic method of
analysis used to determine stormwater runoff from a site.
SCS: Soil Conservation Service, U.S. Department of Agriculture.
Sediment: Fragmented material that originates from weathering and erosion of rocks or
unconsolidated deposits, and is transported by, suspended in, or deposited by water.
Sedimentation: The depositing or formation of sediment.
Seepage: 1 .Water escaping through or emerging from the ground. 2. The process by which
water percolates through soil.
Seepage Length: In sediment basins or ponds, the length along the pipe and around the
antiseep collars that is within the zone of saturation through an embankment.
Setbacks: The minimum distance requirements for locating certain structures in relation to
roads, wells, septic fields, or other structures.
Settleable Solids: Those suspended solids in stormwater that separate by settling when the
stormwater is held in a quiescent condition for a specified time.
Sheetflow: Runoff which flows over the ground surface as a thin, even layer, not concentrated
in a channel.
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Short Circuiting: The passage of runoff through a BMP in less than the design treatment time.
Siltation: The process by which a river, lake, or other water body becomes clogged with
sediment. Silt can clog gravel beds and prevent successful salmon spawning.
Soil Group: A classification of soils by the Soil Conservation Service into four runoff potential
groups. The groups range from A soils, which are very permeable and produce little or no runoff,
to D soils, which are not very permeable and produce much more runoff.
Soil Permeability: The ease with which gases, liquids, or plant roots penetrate or pass through
a layer of soil.
Soil Stabilization: The use of measures such as rock lining, vegetation or other engineering
structures to prevent the movement of soil when loads are applied to the soil.
Source Control BMP: A BMP that is intended to prevent pollutants from entering stormwater.
A few examples of source control BMPs are erosion control practices, maintenance of
stormwater facilities, constructing roofs over storage and working areas, and directing wash
water and similar discharges to the sanitary sewer or a dead end sump.
Spillway: A passage such as a paved apron or channel for surplus water over or around a dam
or similar obstruction. An open or closed channel, or both, used to convey excess water from a
reservoir. It may contain gates, either manually or automatically controlled, to regulate the
discharge of excess water.
Stabilization: Providing vegetative and/or structural measures that will reduce or prevent
erosion.
Stage (Hydraulics): The variable water surface or the water surface elevation above any
chosen datum.
Steep Slope: Slopes of 25 percent gradient or steeper.
Stilling Basin: An open structure or excavation at the foot of an outfall, conduit, chute, drop, or
spillway to reduce the energy of the descending stream of water.
STORET: EPA's computerized STOrage and RETrieval water quality data base that includes
physical, chemical, and biological data measured in waterbodies throughout the United States.
Storm Frequency: The time interval between major storms of predetermined intensity and
volumes of runoff for which storm sewers and other structures are designed and constructed to
handle hydraulically without surcharging and backflooding, e.g., a 2-year, 10-year or 100-year
storm.
Stormwater: That portion of precipitation that does not naturally percolate into the ground or
evaporate, but flows via overland flow, interflow, channels or pipes into a defined surface water
channel, or a constructed infiltration facility.
Stormwater Drainage System: Constructed and natural features which function together as a
system to collect, convey, channel, hold, inhibit, retain, detain, infiltrate, divert, treat or filter
stormwater.
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Stormwater Facility: A constructed component of a stormwater drainage system, designed or
constructed to perform a particular function, or multiple functions. Stormwater facilities include,
but are not limited to, pipes, swales, ditches, culverts, street gutters, detention basins, retention
basins, constructed wetlands, infiltration devices, catchbasins, oil/water separators, sediment
basins and modular pavement.
Stormwater Filtering: Stormwater treatment methods which utilize an artificial media to filter
out pollutants entrained in urban runoff.
Stormwater Ponds: A land depression or impoundment created for the detention or retention
of stormwater runoff.
Stormwater Quality: A term used to describe the chemical, physical, and biological
characteristics of stormwater.
Stormwater Quantity: A term used to describe the volume characteristics of stormwater.
Stormwater Site Plan: A plan which shows the measures that will be taken during and after
project construction to provide erosion and sediment control and stormwater control.
Stormwater Wetlands: Shallow, constructed pools that capture stormwater and allow for the
growth of characteristic wetland vegetation.
Stream Buffers: Zones of variable width which are located along both sides of a stream and
are designed to provided a protective natural area along a stream corridor.
Stream Gaging: The quantitative determination of stream flow using gages, current meters,
weirs, or other measuring instruments at selected locations. See gaging station.
Streams: Those areas where surface waters flow sufficiently to produce a defined channel or
bed. A defined channel or bed is indicated by hydraulically sorted sediments or the removal of
vegetative litter or loosely rooted vegetation by the action of moving water. The channel or bed
need not contain water year-round.
Structural BMPs: Devices which are constructed to provide temporary storage and treatment
of stormwater runoff.
Subbasin: A drainage area which drains to a water course or waterbody named and noted on
common maps and which is contained within a basin.
Subgrade: A layer of stone or soil used as the underlying base for a BMP.
Suspended Solids: Organic or inorganic particles that are suspended in and carried by the
water. The term includes sand, mud, and clay particles (and associated pollutants) as well as
solids in stormwater.
Swale: A shallow drainage conveyance with relatively gentle side slopes, generally with flow
depths less than one foot.
Tailwater: Water, in a river or channel, immediately downstream from a structure.
Technical Release No. 20 (TR-20): A Soil Conservation Service (now NRCS) watershed
hydrology computer model that is used to compute runoff volumes and provide routing of storm
events through stream valleys and/or ponds.
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Glossary BMP Design Considerations
September 2002
Technical Release No. 55 (TR-55): A watershed hydrology model developed by the
SoilConservation Service (nowNRCS) used to calculate runoff volumes and provide a
simplified routing for storm events through stream valleys and/or ponds.
Ten-Year Storm: The 24 hour storm event which exceeds bankfull capacity and occurs on
average once every ten years (or has a likelihood of occurrence of 1/10 in a given year).
TESC: Temporary Erosion and Sediment Control (Plan).
Time of Concentration: The time period necessary for surface runoff to reach the outlet of a
subbasin from the hydraulically most remote point in the tributary drainage area.
Toe of Slope: A point or line of slope in an excavation or cut where the lower surface changes
to horizontal or meets the existing ground slope; or a point or line on the upper surface of a slope
where it changes to horizontal or meets the original surface.
Toe Wall: Downstream wall of a structure, usually to prevent flowing water from eroding
under the structure.
Topography: General term to include characteristics of the ground surface such as plains, hills,
mountains; degree of relief, steepness of slopes, and other physiographic features.
Topsoil: Fertile or desirable soil material used for the preparation of a seedbed.
Total Maximum Daily Load (TMDL): The amount of pollutant, or property of a pollutant,
from point, nonpoint, and natural background sources, that may be discharged to a water quality-
limited receiving water. Any pollutant loading above the TMDL results in violation of applicable
water quality standards.
Total Phosphorus (TP): The total amount of phosphorus that is contained within the water
column.
Total Solids: The solids in water, sewage, or other liquids, including the dissolved, filterable,
and nonfilterable solids. The residue left when the moisture is evaporated and the remainder is
dried at a specified temperature, usually 13OC.
Total Suspended Solids (TSS): A measure of the filterable solids present in a sample, as
determined by the method specified in 40 CFR Part 136.
Toxic Pollutant: Pollutants or combinations of pollutants, including disease-causing agents,
which after discharge and upon exposure, ingestion, inhalation or assimilation into any
organism, either directly from the environment or indirectly by ingestion through food chains,
will, on the basis of information available to the Administrator of EPA, cause death, disease,
behavioral abnormalities, cancer, genetic mutations, physiological malfunctions, (including
malfunctions in reproduction) or physical deformations, in such organisms or their offspring.
Toxic pollutants also include those pollutants listed by the Administrator under CWA Section
307(a)(l) or any pollutant listed under Section 405(d) which relates to sludge management.
Trash Rack: Grill, grate or other device installed at the intake of a channel, pipe, drain or
spillway for the purpose of preventing oversized debris from entering the structure.
Travel Time: The estimated time for surface water to flow between two points of interest.
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Glossary BMP Design Considerations
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Truncated Hydrograph: A method of computing the required design infiltration storage
volume utilizing the differences from post-developed and pre-developed hydrograph volumes
over a specific time frame.
Two-Year Storm: The 24 hour storm event which exceeds bankfull capacity and occurs on
average once every two years (or has a likelihood of occurrence of 1/2 in a given year).
Underdrain: Plastic pipes with holes drilled through the top, installed on the bottom of an
infiltration BMP which are used to collect and remove excess runoff.
Unstable Slopes: Those sloping areas of land which have in the past exhibited, are currently
exhibiting, or will likely in the future exhibit, mass movement of earth.
Urbanized Area: Areas designated and identified by the U.S. Bureau of Census according to
the following criteria: an incorporated place and densely settled surrounding area that together
have a maximum population of 50,000.
USEPA: The United States Environmental Protection Agency.
Ultimate Condition: Full watershed build-out based on existing zoning.
Ultra-Urban: Densely developed urban areas in which little pervious surface exists.
Vactor Waste: The waste material that is found in the bottom of a catch basin.
Values: Wetland processes or attributes that are valuable or beneficial to society (also see
Functions). Wetland values include support of commercial and sport fish and wildlife specie s,
protection of life and property from flooding, recreation, education, and aesthetic enhancement
of human communities.
Vegetative Filter Strip: A facility that is designed to provide stormwater quality treatment of
conventional pollutants but not nutrients through the process of biofiltration.
Velocity Head: Head due to the velocity of a moving fluid, equal to the square of the mean
velocity divided by twice the acceleration due to gravity (32.16 feet per second per
second) [v2/2g].
Volumetric Runoff Coefficient (Rv): The value that is applied to a given rainfall volume to
yield a corresponding runoff volume based on the percent impervious cover in a drainage basin.
Water Quality BMP: A BMP specifically designed for pollutant removal.
Water Quality Criteria: Comprised of numeric and narrative criteria. Numeric criteria are
scientifically derived ambient concentrations developed by EPA or states for various pollutants
of concern to protect human health and aquatic life. Narrative criteria are statements that
describe the desired water quality goal.
Water Quality Standard (WQS): A law or regulation that consists of the beneficial use or uses
of a waterbody, the numeric and narrative water quality criteria that are necessary to protect the use
or uses of that particular waterbody, and an antidegradation statement.
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Glossary BMP Design Considerations
September 2002
Water Quality Volume (Wqv): The volume needed to capture and treat 90% of the average
annual stormwater runoff volume equal to 1" (or 0.9" in Western Rainfall Zone) times the
volumetric runoff coefficient (Rv) times the site area.
Water Quantity BMP: A BMP specifically designed to reduce the peak rate of stormwater
runoff.
Water Surface Profile: The longitudinal profile assumed by the surface of a stream flowing in
an open channel; the hydraulic grade line.
Wedges: Design feature in stormwater wetlands that increases flow path length to provide for
extended detention and treatment of runoff.
Wetlands: Those areas that are inundated or saturated by surface or ground water at a
frequency and duration sufficient to support, and that under normal circumstances do support, a
prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands
generally include swamps, marshes, bogs, and similar areas. This includes wetlands created,
restored or enhanced as part of a mitigation procedure. This does not include constructed
wetlands or the following surface waters of the state intentionally constructed from sites that are
not wetlands: irrigation and drainage ditches, grass-lined swales, canals, agricultural detention
facilities, farm ponds, and landscape amenities.
Wet Pond: A facility that treats stormwater for water quality by utilizing a permanent pool of
water to remove conventional pollutants from runoff through sedimentation, biological uptake,
and plant filtration.
Wet Swale: An open drainage channel or depression, explicitly designed to retain water or
intercept groundwater for water quality treatment.
Wetted Perimeter: The length of the wetted surface of the channel.
Wet Vaults/Tanks: Underground storage facilities that treat stormwater for water quality
through the use of a permanent pool of water that acts as a settling basin.
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References BMP Design Considerations
September 2002
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