A Current Assessment
of Urban Best
Management
Practices _ . . "—
Techniques for
Reducing Non-
Point Source
Pollution in the
Coastal Zone
Prepared by:
Metropolitan
Washington
Council of
Governments
-------
t.l
A Current Assessment of
Urban Best Management Practices
Techniques for Reducing Non-Point Source
Pollution in the Coastal Zone
4, MAY 3 0 J997
^S*OiAww#.9w«j#,WAW0?
Prepared by:
Thomas R. Schueler, Peter A. Kumble, and Maureen A. Heraty
Anacostia Restoration Team
Department of Environmental Programs
Metropolitan Washington Council of Governments
777 North Capitol Street, Suite 300
Washington, DC 20002
Prepared for:
U.S. Environmental Protection Agency
Office of Wetlands, Oceans, and Watersheds
Technical Guidance to Implement Section 6217(g)
of the Coastal Zone Management Act
March, 1992
U.S. EPA LIBRARY REGION 10 MATERIALS
~ ~~~~
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ABSTRACT
TITLE:
A Current Assessment of Urban Best Management Practices: Techniques for Reducing Nonpoint
Source Pollution in the Coastal Zone.
DATE: March 1992
AUTHOR: Anacostia Restoration Team
Department of Environmental Programs
Metropolitan Washington Council of Governments
AGENCY:
REPORT
ABSTRACT:
The Metropolitan Washington Council of Governments is the regional planning organization of the
Washington, D.C. area's major local governments and their governing officials. COG works toward
solutions to such regional problems such as energy shortages, traffic congestion, inadequate housing, air
and water pollution. The US Environmental Protection Agency works toward the reduction and
elimination of national pollution problems. It develops guidelines for the implementation of environ-
mental legislation, enforces national pollution control programs, offers technical assistance to state and
local programs, and provides guidance in determining future national pollution control priorities.
Summarizes the capabilities and limitations of structural best management practices in current use for
the control of the quality of urban runoff. Addresses issues of particular concern to the coastal zone.
Observations are derived from multiple field studies. Includes assessments of extended detention
ponds, wet ponds, stormwater wetlands, multiple pond systems, infiltration trenches, infiltration
basins, porous pavement, sand fil ters, grassed swales, vegetated filter strips and water quality inlets. For
each practice, the report discusses pollutant removal capability, longevity, environmental impacts,
construction and maintenance costs, feasibility in different development contexts, and adaptability to
different geographic settings, particularly coastal. The report also suggests improvements under
development for each practice, intended to correct notable design flaws or increase overall stormwater
management capability.
REPORT
PRICE:
$30.00
PUBLICATION
NUMBER: 92705
ORDER
COPIES
FROM:
Information Center
Metropolitan Washington Council of Governments
777 North Capitol Street, N. E.
Suite 300
Washington, D.C. 20002-4201
(Make checks payable to "MWCOG")
Metropolitan Washington Council of Governments
PROJECT STAFF
~
Anacostia Restoration Team
Tom Schueler, Peter Kumble and Maureen Heraty, principal authors
Illustrations by Peter Kumble and Mark Pfoutz
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Preface
This report has been prepared under Grant X-818188-01-1 from the
Office of Wetlands, Oceans and Watersheds (OWOW) of the U.S.
Environmental Protection Agency. Some data and graphics within the
report were prepared from funding provided by the Water Resources
Committee of the Council of Governments.
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Acknowledgements
The authors would like to thank a number of planners, engineers and program
administrators for their contributions to this report. The quality of this manual
has been greatly enhanced as a result of their sound advice, commentary and
overall assistance.
Technical review and project management:
US EPA: Robert Goo, Ann Beier, Rod Frederick and Tom
Davenport
Sharing of data and reports:
Woodward-Clyde
Federal Services: Dale Lehman and Lynn Mayo
Tetra Tech: Mary Beth Corrigan and John Hochheimer
Florida Department
of Environmental
Regulation: Eric Livingston
Review and technical insights:
COG Staff: Lorrie Herson, John Galli and Dave Shepp
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Table of Contents
1. EXECUTIVE SUMMARY 1
2. TABLE 1: A Comparative Assessment of the Effectiveness of Current
Urban Best Management Practices 3
3. INTRODUCTION 5
4. BMP FACT SHEETS
#1 Extended Detention Ponds ...7
#2 Wet Ponds .....15
#3 Stormwater Wetlands 23
#4 Multiple Pond Systems 31
#5 Infiltration Trench 39
#6 Infiltration Basins 47
#7 Porous Pavement 55
#8 Sand Filters 63
#9 Grassed Swales 71
#10 Filter Strips 79
#11 Water Quality Inlets/Oil Grit Separators 87
5. REFERENCES 93
6. APPENDIX A: Pollutant Removal Table 103
7. APPENDIX B: Glossary of Terms 115
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Executive Summary
This report is intended to define the capabilities and limitations of the current
generation of urban best management practices (BMPs) in order to provide effective
stormwater quality management within the coastal zone. These capabilities and
limitations are extremely important to keep in mind as local communities and state
agencies begin to develop stormwater management programs.
Table 1 provides a comparative assessment of eleven different BMP options.
Several major conclusions can be made regarding the current generation of BMPs:
• Not all urban BMPs can reliably provide high levels of removal for both
particulate and soluble pollutants. Effective BMPs include wet ponds,
stormwater wetlands, multiple pond systems and sand filters. Infiltration
BMPs are presumed to be effective in removing pollutants, but are not
reliable given their poor longevity. Other BMPs, such as grassed swales,
filter strips and water quality inlets, cannot provide reliable levels of
pollutant removal until their basic design is significantly enhanced.
• The longevity of some BMPs is limited to such a degree that their
widespread use is currently not encouraged. Of particular concern are the
infiltration practices, such as basins, trenches and porous pavement. The
poor longevity of these BMPs is attributable to a number of factors: lack
of pretreatment, poor construction practices, application to infeasible sites,
lack of regular maintenance, and in some cases, fundamental difficulties in
basic design. Very often the life-spans of BMPs can be increased to
acceptable lengths if local communities adopt enhanced designs and commit
to strong maintenance and inspection programs.
• BMP options are adaptable to most regions of the country with the
exception of extremely arid regions of the West and the colder climates of
the North. In these regions, conventional BMP designs may need to be
refined to account for high evaporation rates or subfreezing snowmelt
conditions, respectively. New BMP options should also be explored.
• No single BMP option can be applied to all development situations and all
BMP options require careful site assessment prior to design. Pond options
are applicable to the widest range of development situations, but typically
require a minimum drainage area. On the other hand, infiltration practices
have very limited applications, requiring field verification of soils, water
tables, slope and other factors.
A Current Assessment of Urban Best Management Practices
Page - 1
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Executive Summary
• Several BMPs can have significant secondary environmental impacts,
although the extent and nature of these impacts is uncertain and site-
specific. Pond systems, which offer reliable pollutant removal and
longevity, tend to be associated with the greatest number and strongest
degree of secondary environmental impacts. Careful site assessment and
design are often required to prevent stream warming, natural wetland
destruction and riparian habitat modification.
• Relatively limited cost data exists to aid in the assessment of the
comparative cost-effectiveness of urban BMP options. Presently, the
selections of BMPs is based more on longevity, feasibility, and local design
factors than on comparative cost. It is expected that construction costs for
all BMPs will increase in the future due to the enhanced designs needed for
more reliable pollutant removal and longevity. Costs may also increase in
response to increasingly complex permitting requirements. Maintenance
costs may rival construction costs over the design life of many BMPs;
however, many jurisdictions currently do not have very active BMP
maintenance programs.
• Many of the conventional urban BMPs need to be enhanced to provide
more reliable pollutant removal and greater longevity. In many cases, a
systems approach to BMP design is warranted whereby multiple techniques
for runoff attenuation, conveyance, pretreatment, and treatment are utilized.
The report suggests some general directions for enhancing conventional
urban BMP designs and a priority should be placed on demonstrating the
performance of the enhanced BMPs in the field.
• Several fundamental uncertainties still exist with respect to urban BMPs
and need to be resolved through basic research. These uncertainties include
the toxicity of residuals trapped within BMPs; the interaction of
groundwater and BMPs (both ponds and infiltration); and the long-term
performance of urban BMPs.
• This report only discusses structural BMPs. Well-planned development sites
usually incorporate non-structural BMPs as well. These non-structural BMPs
are currently the subject of an ongoing study at COG which will be
available in 1993.
A Current Assessment of Urban Best Management Practices
Page ¦ 2
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TABLE 1
A COMPARATIVE ASSESSMENT OE THE EFKECTIVENESS OE CURRENT UKIIAN IIEST MANAGEMENT PRACTICES
Q)
IQ
CD
URBAN BMP
OPTIONS
RELIABILITY
FOR POLLUTANT
REMOVAL
LONGEVITY*
APPLICABLE
TO MOST
DEVELOPMENTS
REGIONAL
CONCERNS
ENVIRONMENTAL
CONCERNS
COMPARATIVE
COST
SPECIAL
CONSIDERATIONS
Extended
Detention
Ponds
Moderate, but
not always reliable
20+ years, but
frequent clogging
and abort
detention common.
Widely applicable.
Very few.
Possible stream warming
and habitat destruction.
Lowest cost alternative
in size range
Recommended with
design improvements
and with the use
of micropools and wetlands.
Wet Pond
Moderate to high
20+ years
Widely applicable.
Arid and high ET
regions.
Possible stream warming,
trophic shifts, habitat
destruction, safety hazards,
sacrifice of upstream channels.
Moderate to high compared
to conventional
stormwater detention.
Recommended, with
careful site evaluation.
Stormwater
Wetlands
Moderate to High
20+ years
Space may be limiting.
Arid and high ET
regions; short growing
seasons.
Stream warming, natural
wetland alteration.
Marginally higher than wet
ponds.
Recommended.
Multiple
Pond Systems
Moderate to high;
redundancy increases
reliability.
20+ years
Many pond options.
Arid regions.
Selection of appropriate pond
option minimizes overall
environmental impact.
Most expensive pond option.
Recommended.
Infiltration
Trenches
Presumed moderate
%
50 % failure rate
within five years.
Highly restricted (soils,
ground water, slope,
area, sediment input).
Arid and cold regions;
sole-source aquifers.
Slight risk of ground water
contamination.
Cost-effective on smaller
Rehab costs can be
considerable.
Recommended with
pretreatment and geotechnical
evaluation.
Infiltration
Basins
Presumed moderate,
if working
60 - 100 % failure
within five years.
Highly restricted (see
infiltration trench).
Arid and cold regions;
sole-source aquifers.
Slight risk of ground water
contamination.
Construction cost
moderate, but rehab cost
high.
Not widely recommended
until longevity is improved.
Porous
Pavement
High (if working)
75 % failure
within five years.
Extremely restricted
(traffic, soils, ground
water, slope, area,
sediment input).
Cold climates; wind
erosion; sole-source
aquifers.
Possible ground water impacts;
uncontrolled runoff.
Cost-effective compared
to conventional asphalt
when working properly.
Recommended in highly
restricted applications
with careful construction
and effective maintenance.
Sand Filters
Moderate to high
20+ years
Applicable (for smaller
developments).
Few restrictions.
Minor.
Comparatively high
construction costs and
frequent maintenance.
Recommended, with
local demonstration.
Grassed
Swales
Low to moderate,
but unreliable.
20+ years
Low density development
and roads.
Arid and cold regions.
Minor.
Low compared to curb and
gutter.
Recommended, with
cheekdams, as one
element of a BMP
system.
Filter Strips
Unreliable in
urban settings.
Unknown, but may
be limited.
Restricted to low density
areas.
Arid and cold regions.
Minor.
Low.
Recommended as one
element of a BMP system.
Water Quality
Inlets
Presumed low
20+ years
Small, highly impervious
catchment# (< 2 acres).
Few.
Resuspension of hydro-
carbon loadings. Disposal of hydro
carbon and toxic residuals.
High, compared to trenches
and sand filters.
Not currently
recommended as a
primary BMP option.
•losed on current designs and prevailing maintenance pradices
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Introduction
This report presents a technical assessment on the capability of selected urban
best management practices (BMPs) to provide effective control of stormwater quality.
The report is intended to provide guidance in urban BMP selection for coastal
communities that need to comply with Section 6217(g) of the 1990 Coastal Zone
Management Reauthorization Act.
While urban BMPs have been applied in several areas of the country for the
past decade (most notably in the mid-Atlantic region), surprisingly little information
is available about their performance, cost and longevity. Quite simply, the design of
urban BMPs is in its infancy, and improved and more reliable designs are constantly
evolving.
As coastal communities develop stormwater quality management programs,
they .must choose a series of BMP options that can reliably achieve water quality
goals. This report is intended to answer questions that decisionmakers must face in
choosing a particular BMP or combination of BMP options. Those questions are:
Can the BMP reliably remove urban pollutants?
The report reviews available performance monitoring data for each BMP and
outlines primary pollutant removal mechanisms. Design factors that are known
or suspected to improve or degrade pollutant removal efficiency are noted.
How well does the BMP operate over time?
The report reviews studies that indicate the longevity of the BMP over time
and identifies the critical factors that lead to premature failure.
When and where is the BMP feasible?
The use of nearly all BMPs is restricted by a number of site and land-use
conditions. For each BMP, the report identifies key feasibility factors. In
addition, it describes the potential use of a BMP for stormwater quantity, urban
retrofits, or in ultra-urban areas.
How much will the BMP cost?
The economics of urban BMPs are sparsely documented and often depend on
many site-specific factors. To answer this question, the report makes general
observations with respect to the comparative construction and maintenance
costs for BMPs. It also outlines differences in design and permitting
A Current Assessment of Urban Best Management Practices
Page - 5
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Introduction
requirements among different BMPs.
Can the BMP be adapted for all coastal areas?
A frequent concern has been that much of the performance and longevity data
for urban BMPs has been generated within the mid-Atlantic region, and
therefore, may not be applicable to all regions of the country. The report notes
factors that may limit or restrict the application of an urban BMP to other
regions of the country with different climates and/or vegetative cover.
Does the BMP have any environmental benefits or liabilities?
Urban BMPs often generate secondary environmental impacts at the site. Some
of these impacts can be beneficial (such as the creation of wildlife habitat or
wetlands) and others may be negative. Data is often sparse on negative
impacts, but they can nevertheless be a critical factor in whether a BMP option
is selected for a region. The report describes significant environmental benefits
and concerns in relation to each BMP option.
What is not understood about the BMP?
Despite our experience with many BMPs, many aspects of BMP performance
are not well understood. The report lists important gaps in our current
understanding of the limits and capabilities of each BMP option. Further
research to resolve these uncertainties is recommended.
To address these questions, COG staff thoroughly reviewed the extant
literature, consulted with numerous local and state experts around the country, and
analyzed data from ongoing studies. This report represents the conclusions resulting
from that research.
In the report, we review eleven structural BMPs and make a general
recommendation for each one on its application in coastal areas. The BMPs include:
1. Extended Detention Ponds
2. Wet Ponds
3. Stormwater Wetlands
4. Multiple Pond Systems
5. Infiltration Trenches
6. Infiltration Basins
7. Porous Pavement
8. Sand Filters
9. Grassed Swales
10. Filter Strips
11. Water Quality Inlets
A Current Assessment of Urban Best Management Practices
Page - 6
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Extended Detention
Ponds BMP Fact Sheet #1
Definition
Conventional Extended Detention (ED) ponds temporarily detain a portion
of stormwater runoff for up to twenty-four hours after a storm using a fixed
orifice. Such extended detention allows urban pollutants to settle out. The
ED ponds are normally "dry" between storm events and do not have any
permanent standing water.
Enhanced ED ponds are designed to prevent clogging and resuspension.
They provide greater flexibility in achieving target detention times. They are
equipped with plunge pools near the inlet, a micropool at the outlet, and
utilize an adjustable reverse-sloped pipe as the ED control device.
Schematic Design of an Enhanced Dry ED Pond System
existing forested
wetland retained
sediment
aquatic
.bench
torebay
micropool
maintinence
plunge
pool
maximum elevation
of ED pool
maximum elevation
of safety storm
emergency spillway
safety bencl
Source: Schuelor, 1991.
A Current Assessment of Urban Best Management Practices
Page - 7
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Extended Detention Ponds
Capsule Summary:
Pollutant Removal Capability: Conventional ED ponds provide moderate but
variable removal of particulate pollutants, such as sediment, phosphorus and
organic carbon, but provide negligible removal of soluble pollutants.
Longevity: While few conventional ED ponds built to date have totally failed,
many do not operate as designed and a majority are not achieving target
detention times.
Feasibility: The enhanced ED pond can be utilized in most low visibility
development situations, as a retrofit practice, or in combination with wetlands
or permanent pools.
Environmental Concerns: Poorly-maintained conventional ED ponds may
create nuisances and are not popular with nearby residents. If not properly
located, ED ponds can degrade forests, wetlands and other habitat areas.
Environmental Benefits: Enhanced ED ponds provide excellent protection for
downstream channel erosion and, if landscaped properly, can create valuable
wetland and wet meadow habitat.
Costs: ED ponds are generally the least costly stormwater quality ponds to
construct, but also have the greatest regular maintenance burden of any
stormwater ponds.
Adaptability: ED ponds are an adaptable BMP that can be applied to most,
if not all, regions of the country.
Maintenance Burden: Conventional ED pond designs exhibit chronic clogging
and are difficult to mow. Debris and sand deposits quickly accumulate.
A Current Assessment of Urban Best Management Practices
Page - 8
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Extended Detention Ponds
Usefulness as a Coastal Urban NPS Management Practice:
The extended detention pond can be a useful, cost-effective and widely
applied BMP in the coastal zone when:
• appropriate treatment volumes are selected
• design improvements are made to reduce chronic clogging and
to provide greater flexibility and reliability in achieving target
detention times.
In general, enhanced ED ponds that incorporate micropools and wetlands
are recommended for coastal areas. These design features reduce chronic
clogging and enhance nutrient removal.
Can Extended Detention Ponds Reliably Remove Urban Stormwater
Pollutants?
Pollutant Removal Mechanisms: Pollutant removal is primarily accomplished
by gravitational settling that is dependent on the detention time and the
fraction of the annual runoff volume that is effectively detained in the pond.
(1)
Review of Monitoring Studies: Six performance monitoring studies have been
conducted to date. (Appendix A) Reported removal for TSS ranges from 30 -
70 %, but is variable for smaller runoff events. For Total P, removal generally
ranges from 10 - 30 %. For soluble nutrients, removal capability is estimated
as low or negative. For COD, the removal rate ranges from 15 - 40 %. No
data is yet available on the effectiveness of enhanced dry ED ponds.
A Current Assessment of Urban Best Management Practices Page - 9
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Extended Detention Ponds
Factors Influencing Pollutant Removal:
Positive Factors
Negative Factors
• Six to twelve hours of
• Resuspension of previously
deposited pollutants from the
pilot channel of pond floor (2)
(5)
detention (minimum) (2)
• Smaller treatment volumes (e.g.
0.5 watershed inches) provide the
best removal rates (3)
• Large treatment volumes:
acceptable ED times cannot be
achieved over the broad range of
expected storms (6)
• Wetlands in lower stage can
prevent resuspension and
augment removal
• Use of a micropool to protect the
ED pond orifice (4)
• Difficulty in predicting ED
hydraulics (7)
How Well Do Extended Detention Ponds Operate over Time?
Failure Rates: Four of six conventional ED ponds were unable to achieve
target detention times for the entire range of storms monitored. Preliminary
field studies in Maryland suggest that most dry ED ponds constructed are
partially clogged and have standing water and/or wetland plants. (8) Many
ED ponds were not providing adequate ED for the range of expected runoff
events.
Despite their clogging and short ED times, nearly all the conventional
ED ponds surveyed were providing at least partial pollutant removal. (8) The
oldest conventional ED ponds to date are ten to fifteen years old.
Factors Influencing Longevity: Greater longevity and reduced clogging can
be achieved by:
• Two-stage design, utilizing wetlands in the lower stage (1)
• Smaller ED treatment volumes (i.e., avoid two-year ED) (6)
• Use of single orifices located within the permanent micropool
• Avoidance of concrete pilot channels
• Equipping the pond with a drain
• Adjustable ED gate valves to achieve target detention times (4)
A Current Assessment of Urban Best Management Practices
Page - 10
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Extended Detention Ponds
Where and When are Extended Detention Ponds Feasible?
Contributing Watershed Area: In most cases, ED ponds are not practical if
the watershed area is less than ten acres. (1)
Development Feasibility: Can be useful for most development situations, but
may not be appropriate in high visibility residential or commercial settings.
Depth to Bedrock: If bedrock is close to the surface, high excavation costs
may make ED ponds infeasible.
Depth to Water Table: If the water table is within two feet of the bottom of
the ED pond, it can create problems with standing water and also indicate
potential wetland status.
Use in Ultra-urban Areas: Fairly limited due to space constraints.
Retrofit Capability: Frequently used for stormwater retrofits, particularly
within dry stormwater management ponds and at culvert/channel intersections.
Usually used in combination with a micropool, wetland or permanent pool. (9)
(10)
Stormwater Management Capability: Frequently used in combination with
two-year storm control.
What are the Costs Associated with Extended Detention Ponds?
Permitting/Review: Permitting costs are usually less than other stormwater
quality pond options.
Construction: Generally, extended detention ponds are the least expensive
stormwater quality pond option available. The addition of extended detention
to conventional stormwater detention facilities adds zero to twenty-five percent
additional cost. (11)
Maintenance: Estimated to be three to five percent of construction cost each
year. (11) Primary maintenance activities include mowing; unclogging of the
ED control device; and sediment cleanout in the lower stage. The ED pond
has the highest routine maintenance burden of any stormwater quality pond
system, due to mowing and clogging problems.
A Current Assessment of Urban Best Management Practices
Page - 11
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Extended Detention Ponds
Can Extended Detention Ponds Be Easily Adapted for All Coastal Areas?
The basic ED pond can be adapted for use in most coastal areas, particularly
if micropool, wetland or permanent pool features are incorporated into the basic
design.
However, the size of the ED treatment volume and the selection of the target
detention time must be established locally based on the annual rainfall/runoff
frequency relationship for a region.
What are the Environmental Concerns and Benefits of Extended
Detention Ponds?
Positive Impacts:
• Extended detention is the best technique available for reducing the
frequency of bankfull and subbankfull flooding events, and thereby
is very useful in protecting downstream channels (1)
• ED ponds can create both terrestrial and aquatic wildlife habitat with
appropriate pondscaping and vegetation management
• They are less hazardous than other stormwater quality ponds with
deeper permanent pools
Negative Impacts:
• ED ponds can contribute to downstream warming if pilot channels
are not shaded (high delta-t) (12)
• Improper site selection can create wetland, forest and habitat conflicts
(13)
• Poorly maintained ED ponds are not popular with adjacent residents
(14)
• "Dry" ED ponds can create mosquito breeding conditions and other
nuisances
A Current Assessment of Urban Best Management Practices
Page - 12
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Extended Detention Ponds
What is Not Known about Extended Detention Ponds?
Although the ED pond is a well-established BMP, some uncertainties
remain. These include:
• The ability to consistently achieve target detention times across the
broad range of expected storm events using existing
hydrologic/hydraulic design models and existing ED control devices
• The ability to predict (and adjust) treatment volumes and detention
times to best protect downstream channels
• The impact of chronic clogging in ED ponds on their long-term
pollutant removal capability
A Current Assessment of Urban Best Management Practices
Page - 13
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Wet P
BMP Fact Sheet #2
Definition
Conventional wet ponds have a permanent pool of water for treating
incoming stormwater runoff.
In enhanced wet pond designs, a forebay is installed to trap incoming
sediments where they can be easily removed; a fringe wetland is also
established around the perimeter of the pond.
Schematic Design of an Enhanced Wet Pond System
pond buffer 33 feet minimum
forebay
riser in embankment
reverse pipe
irregular pool shape
1.5 to 2.0 meters deep
.aquatic bench
no trees on embankment
native landscaping around pool
safety bench
Source: Schueler, 1991.
Current Assessment of Urban Best Management Practices
Page - 15
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Wet Ponds
Capsule Summary:
Pollutant Removal Capability: Conventional wet ponds provide moderate to
high removal of both particulate and soluble urban stormwater pollutants.
Reliable removal rates can be achieved with pool sizes ranging from 0.5 to 1.0
inches of runoff per impervious acre.
Longevity: Well-designed wet ponds can function for twenty years or more
and very few conventional ponds have ever failed to provide some water
quality benefit. Performance will decline over time, however, unless regular
sediment cleanout is undertaken.
Feasibility: Wet ponds can be utilized in both low and high visibility
development situations if contributing watershed area is greater than ten acres
and/or a reliable source of baseflow exists.
Environmental Concerns: If located improperly, wet ponds can have several
adverse environmental impacts, including downstream warming, trophic shifts,
and a slight risk of poor quality pond effluent during dry weather. Local
impacts include possible wetland and forest destruction, and slight risks of
sediment and/or groundwater contamination. For larger wet ponds, sacrifice
of upstream channels is a concern.
Environmental Benefits: An enhanced wet pond can be an attractive
landscape and community feature, and create warm-water fishery, waterfowl
habitat and wetlands in the urban scene.
Costs: Wet pond costs are twenty-five to forty percent greater than those
reported for conventional stormwater detention. Maintenance costs range from
three to five percent of construction costs annually.
Adaptability: Wet pond designs are not useful in arid regions where
evapotranspiration significantly exceeds precipitation on an annual basis. Also,
the size of the pool will need to reflect the prevailing climate and runoff
frequency for a particular region. Ponds can be used in colder northern
climates, but their performance declines slightly during ice and snowmelt runoff
conditions.
Maintenance Burden: Wet ponds have a modest maintenance burden,
consisting primarily of inspections, mowing of the embankment and buffers,
and removal "of trash and debris from the forebay.
Current Assessment of Urban Best Management Practices
Page - 16
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Wet Ponds
Usefulness as a Coastal Urban NPS Management Practice:
The wet pond is one of the most reliable and attractive BMPs that exists;
moreover, it can be applied in most regions of the country. While wet ponds
confer many community amenities and environmental benefits, they can have
adverse environmental impacts if the ponded areas are not carefully evaluated
and located.
Can Wet Ponds Reliably Remove Urban Stormwater Pollutants?
Pollutant Removal Mechanisms: Achieved by gravitational settling, algal
settling, wetland plant uptake and bacterial decomposition. (15) The degree of
pollutant removal is a function of pool size in relation to contributing
watershed area. Reliable removal can be achieved if the permanent pool is
sized to store between 0.5 to 1.0 inch of runoff per contributing watershed area.
(6)
Review of Monitoring Studies: The polllutant removal capability of
conventional wet ponds is well documented with over twenty performance
monitoring studies in publication. (Appendix A) Reported sediment removal
typically ranges from 50 - 90 %. Total phosphorus removal ranges from 30 -
90 %. Removal of soluble nutrients ranges from 40 - 80 %. Moderate to high
removals of trace metals, coliforms and organic matter are frequently reported.
Current Assessment of Urban Best Management Practices
Page - 17
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Wet Ponds
Factors Influencing Pollutant Removal:
Positive Factors
• Pretreatment by sediment
forebay (16)
• Permanent pool, 0.5 - 1.0
inches per impervious acre
treated (6) (15)
• Fringe wetlands
• Shallow wetlands and/or
extended detention may
improve removal efficiencies
(14)
• High length to width
ratios
Negative Factors
• Small pool size (15)
• Waterfowl populations (17)
• Short-circuiting and
turbulence (18)
• Sediment phosphorus release
• Extremely deep pool depths
( > than 10 feet)
• Snowmelt conditions and/or
ice (19)
How Well Do Wet Ponds Operate over Time?
Failure Rates: Preliminary field assessments of conventional wet ponds indicate that
most wet ponds are functioning as designed and few, if any, have actually failed. (8)
However, pollutant removal capability over time may be diminished by sediment
accumulation. Some wet ponds are still operating effectively fifteen years after
construction.
Factors Influencing Longevity:
• Sediment forebay (6)
• Regular (2-5 year) sediment clean-outs
• Reverse-slope pipes (4)
• On-site sediment disposal area
• Use of concrete riser/barrels rather than corrugated metal pipe
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Wet Ponds
Where and When Are Wet Ponds Feasible?
Contributing Watershed Area: Contributing watershed areas greater than ten
acres and less than one square mile are generally suitable for wet ponds.
Development Situations: Very useful in both low and high visibility
commercial and residential development applications.
Baseflow: Dry-weather baseflow is needed to maintain pool elevations and
prevent pool stagnation.
Available Space: Wet ponds and associated buffer/setbacks can consume from
one to three percent of total site area.
Downstream Impacts: Wet ponds may not be advisable in cold water trout
streams and may create wetland, forest and/or habitat conflicts.
Use in Ultra-urban Areas: Use in ultra-urban areas if fairly limited due to
space constraints, but can provide an attractive urban amenity if open space
or parkland is available.
Retrofit Capability: Occasionally used for stormwater retrofits, particularly
within dry stormwater basins. (20) Often used in combination with wetlands
or extended detention treatment techniques.
Stormwater Management Capability: Most wet ponds can provide two-year
stormwater quantity control, in addition to quality control.
What are the Costs Associated with Wet Ponds?
Permitting/Review: Wet ponds require numerous permits and approvals,
including wetlands permits, water quality certifications, dam safety, sediment
and erosion control plans, and waterway permits. In some cases, permitting
costs may rival design costs and may lead to a determination that a wet pond
cannot be used at the site.
Construction: Little recent cost data is available for wet ponds. Earlier studies
estimate that the addition of a permanent pool to a stormwater management
facility can add twenty-five to forty percent to total construction costs. (11)
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Wet Ponds
Maintenance: Estimated to be three to five percent of construction cost per
year. (11) (21) Mowing and sediment dean-removal are among the most costly
maintenance activities. Ponds have a demonstrated positive impact on the
value of adjacent land.
Can Wet Ponds Be Easily Adapted for All Coastal Areas?
The basic wet pond design is fairly adaptable to most regions of the country;
however, problems may be encountered in climatic regions that:
• experience extremely long periods of dry weather
• have high evaporation rates
• undergo long periods of cold weather during which the pond is frozen
• experience low rainfall
• cannot sustain adequate vegetative cover in contributing watersheds.
Criteria for sizing the permanent pool should reflect the prevailing runoff
frequency spectrum within a region
What are the Environmental Concerns and Benefits of Wet Ponds?
Positive Impacts:
• Creation of wetland features
• Creation of aquatic and terrestrial habitat (particularly for waterfowl)
• Creation of a warm-water fishery
• High community acceptance and landscaping values (14)
• Pollutant removal and downstream channel protection
• All studies to date indicate that pond sediments meet sludge toxicity limits
and can be safely landfilled (23) (53) (54)
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Wet Ponds
Negative Impacts:
• Downstream warming (12)
• Upstream channels are heavily impacted when wet ponds serve large
drainage areas ( > 250 acres) (13)
• Potential loss of wetlands, forest and floodplain habitat associated with poor
site selection for the pool (13)
• Downstream shifts in trophic status (24)
• Limited risk of ground water quality impacts over the long term; all
studies to date indicate that wet ponds do not significantly contribute to
ground water contamination (25)
• Potential hazard for nearby residents
What is Not Known about Wet Ponds?
Few fundamental uncertainties exist with respect to wet ponds. More research
may be needed in the following areas:
• sediment nutrient release within ponds over the long term
• uptake of trace metals by biota (particularly fish)
• downstream impacts of ponds
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Storm water
Wetlands
BMP Fact Sheet #3
Definition
Conventional stormwater wetlands are shallow pools that create growing
conditions suitable for the growth of marsh plants. These stormwater
wetlands are designed to maximize pollutant removal through wetland uptake,
retention and settling. Stormwater wetlands are constructed systems and
typically are not located within delineated natural wetlands. In addition,
stormwater wetlands differ from artificial wetlands created to comply with
mitigation requirements in that they do not replicate all the ecological
functions of natural wetlands.
Enhanced stormwater wetlands are designed for more effective pollutant
removal and species diversity. They also include design elements such as a
forebay, complex microtopography, and pondscaping with multiple species of
wetland trees, shrubs and plants.
gabion wall
micropool
lorebay
waterfowl'
island
Schematic Design of an Enhanced Shallow Marsh System
safety bench
25% of pond perimeter open grass
gale valves provide
flexibility m deplh ron'fi
use of wetland mulch
to create diversity
33 foot wetland buffer landscaped
with native tree/shrubs for habitat
Source: Schueier, 1991.
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Stormwater Wetlands
Capsule Summary:
Pollutant Removal Capability: In general, conventional stormwater wetlands
have a high pollutant removal capability that is generally comparable to that
of conventional wet ponds. Sediment removal may be greater in well designed
stormwater wetlands, but phosphorus removal is more variable.
Longevity: Well-designed conventional stormwater wetlands should function
for many years, but very few stormwater wetlands are yet ten years old.
Feasibility: Enhanced stormwater wetlands can be applied to most
development situations where sufficient baseflow is available to maintain water
elevations.
Environmental Concerns: If located improperly, the construction of stormwater
wetlands may impact existing forests and natural wetlands; shallow wetlands
can also contribute to downstream warming.
Environmental Benefits: With careful design and buffers, enhanced stormwater
wetlands can create unique and valuable habitat for waterfowl and wildlife in
the urban scene.
Costs: Construction costs for stormwater wetlands have not been systematically
analyzed, but are expected to be marginally higher than wet ponds.
Maintenance costs may average three to five percent of construction costs
annually.
Adaptability: Enhanced stormwater wetlands can be adapted for most regions
of the country that are not excessively arid.
Maintenance Burden: Stormwater wetlands require greater maintenance in the
first three years to establish the marsh. Thereafter, the maintenance burden is
similar to other pond systems.
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Stormwater Wetlands
Usefulness as a Coastal Urban NPS Management Practice:
Stormwater wetlands can have great utility in coastal areas in most
regions of the country; hence, their use in that environment should be
encouraged.
Can Stormwater Wetlands Reliably Remove Urban Stormwater
Pollutants?
Pollutant Removal Mechanisms: Wetlands remove pollutants through
gravitational settling, wetland plant uptake, adsorption, physical filtration and
microbial decomposition. The degree of pollutant removal is a function of
aquatic treatment volume, surface area to volume ratio, and the ratio of
wetland surface area to watershed area. (16) (26) (27)
Review of Monitoring Studies: Eighteen studies of the performance of
conventional natural and constructed wetlands are available. (Appendix A)
Removal rates are generally comparable to those reported for conventional wet
ponds of similar treatment volume; however, sediment removal rates are often
slightly higher and nutrient removal rates are somewhat lower. Some cases of
negative removal for ammonia and ortho-phosphorus were reported. Overall
performance is greatest during the growing season and lowest during the
winter months. (28)
Factors Influencing Pollutant Removal:
Positive Factors
Negative Factors
• Constant pool elevations (6)
• Range of microtopography within
• Low removal rate during non-
growing season (29)
• Concentrated inflows (28)
• Wetland area less than two
the wetland (6)
• Sediment forebay (16)
• High surface area to volume
percent of watershed area
• Sparse wetland cover (5)
• Ice cover or snowmelt runoff (19)
ratio (28)
• Constructed wetland performs
better than natural wetland
• Adding greater retention volume
and /or detention time to the
wetland (26) (28)
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Stormzvater Wetlands
How Well Do Stormzvater Wetlands Operate Over Time?
Failure Rates: While most conventional stormwater wetlands have persisted over
time, the quality and coverage of wetland plants may not be optimal for pollutant
removal. It should be noted that few stormwater wetlands meet the strict success
criteria for wetland mitigation, but they are not intended to do so.
Factors Influencing Longevity:
• Sediment forebay
• Ability to regulate water depths
• Reinforcement plantings (6)
• Selection of an experienced wetland contractor for design (27)
Where and When are Stormwater Wetlands Feasible?
Contributing Watershed Area: Stormwater wetlands can be used in
watersheds as small as five acres; however, these pocket wetlands can create
nuisances and are hard to maintain. (8)
Presence of Baseflow: To maintain a constant water level, it is often necessary
to have a reliable dry-weather baseflow to the wetland or a groundwater
supply.
Permeable Soils: It is difficult to establish wetlands at sites with sandy soils,
high soil infiltration rates or high summer evapotranspiration rates.
Available Space: Because of their shallow depths, stormwater wetlands can
consume two to three times the site area compared to other stormwater quality
options (in some cases, as much as five percent of total site area). The land
requirements of stormwater wetlands can be sharply reduced by partially
substituting vertical extended detention storage for horizontal wetland storage.
Use in Ultra-urban Areas: Limited due to space constraints; however,
pollutant removal can be obtained by modifying existing degraded urban
wetlands for stormwater control.
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Stormwater Wetlands
Retrofit Capability: The addition of wetland features to older dry stormwater
basins is an effective retrofit technique. (28) Many retrofits utilize a
combination of extended detention, wetlands and a permanent pool.
Stormwater Management Capability: In most cases, stormwater detention can
be provided in stormwater wetlands.
What are the Costs Associated with Stormwater Wetlands?
Design: Typically, design costs for wetland systems are slightly higher than
for other ponds due to the need for environmental analysis of the proposed
wetland site and the need for specialized planting techniques.
Permitting/Review: Wetland creation may require a 404 permit in some
circumstances, thereby establishing a protected jurisdictional wetland. It is
extremely difficult to construct a stormwater wetland within an existing natural
wetland. (30)
Construction: Very little systematic cost data is available for the construction
of stormwater wetlands. (21) The prevailing viewpoint is that stormwater
wetland construction costs exceed those of wet ponds due to the more complex
grading and wetland planting costs. Also, stormwater wetlands may require
more space than other pond systems, thereby driving up land acquisition costs.
Maintenance: No reliable maintenance cost data is available. It has been
assumed that maintenance costs are comparable to those of other pond systems
over the long term. (6) However, costs may well be higher in the first few
years after construction due to difficulties encountered in wetland establishment
and the possible need for reinforcement plantings.
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Stormwater Wetlands
Can Stormwater Wetlands Be Easily Adapted for All Coastal Areas?
Like other pond systems, the basic stormwater wetland design can be adapted
to other regions of the country; however, alternative designs may be needed in
northern regions with short growing seasons and hard winters. Conversely, regions
with low rainfall, high evaporation and frequent droughts will require extensive
modifications to the basic design in order to maintain the marsh system.
More information is needed on acceptab'e wetland plant species and on optimal
depths for different areas of the country. Also, some regions may not have local
wetland nurseries to supply wetland plant stock.
What are the Environmental Concerns and Benefits of Stormwater
Wetlands?
Positive Impacts:
• Stormwater wetlands can provide an excellent urban habitat for
wildlife and waterfowl, particularly if they are surrounded by a buffer
and have some deeper water area (14) (26)
Negative Impacts:
• Possible impact on wetland biota from trace metal uptake (28)
• Stormwater wetlands have a positive delta-t (12)
• Construction may adversely impact existing wetland or forest areas (6)
(30)
• Possible takeover by invasive aquatic nuisance plants (e.g., loosestrife,
cattails and phragmites) (31)
• Bacterial contamination if waterfowl populations become very
dense (17)
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Stormwater Wetlands
What is Not Known About Stormwater Wetlands?
Stormwater wetlands are no longer an experimental technology. They have
been proven to be effective and provide moderate to high levels of pollutant removal
throughout the year. Stormwater wetland designs are numerous, however, and
research needs to be done on the optimal combination of wetlands, ED and
permanent pool storage.
Other uncertainties include:
• uptake of metals by wetland biota
• ability to maintain wetland target species over the long term
• whether the annual plant dieback exports a pulse of nutrients from
the system
• the degree to which removal rates are reduced during the non-
growing season
• the potential value of annual plant harvesting to increase removal
rates
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Multiple Pond
Systems BMP Fact Sheet #4
Definition
Multiple pond systems is a collective term for a cluster of pond designs
that incorporate redundant runoff treatment techniques within a single pond
or series of ponds. These pond designs employ a combination of two or more
of the following: extended detention, permanent pool, shallow wetlands, or
infiltration. Examples of a multiple pond system include the wet ED pond,
ED wetlands, infilter ponds and pond-marsh systems.
Schematic of a Multiple Pond System Design
aquatic bench —
high marsh
concrete
sp
-------
Multiple Pond Systems
Cross-section View of a Standard ED Pond System Design
IV
safety storm storage
bankfull flood storage (2yr.)
variable EO storage
permanent pool storage
i)ffs.vw,yi ii itsa rrrfe^
emergency spillway
. gate
valves
anti-ssep collars
M
///////////,//•/
pond dram
W.\:. V;:
/////////////////
stablized outfall
LI L! barrel
Source: Schueler, 1991.
Schematic Design of a Dry In-filter System
I
slormwaler detention area
level spreader
Snlilifation!
willows provide shade
! basin;
plunge
Source: Schueler, 1991.
Page - 32
Current Assessment of Urban Best Management Practices
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Multiple Pond Systems
Schematic Design of an Enhanced Wet ED Pond
(orebay
permanent pool
1.5 !o 2.0 meters depth
aquatic bench
maintinance access
around pond
pond buffer 33 feet minimum
riser in
embankment
preserve ripairan
canopy
\'
\ .
\ * • %
s " •
r
ma* salety storm limit
max EO limit
Source: Schueler, 1991.
Schematic Design of a Shallow ED Marsh System
max ED Smil
, yW ^ vV:^W('0 V , high marsh zone
. ,.tK
,SVJ ry.\s }J ^
loronay
micropool
^ ^e,nergent marsh zone
mMcV"
Source: Schueler, 1991.
pond buffer 33 feet minimum
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Multiple Pond Systems
Multiple pond systems (MPS) have evolved as a common approach to provide
stormwater quality control over the past five years. MPS is a collective term for a
wide variety of approaches to stormwater pond design. While many aspects of their
design are unique and site-specific, they do share several common features:
Redundancy: The MPS designs emphasize the use of multiple treatment
mechanisms (such as a permanent pool/ extended detention, wetlands, within
a pond or series of ponds), rather than a single method of treatment. The
redundancy helps to improve both the level and reliability of pollutant removal
provided by the pond system.
Flexibility: Because the location and allocation of treatment mechanisms is not
rigid, the designer of an MPS has a great deal of flexibility in responding to
site-specific conditions. Additionally, the flexibility enables the designer to
minimize or avoid negative environmental impacts that can be created by single
ponds.
Complexity: MPS are inherently more complex in design than single treatment
ponds. Typically, MPS systems have more sophisticated hydrologic control
devices that are targeted toward different patterns of the annual runoff
frequency spectrum. In addition, some MPS have interconnected cells within
a pond.
Pollutant Removal Capacity:
As shown in Appendix A, many MPS are reported to provide incrementally
higher and more consistent levels of urban pollutant removal in comparison to single
treatment systems. This improvement is due to a number of factors:
Multiple-cell Ponds: Studies have shown that multiple cell ponds tend to have
incrementally higher levels of pollutant removal when compared to single cell
ponds. (32) The superior performance of multiple cell ponds can be attributed
to a longer flow path, possible reductions in short circuiting, and increases in
retention time.
Wet Pond/Wetland Systems: MPS that utilize a wet pond cell leading to a
wetland cell have been reported to be very effective in removing pollutants
from urban runoff. (19) (28) (33) (34) The wet pond cell is apparently very
effective in pretreating the incoming runoff; it also reduces its velocity and
distributes it more evenly across the marsh.
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Multiple Pond Systems
Extended Detention/Wetland Systems: Wetlands are believed to improve the
effectiveness of conventional extended detention (ED) in severed ways. The
plants help to stabilize deposited sediments, take up nutrients, and create more
ideal settling conditions. The extent of the improved pollutant removal
attributable to ED-wetland systems is not well documented/ however. Four
performance studies of ED-wetland systems have been reported (see Appendix
A), and these indicate moderate to high removal of particulate pollutants/ and
low to moderate removal of soluble pollutants. However, all four systems that
were studied had inadequate treatment volumes to provide for optimum
pollutant removal (0.08 to 0.15 inches of runoff per contributing acre).
Wet Extended Detention Ponds: The wet extended detention pond system has
been projected to have higher and more reliable pollutant removal man a wet
pond or an ED pond acting alone. This superior performance is due to the
role of the pool in acting as a barrier to resuspension and the role of ED in
increasing retention times for the full range of storms. (4) Limited monitoring
conducted to date (studies 33 - 34 in Appendix A) support this contention.
Longevity:
The longevity of MPS is expected to be at least comparable to conventional
pond systems. Often, one treatment storage component can be used to protect the
long term capacity of another component. For example:
Wet Pond/Wetland Systems: The wet pond cell traps the majority of the
incoming sediment, thereby preserving treatment capacity in the wetland and
maintaining optimum water depths.
Dry Infilter Systems: The plunge pool, grassed swale and filter cloth provide
excellent pretreatment of runoff before it enters the trench, thereby enhancing
its longevity. A dry infilter pond has been operating with only minor clogging
for over six years in Maryland.
All Multiple Pond Systems: The basic design of all MPS has two features that
promote greater longevity for ponds. The first is the subsurface reverse-slope
pipe used as the hydrological control device. This design feature greatly
reduces clogging. The second design feature is the fbrebay, or wet cell, which
concentrates sediment deposition in an area where it can be easily removed
without disturbing the entire system.
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Multiple Pond Systems
Feasibility:
MPS are generally subject to the same feasibility requirements associated with
conventional pond systems.
Environmental Attributes:
The flexibility of MPS enables the designer to minimize or avoid many
secondary impacts commonly associated with ponds. The ability to allocate treatment
storage components, or locate them in series can aid the designer in "fingerprinting"
the MPS to avoid disruption to forests and wetlands. Similarly, by allocating less
storage to the permanent pool (and more to ED), one can reduce the potential delta-
t of the pond. In addition, by combining wetlands with conventional wet ponds or
extended detention ponds, it is possible to significantly enhance the habitat value.
Finally, by adding ED to wetlands or wet ponds, one can provide a greater degree
of downstream channel protection. Some of the comparative attributes of alternative
pond designs are illustrated in Table 1.
Costs:
Due to the wide variety of designs, it is difficult to accurately project the
construction costs associated with MPS. In most cases, they will be incrementally
higher than conventional pond systems, if only because of their more complex design.
However, costs can be somewhat reduced if extended detention is used as a partial
substitute for more expensive wet pond or wetland storage.
Adaptability:
MPS are adaptable for use in most regions of the country. In arid or
extremely cold regions, , more of the total storage in the MPS should be devoted to
extended detention.
Maintenance:
MPS have a maintenance burden similar to that of conventional pond systems.
While the MPS may have more complex operation (e.g., adjustment of valves), their
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Multiple Pond Systems
design incorporates numerous features that can reduce routine and non-routine
maintenance (e.g., mowing and sediment removal).
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Infiltration
Trenches
BMP Fact #5
Definition
A conventional infiltration trench is a shallow, excavated trench that has
been backfilled with stone to create an underground reservoir. Stormwater
runoff diverted into the trench gradually exfiltrates from the bottom of the
trench into the subsoil and eventually into the water table.
Enhanced infiltration trenches have extensive pretreatment systems to
remove sediment and oil. They require on-site geotechnical investigations to
determine appropriate design and location.
Schematic Design of a Conventional Infiltration Trench
O o ."'T,
% Runoff Fillers Through
20 Foot Wide Grass Butter Strip
' ° V ' • o •.• »'a
-------
Infiltration Trenches
Capsule Summary:
Pollutant Removal Capability: Although actual performance data on
conventional infiltration trenches is rare, trenches are believed to have
high capability to remove particulate pollutants and a moderate
capability to remove soluble pollutants.
Longevity: Thus far, conventional trenches have proved to have short
life spans. Slightly over half partially or totally fail within five years of
construction. Longevity could be greatly improved through the
utilization of enhanced trenches (i.e., runoff pretreatment, better
geotechnical evaluation and regular maintenance).
Feasibility: The application of trenches, like other infiltration practices,
is severely restricted by soils, water table, slope and contributing area
conditions. These conditions must be carefully investigated in the field
before proceeding with design.
Environmental Concerns Concerns persist about the possibility of
groundwater contamination by trenches. Studies to date do not indicate
a major risk, but have noted migration of nitrate and chlorides.
Environmental Benefits: The widespread use of infiltration in a
watershed helps to replicate predevelopment hydrology, increase dry-
weather baseflow, and reduce bankfull flooding frequency. This benefit
may not be realized in practice, however, given the short lifetimes of
conventional trenches.
Costs: While infiltration trenches are more costly than pond systems in
terms of cost per unit of runoff treated, they are a cost-effective option
for smaller sites where ponds cannot be applied.
Adaptability: The widespread use of infiltration trenches may be limited
in colder or more arid climates and in regions where soils are
predominantly clays or silts.
Maintenance Burden: To enhance longevity and maintain performance,
trenches and associated pretreatment systems do require significant
maintenance. Most conventional trenches do not appear to be regularly
maintained in the field and thus many will require costly rehabilitation
or replacement to maintain their function.
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Infiltration Trenches
Usefulness as a Coastal Urban NPS Management Practice: The use of
conventional infiltration trenches in the coastal zone is limited by site
constraints and poor longevity. While trenches are believed to provide
excellent pollutant removal and can replicate predevelopment hydrology,
their current design must be significantly improved to prevent clogging.
In addition, if local governments commit to widespread use of infiltration,
then they will need to require careful geotechnical investigations and
develop an aggressive trench maintenance/protection program.
Can Infiltration Trenches Reliably Remove Urban Stormwater Pollutants?
Pollutant Removal Mechanisms: Include adsorption, straining and
microbial decomposition in the soil below the trench and trapping of
particulate matter within pretreatment areas (i.e., grass filter strips, sump
pits and plunge pools). (1)
Review of Monitoring Studies: Very few studies monitoring the
performance of conventional infiltration trenches have been conducted
to date. (21) Estimates of performance have been inferred from studies
of rapid infiltration land wastewater treatment systems or by modeling.
(1) (21) For sediment removal, rates in excess of 90 % are cited; for
phosphorus and nitrogen removal, the rate is estimated at 60 %.
Removal rates for trace metals, coliforms and organic matter are
estimated at 90 %. Lower rates are expected for nitrate, chlorides and
soluble trace metals, particularly in sandy soils. (25) (35)
l
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Infiltration Trenches
Factors Influencing Pollutant Removal:
Positive Factors
• Bank run or washed
aggregate
• High organic matter and loam
content of subsoil (35)
• Capture of a large fraction of
annual runoff volume
• Effective pretreatment system,
e.g., a sump pit (20)
Negative Factors
• Sandy sdils
• Trench clogging
• High water table
• Long de-watering times (8)
How Well Do Infiltration Trenches Operate over Time?
Failure Rates: According to data from Maryland, about one in five
conventional trenches fails to operate as designed immediately after
construction; furthermore, barely half of all conventional infiltration trenches
operated as designed after five years. (Many of these had become partially or
totally clogged.) Based on these data, it would appear that conventional
trenches have a design life-span of less than five years without adequate
pretreatment.
A second study of infiltration trench longevity in Maryland indicated
that approximately fifty-five percent of trenches are not operating as
designed. (8) According to the study, one-third of the trenches were
partially or totally clogged; another twenty percent had significant inflow
problems. The oldest trench surveyed was five years old.
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Infiltration Trenches
Factors Influencing Longevity. The relatively short life span of conventional
trenches could be significantly increased by the following:
• Field verification of soil infiltration rates and water table location (8)
• Use of pretreatment systems that provide some degree of storage
(e.g. sump pits, swales with check dams, plunge pools) (8)
• A layer of filter fabric one foot below surface of trench (1)
• Use of a sand layer rather than filter fabric at the bottom of a trench (8)
• Avoiding construction until all contributing watershed disturbances
and construction activities are completed (39)
• Rototilling of trench bottom to preserve infiltration rates (8)
Where and When are Infiltration Trenches Feasible?
Soils: Trenches are not practical in soils with field-verified infiltration
rates of less than 1/2" per hour. (Soil borings should be taken well
below the proposed bottom of the trench to identify any restricting
layers.) (39)
Area: Maximum contributing drainage area to an individual trench
should not exceed five acres.
Slope: The effectiveness of surface trenches is sharply reduced if slopes
are greater than five percent.
Depth to Bedrock and Depth to Water Table: Three feet of clearance
from bottom of trench is recommended.
Sediment Inputs: Conventional trenches may not be advisable on sites
expected to provide high levels of sediment input.
Use in Ultra-urban Areas: Very limited due to unsuitable soils.
Retrofit Capability: Very limited due to unsuitable soils. (10)
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Infiltration Trenches
Stoxmwater Management Capability: Some trench designs can provide
stormwater quantity requirements; however, most trenches function only
as water quality BMPs.
What are the Costs Associated with Infiltration Trenches?
Permitting/Review: Relatively minor permitting and review
requirements at present. An EPA groundwater injection permit,
however, may be required in the future. Moreover, significant costs
may be incurred conducting geotechnical and soils investigations to
determine the feasibility of infiltration at the site.
Construction: On a unit of runoff per volume treated basis, infiltration
trenches are not as cost-effective as pond systems; however, ponds
cannot be used in small watershed areas. (11) Trenches are usually
more cost effective than sand filters. (21)
Maintenance: Very limited data is available on the long-term maintenance
costs associated with trenches. (21) Field studies indicate that regular
maintenance is not being conducted on most infiltration trenches. (8) Sixty to
seventy percent of trenches inspected were found to require maintenance. Few,
if any of the trenches inspected in the field, appeared to ever have been
maintained. (8)
Routine maintenance activities should include inspection and maintenance
of the pretreatment system. Based on the longevity statistics for conventional
trenches, trench replacement or rehabilitation may be required every ten years.
The cost of this may be equal to the initial construction cost.
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Infiltration Trenches
Can Infiltration Trenches Be Easily Adapted for All Coastal Areas?
Trenches may not be easily adapted to perform in:
• regions with long cold winters and deep freeze thaw
levels
• more arid regions with sparse vegetative cover in
upland areas that might contribute high sediment levels
• regions where groundwater is used locally for human
consumption.
• regions with day or silty soils
What are the Environmental Concerns and Benefits of Infiltration
Trenches?
Positive Impacts:
• Groundwater recharge
• Reduction in downstream bankfull flooding events
Negative Impacts:
• Slight to moderate risk of groundwater contamination depending
on soil conditions
• No habitat is created
• High failure rates of conventional trenches sharply limit the ability
to meet stormwater and water quality goals at the watershed scale
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Infiltration Trenches
What is Not Known about Infiltration Trenches?
• The most effective pretreatment system(s) to, protect infiltration
capability over the long term
• The pollutant removal performance of trenches in sandy soils near
the water table
• The performance of trenches in subfreezing weather and during
snowmelt runoff conditions
• Maintenance programs and schedules that can be developed to
improve trench performance
• Further development of experimental methods to accurately measure
soil infiltration rates is also needed
Current Assessment of Urban Best Management Practices
Page - 46
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Infiltration Basins bmp Fact Sheet #6
Definition
Infiltration basins are impoundments where incoming stormwater runoff is
stored until it gradually exfiltrates through the soil of the basin floor.
Schematic Design of an Infiltration Basin
Top View
Riprap
Outlall
Protection
Side View
Embankment
Flat Basin Floor with
Dense Grass Turl
Riprap
Settling
Basin and
Level Spreader
A
Back-up Underdrain
Emergency Spillway
Valve
Exfiltration Storage
Back-up Underdrain Pipe in Case ot Standing Water Problems
Source: Schueler, 1987.
Current Assessment of Urban Best Management Practices
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Infiltration Basins
Capsule Summary:
Pollutant Removal Capability: No performance data on infiltration basins is
available; however, they are presumed to have the same general removal
efficiencies reported for infiltration trenches: high removal for particulate
pollutants and moderate removal for soluble pollutants.
Longevity: Infiltration basins do not have long life spans. Sixty to one
hundred percent of basins studied could no longer exfiltrate runoff after five
years. Major design refinement and site investigation will be required to
achieve sufficient longevity.
Feasibility: The application of basins is restricted by numerous site factors
(soils, slope, water table and contributing watershed area).
Environmental Concerns: The greatest environmental concern relative to
infiltration basins is the fact that their environmental benefits may not be
realized due to widespread failure. Groundwater contamination is frequently
cited as a risk, yet studies to date indicate that pollutant migration is very
localized in scope.
Environmental Benefits: When infiltration basins work, they can replicate
predevelopment hydrology more closely than other BMP options. Basins also
provide more habitat value than other infiltration systems (but less than pond
systems).
Costs: An attractive feature of infiltration basins is their cost-effectiveness.
They are projected to cost only ten to twenty percent more than dry detention
ponds; however, the cost of an improved infiltration basin may be much
higher.
Adaptability: Infiltration basin may not be applicable in areas of cold winters,
arid growing seasons or impermeable soils.
Maintenance Burden: Regular maintenance activities apparently cannot prevent
rapid clogging of infiltration basins. Once clogged, it has been very difficult
to restore thetf original "function; thus, many have been converted to retention
basins or wetlands.
Current Assessment of Urban Best Management Practices
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Infiltration Basins
Usefulness as a Coastal Urban NPS Management Practice: Until a better
infiltration basin design is demonstrated, infiltration basins are not
recommended for widespread application in the coastal zone.
Can Infiltration Basins Reliably Remove Urban Stormwater Pollutants?
Pollutant Removal Mechanisms: As with other infiltration systems, removal
is accomplished by adsorption, straining, and microbial decomposition in the
basin subsoils as well as the trapping of particulate matter within pretreatment
areas. (1)
Review of Monitoring Studies: No actual performance data is available to
evaluate the pollutant removal capability of infiltration basins. (21) Estimates
have been inferred from studies of rapid infiltration of land-applied wastewater
effluent and from modeling studies. (1) Removal efficiencies are presumed to
be high for particulate pollutants and moderate for soluble pollutants. Lower
rates are expected for nitrate, chlorides and soluble trace metals, particularly
in sandy soils. (25) Actual pollutant removal is projected to be related to the
proportion of the annual runoff volume successfully exfiltrated into the subsoil.
Factors Influencing Pollutant Removal:
Positive Factors (8)
Negative Factors
• Forebay
• Short dewatering times
• Back-up underdrain systems
• Small contributing watershed
• Dense vegetative cover
• Non-concentrated flow
• Basin clogging
• High water tables
• Clay and silt soils
• High sediment inputs
• Large contributing watershed
area
• Long dewatering times
• Algal growth
• Large depth of standing water
(8)
Current Assessment of Urban Best Management Practices
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Infiltration Basins
How Well Do Infiltration Basins Operate over Time?
Failure Rates: The failure rates for infiltration basins in the mid-Atlantic region
range from sixty to one hundred percent in the first five years, according to
two recent studies. (36) (37) (8) Up to fifty percent had failed shortly after
construction. The primary reason for failure is clogging. Of twelve basins in
Maryland, none were able to exfiltrate runoff after five years. (8) These basins
had an average standing water depth of one foot.
Astonishingly, all these basins were partially covered by wetland
vegetation and/or algal mats. The basins had become defacto retention ponds;
some sixty percent were still providing partial water quality treatment. About
twenty percent of infiltration basins studied in Maryland have been retrofit into
pond systems. (36)
Factors Influencing Longevity: Clearly, current infiltration basin designs do
not perform adequately. The following factors appear to contribute heavily to
improved life-span for infiltration basins (8):
• Shorter dewatering rate (24 hours rather than 72 hours)
• Pretreatment fbrebays to control sediment inputs
• Small contributing watershed areas
• Shallow basin depths (standing water appears to promote soil
compaction)
• Off-line designs that bypass large storms and sediment inputs
• More efficient dewatering mechanisms in basins (e.g., stone trenches
rather than soil)
• Careful geotechnical investigation of soil conditions prior to
excavation (40)
• Use erf sand as a surface layer in the basin
• Installation of underdrains into the basin
Current Assessment of Urban Best Mahagement Practices
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Infiltration Basins
It is difficult to determine whether the design changes, as proposed
above, would achieve sufficient longevity. Local communities should be
cautious in promoting infiltration basins until:
• the longevity and performance of the new generation of infiltration
basins is adequately demonstrated
• the basic infiltration basin design is readily convertible into a
retention basin.
Where and When are Infiltration Basins Feasible?
Soils: Basins are not feasible at sites with field-verified soil infiltration rates
of less than 0.5 inches/hour. Soil borings should be taken well below the
proposed bottom of the basin to identify any restricting layers. (39)
Contributing Watershed Area: Normal contributing drainage area ranges from
two to fifteen acres. Larger drainage areas are not generally recommended.
Depth to Bedrock/Seasonally High Water Table: Minimum of three feet.
Pre treatment: Basins are not recommended unless upland sediment inputs can
be pretreated.
Land Use: Some caution should be exercised when applying a basin in a
watershed with a risk of chronic oil spills.
Use in Ultra-urban areas: Not recommended.
Retrofit Capability: Not recommended. (20)
Stormwater Management Capability: In some instances, a basin can provide
stormwater management detention, but it is not generally recommended.
Current Assessment of Urban Best Management Practices
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Infiltration Basins
What are the Costs Associated with Infiltration Basins?
Permitting/Review: At present, infiltration basins have few permitting
problems, but there may be a need in the future to secure groundwater
injection permits. Basins should nfit be approved until soil and water table
behavior have been confirmed at the site through geotechnical investigation and
a sound and redundant runoff pretreatment system has been devised.
Construction: Construction of infiltration basins has been estimated to cost ten
to twenty percent more than a conventional dry stormwater pond; however,
the design improvements needed to enhance basin longevity may significantly
escalate cost figures. (1)
Maintenance: Regular maintenance for effective infiltration basins is estimated
to be about five percent of initial construction cost; however, the actual
maintenance cost of current designs is much higher, reflecting rehabilitation into
retention or ED ponds. (11)
Can Infiltration Basins Be Easily Adapted for All Coastal Areas?
Infiltration basins are not widely recommended for other coastal areas
outside the Mid-Atlantic region, until sufficient longevity can be demonstrated.
In addition, basins may fare poorly in:
• regions with long cold winters and snowmelt/freeze thaw conditions
• arid regions where a dense vegetative cover in the contributing
watershed cannot be reliably maintained
• regions with sole-source aquifers
• regions with predominantly clay or silt soils
Current Assessment of Urban Best Management Practices
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Infiltration Basins
What are the Environmental Concerns and Benefits of Infiltration
Basins?
Positive Impacts:
• Groundwater recharge helps to maintain dry-weather flows in streams
• Reduction in downstream bankfull flooding events
(Note: The short lifetimes of basins as currently designed suggest
that the positive hydrological and water quality impacts may not be
realized in practice.)
Negative Impacts:
• Slight to moderate risk of local groundwater contamination
(particularly if contributing watershed is industrial or has heavy
vehicular petroleum washoff).
• Infiltration basins provide some habitat value, but this is quite modest
in comparison to that provided by pond systems. Failed basins
provide better habitat than functioning basins.
Current Assessment of Urban Best Management Practices
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Infiltration Basins
What is Not Known about Infiltration Basins?
Significant uncertainty exists as to the fundamental feasibility of the infiltration
basin approach. The basic issue is whether the temporary but deep ponding in
basins makes clogging inevitable. Other certainties include:
• what, if any, pretreatment system can assure basin longevity
• the impact of basins on the local water table and the risk of
groundwater migration
• performance of basins in freezing conditions and during snowmelt
conditions
• the accuracy of current field methods for estimating long-term local
soil infiltration rates
• the actual monitored pollutant removal performance of a fully
operational basin, particularly with respect to soluble nutrients
Current Assessment of Urban Best Management Practices
Page • 54
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Porous Pavement bmp Fact sheet #7
Definition
Porous pavement is an alternative to conventional pavement whereby runoff
is diverted through a porous asphalt layer and into an underground stone
reservoir. The stored runoff then gradually infiltrates into the subsoil.
Schematic Design of a Porous Pavement System
Berm Keeps Oft site
Runoff and Sediment
Out. Provides
Temporary Storage
Asphalt is Vacuum Swept
Followed by Jet Hosing
to Keep Pores Free
Site Posted to Prevent
Resurfacing and Use of
Abrasives, and to
Restrict Truck Parking
Porous Asphall
Overflow
( Reverse Perforated Pipe
Observation
Well
Gravel
Course or
b Inch
Sand Layer
Filter Fabric
Line* Sides _
ot Reservoir
to Prevent
Sediment Entry <
Undisturbed Soils with an fc Greater Than 0 27 »nches/Mour,
Preferably 0 50 Inches/Hour or More
Side View
Source: Schueler, 1987
Porous Pavement Course
(2.5-4.0 inches Thick)
Filter Court*
(0 5 inch Oiameter Gravel.
10 inch Thick)
Stone Reservoir
(1.5-3.0 Inch
Diameter Stone)
Oepth variable Oepending
on the Storage Volume
Needed Storage Provided
by the Void Space Between
Stones
Filter Course (Gravel. 2 inch Deep)
Filter Fabric Layer
Undisturbed Son
Current Assessment of Urban Best Management Practices
Page - 55
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Porous Pavement
Capsule Summary:
Pollutant Removal Capability. Operating porous pavement systems have been
shown to have high removal rates for sediment, nutrients,, organic matter, and
trace metals. & majority of the removal occurs as the result of the
exfiltration of runoff into the subsoil, and subsequent adsorption or straining
of pollutants within the subsoil.
Longevity: Porous pavement sites have a high failure rate (75 %). Failure is
due to partial or total clogging of the facility that occurs:
• Immediately after construction
, Over time, when porous asphalt is clogged by sediment and
• When pavement is resurfaced with non-porous materials
Feasibility: The use of porous pavement is highly constrained, requiring deep
and permeable soils, restricted traffic, and suitable adjacent land uses.
Environmental Concerns: Concerns range from possible a-oundwater
contamination (exacerbated by leaching of asphalt materials and hydro-carbons)
to the loss of benefits due to premature failure.
Environmental Benefits: When operating properly, porous pavement can
replicate pre-development hydrology, increase groundwater recharge, and
provide excellent pollutant removal.
Costs: Porous pavement can be a very cost-effective BMP in the commercial
areas where it can be applied. While the asphalt is more expensive than
conventional pavement, porous pavement eliminates the need for stormwater
drainage, conveyance, and treatment systems off-site.
Adaptability: Use of porous pavement may he restricted in regions with
colder climates, arid regions or regions with high wind erosion rates, and in
areas of sole-source aquifers.
Maintenance Purdeiu Quarterly vacuum sweeping and/or jet hosing is needed
to maintain porosity. Field data however indicate that this routine maintenance
practice is not frequently followed.
Current Assessment of Urban Best Management Practices
Page - 56
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Porous Pavement
Usefulness as a Coastal Urban NPS Management Practice: While porous
pavement can be a very effective urban BMP in certain restricted
applications, it cannot be used on a widespread basis in the coastal zone.
Local communities need to determine if the high failure rate of porous
pavement can be alleviated by better design, inspection, sediment control,
and maintenance within their jurisdiction.
Can Porous Pavement Reliably Remove Urban Stormwater Pollutants?
Pollutant Removal Mechanisms: Include adsorption, straining, and microbial
decomposition in the sub-soil below the aggregate chamber, and trapping of
particulate matter within the aggregate chamber. In addition, up to ninety of
the annual rain fall volume is diverted to groundwater rather than surface
runoff. (1)
Review of Monitoring Studies: Two monitoring studies have been conducted
that indicate high long-term removal of sediment (up to 80 %), phosphorous
(up to 60 %), and nitrogen (up to 80 %), as well as high removal rates for
trace metals and organic matter. (39) (41) The majority of pollutant removal
at porous pavement sites is due to the reduction of mass loadings via the
groundwater. Measured concentrations of sediment, phosphorous, and nitrogen
are only slightly reduced in the small, measured outflows from porous
pavement.
Factors Influencing Pollutant Removal (1):
Positive Factors
Negative Factors
• High exfiltration volumes
• Routine vacuum sweeping
• Maximum drainage time of
• Poor construction practices
• Inadequate surface
maintenance
• Use of sand for snow removal
• Low exfiltration volumes
two days
• Highly permeable soils
• Clean-washed aggregate
• Organic matter in subsoils
• Pretreatment of off-site runoff
Current Assessment of Urban Best Management Practices
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Porous Pavement
How Well Does Porous Pavement Operate Over Time?
Failure Rates: Seventy-five percent of all porous pavement systems surveved
in Maryland have partially or totally clogged within five years. (36) Failure
has been attributed to inadequate construction techniques, low permeable soils
and/or restricting layers, heavy vehicular traffic, and resurfacing with non-
porous pavement materials. Some fraction of the clogged porous pavement
sites could be rehabilitated with drop inlets and daylighting from the aggregate
chamber. The oldest operating porous pavement systems are about ten years
old.
Factors Influencing Longevity;
• Routine vacuum sweeping
• Use in low intensity parking areas
• Restrictions on access by heavy trucks, use of de-icing chemicals and
sand
• Resurfacing
• Inspection and enforcement of specifications during construction
• Pretreatment of off-site runoff
• Extra-ordinary sediment control during construction
Where and When is Porous Pavement Feasible?
Soils: Porous pavement is not practical in soils with field verified infiltration
of lei than 0.5 inches per hour. Soil borings must be taken two to four feet
below the aggregate to identify any restricting layers.
Area: Most porous pavement sites are less than ten acres in size. This
primarily reflects the perceived economic and liability problems associated with
larger applications.
Slope: Less than five percent.
Depth to Bedrock and Water Table: Three feet minimum clearance from
bottom of system.
Current Assessment of Urban Best Management Practices
Page • 58
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Porous Pavement
Traffic Volumes: Porous pavement is not recommended for most roadways
and cannot withstand the passage of heavy trucks. Typically, porous pavement
is recommended for lightly used satellite parking areas and access roads.
Sediment Inputs: Porous pavement is not advisable in areas expected to
provide high levels of off site sediment input (e.g., upland construction,
sparsely vegetated upland areas and areas with high wind erosion rates)
Cold Climates: While the standard porous pavement design is believed to
withstand freeze/thaw conditions normally encountered'in most regions of the
country, the porous pavement system is very sensitive to clogging during snow
removal operation (e.g., application of sand and de-icing chemicals and
scrapping by snow plows).
Use in Ultra-urban Areas: Some possibilities exist for the use of porous
pavement during infill development provided that suitable soils are present.
Retrofit Capability: Extremely limited. Most soils in urbanized watersheds
have been previously modified and so are not capable of providing adequate
infiltration rates.
Stormwater Management Capability: Porous pavement sites can meet
stormwater management requirements in many cases.
What are the Costs Associated with Porous Pavement?
Permitting/Review: To prevent premature failure, local governments need to
extensively review proposed porous pavement applications. In addition, the
use of porous pavement may require a groundwater injection permit. Plans
need to be reviewed to assure that
• geo-technical data confirm exfiltration capability
• porous asphalt is correctly mixed and installed
• a long-term maintenance/resurfacing plan exists
Furthermore, local governments must conduct strict inspections dining
pavement installation with an emphasis on sediment control.
Current Assessment of Urban Best Management Practices
Page ¦ 59
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Porous Pavement
Construction: The economics of porous pavement systerr^s are difficult to
evaluate. (38) On the positive side, porous pavement eliminates the need for
most inlets, outlets, and some curb and gutters in the parking lot, as well as
reducing off-site conveyance / storm water treatment costs, and may preserve
land with a high economic value for future use. (38) On the negative side,
porous pavement has higher engineering and installation costs as compared to
conventional pavement and may require greater excavation. In addition,
porous asphalt can be up to fifty percent more expensive than conventional
asphalt and may be difficult to obtain in some regions of the country.
Maintenance: The routine maintenance of porous pavement consists of
quarterly vacuum sweeping and may constitute one to two percent of the
initial construction costs. However, given the high rates of failure, the total
maintenance costs must include rehabilitation of the clogged system. These
costs can be extremely high. In addition, the costs and responsibility for
periodic resurfacing of porous pavement sites must be addressed.
Can Porous Pavement be Easily Adapted for All Coastal Areas?
Porous pavement may not be easily adapted to reliably perform in:
• regions with long cold winters and deep freeze/thaw conditions
• arid regions with sparse vegetative cover and high wind erosion
rates
• regions with predominantly high clay or silty soils
• sole-source aquifers
What are the Environmental Concerns and Benefits of Porous Pavement?
Positive Impacts:
• Porous pavement can divert large volumes of potential surface runoff
to groundwater recharge and, in some cases, provide even greater
recharge than pre-development conditions (41)
• Porous pavement can reduce downstream bank-full flooding
• Provides stormwater quantity and quality treatment on-site, thereby
protecting woodland, wetland, and stream valleys elsewhere on the site
(38)
Current Assessment of Urban Best Management Practices
Page - 60
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Porous Pavement
Negative Impacts:
• Slight to moderate risk of groundwater contamination depending on
soil conditions and aquifer susceptibility
• Possible transport of hydrocarbons from vehicles and leaching of toxic
chemicals from asphalt/or binder surface
• The high failure rate of porous pavement sharply limits the ability to
meet watershed storm water quality and quantity goals
What is Not Known About Porous Pavement?
• The ability to maintain pavement porosity over the long term, particularly
with resurfacing needs and snow removal
• Pollutant removal capability of porous pavement during sub-freezing
weather and snow removal conditions
• The interaction of porous pavement with groundwater in sandy soils and
high water table conditions typical of coastal areas
• Low cost maintenance and rehabilitation options for restoration of porosity
Current Assessment of Urban Best Management Practices
Page • 61
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Sand Filters
BMP Fact Sheet #8
Definition
Sand filters are a relatively new technique for treating stormwater, whereby
the first flush of runoff is diverted into a self-contained bed of sand. The
runoff is then strained through the sand, collected in underground pipes and
returned back to the stream or channel.
Enhanced sand filters utilize layers of peat, limestone, and/or topsoil, and
may also have a grass cover crop. The adsorptive media of enhanced sand
filters is expected to improve removal rates.
In addition, sand-trench systems have been developed to treat parking lot
runoff.
Conceptual Design of a Sand Filter System
Otmova pipe
1 n
n
GeocenUe Fabric
//
n
::::::: ;
v.v » ¦ •• -t
!»• FINE SAND
• '** ^ ^ £ V- /
— v>* ' -j- .
P* r r .¦
v?."-
v!iv
*•*•*•* ^ fnOCK^ «Vl
W>:
«" Performed pipe GromoninM
Source: Austin, Texas. 1991.
Current Assessment of Urban Best Management Practices
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Sand Filters
Capsule Summary:
Pollutant Removal Capability: Sand filter removal rates are high for
sediment and trace metals, and moderate for nutrients, BOD and fecal
coliform. The untested peat sand filter is projected to achieve
significantly higher removal rates.
Longevity: Sand filters appear to have excellent longevity due to their
off-line design and the high porosity of sand as a filtering media;
however, relatively simple but frequent maintenance is required to
maintain performance.
Feasibility: Because sand filters are a self-contained man-made soil
system, they can be applied to most development sites and have few
constraining factors. Most sand filters have been used on small parking
lots.
Environmental Concerns: Sand filters have very few environmental
concerns because they are an off-line self-contained system. Surface sand
filters can be an eyesore; hence, they may enjoy limited community
acceptance in some regions.
Environmental Benefits: Particularly useful for groundwater protection.
Little or no wildlife habitat value is provided.
Costs: Sand filters are more costly than infiltration trenches (by a factor
of two or three), but have lower regular maintenance/rehab costs.
Adaptability: To date, sand filters have only been widely applied in
one region of the country, and some localities may experience some
initial problems in importing the technology. Performance of sand filters
in colder climates is unknown.
Maintenance Burden: Sand filters require frequent manual maintenance,
primarily raking, surface sediment removal, and removal of trash, debris
and leaf litter.
Current Assessment of Urban Best Management Practices
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Sand Filters
Usefulness as a Coastal Urban NPS Management Practice:
Sand filters are a widely applicable and adaptable urban BMP that can
provide significant pollutant removal at the small sites that often
characterize coastal development. Sand filters are a costly practice, but
appear to be long-lived and possess a reasonable maintenance burden. At
the present time, only one locality in the country extensively utilizes the
sand filter. Consequently, local governments may need to demonstrate the
sand filter before it becomes widely adopted.
Can Sand/Peat Sand Filters Reliably Remove Urban Stormwater
Pollutants?
Pollutant Removal Mechanisms: Pollutant removal is primarily
achieved by straining pollutants through the filtering medium (i.e., sand
or peat) and by settling on top of the sand-bed and/or pretreatment
pool. Additional nutrient removal can be accomplished by plant uptake
if the filter has a grass cover crop.
Review of Monitoring Studies: Performance monitoring has been
conducted on three sand filter systems in the Austin, Texas area. (42)
Average removal rates of 85 % for sediment, 35 % for nitrogen, 40 %
for dissolved phosphorus, 40 % for fecal coliforms, and 50 to 70 % for
trace metals were reported. Negative removal was reported for nitrate-
N which may reflect the nitrification process.
Slightly higher pollutant removal performance has been projected
for peat sand filters due to the adsorptive properties of peat. (43) These
are an estimated 50 % for TN, 70 % for TP and 90 % for BOD. The use
of grass on the surface of a sand filter may also augment pollutant
removal.
Current Assessment of Urban Best Management Practices Page - 65
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Sand Filters
Factors Influencing Pollutant Removal:
Positive Factors
• Off-line systems (42)
• Peat and/or calcitic limestone
Negative Factors
• On-line systems (42)
• Freezing weather (43)
layer (43)
• Grass cover (42)
• Longer drawdown times (24
to 40 hours) (42)
• Pretreatment pool (43)
• Minimum depth of eighteen
inches (42)
How Well Do Sand/Peat Sand Filters Operate over Time?
Failure Rates: Nearly one thousand sand filters have been installed in
the Austin, Texas area. (44) According to the Austin Department of
Public Works, the vast majority are working as designed and very few
have failed. The oldest operating sand filter is almost ten years old.
Sand filters have not been widely applied elsewhere in the country,
although they have been used effectively in dense urban areas within
the District of Columbia. (57)
Factors Influencing Longevity:
• Quarterly maintenance of the sand filter to maintain porosity
• Flow splitter designs that will not clog frequently
• Pretreatment pool
• Adequate access to the sand filter
• Regular removal of surface sediments (frequency variable)
Most of the maintenance for sand filters is done by manual rather
than mechanical means; consequently, the design should be oriented to
make access and manual sediment removal an easier proposition. (44)
Current Assessment of Urban Best Management Practices
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Sand Filters
Where and When are Sand/Peat Sand Filters Feasible?
Sand filters are a very adaptable practice; they can be used on
areas with thin soils, high evaporation rates, low soil infiltration rates,
and limited space. (45)
Watershed Size: The upper limit on sand filters appears to be about
fifty acres; however, most have a contributing watershed between a half
and ten acres. The Delaware parking lot sand trench (Shaver, 1991) is
restricted to five acres or less.
Head Requirements: Two to four feet of available head needed for
most off-line sand filter applications.
Use in Ultra-urban Areas: Sand filters and peat sand filters can be used
to treat stormwater runoff from small infill developments and from small
parking lots (i.e., gas stations, convenience stores). They are widely
applied in ultra-urban areas within the District of Columbia. (57)
Retrofit Capability: Sand filters and peat sand filters have been
designed as end-of-pipe retrofits in several applications. The Delaware
sand filter system may be of particular value for older parking lots.
Stormwater Management Capability: Sand filters have a limited ability
to reduce peak discharges; they are usually designed solely to improve
water quality.
What are the Costs Associated with Sand/Peat Sand Filters?
Design: A number of standard sand filter designs are available;
however, the technology is still developing. Sand filters have few
permitting constraints. Sand filters with concrete walls, or parking lot
designs, may require additional structural engineering.
Construction: $3 to $10 per cubic foot of runoff treated. (21) For
comparison purposes, this is about two to three times the cost of
constructing a similarly sized infiltration trench. Significant economies
of scale exist as sand filter size increases.
Current Assessment of Urban Best Management Practices
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Sand Filters
Maintenance: Estimated to be approximately five percent of construction
cost per year. (44) Much of the maintenance requires manual rather
than mechanical labor, such as raking/ disposal of contaminated sand, and trash
and debris removal.
Can Sand/Peat Sand Filters Be Easily Adapted for All Coastal Areas?
The sand filter holds great promise as a BMP for coastal areas.
It can be applied to smaller sites that cannot effectively be served by
ponds or to regions where poor soil infiltration rates or groundwater
concerns restrict the use of infiltration. The sand filter may not be
applicable in regions of colder climate, however. Sand filters are useful
in waterfront situations where limited space and small contributing
watershed areas eliminate other BMP options.
What are the Environmental Concerns and Benefits of Sand/Peat Sand
Filters?
Positive Impacts:
• Sand filters are useful in watersheds where concerns over
groundwater quality prevent use of infiltration.
* Disposal of surface sediments from sand filters does not appear to be
a problem. Testing by the Austin Department of Public Works indicates
that the sediments are not toxic and can be landfilled.
Current Assessment of Urban Best Management Practices
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Sand Filters
Negative Impacts:
• Larger sand filter designs, without grass cover, may not be attractive
in residential areas. The surface of sand filters can be extremely
unattractive; some sand filters have caused odor problems.
• The concrete walls that surround the sand filter represent a safety
hazard and thus should be fenced.
• Sand filters generally function only as a stormwater quality practice
and do not provide detention for downstream areas.
What is Not Known about Sand/Peat Sand Filters?
• How well sand filters operate in colder climates and freezing
conditions
• Recommended frequency for disposal of the surface sediments of a
sand filter
• Whether the peat sand filter can improve pollutant removal
• Availability of appropriate peat materials in all areas of the country
• What types of filtering media, other than sand and peat, might
promote greater nutrient removal
• Delta-t of the sand filters
Sand filters have only been extensively used in one region of the
country. The basic design may need to be modified to suit local
conditions. Initial performance of sand filters and peat sand filters may
be limited until local governments and engineers gain more experience
with them.
Current Assessment of Urban Best Management Practices
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Grassed Swales BMP Fact Sheet #9
Definition
Conventional grassed swales are earthen conveyance systems in which
pollutants are removed from urban stormwater by filtration through grass and
infiltration through soil.
Enhanced grassed swales, or biofilters, utilize check dams and wide
depressions to increase runoff storage and promote greater settling of
pollutants.
Schematic Design of an Enhanced Grassed Swale
Swale Slopes
as Close to
Zero as Drainage
Will Permit
Side-slopes
3:1 or Less
Railroad Tie
Check-dam
(Increases Infiltration)
i o-xj
i-o-.i'-;':
Dense Growth '-;-:
ot Grass (Reed
Canary or KY-31
Tall Fescue)
Stone Prevents
Downstream Scour
Source: Schueler, 1987.
Current Assessment of Urban Best Management Practices
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Grassed Swales
Capsule Summary:
Pollutant Removal Capability: Conventional grassed swale designs have
achieved mixed performance in removing particulate pollutants such as
suspended solids and trace metals. They are generally unable to remove
significant amounts of soluble nutrients. Biofilters that increase detention,
infiltration and wetland uptake within the swale have the potential to
substantially improve swale removal rates.
Longevity: Conventional swales can last an indefinite period of time if
properly designed, periodically mowed, and if sediment deposits are removed
from time to time.
Feasibility: Swales can provide sufficient runoff control to replace curb and
gutter in single-family residential subdivisions and on highway medians;
however, their ability to control large storms is limited. Therefore, in most
cases, swales must be used in combination with other BMPs downstream.
Environmental Concerns: Leaching from culverts and fertilized lawns may
actually increase the presence of trace metals and nutrients, in some instances.
Environmental Benefits: Swales eliminate curbs and gutters and provide some
infiltration and habitat benefits.
Costs: Swales are usually less expensive to construct than curb and gutter but
may require more land.
Adaptability: Swale performance diminishes sharply in highly urbanized
settings. Also, swales should generally not receive construction stage runoff.
Maintenance Burden: Mowing and periodic sediment cleanout are the primary
maintenance activities. In residential subdivisions, adjacent homeowners will
undertake these responsibilities.
Current Assessment of Urban Best Management Practices
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Grassed Swales
Usefulness as a Coastal Urban NPS Management Practice:
Traditionally, swales have been used as a stormwater conveyance system
rather than a stormwater treatment method. Given the appropriate site
conditions, swales can complement (but seldom substitute for) other BMPs.
Can Grassed Swales Reliably Remove Urban Stormwater Pollutants?
Pollutant Removal Mechanisms: Grassed swales act to remove pollutants by
the filtering action of grass, by settling, and in some instances, by infiltration
into the subsoil.
Review of Monitoring Studies: The pollutant removal capability of ten
conventional residential and highway swale systems has been monitored by six
researchers. (21) The results are mixed. Half of the swales studied
demonstrated a moderate to high pollutant removal capability and the other
half showed no removal or negative removal capability. (2)
The expected removal efficiency of a well-designed, well-
maintained conventional swale is projected to 70 % for TSS, 30 % for
TP, 25 % for TN, and 50 to 90 % for various trace metals. Swales
appear to be more effective at removing metals than nutrients; a number
of researchers have observed trace metal accumulation in swale
sediments. (46) (47) Some evidence has also been offered that
resuspension or remobilization of nutrients may occur. (46) No
performance data exists on the effect of check dams in swales; however,
the detention and trapping capability that they add is projected to be
quite useful. (1)
Current Assessment of Urban Best Management Practices Page - 73
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Grassed Swales
Factors Influencing Pollutant Removal (32):
Positive Factors
Negative Factors
• Check dams
• Low slopes
• Permeable subsoils
• Dense grass cover
• Long contact time
• Smaller storm events
• Coupling swales with plunge
• Compacted subsoils
• Short runoff contact
storms
• Large storm events
• Snow melt events
• Short grass heights
• Steep slope (six percent or
pools, infiltration trenches or
greater)
• Runoff velocities (1.5
pocket wetlands
• Swale length greater than two
fps or more)
• Peak discharge (5 cfs or
hundred feet
more)
• Dry-weather flow
How Well Do Grassed Swales Operate over Time?
Failure Rates: Surveys by Horner (1988) and in the Washington area indicate
that the vast majority of conventional swales operate as designed with
relatively minor maintenance (grass mowing). The primary maintenance
problem is the gradual build-up of soil and grass adjacent to roads which
prevents entry of runoff in swales. Surprisingly, gully erosion is not a
problem in well-designed swales in areas where climate permits the
establishment of a dense turf.
Factors Influencing Longevity:
• Runoff velocity that is consistently high (i.e., > 5 fps) will increase
the tendency for the swale to erode
• The rate of erosion also diminishes as side slopes become flatter (32)
Current Assessment of Urban Best Management Practices
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Grassed Swales
Where and When are Grassed Swales Feasible?
Contributing Watershed Area: Grassed swales can only be applied in areas
where maximum flow rates are not expected to exceed 1.5 fps. (32) The
suitability of a swale at a site will depend on the area, slope and
imperviousness of the contributing watershed as well as the dimensions and
slope of the swale system.
Dry-Weather Flow: Pollutant removal will be reduced if dry-weather flow is
present in the swale.
Peak Discharge: Swales generally do not have the capacity to control runoff
effectively in areas where peak discharge exceeds 5 ds or where velocity is
over 3 fps. To decrease velocity, the swale should be designed to be as wide
as available space allows.
Construction Areas: The high sediment loads from unstabilized construction
sites can overwhelm the system.
Slope: To increase infiltration rates, longitudinal slopes should be as close to
zero as possible and not greater than five percent. (1)
Grass Height: A vertical stand of dense vegetation higher than the water
surface is most effective (a minimum of six inches). (48)
Swale Contact Time: In general, pollution removal capacity increases with
contact time of runoff through the swale. Swale contact time varies with the
depth, width and length of the swale as well as longitudinal slope and type
of vegetation. Any one of these variables or sets of variables can be
manipulated to meet water quality objectives. In addition, check dams can
further increase contact time. (32)
Use in Ultra-urban Areas: It is very difficult to prevent erosion in swales
located in highly impervious, ultra-urban area.
Retrofit Capability: Many residential developments and highways have
existing grass channels. An attractive retrofit option is to install check dams
to increase contact time and promote settling using ported weirs.
Current Assessment of Urban Best Management Practices
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Grassed Swales
Stoxmwater Management Capability: Conventional grassed swales are
primarily a stormwater conveyance system and rarely provide sufficient
detention to attenuate storm flows. The exception is when detention storage
is provided behind check dams in very long swale systems.
Whot ore the Costs Associated with Grossed Swciles?
Permitting/Review; Swale design is relatively standard and no special penruts
are required. During review, however, the plan reviewer should carefully
evaluate the swale design to see if it will act as a BMP or merely as an open
channel conveyance system.
Construction: Typically, grassed swales cost less to construct, than curb,
gutters, and underground pipe. Costs may ran from $5 to $15 per linear foot,
depending on swSe dimensions and the degree of internal storage (check
dams) provided. (1)
Regular maintenance costs for conventional swales are minimal.
r£r™,,t of sediments, trapped behind check dams, and spot vegetation repair
may be required.
Grassed swales also require general lawn maintenance such as mowing,
waterinz and chemical appHcation. In residential subdivisions, adjacent
homeowners will manage this responsibility. Also, inspection after large storms
for erosional failures aid special maintenance should occur regularly.
Current Assessment of Urban Best Management Practices
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Grassed Swales
Can Grassed Swales Be Easily Adapted for All Coastal Areas?
Grassed swales can be used in all regions of the country where climate
and soils permit the establishment and maintenance of dense vegetative cover.
The performance of swales in removing pollutants may be reduced in:
• regions with long, cold winters and snowmelt conditions, particularly
where salts and other de-icing chemicals are applied or where snow
plowing scrapes the shoulder
• regions with sandy soils (Sandy soils make it difficult to maintain
the side slopes of the swale.)
It should be noted that the highest removal rates for swales have been
reported in Florida where the climate supports lush vegetation year-round.
What are the Environmental Concerns and Benefits Associated with
Grassed Swales?
Positive Impacts:
• When grassed swales are substituted for curbs and gutters, they can
slightly reduce impervious areas, and more importantly, eliminate a
very effective pollutant collection and delivery system
• Low slope swales can create wetland acreage
• Unmowed swale systems that are not adjacent to roadways can
provide valuable "wet meadow" habitat
• Swales can act to partially infiltrate runoff from small storm events
if underlying soils are not compacted
Current Assessment of Urban Best Management Practices
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Grassed Swales
Negative Impacts:
• Culverts may leach trace metals into runoff
• Lawn fertilization may increase runoff nutrient levels
• Possible impact on local groundwater quality
• Standing water in residential swales will not be popular with adjacent
residents for aesthetic reasons and because of potential safety, odor
and mosquito problems.
What I$ Not Known about Grassed Swales?
• The degree to which certain design factors can enhance removal
capability
• The effect of combining swales with micropools, pocket wetlands and
infiltration trenches on pollutant removal efficiency
• Whether pollutant removal rates of swales decline with age (e.g.,
when adsorption sites are used up)
• The effect of slope on the filtration capacity of vegetation
• The benefit of check dams in preventing downstream migration of
pollutants
Current Assessment of Urban Best Management Practices
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Filter Strip BMP Fa
Definition
Filter strips are vegetated sections of land designed to accept runoff as
overland sheet flow from upstream development. They may adopt any natural
vegetated form, from grassy meadow to small forest. The dense vegetative
cover facilitates pollutant removal. Filter strips cannot treat high velocity
flows; therefore, they have generally been recommended for use in agriculture
and low density development.
Filter strips differ from natural buffers in that strips are not "natural";
rather, they are designed and constructed specifically for the purpose of
pollutant removal. A filter strip can also be an enhanced natural buffer,
however, whereby the removal capability of the natural buffer is improved
through engineering and maintenance activities such as land grading or the
installation of a level spreader.
Filter strips also differ from grassed swales in that swales are concave
vegetated conveyance systems, whereas filter strips have fairly level surfaces.
Schematic Design of a Filter Strip
Berms Placed Perpendicular
to Top of Strip Prevent
Concentrated Flows
Elevation
Same Contour.
Abuts Trench
Source: Schueler, 1987.
Stone Trench
Acts as
Level Spreader
5% Strip Slope or Less
Current Assessment of Urban Best Management Practices
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Filter Strips
Capsule Summary:
Pollutant Removal Capability: Filter strips can effectively reduce particulate
pollutant levels, e.g. sediment, organic materials and trace metals, in areas
where runoff velocity is low to moderate; however, studies show that, under
these same conditions, their ability to remove soluble pollutants is highly
variable.
Longevity: Urban filter strips that are not regularly maintained may quickly
become nonfunctional. Field studies indicate that strips tend to have short life
spans because of lack of maintenance, improper location and poor vegetative
cover.
Feasibility: Vegetated filter strips have limited feasibility in highly urbanized
areas where runoff velocities are high and flow is concentrated. Therefore their
use is primarily restricted to low and medium density residential areas where
they can accept rooftop runoff and runoff from pervious areas such as lawns.
Environmental Concerns: Few.
Environmental Benefits: Filter strips can preserve the character of riparian
zones and prevent erosion along streambanks. They also provide excellent
urban wildlife habitat.
Costs: Low, almost negligible costs if established before site development.
Adaptability. Filter strips do not provide adequate pollutant removal on slopes
over fifteen percent, moreover, they require climates that can sustain vegetative
cover on a year-round basis.
Furthermore, contributing upland areas must be small (one to five acres)
so that runoff arrives at the filter strip as overland sheet flow.
Maintenance Burden: Filter strips require periodic repair, regrading and
sediment removal to prevent channelization.
Current Assessment of Urban Best Management Practices
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Filter Strips
Usefulness as a Coastal Urban NPS Management Practice:
For several years, filter strips have been used with relative success
to control surface runoff from agriculture. Agricultural runoff is generally
not concentrated which allows effective pollutant removal in the receiving
filter strip. Increases in impervious surface area in urban areas tend to
cause runoff to become concentrated, rendering the filter strip ineffective.
Hence, although filter strips can be useful stream buffers, they are not likely
to function well as the sole water quality control in highly impervious areas.
Can Filter Strips Reliably Remove Urban Stormwater Pollutants?
Pollutant Removal Mechanisms: Pollutants are removed by the filtering action
of vegetation, deposition in low velocity areas, or by infiltration into the
subsoil.
Review of Monitoring Studies: Two studies of filter strips in urban areas
have indicated that filter strips do not trap pollutants efficiently in urban
settings due to high runoff velocity. (21) Of these studies, one is ongoing and
final results are not yet reported. The other study indicated an average TSS
removal rate of only 28 % and did not report removal rates for either TN or
TP.
Research to date on vegetated filter strips has largely focused on filter
strips in agricultural settings. Most of these studies indicate that, when
functioning properly, filter strips can remove particulate pollutants with some
reliability, but are less dependable for nutrient removal.
Current Assessment of Urban Best Management Practices
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Filter Strips
Factors Influencing Pollutant Removal:
Positive Factors
• Minimum strip width
of fifty feet (1)
• Slope of five percent or less
(1)
• Forested filter strips (1)
• Clay soil or oiganic matter
matter surface (49)
• Contributing area of less than
five acres (1)
• Grass height of six to twelve
inches (48)
Negative Factors
• High runoff velocity ( > 2.5
fps, depending on site
conditions) (32)
• Slopes greater than fifteen
percent (48)
• Hilly terrain
• Unmowed filter strips
How Well Do Filter Strips Operate over Time?
Failure Rates: Studies in agricultural settings (21) (51), where peak discharge
rates tend to be much lower, show that filter strips have generally failed when:
• design slope has exceeded the recommended fifteen percent
• design width of slopes has been too narrow to adequately service the
contributing area
• strips have poor vegetative cover
• uneven terrain has caused channelization.
In a study of thirty-three farms in Virginia, researchers found that the
majority of filter strips in use were ineffective because most flow had become
channelized. (32) The study suggests, moreover, that poor design or
maintenance may cause a strip to fail within a fairly short period of time (six
months or less).
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Filter Strips
Factors Influencing Longevity (1):
• Use of a level spreader at the top of the strip will help to distribute
flow evenly as well as protect the strip from man-made damage.
• Regular removal of sediment will help to maintain the original
filtration capacity of the strip as well as assure that build-up of
sediment does not alter design features such as contour or slope.
• Periodic repair of eroded areas and regrading around the strip may
be necessary to assure that flows do not concentrate through or
around the strip.
• Periodic weeding and replanting, particularly in the first few years
of life, will allow the vegetative cover to stabilize and become
permanent.
• If a filter strip is used for sediment control, it should be reseeded
and regraded after construction.
Where and When are Filter Strips Feasible?
Contributing Watershed Area: To prevent concentrated flows from forming,
maximum contributing area to an individual filter should be less than five
acres. (1)
Land Use. In urban settings, it is likely that filter strips will be most effective
in treating rooftop runoff and runoff generated from lawns and other pervious
areas. Filter strips should not be used to control large impervious areas, such
as parking lots.
Peak Discharge Rates: High flow velocity will prevent the strip from trapping
pollutants and will cause erosion and channelization.
Soils: The ability of filter strips to remove nutrients from surface runoff
improves where clay soils or organic matter surfaces are present.
Length: Minimum length should be no less than fifty to seventy-five feet plus
four feet for any one percent increase in slope. (1)
Current Assessment of Urban Best Management Practices
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Filter Strips
Depth to Water Table: Greater removal of soluble pollutants can be achieved
where the water table is within three feet of the surface/ i.e. within the root
zone. (50)
Use In Ultra-urban Areas: The high percentage of impervious surface in urban
areas creates high peak discharge rates, and thus, limits the usefulness of this
practice as a water quality control in ultra-urban settings.
Retrofit Capability: Retrofit is relatively simple if enough land area is
available to adequately service the contributing watershed area, and soil and
slope conditions are favorable.
Stormwater Management Capability: Filter strips cannot reduce peak
discharges to predevelopment levels. They function primarily as a water
quality BMP. The limited ability of filter strips to control runoff and to remove
nutrients suggests their most elective use is in combination with pretreatment
and detention systems.
What are the Costs Associated with Filter Strips?
PermittinK/Review: The creation of vegetated filter strips does not require any
permits; however, it is important that the plan reviewer notes whether site
conditions will permit the strip to remove the pollutants of concern effectively.
Construction: Low, especially if established before site development.
Maintenance. Routine maintenance activities include inspection, sediment
removal, replanting and reseeding, and regrading. Mowing may be required
for smaller strips. Inspections and corrective maintenance, such as weeding or
replanting should take place more frequently in the first couple of years to
assure stabilization. Removal of dead vegetation may also improve strip
performance.
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Filter Strips
Can Filter Strips Be Easily Adapted for All Coastal Areas?
Vegetated filter strips are adaptable in most climates where it is possible
to grow a dense vegetative cover. Filter strips are not recommended for arid
regions where vegetation in upland areas is sparse.
They may function adequately in regions with long, cold winters, but
may be ineffective in treating runoff during snowmelt conditions.
What are the Environmental Concerns and Benefits of Filter Strips?
Positive Impacts:
• Filter strips can be combined with stream buffers to protect the
riparian corridor
• Groundwater recharge
• Urban wildlife habitat
• Streambank stabilization
• Aesthetically pleasing
• Can serve as a buffer between incompatible uses
What is Not Known about Filter Strips?
• The relative effectiveness of forested filter strips versus grassed buffer
strips
• Whether concentration of runoff through filter strips can be reduced
by adding structural complexity or by employing level spreaders
• Effects of mowing on pollutant removal
Current Assessment of Urban Best Management Practices
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Water Quality
Oil Grit Separators
Fact Sheet #11
Definition
A water quality inlet is a three-stage underground retention system
designed to remove heavy particulates and absorbed nydrocarbons
stormwater runoff. Also known as an oil/grit separator.
Schematic Design of a Water Quality Inlet/Oil Grit Separator
Side View
Stormdrain
Permanent Pool
400 Cubic Feel
of Storage Per
Contributing
Acre. 4 Feet
Deep
Access
Manholes
Trash Rack Protects
Two 6 Inch Orifices
First Chamber
(Sediment Trapping)
Inverted Elbow J"~
Pipe Regulates |
Water
Levels
Second Chamber
(Oil Separation)
Overflow
Pipe
Reinforced
Concrete
Construction
Third Chamber
Source: Schuster, 1987.
Current Assessment of Urban Best Management Practices
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Water Quality Inlets
Capsule Summary:
Pollutant Removal Capability: Current designs of water quality inlets have
limited pollutant removal capability and only appear to trap coarse-grained
sediments and some hydrocarbons. Removal of silt, clay, nutrients, trace
metals, and organic matter is expected to be slight. Re-suspension also appears
to limit long-term removal. Actual removal only occurs when inlets are
cleaned out. Currently, a lack of effective clean-out and disposal methods
prevents the required quarterly clean-out of trapped residuals.
Longevity: Longevity of water quality inlets is high. Over ninety-five percent
of all inlets are operating as designed in their first five years of operation.
Feasibility: Inlets are restricted to small, highly impervious catchments of
two acres or smaller (such as gas stations, parking lots, fast food outlets, and
convenience stores).
Environmental Concerns: The greatest concern is the pollutant toxicity of
trapped residuals and oily waters, and how the toxicity influences the ultimate
disposal of the residuals. A secondary concern is the possibility of pulse
loadings of the trapped residuals during longer storm events (due to re-
suspension).
Environmental Benefits: Inlets show some capacity to trap trash, debris, and
other floatables, thereby preventing their discharge to receiving waters.
Costs: The cost of inlets averages about $8,000 per unit. The inlets are costly
on a runoff volume treated basis, averaging three to four times the unit cost
of trenches or sand filters.
Adaptability: Inlets can be adapted to all regions of the country.
Maintenance Burden: Inlets require quarterly clean-outs. However, no
acceptable clean-out and disposal techniques currently exist, rendering
maintenance impossible.
Current Assessment of Urban Best Management Practices
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Water Quality Inlets
Usefulness As a Coastal Urban NPS Management Practice: Water quality
inlets are not recommended at the present time for widespread application
in coastal areas due to:
• limited pollutant removal capability
• a lack of clean-out and disposal methods
• high cost in comparison to other BMP alternatives
Until basic design and clean-out methods are greatly improved, water
quality inlets should only be considered as a pretreatment system and not
a primary urban BMP.
Can Water Quality Inlets Reliably Remove Urban Stormzvater
Pollutants?
Pollutant Removal Mechanisms: Gravitational settling within the first two
chambers can achieve partial removal of grit and sediments. An inverted pipe
elbow can remove oil; the pipe elbow keeps the less dense oil near the surface,
where it can bind with sediments, and ultimately settle. It should be noted
that actual pollutant removal is accomplished when trapped residuals are
cleaned out of the inlet. (1)
Review of Monitoring Studies: The pollutant removal performance of water
quality inlets has never been monitored in the field, however, design factors
suggest that removal capability is limited. Recent field studies confirm the
limited effectiveness of water quality inlets. (52) For example, the average
depth of sediments trapped in over 120 water quality inlets was less than two
inches, and more than eighty percent of the trapped sediments were coarse
grained grit and organic matter. Disturbingly, sediment accumulation did not
increase with age, suggesting that re-suspension was a significant problem.
Water quality inlets did partially trap floatable debris, and the sediments
trapped (average of 10 cubic feet per structure) were extremely oily in nature.
(8)
Current Assessment of Urban Best Management Practices
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Water Quality Inlets
Factors Influencing Pollutant Removal (8):
Positive Factors
• Off-line designs
• Adsorptive media (peat, sand)
• Adsorptive pads (tor oil)
• Elevated orifices between
chambers one and two
• Baffle plates
• Other methods to prevent
re-suspension
• Regular clean-out
How Well Do Water Quality Inlets Operate Over Time?
Failure Rates: Nearly four hundred water quality inlets have been installed
in the Baltimore/Washington area. Field studies of over one hundred water
quality inlets indicated that over ninety-five percent are operating as designed,
and very few clogging problems have been noted. (8) The oldest operating
inlets are five years old.
Factors Influencing Longevity; The basic design is very robust, and very few
structural or clogging problems appear to have occurred in the first five years
of operation; however, regular clean-outs are not being performed at the vast
majority of inlets. (8) Therefore, the actual pollutant removal is very low at
present.
Where and When are Water Quality Inlets Feasible?
Physical Factors: Water quality inlets can be applied in most small
development situations, such as parking lots, gas stations, convenience stores,
and along some road-ways. The primary limitation is contributing area. Most
systems are applied to contributing watershed areas of two acres or less. The
contributing areas typically are mostly or entirely impervious. The inlet must
be connected to the storm drain pipe.
Use In Ultra-Urban Axeas: Water quality inlets are frequently applied in ultra-
urban areas, where space or storage are not available for other, more effective
urban BMPs.
Negative Factors
On-line systems
Low volume
Low orifices
Current Assessment of Urban Best Management Practices
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Water Quality Inlets
Retrofit Capability: Very limited. Low removal capability coupled with
disposal problems limits the utility of the water quality inlets as a retrofit
practice. If the current design is improved, it may become a better retrofit
alternative.
Stormwater Management Capability: None. Limited storage of water quality
inlets cannot meet stormwater requirements.
What Are the Costs Associated With Water Quality Inlets?
Permitting/Review: Permitting and design review for water quality inlets is
minimal, and the basic design and construction had been standardized.
Construction: Costs for water quality inlets range from $5,000 to $15,000 per-
inlet, with a range of $7,000 to $8,000. This translates to a cost of $10 to $40
per cubic foot treated. (53)
Maintenance: Since no acceptable dean-out and disposal methods currently
exist for water quality inlets, it is impossible, at this time; to estimate the costs
for this critically important maintenance function.
Can Water Quality Inlets be Easily Adapted for All Coastal Areas?
Because of the standardized design, the water quality inlet can be utilized in
all regions of the country. Until the regular clean-out/disposal procedures are
developed, however, the use of water quality inlets should not be promoted.
What are the Environmental Concerns and Benefits of Water Quality
Inlets?
Positive Impacts:
• Trapping of floatable trash and debris
• Potential reduction of hydrocarbon load from areas with high
traffic/parking use
Current Assessment of Urban Best Management Practices
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Water Quality Inlets
Negative Impacts:
• Toxicity of trapped residuals
• Possibility of pulse hydrocarbon loadings due to re-suspension during
large storms
• In some regions, it may be difficult to find environmentally acceptable
disposal methods. The slurry of oily water and trapped residuals
cannot be land-filled, land-applied, or introduced into the sanitary
sewer system due to hazardous waste, pretreatment or groundwater
regulations
What is Not Know About Water Quality Inlets?
• The actual pollutant removal capability of water quality inlets
• Disposal methods that are environmentally acceptable, practical for
small sites, rapid and cost-effective
• Actual toxicity of trapped residuals
• Degree of re-suspension and export of trapped residuals, and design
methods that minimize this
• The effect of sand, peat, or other adsorbent media in improving
pollutant removal efficiency
• Possibility of regional treatment of inlet slurries
Note: COG is currently conducting a long-term study to provide data on these
questions. It will be completed in early 1993.
Current Assessment of Urban Best Management Practices
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Cited References
1. Schueler, T. R. 1987. Controlling Urban Runoff: A Practical Manual for Planning
and Designing Urban Best Management Practices. Metropolitan Washington
Council of Governments. 213 pp. + appendices.
2. Metropolitan Washington Council of Governments (MWCOG). 1983. Final
Report: Pollutant Removal Capability of Urban Best Management Practices in the
Washington Metropolitan Area. Prepared for the U.S. Environmental Protection
Agency. 64 pp.
3. Pope, L. M. and L. G. Hess. 1988. Load-Detention Efficiencies in a Dry Pond
Basin, (in): Design of Urban Runoff Quality Controls. American Society of Civil
Engineers. New York, New York. pp. 258-267.
4. Schueler, T. R. and M. Helfrich. 1988. Design of Extended Detention Wet Pond
Systems, (in): Design of Urban Runoff Controls. L. Roessner and B. Urbonas, eds.
American Society or Civil Engineering. New York, New York. pp. 180 -200.
5. Occoquan Watershed Monitoring Laboratory and George Mason University.
Department of Biology. 1990. Final Project Report: The Evaluation of a Created
Wetland as an Urban Best Management Practice. Prepared for the Northern
Virginia Soil and Water Conservation District. 175 pp. + appendices.
6. Schueler, T. R. 1992. Design of Stormwater Pond Systems. Metropolitan
Washington Council of Governments. Washington, DC.
7. GKY. 1989. Outlet Hydraulics of Extended Detention Facilities. Northern
Virginia Planning Commission. 48 pp.
8. Galli, F. J. 1992. Preliminary Analysis of the Performance and Longevity of
Urban BMPs installed in Prince George County, Maryland. Prepared for the
Department of Environmental Resources. Prince George's County, Maryland.
9. Galli, F. J. and L. Herson. 1988. Montgomery County Anacostia Watershed
Retrofit inventory. Anacostia Restoration Team. 400 pp.
10. Galli, F. J. and L. Hereon. 1989. Prince George's County Anacostia Watershed
Restoration Inventory. Anacostia Restoration Team. 516 pp.
Current Assessment of Urban Best Management Practices
Page - 93
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Cited References
11. Wiegand, C. W., W. C. Chittenden, and T. R. Schueler. 1986. Cost of Urban
Runoff Controls, (in): Urban Runoff Quality : Impact and Quality Enhancement
Technology. B. Urbonas and L. Roesner, eds. American Society of Civil
Engineers, pp. 366 - 380.
12. Galli, F. J. 1991. Thermal Impacts Associated With Urbanization and Stormwater
BMPs in Maryland. Anacostia Restoration Team. Prepared for Maryland Dept.
of the Environment. 150 pp.
13. Schueler, T. R. 1991. Mitigating the Adverse Impacts of Urbanization on
Streams: A Comprehensive Strategy for Local Governments. Proceedings of the
National Conference Integration of Stormwater and Local Nonpoint Source Issues.
Northern Illinois Planning Commission, pp. 25 - 36.
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17. Wu, J. S., B. Holman and J. Dorney. 1988. Water Quality Study on Urban Wet
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York, New York. pp. 280 - 289.
18. Martin, E. H. 1988. Mixing and Stormwater Residence Times of Stormwater
Runoff in a Detention System, (in): Design of Urban Runoff Quality Controls. L.
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Engineers. New York, New York. pp. 164-178.
19. Oberts, G. L., P.J. Wotzka and J.A. Hartsoe. 1989. The Water Quality
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Minnesota. 81 pp. + appendix.
Current Assessment of Urban Best Management Practices
Page • 94
-------
Cited References
20. Schueler, T. R., F. J. Galli, L. Hereon, P. Kumble and D. Shepp. 1991.
Developing Effective BMP Systems for Urban Watersheds. Urban Nonpoint
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21. Woodward-Clyde Federal Services. 1991. Draft Summary of Urban BMP Cost
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22. Livingston, E. H. 1991. Environmental Administrator. Nonpoint Source
Management Section. Florida Department of Environmental Regulation. Personal
communication.
23. Dewbeny and Davis. 1989. Toxicity of Sediments from BMP Ponds. Prepared
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24. Galli, F. J. 1988. A Limnological Study of an Urban Stormwater Management
Pond and Stream Ecosystem. Master's Thesis. George Mason University. 153
pp.
25. US Environmental Protection Agency. 1991. Detention and Retention Effects on
Groundwater: Literature Review. Region V. Water Quality Section. 24 pp.
26. Athanas, C. 1986. Wetland Basins for Stormwater Treatment: Analysis and
Guidelines. Prepared for the Water Resources Administration. Maryland
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27. Rhode Island Department of Environmental Management. Office of
Environmental Coordination. 1989. Artificial Wetlands for Stormwater
Treatment: Processes and Design. Prepared for the Rhode Island Nonpoint
Source Management Program. Providence, Rhode Island. 63 pp.
28. Strecker, W. E., J. M. Kersnar, and E. D. Driscoll. 1990. The Use of Wetlands
for Controlling Stormwater Pollution. Woodward-Clyde Consultants. Prepared
for the US Environmental Protection Agency. 61 pp. + appendix.
29. Athanas, C. and C. Stevenson. 1991. The Use of Artificial Wetlands in Treating
Stormwater Runoff. Prepared for the Sediment and Stormwater Administration.
Maryland Department of the Environment. 96 pp.
Current Assessment of Urban Best Management Practices
Page • 95
-------
Cited References
30. Tetra Tech. 1992. Natural Wetlands and Urban Stormwater: Potential Impacts
and Management. Prepared for the Office of Wetlands, Oceans and Watersheds.
US Environmental Protection Agency. Washington, DC. 54 pp.
31. Stockdale, E. C. 1991. Freshwater Wetlands, Urban Stormwater, and Nonpoint
Pollution Control: A Literature Review and Annotated Bibliography. Second
Edition. Washington State Department of Ecology. Olympia, Washington. 273
pp.
32. Horner, R. R. 1988. Biofiltration Systems for Storm Runoff Water Quality
Control. Prepared for the Washington State Department of Ecology. 46 pp. +
appendices.
33. Wotzka, L. and G. Oberts. 1988. The Water Quality Performance of a Detention
Basin Wetland Treatment System in an Urban Area. Nonpoint Source Pollution:
1988. Economy, Policy, Management and Appropriate Technology. American Water
Resources Association.
34. Esry and Cairns. 1988. Cited in Strecker et al., 1990.
35. Kuo, C. Y., G. D. Boardman, and K. T. Laptos. 1990. Phosphorus and Nitrogen
Removal Efficiencies of Trenches. Virginia Polytechnic and State University.
Prepared for the Northern Virginia Planning District Commission.
36. Maryland Department of the Environment. 1991. Stormwater Management
Infiltration Practices in Maryland: A Second Survey. Sediment and Stormwater
Administration. 75 pp.
37. Maryland Department of the Environment. 1986. Maintenance of Stormwater
Management Facilities: A Departmental Summary. Sediment and Stormwater.
Administration. 60 pp.
38. Cahill, T. H., W. R. Horner, J. McGuire and C. Smith. 1991. Interim Report:
Infiltration Technologies. Prepared for the Nonpoint Source Control Branch. US
Environmental Protection Agency.
39. Maryland Department of the Environment. 1983. Standards and Specifications
for Infiltration. Sediment and Stormwater Administration. 100 pp.
40. Bergling, T. R. 1991 (unpublished). Evaluation of Soil Infiltration Rates.
Schnabel Engineering Associates. Northern Virginia.
Current Assessment of Urban Best Management Practices
Page - 96
-------
Cited References
41. Occoquan Watershed Monitoring Laboratory (OWML). 1986. An Evaluation of
the Performance of Porous Pavement for Stormwater Quality Control. Davis
Foundation. Northern Virginia Water Control Board.
42. City of Austin. 1991. Design Guidelines for Water Quality Control Basins.
Public Works Department. Austin, Texas. 64 pp.
43. Galli, F. J. 1990. The Peat Sand Filter: An Innovative BMP for Controlling
Urban Stormwater. Anacostia Restoration Team. 45 pp.
44. City of Austin. Environmental Resource Management Division. 1991. Personal
Communication and Site Visit. Austin, Texas. November 13 - 15, 1991.
45. City of Austin. 1988. Environmental Criteria Manual. Environmental Resource
Management Division P.O. Box 1088. Austin, Texas. 78767.-
46. Dorman, M.E., J. Hartigan, R F. Steg, and T. Quasebarth. 1989. Retention,
Detention, and Overland Flow for Pollutant Removal from Highway Stormwater
Runoff. Prepared for the Federal Highway Administration. McLean, Virginia.
168 pp.
47. Yousef, Y. A., M. P. Wanielista and H. H. Harper. 1986. Design and
Effectiveness of Urban Retention Basins, (in): Urban Runoff Quality-Impact and
Quality Enhancement Technology. B. Urbonas and L. A. Roesner, eds. American
Society of Civil Engineers. New York, New York. pp. 338 - 350.
48. Southeastern Wisconsin Regional Planning Commission. 1991. Technical Report
Number 31: Costs of Urban Nonpoint Water Pollution Control Measures.
Waukesha, Wisconsin. 109 pp.
49. IEP, Inc. 1990. Vegetated Buffer Strip Designation Method Guidance Manual.
Narragansett Bay Project. 30 pp.
50. Groffman, P. M., A. J. Gold, T. P. Husband, R. C. Simmons, and W. R.
Eddleman. Final Report: Narragansett Bay Project. An Investigation into
Multiple Uses of Vegetated Buffer Strips. University of Rhode Island. 150 pp.
51. Dillaha, T. A., J. H. Sherrard, and D. Lee. 1989. Long-Term Effectiveness of
Vegetative Filter Strips. Water Environment and Technology. November 1989, pp.
419 -421.
Current Assessment of Urban Best Management Practices
Page - 97
-------
Cited References
52. Shepp, D., D. Cole, and F. J. Galli. 1992. A Field Survey of the Performance of
Oil/Grit Separators. Metropolitan Washington Council of Governments.
Prepared for the Maryland Department of the Environment.
53. Galli, F. J. 1992. Metropolitan Washington Council of Governments. Personal
communication.
54. Yousef, Y. A., L. Lin, J. V. Sloat and K. Y. Kaye. 1991. Maintenance Guidelines
for Accumulated Sediments in Retention/Detention Ponds Receiving Highway
Runoff. Final Report. Prepared by University of Central Florida. Department
of Civil and Environmental Engineering. Prepared for the Florida Department
of Transportation. 210 pp.
55. Yousef, Y. A., M. P. Wanielista, J. D. Dietz, L. Y. Lin and M. Brabham. 1990.
Energy Optimization of Wet Detention Ponds for Urban Stormwater Management.
Final Report. Prepared the University of Central Florida. Department of Civil
Engineering. Prepared for the Florida Department of Environmental Regulation.
198 pp.
56. Harper, H. H. 1988. Effects of Stormwater Management Systems on
Groundwater Quality. Final Report to the Florida Department of Environmental
Regulation. 458 pp.
57. Troung, H. V. 1989. The Sand Filter Water Quality Structure. District of
Columbia. Environmental Regulation Administration. 26 pp.
Current Assessment of Urban Best Management Practices
Page • 98
-------
Other References
Athanas, C. and C. Stevenson. 1991. Chris Athanas, Ph.D. and Associates, Inc.
Personal Communication.
Baltimore Department of Public Works. 1989. Detention Basin Retrofit Project and
Monitoring Study Results. Water Quality Management Office. Baltimore,
Maryland. 42 pp + appendices.
Bannerman, R. 1991. Unpublished data. Bureau of Water Resources Management.
Wisconsin Department of Natural Resources. Madison, Wisconsin.
Boto, K. G. and W. H. Patrick, Jr. 1978. The Role of Wetlands in the Removal of
Suspended Sediments. The American Water Resources Association. 10 pp.
Bryant, G. and D. Andrews. 1990. Draft Final Report on Stormwater Quality Best
Management Practices. Marshall Macklin Monaghan Limited. Prepared for the
Ontario Ministry of the Environment. Toronto, Ontario. 129 pp.
Cullum, Michael. 1985. Stormwater Runoff Analysis at a Single Family Residential
Site. University of Central Florida at Orlando. Publication 85-1: 247-256.
Driscoll, E.D. 1983. Detention and Retention Controls for Urban Runoff, (in): Urban
Runoff Quality: Impact and Quality Enhancement Technology. B. Urbonas and L.
Roesner, eds. American Society of Civil Engineers. 477 pp.
Fish, W. 1988. Behavior of Runoff-Derived Metals in a Weil-Defined Paved-
Catchment/Retention Pond System. Water Resources Research Institute. Oregon
State University. Corvallis, Oregon. 53 pp.
Horner, R. R., J. Guedry and M. H. Kortenhoff. 1990. Final Report: Improving the
Cost Effectiveness of Highway Construction Site Erosion and Pollution Control.
Prepared for the Washington State Transportation Commission. 51 pp. +
appendices.
Kumble, P. 1991. Management Measures for Coastal Urban Nonpoint Source
Pollution Control. Anacostia Restoration Team. Prepared for U.S. EPA Nonpoint
Source Branch.
Lakatos, D. F. and L. J. McNemar. Wetlands and Stormwater Pollution Management.
Walter B. Sattherwaite Associates, Inc. 9 pp.
Current Assessment of Urban Best Management Practices
Page • 99
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Other References
Lane Council of Governments. 1982. Final Report: Nationwide Urban Runoff
Program Study for Eugene and Springfield, Oregon. Prepared for the US
Environmental Protection Agency. Eugene, Oregon. 494 pp.
Maristany, A. E. and R. L. Bartel. 1989. Wetlands and Stormwater Management:
A Case Study of Lake Munson. Part I: Long-term Treatment Efficiencies.
Wetlands: Concerns and Successes. American Water Resources Association,
pp. 215 - 229.
Metroplan. 1983. Fourche Creek Urban Runoff Project. Volume 2. Prepared for the
Environmental Protection Agency. Little Rock, Arkansas. 222 pp.
Myers, J. C. 1989. Evaluation of. Best Management Practices Applied to Control of
Stormwater-Borne Pollution in Mamaroneck Harbor, New York. Draft. Prepared
for the Long Island Sound Study. US EPA Region n. 50 pp + appendix.
Oberts, G. L. and R. A. Osgood. 1988. Lake McCarrons Wetland Treatment System:
Final Report on the Function of the Wetland Treatment System and the Impacts
on Lake McCarrons. Metropolitan Council of the Twin Cities Area. St. Paul,
Minnesota. 94 pp + appendices.
Occoquan Watershed Monitoring Laboratory (OWML). 1983(a). Evaluation of
Management Tools in the Occoquan Watershed. Final Contract Report to the
Virginia State Water Control Board. Grant #R806310010.
Occoquan Watershed Monitoring Laboratory (OWML). 1983(b). Final Report
Metropolitan Washington Urban Runoff Project. Prepared for the Metropolitan
Washington Council of Governments. Manasssas, Virginia. 260 pp.
Occoquan Watershed Monitoring Laboratory (OWML). 1987. Final Report. London
Commons Extended Detention Facility Urban BMP Research and Demonstration
Project. Virginia Tech University. Manassas, Virginia. 68 pp. + appendix.
Ontario Ministry of the Environment. 1991. Stormwater Quality Best Management
Practices. Marshall Macklin Monaghan Limited. Toronto, Ontario. 177 pp. +
appendices.
Palmer, C. N. and J. D. Hunt. 1989. Greenwood Urban Wetland and a Manmade
Stormwater Treatment Facility. American Water Resources Association. 11 pp.
Current Assessment of Urban Best Management Practices
Page - 100
-------
Other References
Prince George's County Department of Environmental Resources. 1989. Tree Cover
Ordinance and Handbook. County Administration Building. Upper Marlboro, MD.
Schueler, T. R. 1983. Urban Runoff in the Washington Metropolitan Area. Water
Resources Planning Board. Metropolitan Washington Council of Governments.
Schueler, T. R. and J. Lugbill. 1989. Performance of Current Sediment Control
Measures at Maryland Construction Sites. Prepared for the Maryland Dept. of
Environment by the Metropolitan Washington Council of Governments. 90 pp.
Shaver, E. 1991. Sand-Filter Design for Water Quality Treatment. Department of
Natural Resources and Environmental Control. Dover, Delaware. 18 pp.
US Environmental Protection Agency. 1986. Methodology for Analysis of Detention
Basins for Control of Urban Runoff Quality. Washington, DC. 51 pp. +
appendix.
Walker, Jr., W. W. 1987. Phosphorus Removal by Urban Runoff Detention Basins.
Lake and Reservoir Management. Vol. HI: 314-326.
Whalen, P. J. and M. G. Cullum. 1988. Technical Publication 88-9: An Assessment
of Urban Land Use/Stormwater Runoff Quality Relationships and Treatment
Efficiencies of Selected Stormwater Management Systems. South Florida Water
Management District. Resource Planning Department. 56 pp
Current Assessment of Urban Best Management Practices
Page • 101
-------
Appendix A
-------
TIIK POLLUTANT REMOVAL CAPABILITY OF POND ANl) WETLAND SYSTEMS: A REVIEW
NOTE: The table below provides summary data on the pollutant removal capability of nearly sixty stormwater pond and wetland systems. Each
study differs with respect to pond design, number of storms monitored, pollutant removal calculation technique, and monitoring
technique, so exact comparisons between studies are not appropriate.
NO. OF
WATER-
SHED
TREAT-
MENT
RKMOVAL kkkhikn*
i <%t
TYPE
NO.
NAME
STATE
STORMS
AREA
(Acre.)
VOL.
(In./Acre)
TSS
TP
SP
TN
N03
COD
Pb
Zn
OTHER
DRY ED
1
Lakeridge
VA
28
88.0
0.00
14.0
20.0
(-6.0)
10.0
9.0
(-1.0)
(-10.0)
2
London Commons
VA
27
11.4
0.22
A: 29.0
B: 74.0
40.0
56.0
25.0
60.0
17.0
41.0
39.0
25.0
24.0
40.0
3
Stedwick
MD
25
34.0
0.30
70.0
13.0
24.0
27.0
62.0
57.0
TKN: 30.0
4
Maple Run III
TX
17
28.0
0.50
30.0
18.0
35.0
52.0
22.0
29.0
(-38.0)
TOC: 30.0
Cu: 31.0
BOD: 35.0
NH3: 55.0
FColi: 78.0
5
Oakhampton
MD
16.8
0.50*
87.0
26.0
(-12.0)
(-10.0)
NH4: 53.5
6
None given
KS
19
12.3
3.42
3.0
19.0
0.0
20.0
16.0
66.0
65.0
Note: An asterisk <•> denotes an Inferred value
-------
THE POLLUTANT REMOVAL CAPABILITY OF POND AND WETLAND SYSTEMS.-
£
-------
TIIE POLLUTANT REMOVAL CAPABILITY OK POND AND WETLAND SYSTEMS—
NO. OF
WATER-
SHED
TREAT-
MENT
REMOVAL EFFICIENCY <*)
TYPE
NO.
NAME
STATE
STORMS
AREA
(Acre.)
VOL.
(In./Acre)
TSS
TP
SP
TN
N03
COD
n>
Zn
OTHER
20
Buckland
CT
7
20.0
0.40
61.0
45.0
22.0
18.0-59.0
51.0
Cd: <0
TKN: 24.0
TOC: 33.0
Cu: 38.0
21
Highway Site
FL
13
41.6
0.55
65.0
17.0
21.0
7.0
41.0
37.0
22
Woodhollow
TX
14
381.0
0.55
54.0
46.0
39.0
45.0
41.0
76.0
69.0
TKN: 26.0
NH3: 28.0
BOD: 39.0
FColi: 46.0
23
SR 204
WA
5
1.8
0.60
99.0
91.0
69.1
88.2
87.0
Cu: 90.0
24
Farm Pood
VA
51.4
1.13
85.0
86.0
73.0
34.0
NH3: (-107.0)
25
Burke
VA
29
27.1
1.22
(-33.3)
39.0
77.0
32.0
21.0
84.0
38.0
WET PONDS
26
Wcrtleigh
MD
32
48.0
1.27
81.0
54.0
71.0
37.0
35.0
82.0
26.0
TKN: 27.0
(Cont'd)
27
Mercer
WA
5
7.6
1.72
75.0
67.0
76.9
23.0
38.0
Cu: 51.0
28
1-4
FL
6
26.3
2.35
54.0
69.0
97.0
41.0-94.0
69.0
TOC: 45.0
TKN: 68.0
Cd: 43.0-51.0
Cu: 66.0-81.0
29
Timber Crock
FL
9
122.0
3.11*
64.0
60.0
80.0
15.0
80.0
30
Maitland
FL
30-40
49.0
3.65
90.0
87.0
95.0
96.0
PP: 11.0
Cu: 77.0
NH3: 82.0
|
31
Lakeside
NC
5
65.0
7.16
91.0
23.0
82.0
TKN: 6.0
Note: An ailcrfak (*) m tafcrrc* value
-------
TIIK POLLUTANT REMOVAL CAPABILITY OF PONI) ANI) WETLAND SYSTEMS..
c?
-------
TIIK POLLUTANT REMOVAL CAPABILITY OF POND AND WETLAND SYSTEMS...
TYPE
NO.
NAME
STATE
NO. OF
STORMS
WATER-
SHED
AREA
(Acres)
TREAT-
MENT
VOL.
(In./Acre)
REMOVAL EFFICIENCY (%)
TSS
TP
SP
TN
N03
COD
Pb
Zn
OTHER
ED WETLANDS
(Cont'd)
48
Tanner's Lake
MN
10
413.0
0.10
A: 62.0
B: 63.0
24.0
7.0
10.0
(-14.0)
36.0
5.0
23.0
1.0
63.0
59.0
TKN: 40.0
TKN: 7.0
49
Mays Chapel
MD
97.0
0.10*
24.0
16.0
24.0
35.0
NH3: 43.0
50
Clear Lake
MN
1070.0
0.15*
76.0
54.0
40.0
TKN: 25.0
NH3: 55.0
NATURAL
WETLANDS
51
Hidden Lake
FL
55.4
1.08*
83.0
7.0
(-109.0)
(-1.6)
80.2
54.0
40.0
ON: (-24.0)
Cu: 40.0
NH3: 62.0
Cd: 70.0
BOD: 81.0
52
Wayzata
MN
73.0
1.25*
94.0
78.0
94.0
82.0
NH3: (-44.0)
Cd: 67.0
Cu: 80.0
POND/WETLAND
SYSTEMS
53
Lake Munson
FL
3
23393.0
92.0
64.0
11.0
15.0
28.0
55.0
59.0
NH4: (-39.0)
TKN: 11.0
N03: 15.0
BOD: 42.0
54
Carver Ravine
MN
15
170.0
.30*
A: 46.0
B: 20.0
24.0
1.0
21.0
1.0
15.0
(-6.0)
18.0
9.0
42.0
6.0
TKN: 14.0
TKN: (-10.0)
55
McCarrons
MN
21
608.0
>0.50
94.0
78.0
83.0
93.0
90.0
56
Lake Jackson
FL
2230.0
.88*
96.0
90.0
75.0
70.0
NH4: 37.0
57
Highway Site
FL
13
41.6
>1.35
89.0
36.0
43.0
84.0
67.0
58
Long Lake
ME
11
18.0
2.0*
95.0
92.0
Note: An asterisk (•) denotes an Inferred value
-------
Pollutant Removal Capability Table: References, Notes and Notation
I. Notes and References
Study No. Reference and Notes
1 MWCOG, 1983. Minor ED provided (1-2 hours). Frequent resuspension by clogging of lowflow
orifice.
2 OWML, 1987. A: Pre-adjustment, less than 10 hours of ED (maximum). B: Post-adjustment, less
than 20 hours of ED (maximum). Exfiltration of runoff accounts for some removal.
3 Schueler and Helfrich, 1988. Achieved ED times of 6 to 8 hours. Prone to suspension.
4 City of Austin, 1991. Originally a dry stormwater pond, but due to poor maintenance, 3 to 6 hours
of ED was achieved.
5 Baltimore Department of Public Works, 1989.
6 Pope and Hess, 1988. Resuspension. ED volume was very high.
7 Horner et al., 1990.
8 HoUer, 1987.
9 Driscoll, 1983. NURP pond.
10 Driscoll, 1983. NURP pond.
11 Driscoll, 1983. NURP pond.
12 Driscoll, 1983. NURP pond.
13 Driscoll, 1983. NURP pond.
14 Oberts et al., 1989. A: Rainfall only. B: Rainfall and snowmelt.
15 Dorman et al., 1989. Highway runoff.
16 Wotzka and Oberts, 1988. Some ED provided.
17 Oberts et al., 1989. A: Rainfall only. Rainfall and snowmelt.
18 Bannerman, forthcoming March 1992.
19 Wu et al., 1988.
A Current Assessment of Urban Best Management Practices
Page - 110
-------
Pollutant Removal Capability Table: References, Notes and Notation
Study No. Reference and Notes
20 Dorman et al., 1989. 8000 feet of grassed swale treatment prior to pond. Very shallow permanent
pool.
21 Martin, 1988.
22 City of Austin, 1990. Negative removal for TDS off-line facility.
23 Horner et al., 1990.
24 OWML, 1983(b). Farm pond. No urban development
25 OWML, 1983(b). TSS removal estimate appears to be a serious underestimate due to the use of
the median storm EMC calculations.
26 Driscoll, 1986. High algal uptake.
27 Horner et al., 1990.
28 Dorman et al., 1989. Highway runoff.
29 Cull urn, 1985. Commercial area.
30 Yousef et al., 1986. Multiple cell wet pond.
31 Wu et al., 1988. Goose excrement cited for poor nutrient removal.
32 Ontario Ministry of the Environment, 1991. No winter data. Manual ED.
33 Ontario Ministry of the Environment, 1991. No winter data. Manual ED.
34 Ontario Ministry of the Environment, 1991. No winter data. Manual ED.
35 Hey and Barrett, 1991. Cited in Strecker et al., 1990. Resuspension.
36 Hey and Barrett, 1991. Cited in Strecker et al., 1990.
37 Hey and Barrett, 1991. Cited in Strecker et al;, 1990.
38 Hey and Barrett, 1991. Cited in Strecker et al., 1990. No surface discharge during 6 months of
the year.
39 Reinelt et al., 1990. Cited in Strecker et al., 1990. Channelization reduced effectiveness.
40 Reinelt et al., 1990. Cited in Strecker et al., 1990. Chanellization reduced effectiveness.
41 Wotzka and Oberts, 1988. Runoff pretreated by pond.
A Current Assessment of Urban Best Management Practices
Page - 111
-------
Pollutant Removal Capability Table: References, Notes and Notation
Study No. Reference and Notes
42 Athanas and Stevenson, 1991.
43 Driscoll, 1983. Shallow pond with wetlands.
44 Rushton and Dye, 1990. Cited in Strecker et al., 1990.
45 Wotzka and Oberts, 1988. Runoff pretreated by pond.
46 Blackburn et al., 1986. Cited in Strecker et al., 1990. Residential golf course. Polish runoff to
natural wetland.
47 OWML and GMU, 1990. Poor removal for large storm in excess of treatment capacity.
48 Oberts et al., 1989. A: Rainfall only. B: Rainfall and snowmelt.
49 OWML and GMU, 1990.
50 Barten, 1983. Cited in Strecker et al., 1990.
51 Harper et al., 1986. Cited in Strecker et al., 1990. Stormwater introduced to small wooded wetland.
52 Hickock et al., 1977. Cited in Strecker et al., 1990.
53 Maristany and Bartell, 1989. 30-year old lake wetland systems.
54 Oberts et al., 1989. A: Rainfall only. B: and snowmelt
55 Wotzka and Oberts, 1988. Wetpond to wetland.
56 Esry and Cairns, 1988. Cited in Strecker et al., 1990. Pond to filter to wetland.
57 Martin, 1988. Wetpond to wetland.
58 Jolly, 1990. Study period did not cover periods of high phosphorus loading or spring thaw
(snowmelt) and was primarily in an agricultural watershed.
A Current Assessment of Urban Best Management Practices
Page - 112
-------
Pollutant Removal Capability Table: References, Notes and Notation
II. Notation
BOD:
Biological
FColi: Fecal Coli
Pest:
Pesticides
TOC:
Total Organic
Oxygen
Fe: Iron
PP:
Particulate
Carbon
Demand
Hydro: Hydrocarbons
Phosphorus
TP:
Total Phosphorus
Cd:
Cadmium
NH3: Ammonium
SP:
Soluble
TSS:
Total Suspended
COD:
Chemical
NH4: Ammonia
Phosphorus
Solids
Oxygen
N03: Nitrate
TKN:
Total Kgeldahl
Zn:
Zinc
Demand
ON: Organic
Nitrogen
Cr:
Chromium
Nitrogen
TN:
Total Nitrogen
Cu:
Copper
Pb: Lead
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Appendix B
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Glossary
to^s^rfare 't* moI®cu'es.°f a gas, liquid or dissolved substance
nr inmrnnraHnn Offers from absorption in that absorption is the assimilation
P a gas/ liquid or dissolved substance into another substance.
VALVE - A knife gate valve, activated by a handwheel, used
pond drainpipe 3 G °f reverse sloPe V[Pes or allow raPid opening of the
riup " I0rmufOr ^ stone or rock 8ravel needed to fill in an infiltration
aotrranato n, l l fn or P°j0uf Paven^ent. Clean-washed aggregate is simply
aggregate that has been washed clean so that no sediment is associated with.
AQUATIC BENCH - A ten to fifteen foot bench around the inside perimeter of a
permanent pool that is approximately one foot deep. Normally vegetated with
emergent plants, the bench augments pollutant removal, provides habitat, conceals
trash and water level drops, and enhances safety.
ARTIFICIAL MARSH CREATION - Simulation of natural wetland features and
functions via topographic and hydraulic modifications on non-wetland landscapes.
Typical objectives for artificial marsh creation include ecosystem replacement or
stormwater management.
BMP FINGERPRINTING - Term refers to a series of techniques for locating BMPs
(particularly ponds) within a development site so as to minimize their impacts to
wetlands, forest and sensitive stream reaches.
BACTERIAL DECOMPOSITION OR MICROBIAL DECOMPOSITION -
Microorganisms, or bacteria, have the ability to degrade organic compounds as food
resources and to absorb nutrients and metals into their tissues to support growth.
BANK RUN - Gravelly deposits consisting of smooth round stones, generally
indicative of the existence of a prehistoric sea. Such deposits are normally found in
coastal plain regions.
BANK STABILIZATION - Methods of securing the structural integrity of earthen
stream channel banks with structural supports to prevent bank slumping and
undercutting of riparian trees, and overall erosion prevention. To maintain the
ecological integrity of the system, recommended techniques include the use of willow
stakes, imbricated riprap or brush bundles.
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Glossary
BANKFULL DISCHARGE - A flow condition where streamflow completely fills the
stream channel up to the top of the bank. In undisturbed watersheds, the discharge
condition occurs on average every one and a half to two years and controls the shape
and form of natural channels.
BASEFLOW - The portion of stream flow that is not due to storm runoff, and is
supported by groundwater seepage into a channel.
BERM, EARTHEN - An earthen mound used to direct the flow of runoff around or
through a BMP.
BEST MANAGEMENT PRACTICE (BMP) - Structural devices that temporarily store
or treat urban stormwater runoff to reduce flooding, remove pollutants, and provide
other amenities.
BIOFILTRATION - The use of a series of vegetated swales to provide filtering
treatment for stormwater as it is conveyed through the channel. The swales can be
grassed, or contain emergent wetlands, or high marsh plants.
BIOLOGICAL MONITORING - Periodic surveys of aquatic biota as an indicator of
the general health of a waterbody. Biological monitoring surveys can span the
trophic spectrum, from macro-invertebrates to fish species.
CATCHMENT - See CONTRIBUTING WATERSHED AREA.
CHANNEL EROSION - The widening, deepening, and headward cutting of small
channels and waterways, due to erosion caused by moderate to larger floods.
CHECK DAM - (a) A log or gabion structure placed perpendicular to a stream to
enhance aquatic habitat, (b) An earthen or log structure, used in grass swales to
reduce water velocities, promote sediment deposition, and enhance infiltration.
CONTRIBUTING WATERSHED AREA - Portion of the watershed contributing its
runoff to the BMP in question.
DELTA-t - The magnitude of change in the temperature of downstream waters.
DESIGN STORM - A rainfall event of specified size and return frequency (e.g., a
storm that occurs only once every 2 years) that is used to calculate the runoff volume
and peak discharge rate to a BMP.
DE-WATERING - Refers to a process used in detention/retention facilities, whereby
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Glossary
water is completely discharged or drawn down to a pre-established pool elevation by
way of a perforated pipe. De-watering allows the facility to recover its design
storage capacity in a relatively short time after a storm event.
DOWNSTREAM SCOUR - Downstream channel erosion usually associated with an
upstream structure that has altered hydraulic conditions in the channel.
DROP STRUCTURE - Placement of logs with a weir notch across a stream channel.
Water flowing through the weir creates a plunge pool downstream
of the structure and creates fish habitat.
DRAWDOWN - The gradual reduction in water level in a pond BMP due to the
combined effect of infiltration and evaporation.
DRY POND CONVERSION - A modification made to an existing dry stormwater
management pond to increase pollutant removal efficiencies. For example, the
modification may involve a decrease in orifice size to create extended detention times,
or the alteration of the riser to create a permanent pool and/or shallow marsh
system.
EXTENDED DETENTION (ED) PONDS - A conventional ED pond temporarily
detains a portion of stormwater runoff for up to twenty-four hours after a storm
using a fixed orifice. Such extended detention allows urban pollutants to settle out.
The ED ponds are normally "dry" between storm events and do not have any
permanent standing water.
An enhanced ED pond is designed to prevent clogging and resuspension. It
provides greater flexibility in achieving target detention times. It may be equipped
with plunge pools near the inlet, a micropool at the outlet, and utilize an adjustable
reverse-sloped pipe at the ED control device.
ED CONTROL DEVICE - A pipe or series of pipes that extend from the riser of a
stormwater pond that are used to gradually release stormwater from the pond over
a 12 to 48 hour interval.
EMBANKMENT - A bank (of earth or riprap) used to keep back water.
EMERGENT PLANT - An aquatic plant that is rooted in the sediment but whose
leaves are at or above the water surface. Such wetland plants provide habitat for
wildlife and waterfowl in addition to removing urban pollutants.
END OF PIPE CONTROL - Water quality control technologies suited for the control
of existing urban stormwater at the point of storm sewer discharge to a stream. Due
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Glossary
to typical space constraints, these technologies are usually designed to provide water
quality control rather than quantity control.
EXFILTRATION - The downward movement of runoff through the bottom of an
infiltration BMP into the subsoil.
EXTENDED DETENTION - A stormwater design feature that provides for the
gradual release of a volume of water (0.25 - 1.0 inches per impervious acre) over a
12 to 48 interval times to increase settling of urban pollutants, and protect channel
from frequent flooding.
FILTER FABRIC - Textile of relatively small mesh or pore size that is used to (a)
allow water to pass through while keeping sediment out (permeable), or (b) prevent
both runoff and sediment form passing through (impermeable).
FLOW SPLITTER - An engineered, hydraulic structure designed to divert a portion
of stream flow to a BMP located' out of the channel, or to direct stormwater to a
parallel pipe system, or to bypass a portion of baseflow around a pond.
FOREBAY - An extra storage area provided near an inlet of a BMP to trap incoming
sediments before they accumulate in a pond BMP.
FREQUENT FLOODING - A phenomenum in urban streams whereby the number
of bankfull and sub-bankfull flood events increases sharply after development. The
frequency of these disruptive floods is a direct function of watershed imperviousness.
FRINGE WETLAND CREATION - Planting of emergent aquatic vegetation along the
perimeter of open water to enhance pollutant uptake, increase forage and cover for
wildlife and aquatic species, and improve the appearance of a pond.
GABION - A large rectangular box of heavy gauge wire mesh which holds large
cobbles and boulders. Used in streams and ponds to change flow patterns, stabilize
banks, or prevent erosion.
GEOMEMBRANE - Lining of filter fabric on the bottom and sides of porous
pavement to prevent lateral or upward movement of soil into the stone reservoir.
GEOTEXTILE FABRIC - See FILTER FABRIC.
GRASSED SWALE - A conventional grass swale is an earthen conveyance system in
which the filtering action of grass and soil infiltration are utilized to remove
pollutants from urban stormwater. An enhanced grass swale, or biofilter, utilizes
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Glossary
checkdams and wide depressions to increase runoff storage and promote greater
settling of pollutants.
GRAVITATIONAL SETTLING - The tendency of particulate matter to "drop out" of
stormwater runoff as it flows downstream when runoff velocities are moderate
and/or slopes are not too steep.
HEAD - Pressure.
HIGH MARSH - Diverse wetland type found in areas that are infrequently inundated
or have wet soils. In pond systems, the high marsh zone extends form the
permanent pool to the maximum ED water surface elevation.
INFILTRATION BASIN - An impoundment where incoming stormwater runoff is
stored until it gradually exfiltrates through the soil of the basin floor.
INFILTRATION TRENCH - A conventional infiltration trench is a shallow, excavated
trench that has been backfilled with stone to create an underground reservoir.
Stormwater runoff diverted into the trench gradually exfiltrates from the bottom of
the trench into the subsoil and eventually into the water table.
An enhanced infiltration trench has an extensive pretreatment system to remove
sediment and oil. It requires an on-site geotechnical investigation to determine
appropriate design and location.
LEVEL SPREADER - A device used to spread out stormwater runoff uniformly over
the ground surface as sheet flow (i.e., not through channels). The purpose of level
spreaders are to prevent concentrated, erosive flows from occurring, and to enhance
infiltration.
LOW MARSH - Wetland type with emergent plant species that require some depth
of standing water throughout the year. The low marsh zone in pond systems is
created in areas where the permanent pool is zero to twelve inches deep.
LOWFLOW 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.
MICROPOOL - A smaller permanent pool used in a stormwater pond due to
extenuating circumstances, i.e. concern over the thermal impacts of larger ponds,
impacts on existing wetlands, or lack of topographic relief.
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Glossary
MICROTOPOGRAPHY - Refers to the contours along the bottom of a shallow marsh
system. A complex microtopography creates a great variety of environmental
conditions that favor the unique requirements of many different species of wetland
plants.
MULTIPLE POND SYSTEM - A collective term for a cluster of pond designs that
incorporate redundant runoff treatment techniques within a single pond or series of
ponds. These pond designs employ a combination of two or more of the following:
extended detention, permanent pool shallw wetlands, or infiltration. Examples of a
multiple pond system include the wet ED pond, ED wetlands, infilter ponds and
pond-marsh systems.
NATURAL BUFFER - A low sloping area of maintained grassy or woody vegetation
located between a pollutant source and a waterbody. A natural buffer is formed
when a designated portion of a developed piece of land is left unaltered from its
natural state during development. A natural vegetative buffer differs from a
vegetated filter strip in that it is "natural" and in that they need not be used solely
for water quality purposes.
To be effective, such areas must be protected against concentrated flow.
OBSERVATION WELL - A test well installed in an infiltration trench to monitor
draining times after installation.
OFF-LINE BMP - A water quality facility designed to treat a portion of stormwater
(usually 0.5 to 1.0 inches per impervious acre) which has been diverted from a stream
or storm drain.
OFF-LINE TREATMENT - A BMP system that is located outside of the stream
channel or drainage path. A flow splitter is used to divert runoff from the channel
and into the BMP for subsequent treatment.
OIL/GRIT SEPARATOR - A best management practice consisting of a three-stage
underground retention system designed to remove heavy particulates and absorbed
hydrocarbons. Also known as a WATER QUALITY INLET.
OUTFALL - The point of discharge for a river, drain, pipe, etc.
PARALLEL PIPE SYSTEM - A technique for protecting sensitive streams. Excess
stormwater runoff is piped in a parallel direction along the stream buffer instead of
being discharged directly into the stream.
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Glossary
PEAT SAND FILTER - Best management practice, utilizing the natural adsorptive
features of fabric or hemic peat, which consists of a vertical filter system with a grass
cover crop/ alternating layers of peat and sand and a sediment forebay feature. The
peat sand filter is presently used for municipal waste treatment systems and is being
adapted for use in stormwater management.
PERMANENT POOL - A three to ten foot meter deep pool in a stormwater pond
system, that provides removal of urban pollutants through settling and biological
uptake. (Also referred to as a wet pond).
PHYSICAL FILTRATION - As they pass across or through a surface, particulates are
separated from runoff by grass, leaves and other organic matter on the surface.
PILOT CHANNEL - A riprap or paved channel that routes runoff through a BMP
to prevent erosion of the surface.
PLUNGE POOL - A small permanent pool located at either the inlet to a BMP or at
the outfall form a BMP. The primary purpose of the pool is to dissipate the velocity
of stormwater runoff, but it also can provide some pre-treatment, as well.
PONDSCAPING - A method of designing the plant structure of a stormwater
wetland or pond using inundation zones. The proposed wetland or pond system is
divided into zones which differ in the level and frequency of inflow. For each zone,
plant species are chosen based on their potential to thrive, given the inflow pattern
of the zone.
POROUS PAVEMENT - An alternative to conventional pavement whereby runoff is
diverted through a porous asphalt layer and into an underground stone reservoir.
The stored runoff then gradually infiltrates into the subsoil.
RETROFIT - The creation/modification of stormwater management systems in
developed areas through the construction of wet ponds, infiltration systems, wetland
plantings, stream bank stabilization, and other BMP techniques for improving water
quality and creating aquatic habitat. A retrofit can consist of the construction of a
new BMP in the developed area, the enhancement of an older stormwater
management structure, or a combination of improvement and new construction.
REVERSE SLOPE PIPE - A pipe that extends downwards from the riser into the
permanent pool that sets the water surface elevation of pool. The lower end of the
pipe is located up to 1 foot below the water surface. Very useful technique for
regulating ED times, and it seldom clogs.
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Glossary
RIPARIAN - A relatively narrow strip of land that borders a stream or river, often
coincides with the maximum water surface elevation of the one-hundred year storm.
RIPARIAN REFORESTATION - The replanting of the banks and floodplain of a
stream with native forest and shrub species to stabilize erodible soils, improve both
surface and ground water quality, increase stream shading, and enhance wildlife
habitat.
RIPRAP - A combination of large stone, cobbles and boulders used to line channels,
stabilize banks, reduce runoff velocities, or filter out sediment.
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.
ROTOTILLING - Mechanical means of tilling, or rotating, the soil.
RUNOFF CONVEYANCE - Methods for safely conveying stormwater to a BMP to
minimize disruption of the stream network, and promote infiltration or filtering of the
runoff.
RUNOFF FREQUENCY SPECTRUM - The frequency distribution of unit area runoff
volumes generated by a long, term continuous time-series of rainfall events. Used
to develop BMP and stormwater sizing rules.
RUNOFF PRETREATMENT - Techniques to capture or trap coarse sediments before
they enter a BMP to preserve storage volumes or prevent clogging within the BMP.
Examples include forebays and micropools for pond BMPs, and plunge pools, grass
filter strips and filter fabric for infiltration BMPs.
SAFETY BENCH - A ten to fifteen foot bench located just outside the perimeter of
a permanent pool. The bench extend around the entire shoreline to provide for
maintenance access and eliminate hazards.
SAND FILTER - A relatively new technique for treating stormwater, whereby the
first flush of runoff is diverted into a self-contained bed of sand. The runoff is then
strained through the sand, collected in underground pipes and returned back to the
stream or channel.
An enhanced sand filter utilizes layers of peat, limestone, and/or topsoil, and
may also have a grass cover crop. The adsorptive media of an enhanced sand filter
is expected to improve removal rates.
SEDIMENT FOREBAY - Stormwater design feature that employs the use of a small
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Glossary
settling basin to settle out incoming sediments before they are delivered to a
stormwater BMP. Particularly useful in tandem with infiltration devices, wet ponds
or marshes.
SHORT CIRCUITING - The passage of runoff through a BMP in less than the
theoretical or design treatment time.
SLURRY - Thin mixture of water and any of several fine, insoluble materials;
therefore, an OIL SLURRY is a thin mixture of water and oil.
STORMWATER TREATMENT - Detention, retention, filtering or infiltration of a
given volume of stormwater to remove urban pollutants and reduce frequent flooding.
STORMWATER WETLAND - A conventional stormwater wetland is a shallow pool
that creates growing conditions suitable for the growth of marsh plants. A
stormwater wetland is designed to maximize pollutant removal through wetland
uptake, retention and settling.
A stormwater wetland is a constructed system and typically is not located
within delineated a natural wetland. In addition, a stormwater wetland differs from
an articifial wetland created to comply with mitigation requirements in that the
stormwater wetland does not replicate all the ecological functions of natural wetlands.
An enhanced stormwater wetland is designed for more effective pollutant
removal and species diversity. It also includes design elements such as a forebay,
complex microtopography, and pondscaping with multiple species of wetland trees,
shrubs and plants.
STREAM BUFFER - A variable width strip of vegetated land adjacent to a stream
that is preserved from development activity to protect water quality, aquatic and
terrestrial habitats.
SUBSOIL - The bed or stratum of earth lying below the surface soil.
SUBSTRATE AMENDMENTS - A technique to improve the texture, and organic
content of soils in a newly excavated pond system. The addition of organic rich soils
is often required to ensure the survival of aquatic and terrestrial landscaping around
ponds.
SUMP PIT - A single-chamber oil/grit separator used to pretreat runoff before it
enters an infiltration trench.
SWALE - A natural depression or wide shallow ditch used to temporarily store route,
or filter runoff.
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Glossary
TRASH AND DEBRIS REMOVAL - Mechanical removal of debris, snags, and trash
deposits from the streambanks to improve the appearance of the stream.
UNDERDRAIN - Plastic pipes with holes drilled through the top, installed on the
bottom of an infiltration BMP, or sand filter, which are used to collect and remove
excess runoff.
VACUUM SWEEPING - Method of removing quantities of coarse-grained sediments
from porous pavement in order to prevent clogging. Not effective in removing fine-
grained pollutants.
VEGETATED FILTER STRIP - A vegetated section of land designed to accept runoff
as overland sheet flow from upstream development. It may adopt any natural
vegetated form, from grassy meadow to small forest. The dense vegetative cover
facilitates pollutant removal.
A filter strip cannot treat high velocity flows; therefore, they have generally been
recommended for use in agriculture and low density development.
A vegetated filter strip differs from a natural buffer in that the strip is not "natural";
rather, it is designed and constructed specifically for the purpose of pollutant
removal. A filter strip can also be an enhanced natural buffer, however, whereby the
removal capability of the natural buffer is improved through engineering and
maintenance activities such as land grading or the installation of a level spreader.
A filter strip also differs from a grassed swale in that a swale is a concave
vegetated conveyance system, whereas a filter strip has a fairly level surface.
WATER QUALITY INLET - Best management practice consisting of a three-stage
underground retention system designed to remove heavy particulates and absorbed
hydrocarbons. Also, known as an OIL/GRIT SEPARATOR.
WEIR - A structure that extends across the width of a channel and is intended to
impound, delay or in some way alter the flow of water through the channel. A
CHECK DAM is a type of weir as is any kind of dam.
A PORTED WEIR is a wall or dam that contains openings through which water
may pass. Ported weirs slow the velocity of flow and therefore, can assist in the
removal of pollutants in runoff by providing opportunities for pollutants to settle,
infiltrate or be adsorbed.
WET POND - A conventional wet pond has a permanent pool of water for treating
incoming stormwater runoff.
In enhanced wet pond designs, a forebay is installed to trap incoming sediments
where thay can be easily removed; a fringe wetland is also established around the
perimeter of the pond.
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Glossary
WETLAND MITIGATION - Regulatory requirement to replace wetland areas
destroyed or impacted by proposed land disturbances with artificially created wetland
areas.
WETLAND MULCH - A technique for establishing low or high marsh areas where
the top twelve inches of wetland soil form a donor wetland are spread thinly over
the surface of a created wetland site as a mulch. The seedbank and organic matter
of the mulch helps to rapidly establish a diverse wetland system.
WETLAND PLANT UPTAKE - Wetland plant species rely on nutrients (i.e.,
phosphorus and nitrogen) as a food source; thus, they may intercept and remove
nutrients from either surface or subsurface flow.
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