United States Office of Water (4203) EPA-841-B-00-005
Environmental Protection Agency Washington, DC 20460 October 2000
Low Impact
Development (LID)
LOW-IMPACT
DEVELOPMENT
A Literature Review
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EXECUTIVE SUMMARY
A literature review was conducted to determine the availability and reliability of data to
assess the effectiveness of low impact development (LID) practices for controlling
stormwater runoff volume and reducing pollutant loadings to receiving waters.
Background information concerning the uses, ownership and associated costs for LID
measures was also compiled. In general LID measures are more cost effective and lower
in maintenance than conventional, structural stormwater controls. Not all sites are
suitable for LID. Considerations such as soil permeablility, depth of water table and slope
must be considered, in addition to other factors. Further, the use of LID may not
completely replace the need for conventional stormwater controls.
Maintenance issues can be more complicated than for conventional stormwater controls
because the LID measures reside on private property. In most instances, homeowners
agree to only the first year of maintenance. Homeowner associations could be a
mechanism for providing long-term maintenance to these areas. Generally, bioretention
facilities require replacement of dead or diseased vegetation, remulching as needed, and
replacement of soils after 5-10 years. Grass swales require periodic mowing and removal
of sediments. Maintenance of permeable pavements requires annual high-powered
vacuuming of the area to remove sediments.
Several studies have been conducted to analyze the effectiveness of various LID practices
based on hydrology and pollutant removal capabilities. Bioretention areas, grass swales,
permeable pavements and vegetated rooftops were the most common practices studied.
These techniques reduce the amount of Effective Impervious Area (EIA) in a watershed.
EIA is the directly connected impervious area to the storm drain system and contributes
to increased watershed volumes and runoff rates. There are documented case studies that
conclusively link urbanization and increased watershed imperviousness to hydrologic
impacts on streams. Existing reports and case studies provide strong evidence that
urbanization negatively affects streams and results in water quality problems such as loss
of habitat, increased temperatures, sedimentation and loss offish populations (USEPA,
1997)
In general bioretention areas were found to be effective in reducing runoff volume and in
treating the first flush (first !/2 inch) of stormwater. Results from three different studies
indicate that removal efficiencies were quite good for both metals and nutrients. Removal
rates for metals were more consistent than for nutrients. Removal rates for metals ranged
from 70-97% for lead, 43-97% for copper and 64-98% for zinc. Nutrient removal was
more variable and ranged from 0-87% for phosphorus, 37-80% for Total Kjeldahl
Nitrogen, <0-92% for ammonium and for nitrate <0-26%. Effluent volumes were lower
than influent volumes. These studies were conducted by means of simulated rainfall
events. Analysis of actual long-term rainfall events would produce more reliable data.
The effectiveness of grass swales was also quite good for both pollutant removal and
runoff volume reduction. A study of three different sites in the United States reveal
similar results despite the differences in location. In general, performance of swales is
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dependant on not only channel length, but also longitudinal slope and the use of check
dams to slow flows and allow for greater infiltration. Further, the removal of metals was
found to be directly related to the removal rate of total suspended solids, and the removal
rate of metals was greater than removal of nutrients.
Reduction of impervious surfaces can greatly reduce the volume of runoff generated by
rainfall. Several methods can be employed to reduce total impervious surface area.
Permeable pavements and vegetated rooftops are two methods to accomplish this goal.
Vegetated rooftops have been used extensively in Germany for more than 25 years and
results show up to 50% reduction in annual runoff in temperate climates. Many
opportunities exist to retrofit these systems into older highly urbanized areas of the
United States. The Philadelphia project case study provides an example of this practice.
Permeable pavements can also reduce impervious surfaces. However, they are more
expensive to construct than traditional asphalt pavements. Costs of these systems may be
off set by the reduction of traditional curb and gutter systems to convey stormwater.
Benefits of these alternate pavement types include better infiltration, ground water
recharge, reduction in runoff volume and treatment of stormwater for pollutants. The
study conducted in Tampa, Florida outlines these benefits as well as the opportunity to
retrofit permeable pavements into existing parking lots with little or no loss of parking
space. Less than 20% of rainfall was converted to runoff when using permeable
pavements. Study results from the University of Washington, compare several different
treatments of varying permeablility. The study shows that the higher the amount of
perviousness of the treatment, the greater the reduction of runoff volume and pollutant
loadings.
The use of LID is relatively new and not widespread. Most of the available data are from
Prince George's County, Maryland, which pioneered the use of LID. The data available
for bioretention analysis were from single simulated storm events in actual bioretention
facilities or from laboratory constructed and tested bioretention systems. The data for
grass swales were for only a few storm events, collected over a short period of time. The
only available data for a long-term study came from the Aquarium parking lot in Tampa,
Florida and the Washington permeable pavement project. More long-term analysis is
required to more accurately assess the effectiveness of LID and to determine long term
trends.
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TABLE OF CONTENTS
Executive Summary i
Table of Contents iii
List of Figures iv
List of Tables v
1 Low Impact Development 1
1.1 Introduction 1
1.2 Benefits and Limitations 2
2 Low Impact Development Practices 4
2.1 Bioretention 4
2.2 Grass Swales 7
2.3 Vegetated Roof Covers 7
2.4 Permeable Pavements 8
2.5 Other LID Strategies 8
3 Evaluation of LID Effectiveness 9
3.1 Hydrological Measures 9
3.2 Pollutant Removal Measures 10
4 Case Studies 11
4.1 Bioretention Facility, Laboratory and Field Study, Beltway Plaza Mall
Parking Lot, Greenbelt, MD 12
4.2 Bioretention Facility, Field Study, Peppercorn Plaza Parking Lot at
Inglewood Center, Landover, MD 15
4.3 Permeable Pavements and Swales, Field Study, Stormwater Management,
Florida Aquarium Parking Lot, Tampa, FL 18
4.4 Vegetated Roof Covers, Field Study, Green Rooftop, Philadelphia, PA 23
4.5 Permeable Pavements, Field Study, Permeable Pavements for Stormwater
Management, Olympia, WA 25
4.6 Grass Swales, Field Study, Highway Grass Channels, Northern Virginia,
Maryland, and Florida 29
5 Conclusion 31
6 Recommendations 33
7 References 34
in
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LIST OF FIGURES
Figure 1: Changes in Stormwater Hydrology as a Result of Urbanization 2
Figure 2: Typical Bioretention System (Prince George's County Department of
Environmental Resources, 1993) 4
Figure 3: Pollutant Removal Rates for All Systems 14
Figure 4: Bioretention System at Peppercorn Place, Inglewood Plaza (Davis, 1999) 16
Figure 5: Florida Parking Lot Study Site (Rushton, 1999) 19
Figure 6: Percent of Rainfall Volume Converted to Runoff Volume for Events
Less Than 2cm 20
Figure 7: Structure of the Philadelphia Vegetated Roof Cover (Miller, 1998) 23
Figure 8: A Rainfall Event of 0.4 inches with Media Completely Saturated
(Miller, 1998) 24
Figure 9: Surface Runoff from 60% Impervious Pavement vs. Asphalt (Booth, 1996) ... 26
Figure 10: Subsurface Runoff From Pavement Less Than 5% Impervious
Compared to Precipitation (Booth, 1996) 27
Figure 11: Surface Runoff From Asphalt Compared to Precipitation (Booth, 1996) 27
Figure 12: Pollutant Removal Rates for Laboratory and Field Experiments of
Bioretention Systems 32
IV
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LIST OF TABLES
Table 1: Example Maintenance Schedule for Bioretention Areas (Prince George's
County, Department of Environmental Resources, 1993) 6
Table 2: Low Impact Hydrologic Design and Analysis Components (Coffman, 2000) 9
Table 3: Summary of Results for Smaller System—Standard Conditions 13
Table 4: Summary of Results for Large System—Standard Conditions 13
Table 5: Summary of Results for Field Bioretention Study 14
Table 6: Summary of Pollutant Removal Results of Bioretention System at Inglewood
Place 17
Table 7: Summary of Pollutant Removal Efficiency for the Various Treatment Types... 21
Table 8: Long Term Pollutant Removal Estimates for Grassed Swales 30
Table 9: Pollutant Removal Efficiencies for Laboratory and Field Bioretention Studies 31
v
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1 LOW IMPACT DEVELOPMENT
1.1 Introduction
Low impact development (LID) is a relatively new concept in stormwater management.
LID techniques were pioneered by Prince George's County, Maryland, in the early
1990's, and several projects have been implemented within the state. Some LID
principles are now being applied in other parts of the country, however, the use of LID is
infrequent and opportunities are often not investigated. The purpose of this report is to
conduct a literature review to determine existing information about the application of LID
in new development and existing urbanized areas, including ownership, operation and
maintenance issues. A related objective was to locate relevant studies of LID projects,
which would provide evidence of the effectiveness of LID in retaining predevelopment
hydrology and as a mechanism for pollutant removal for stormwater. The data from the
studies were analyzed for usefulness and validity and the findings are summarized.
LID is a site design strategy with a goal of maintaining or replicating the pre-
development hydrologic regime through the use of design techniques to create a
functionally equivalent hydrologic landscape. Hydrologic functions of storage,
infiltration, and ground water recharge, as well as the volume and frequency of
discharges are maintained through the use of integrated and distributed micro-scale
stormwater retention and detention areas, reduction of impervious surfaces, and the
lengthening of flow paths and runoff time (Coffman, 2000). Other strategies include the
preservation/protection of environmentally sensitive site features such as riparian buffers,
wetlands, steep slopes, valuable (mature) trees, flood plains, woodlands and highly
permeable soils.
LID principles are based on controlling stormwater at the source by the use of micro-
scale controls that are distributed throughout the site. This is unlike conventional
approaches that typically convey and manage runoff in large facilities located at the base
of drainage areas. These multifunctional site designs incorporate alternative stormwater
management practices such as functional landscape that act as stormwater facilities,
flatter grades, depression storage and open drainage swales. This system of controls can
reduce or eliminate the need for a centralized best management practice (BMP) facility
for the control of stormwater runoff. Although traditional stormwater control measures
have been documented to effectively remove pollutants, the natural hydrology is still
negatively affected (inadequate base flow, thermal fluxes or flashy hydrology), which can
have detrimental effects on ecosystems, even when water quality is not compromised
(Coffman, 2000). LID practices offer an additional benefit in that they can be integrated
into the infrastructure and are more cost effective and aesthetically pleasing than
traditional, structural stormwater conveyance systems.
Conventional stormwater conveyance systems are designed to collect, convey and
discharge runoff as efficiently as possible. The intent is to create a highly efficient
drainage system, which will prevent on lot flooding, promote good drainage and quickly
convey runoff to a BMP or stream. This runoff control system decreases groundwater
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recharge, increases runoff volume and changes the timing, frequency and rate of
discharge. These changes can cause flooding, water quality degradation, stream erosion
and the need to construct end of pipe BMPs. Discharge rates using traditional BMPs may
be set only to match the predevelopment peak rate for a specific design year. This
approach only controls the rate of runoff allowing significant increases in runoff volume,
frequency and duration of runoff from the predevelopment conditions and provides the
mechanisms for further degradation of receiving waters (Figure 1).
LID has often been compared to other innovative practices, such as Conservation Design,
which uses similar approaches in reducing the impacts of development, such as reduction
of impervious surfaces and conservation of natural features. Although the goals of
Conservation Design protect natural flow paths and existing vegetative features,
stormwater is not treated directly at the source. Conservation Design protects large areas
adjacent to the development site and stormwater is directed to these common areas.
P re-develop-menl
Post-dewa lop merr.
TIME
» a re-sul: of
Figure 1: Changes in Stormwater Hydrology as a Result of Urbanization
Although this approach protects trees and does reduce runoff, there is still potentially a
significant amount of connected impervious area and centralized stormwater facilities
that may contribute to stream degradation through stormwater volume, frequency and
thermal impacts. Therefore, the hydrologic and hydraulic impacts of this approach on
receiving waters may still be significant, although the volume and flows will be less than
without the conservation design. The stormwater control measures used in Conservation
Design are off-site and therefore not the individual property owner's responsibility.
However, maintenance is generally provided by the homeowners association and
financed through association fees.
1.2 Benefits and Limitations
The use of LID practices offers both economical and environmental benefits. LID
measures result in less disturbance of the development area, conservation of natural
features and can be less cost intensive than traditional stormwater control mechanisms.
Cost savings for control mechanisms are not only for construction, but also for long-term
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maintenance and life cycle cost considerations. For example, an alternative LID
stormwater control design for a new 270 unit apartment complex in Aberdeen, NC will
save the developer approximately 72% or $175,000 of the stormwater construction costs.
On this project, almost all of the subsurface collection systems associated with curb and
gutter projects have been eliminated. Strategically located bioretention areas, compact
weir outfalls, depressions, grass channels, wetland swales and specially designed storm
water basins are some of the LID techniques used. These design features allow for longer
flow paths, reduce the amount of polluted runoff and filter pollutants from stormwater
runoff (Blue Land, Water and Infrastructure, 2000).
Today many states are facing the issue of urban sprawl, a form of development that
consumes green space, promotes auto dependency and widens urban fringes, which puts
pressure on environmentally sensitive areas. "Smart growth" strategies are designed to
reconfigure development in a more eco-efficient and community oriented style. LID
addresses many of the environmental practices that are essential to smart growth
strategies including the conservation of open green space. LID does not address the
subject of availability of public transportation.
LID provides many opportunities to retrofit existing highly urbanized areas with
pollution controls, as well as address environmental issues in newly developed areas. LID
techniques such as rooftop retention, permeable pavements, bioretention and
disconnecting rooftop rain gutter spouts are valuable tools that can be used in urban
areas. For example, stormwater flows can easily be directed into rain barrels, cisterns or
across vegetated areas in high-density urban areas. Further, opportunities exist to
implement bioretention systems in parking lots with little or no reduction in parking
space. The use of vegetated rooftops and permeable pavements are 2 ways to reduce
impervious surfaces in highly urbanized areas.
LID techniques can be applied to a range of lot sizes. The use of LID, however, may
necessitate the use of structural BMPs in conjunction with LID techniques in order to
achieve watershed objectives. The appropriateness of LID practices is dependent on site
conditions, and is not based strictly on spatial limitations. Evaluation of soil permeability,
slope and water table depth must be considered in order to effectively use LID practices.
Another obstacle is that many communities have development rules that may restrict
innovative practices that would reduce impervious cover. These "rules" refer to a mix of
subdivision codes, zoning regulations, parking and street standards and other local
ordinances that determine how development happens (Center for Watershed Protection,
1998). These rules are responsible for wide streets, expansive parking lots and large-lot
subdivisions that reduce open space and natural features. These obstacles are often
difficult to overcome.
Additionally, community perception of LID may prevent its implementation. Many
homeowners want large-lots and wide streets and view reduction of these features as
undesirable and even unsafe. Furthermore, many people believe that without
conventional controls, such as curbs and gutters and end of pipe BMPs, they will be
required to contend with basement flooding and subsurface structural damage.
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2 LOW IMPACT DEVELOPMENT PRACTICES
LID measures provide a means to address both pollutant removal and the protection of
predevelopment hydrological functions. Some basic LID principles include conservation
of natural features, minimization of impervious surfaces, hydraulic disconnects,
disbursement of runoff and phytoremediation. LID practices such as bioretention
facilities or rain gardens, grass swales and channels, vegetated rooftops, rain barrels,
cisterns, vegetated filter strips and permeable pavements perform both runoff volume
reduction and pollutant filtering functions.
2.1 Bioretention
Bioretention systems are designed based on soil types, site conditions and land uses. A
bioretention area can be composed of a mix of functional components, each performing
different functions in the removal of pollutants and attenuation of stormwater runoff
(Figure 2).
CROWD COVER OR
MULCH
WAX, fQNoa *ATO
DD»TH (i INCHES)
PROPOSED 4' MN
GRADE
PLANTING
sot
(NOT TO SCAlf)
Figure 2: Typical Bioretention System (Prince George's County Department of Environmental
Resources, 1993)
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Six typical components found in bioretention cells:
- Grass buffer strips reduce runoff velocity and filter paniculate matter.
- Sand bed provides aeration and drainage of the planting soil and assists in the
flushing of pollutants from soil materials.
- Ponding area provides storage of excess runoff and facilitates the settling of
particulates and evaporation of excess water.
- Organic layer performs the function of decomposition of organic material by
providing a medium for biological growth (such as microorganisms) to degrade
petroleum-based pollutants. It also filters pollutants and prevents soil erosion.
- Planting soil provides the area for stormwater storage and nutrient uptake by
plants. The planting soils contain some clays which adsorb pollutants such as
hydrocarbons, heavy metals and nutrients.
- Vegetation (plants) functions in the removal of water through evapotranspiration
and pollutant removal through nutrient cycling.
Bioretention facilities are less cost intensive than traditional structural stormwater
conveyance systems. Construction of a typical bioretention area in Prince George's
County, Maryland is between $5,000 and $10,000 per acre drained, depending on soil
type (Weinstein, 2000). Other sources estimate the costs for developing bioretention sites
at between $3 and $15 per square foot of bioretention area. Design guidelines recommend
that bioretention systems occupy 5-7% of the drainage basin. Additional savings can be
realized in reduced construction costs for storm drainpipe. For example, bioretention
practices reduced the amount of storm drain pipe at a Medical Office building in Prince
George's County, Maryland from 800 to 230 feet, which resulted in a cost savings of
$24,000 or 50% of the overall drainage cost for the site (Dept. of Env. Resources, 1993).
Components of the bioretention area should meet required guidelines in order to provide
the most productive system possible. The mulch layer should be approximately 2-3
inches thick and replaced annually. Soil should be tested for several criteria before being
used.
- pH range 5.5-6.5
- Organic matter 1.5-3.0%
- Magnesium (Mg) 351bs/acre
- Phosphorus (P2O5) lOOlbs/acre
- Potassium (K2O) 851bs/acre
- Soluble salts < 500 ppm
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Plant material should be obtained from certified nurseries that have been inspected by
state or federal agencies (Dept. of Env. Resources, 1993). Native species should be used
and selected according to their moisture regime, morphology, susceptibility to pests and
diseases and tolerance to pollutants. Selection of plant species should be based on site
conditions and ecological factors. A minimum of three species of trees and three species
of shrubs should be selected to insure diversity, differing rates of transpiration and ensure
a more constant rate of evapotransportation and nutrient and pollutant uptake throughout
the growing season (Dept. of Env. Resources, 1993). Species that require regular
maintenance should be avoided or restricted. Prince George's County recommends a
warranty be established with the nursery as part of the plant installation, and should
include care and 80% replacement of plants for the first year.
Table 1: Example Maintenance Schedule for Bioretention Areas (Prince George's County,
Department of Environmental Resources, 1993)
Description
Method
Frequency
Time of Year
Soil
Inspect and Repair Erosion
Visual
Monthly
Monthly
Organic Layer
Remulch void areas
Remove previous mulch
layer before applying new
layer (optional)
Additional mulch added
(optional)
By Hand
By Hand
By Hand
As Needed
Once a Year
Once a Year
As Needed
Spring
Spring
Plants
Remove and replace all dead
and diseased vegetation that
cannot be treated
Treat all diseased trees and
shrubs
Water of plant materials, at
the end of the day for 14
consecutive days after
planting
Replace stakes after one
year
Replace deficient stakes or
wires
See Planting
Specifications
Mechanical or by
Hand
By Hand
By Hand
By Hand
Twice a Year
N/A
Immediately after
Completion of
Projects
Once a Year
N/A
Mar 15-Apr 30 and
Oct 1-Nov 30
Varies, depends on
insect or disease
infestation
N/A
Remove only in the
Spring
As Needed
Annual maintenance is required for the overall success of bioretention systems. This
includes maintenance of plant material, soil layer and the mulch layer. A maintenance
schedule outlining methods, frequency and time of year for bioretention maintenance
should be developed. Table 1 is a typical maintenance checklist. Plants will provide
enhanced environmental benefit over time as root systems and leaf canopies increase in
size and pollutant uptake and removal efficiencies. Soils, however, begin filtering
pollutants immediately and can lose their ability to function in this capacity over time.
Therefore, evaluation of soil fertility is important in maintaining an effective bioretention
system. Substances in runoff such as nutrients and metals eventually disrupt normal soil
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functions by lowering the cation exchange capacity (CEC) (Dept. of Env. Resources,
1993). CEC is the soil's ability to adsorb pollutant particles through ion attraction and
will decrease over time. It is recommended that soils be tested annually and replaced
when soil fertility is lost. Depending on environmental factors, this usually occurs within
5-10 years of construction. Replacement of soil can be accomplished in 1-2 days for
approximately $1,000-$2,000 for a typical system which will drain one acre in the
northeastern U.S. (Weinstein, 2000).
2.2 Grass Swales
Grass swales or channels are adaptable to a variety of site conditions, are flexible in
design and layout, and are relatively inexpensive (USDOT, 1996). Generally open
channel systems are most appropriate for smaller drainage areas with mildly sloping
topography (Center for Watershed Protection, 1998). Their application is primarily along
residential streets and highways. They function as a mechanism to reduce runoff velocity
and as filtration/infiltration devices. Sedimentation is the primary pollutant removal
mechanism, with additional secondary mechanisms of infiltration and adsorption. In
general grass channels are most effective when the flow depth is minimized and detention
time is maximized. The stability of the channel or overland flow is dependant on the
erodibility of the soils in which the channel is constructed (USDOT, 1996). Decreasing
the slope or providing dense cover will aid in both stability and pollutant removal
effectiveness.
Engineered swales are less costly than installing curb and gutter/storm drain inlet and
storm drain pipe systems. The cost for traditional structural conveyance systems ranges
from $40-$50 per running foot. This is two to three times more expensive than an
engineered grass swale (Center for Watershed Protection, 1998). Concerns that open
channels are potential nuisance problems, present maintenance problems, or impact
pavement stability can be alleviated by proper design. Periodic removal of sediments and
mowing are the most significant maintenance requirements.
2.3 Vegetated Roof Covers
Vegetative roof covers or green roofs are an effective means of reducing urban
stormwater runoff by reducing the percentage of impervious surfaces in urban areas.
They are especially effective in older urban areas with chronic combined sewer overflow
(CSO) problems, due to the high level of imperviousness. The green roof is a
multilayered constructed material consisting of a vegetative layer, media, a geotextile
layer and a synthetic drain layer. Vegetated roof covers in urban areas offer a variety of
benefits, such as extending the life of roofs, reducing energy costs and conserving
valuable land that would otherwise be required for stormwater runoff controls. Green
roofs have been used extensively in Europe to accomplish these objectives. Many
opportunities are available to apply this LID measure in older U.S. cities with stormwater
infrastructures that have reached their capacities.
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Green roofs are highly effective in reducing total runoff volume. Simple vegetated roof
covers, with approximately 3 inches of substrate can reduce annual runoff by more than
50 percent in temperate climates (Miller, 2000). Research in Germany shows that the 3-
inch design offers the highest benefit to cost ratio. Properly designed systems not only
reduce runoff flows, but also can be added to existing rooftops without additional
reinforcement or structural design requirements. The value of green roofs for reducing
runoff is directly linked to the design rainfall event considered. Design should be
developed for the storm events that most significantly contribute to CSOs, hydraulic
overloads and runoff problems for a given area.
2.4 Permeable Pavements
The use of permeable pavements is an effective means of reducing the percent of
imperviousness in a drainage basin. More than thirty different studies have documented
that stream, lake and wetland quality is reduced sharply when impervious cover in an
upstream watershed is greater than 10%. Porous pavements are best suited for low traffic
areas, such as parking lots and sidewalks. The most successful installations of alternative
pavements are found in coastal areas with sandy soils and flatter slopes (Center for
Watershed Protection, 1998). Permeable pavements allow stormwater to infiltrate into
underlying soils promoting pollutant treatment and recharge, as opposed to producing
large volumes of rainfall runoff requiring conveyance and treatment. Costs for paving
blocks and stones range from $2 to $4, whereas asphalt costs $0.50 to $1 (Center for
Watershed Protection, 1998).
2.5 Other LID Strategies
Another strategy to minimize the impacts of development is the implementation of rain
gutter disconnects. This practice involves redirecting rooftop runoff conveyed in rain
gutters out of storm sewers, and into grass swales, bioretention systems and other
functional landscape devices. Redirecting runoff from rooftops into functional landscape
areas can significantly reduce runoff flow to surface waters and reduce the number of
CSO events in urban areas. As long as the stormwater is transported well away from
foundations, concerns of structural damage and basement flooding can be alleviated. As
an alternative to redirection of stormwater to functional landscape, rain gutter flows can
be directed into rain barrels or cisterns for later use in irrigating lawns and gardens.
Disconnections of rain gutters can effectively be implemented on existing properties with
little change to present site designs.
Many strategies exist to reduce the amount of impervious surface in development areas.
Designing residential streets for the minimum required width needed to support traffic,
on-street parking and emergency service vehicles, can reduce imperviousness. Other
practices include shared driveways and parking lots, alternative pavements for overflow
parking areas, center islands in cul-de-sacs, alternative street designs rather than
traditional grid patterns and reduced setbacks and frontages for homes.
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3 EVALUATION OF LID EFFECTIVENESS
3.1 Hydrological Measures
Enhancements in site drainage from traditional stormwater control measures, such as
curbs and gutters that eliminate potential on-site flooding, often result in an increase in
surface runoff. These alterations can cause an increase in volume, frequency and velocity
of runoff flows, resulting in flooding, high erosion and a reduction in groundwater
infiltration, as well as a reduction in water quality and habitat degradation. Four
hydrological functions should be considered when investigating the effectiveness of LID
practices. The runoff curve number (CN), time of concentration, retention and detention.
LID techniques and the hydrological design and analysis components are represented in
(Table 2).
Table 2: Low Impact Hydrologic Design and Analysis Components (Coffman, 2000)
LID Practice
Flatten Slopes
Increase Flow Path
Increase Roughness
Minimize Disturbances
Flatten Slopes on Swale
Infiltration Swales
Vegetative Filter Strips
Disconnected Impervious Areas
Reduce Curb and Gutter
Rain Barrels
Rooftop Storage
Bioretention
Revegetation
Vegetation Presentation
Low Impact Hydrologic Design and Analysis Components
Lower Post-
Development
CN
X
X
X
X
X
X
X
X
Increase Tc
X
X
X
X
X
X
X
X
X
X
X
X
Retention
X
X
X
X
X
X
X
Detention
X
X
X
The runoff potential for a site is characterized by the runoff curve number or CN. One
method of measuring hydrological function on a developed site is to compare the pre and
post developed curve number. The CN method is used extensively in the analysis of
environmental impact and design rainfall-runoff hydrology. The curve number measures
a watershed or sub water shed's hydrological response and is determined based on soil
type, land cover and amount of impervious surfaces (Hawkins 1998). A detailed
evaluation of both proposed and existing land cover is the basis for determining the low-
impact development CN, which is a calculation of the potential for runoff at a
development site. One of the goals of LID is to design a system so that the post-
developed CN is as close as possible to the predevelopment CN for the site. Limiting the
percent of imperviousness is one technique to accomplishing this. The runoff coefficient,
which can be derived from the CN, calculates the percent of rainfall converted to runoff.
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The time of concentration (Tc) refers to the amount of time it takes for water to travel
from the most distant point to the watershed outlet. By retaining predevelopment Tc,
negative impacts associated with development can be reduced. Retention and detention of
rainfall are the key components of increases in Tc. As the amount of impervious surface
increases within a site, altering drainage paths, the contribution of total land area to
excess rainfall increases, causing the time for stormwater to reach downstream outlets to
decrease. This decrease in Tc reduces the pollutant removal capabilities of the site as well
as resulting in an increase in the peak runoff rate. Maintaining Tc can be achieved by:
- Maintaining flow path lengths
- Increasing surface roughness
- Detaining flows
- Minimizing disturbances at the site
- Flattening grades in impact areas
- Disconnecting impervious surfaces
- Connecting pervious surfaces
3.2 Pollutant Removal Measures
Changes in site runoff characteristics can contribute to a reduction in water quality and
degradation of aquatic and terrestrial habitats. LID practices provide a high level of water
quality treatment controls due to runoff volume control of the "first flush" (first 1A inch)
of runoff, which contains the highest pollutant loadings. Often LID practices control up
to the first 2 inches of runoff and therefore treat a much greater volume of annual runoff
(Coffman, 2000). By increasing the Tc and decreasing the flow velocity, LID practices
result in a reduction in pollutant transport capacity and overall pollutant loading. Further,
LID practices support pollution prevention by modifying human activities, which lower
the introduction of pollutants into the environment.
LID practices such as bioretention facilities or rain gardens can be used as a mechanism
for infiltration and pollutant removal, which is performed through physical and biological
treatment processes occurring in the plant and soil complex. These processes include
filtration, decomposition, ion exchange, adsorption and volatilization (Dept. of Env.
Resources, 1993). Pollutant loadings are concentrated in the "first flush" of runoff from
impervious surfaces and contain grease and oil, nutrients (nitrogen and phosphorous),
sediments and heavy metals. Pollutant loadings and water quality impacts from
development have been well documented in numerous studies. Concentrations of
pollutants are appropriate to look at bio affects, but pollutant loads are better for
assessing impacts to downstream habitats when cumulative effects are considered
(Rushton, 1999). Studies should consider investigating both total metals and dissolved
metals, when analyzing LID practice's effectiveness.
10
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4 CASE STUDIES
The LID "functional landscape" is designed to mimic the predevelopment hydrological
conditions through runoff volume control, peak runoff rate control, flow
frequency/duration control and water quality control. Determining effectiveness of LID
practices can be achieved by evaluating hydrological function and pollutant removal
capabilities. Little investigation has been done to prove the actual effectiveness of LID in
retaining predevelopment hydrology and preventing or reducing pollutant loadings
caused by stormwater runoff on developed sites. LID is a relatively new concept in
stormwater management and not widely implemented in all areas and climates in the
United States. Limited research and analysis has been conducted on the various practices,
due to this limited application.
The following case studies, though limited, represent the best examples of projects that
use LID concepts for stormwater management. Both hydrologic and pollutant removal
effectiveness are investigated. The most significant source for data is Prince George's
County, Maryland where many of the LID practices were developed and first
implemented. The Low-Impact Development Center, also located in Maryland, has done
significant work in design and planning of LID sites. First year data from a two-year
study of a Tampa, Florida, retrofit parking lot and an on-going permeable pavement
project in Washington state provide the only long term analysis for the effectiveness of
LID concepts (permeable pavements and swales) currently available.
11
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4.1 Bioretention Facility
Laboratory and Field Study
Beltway Plaza Mall Parking Lot, Greenbelt, MD
Introduction
Land development results in increased stormwater runoff at the expense of infiltration.
Additionally, surface runoff contains a broad range of pollutants and has been identified
as one of the major sources for pollution of natural waters. Detention basins are
commonly used for stormwater quality improvement and to optimize the infiltration of
stormwater for recharge. A simple, yet effective method to control stormwater is through
the use of bioretention areas or rain gardens.
Bioretention systems generally require less space, are more economical to build and
require less maintenance than large-scale detention ponds. In addition these landscaped
areas have aesthetic value. The design capacity for the system is generally for a typical
storm event (0.5-0.7 inches per hour of rainfall over six hours) and to handle runoff from
a small development area. The goal of this study is to compare field results with baseline
data obtained through a laboratory constructed and tested bioretention systems.
Study Site
This study was conducted in two phases. The first phase took place at the University of
Maryland, Department of Civil Engineering, Stormwater Lab in College Park, Maryland.
Two different-sized bioretention prototypes were constructed and fitted with ports at
varying depths in order to collect and analyze water quality and infiltration data. The
small prototype was 2.5 ft wide and 3.5 ft long with a depth of 24 inches of material. The
small bioretention system was fitted with two port depths. The large prototype was 10 ft
long, 5 ft wide with a depth of 36 inches, and was fitted with three ports at various depth
levels. Both systems had a freeboard of 6 inches, to allow water to accumulate if
necessary. The soil, organic mulch layer and vegetation, were analyzed prior to
construction to assure that the system was constructed according to design
recommendations. Simulated runoff was applied to both systems at a rate of 1.6 inches
per hour for six hours. A total of 16 simulations were tested on the small box, and four on
the large prototype. The total volume of runoff applied to the small system was 200 L,
and 1,000 L for the large system. These volumes represent the bioretention prototypes
occupying 5% of a drainage area.
The second phase, a field study, took place at an existing bioretention facility located in
the parking lot of Beltway Plaza in Greenbelt, Maryland. The depth of the system is 42"
and is designed so that runoff infiltrates through the system and is collected by a 6-inch
diameter perforated pipe underdrain, which feeds into the main storm drain system. A
7.5-ft x 7.5-ft area of the bioretention facility was used to conduct the study.
Approximately 1,000 L of synthetic runoff, with characteristics similar to those used in
the laboratory, were applied to the system over a 6-hour period. Effluent samples were
collected from the main storm drain at 25-30 minutes intervals.
12
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Study Results Summary
The laboratory results for the smaller prototype showed overall that the removal of heavy
metals by the system was good. Cooper, lead and zinc levels in both upper and lower
effluents had removal of more than 90%. Copper removal from samples taken from both
ports was 94%. Lead removal was more effective from lower ports at 98%, but still good
from upper ports at 94%. The average zinc removal from upper and lower ports was
>96% (Table 3). No major variation of removal of metals occurred over time and all
samples were less than EPA standards for freshwater. Nutrient removal for phosphorous
was 65-75% from lower ports and approximately 40% from upper ports. The Total
Kjeldahl Nitrogen (TKN) removal is 45-60% for the upper ports and 65-80% for the
lower ports. Ammonium and nitrate removal followed no pattern and ranged from zero to
90%.
Table 3: Summary of Results for Smaller System—Standard Conditions
Removal
Upper
Removal
Lower
Cu
94%
94%
Pb
94%
98%
Zn
97%
98%
P
25%
83%
TKN
55%
80%
NH4+
60%
83%
N03-
11%
26%
Tn
60%
75%
Results from the large prototype correlated with those of the smaller constructed system.
Experimental results indicated that removal of metals in most cases was more than 90%.
Average copper removal for upper ports was 90% and 93% for middle and lower ports.
Lead removal from upper ports was 93%, and >97% for middle and lower ports. The
removal of zinc was 87% for upper ports and >96% for middle and lower ports. The data
showed a trend of greater metal removal with depth. Nutrient removal was better from
lower ports in most cases compared to removal of middle and upper ports. Phosphorous
removal for lower ports was about 70-80% and 50-60% for middle ports. The upper ports
showed a 10-15% increase in phosphorous levels above the influent amounts. The TKN
removal was 50-75% for the lower and middle ports and a 45-30% increase was noted for
upper ports. Removal of ammonium was 54% at upper ports, 86% for middle ports and
79% at lower ports (Table 4). Doubling or halving the influent pollutant levels during the
laboratory testing had little effect on the effluent pollutant levels. Higher levels of
phosphorous and TKN in effluent at the upper ports can be attributed to the vegetation.
Table 4: Summary of Results for Large System—Standard Conditions
Removal
Upper
Removal
Middle
Removal
Lower
Cu
90%
93%
93%
Pb
93%
>97%
>97%
Zn
87%
>96%
>96%
P
0%
73%
81%
TKN
37%
60%
68%
NH4+
54%
86%
79%
NCV
(-97%)
(-194%)
23%
TN
(-29%)
0%
43%
13
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During the field test at Beltway Plaza, a total of 1,000 L of synthetic runoff were applied
to the bioretention area over a 6 hour period at a rate of approximately 0.5 inches per
hour. Of the 1,000 L of influent, only 39% left the system. The remaining water leaked
through cracks into the manhole, was held in the facility, or infiltrated. Effluent samples
were analyzed for removal of nutrients and heavy metals (copper, lead and zinc).
The TKN removal was about 50% and the phosphorous removal was observed at
approximately 65%. Nitrate concentrations were below input levels, with a removal of
about 17%. The removal for ammonia was very good at >95%. Removal of metals was
very good and was consistent with the laboratory results. The removal of copper was
97% and for lead, and zinc, the removal was >95% (Table 5).
Table 5: Summary of Results for Field Bioretention Study
Removal
Cu
97%
Pb
>95%
Zn
>95%
P
65%
TKN
52%
NH4+
92%
NCV
16%
TN
49%
Removal rates for the field study corresponded with the rates observed for the two
laboratory constructed bioretention systems. In all cases pollutant removal rates
approached 100% for the metals copper, zinc and lead. Doubling or halving the
concentration levels of the influent had no effect on removal efficiencies and were
statistically equivalent in nearly all cases. Pollutant removal rates for all systems are
compared in the above graph (Figure 3). The negative removal rate for nitrate in the large
prototype, upper and middle ports, was attributed to the release of previously captured
nitrated or nitrate from nitrification processes.
150
1 nn
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r-ft
p bu
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on
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Figure 3: Pollutant Removal Rates for All Systems
14
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4.2 Bioretention Facility
Field Study
Peppercorn Plaza Parking Lot at Inglewood Center, Landover, MD
Introduction
Impervious surfaces, such as parking lots, are a major contributor to pollutant loads in
receiving waters in urban areas. These surfaces provide a place for pollutants to
accumulate and later wash-off in the first flush of rainfall events. Parking lots are good
site locations for bioretention systems, since they can be retrofit into existing lots with
little or no loss of parking space. In addition, patrons have expressed appreciation of
green space within parking areas. Bioretention areas are a natural means of controlling
pollutants from entering urban water bodies. The hydrologically functional landscape,
can be used as a mechanism for pollutant removal, through physical and biological
treatment processes occurring in the plant and soil complex. The bioretention area in the
Inglewood Center Parking lot, was analyzed for pollutant removal efficiency during a
simulated rainfall event.
Study Site
The study was conducted at one of the two bioretention areas in the Inglewood Plaza
parking lot. An area of 50 ft was used in the south facility for the simulated rainfall
event. The bioretention facility contains a T-shaped under drain that runs the entire length
of the system and is located 32.5 inches below the surface (Figure 4). The under drain
directly connects with the storm drainage system. Samples were collected from a pool of
water in the storm drain observation area. Output samples were collected every 30
minutes. The soil was dry at the onset of the experiment, due to lack of rainfall for a
period of several days prior to the experiment. The synthetic rainfall was applied at a rate
of 1.6 inches per hour for a duration of six hours. A total of 300 gallons (1100L) was
applied over the course of the experiment.
15
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Observation Well
Observation Well
Runoff Application Area 32 J"
'-\
Figure 4: Bioretention System at Peppercorn Place, Inglewood Plaza (Davis, 1999)
16
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Project Results Summary
Effluent concentrations for metals were fairly constant over the sampling period, with
zinc being the exception by showing improved removal over time. Average removals for
total copper was 43%, total lead was 70% and total zinc 64%. The removals were 5-14%
better for dissolved metals. Nutrient concentrations were all below input levels. Removal
of phosphorous was very good at 87%. Removal of TKN was observed at 67% and
nitrate averaged 15% (Table 6). Ammonium was not detected in either the influent or the
effluent. In addition, the bioretention facility removed some calcium, however chloride
concentrations were higher in the effluent than in the influent, which is attributed to
salting of the parking lot in the winter. Also, temperature variations during the
experiment showed evidence of the system cooling the runoff water temperature.
Table 6: Summary of Pollutant Removal Results of Bioretention System at Inglewood Place
Removal
Cu
43%
Pb
70%
Zn
64%
Ca
27%
P
87%
TKN
67%
NCV
15%
By using synthetic runoff, the concentrations of applied pollutants could be controlled
and accurately measured and compared to levels found in the effluent. However, testing
has not been done on an actual rainfall event to determine effectiveness of the system for
reducing runoff volume and pollutant loads.
17
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4.3 Permeable Pavements and Swales
Field Study
Stormwater Management, Florida Aquarium Parking Lot, Tampa, FL
Introduction
Impervious surfaces are responsible for more Stormwater runoff than any other type of
land use. Paved surfaces that often replace vegetated areas increase the volume and
frequency of rainfall runoff. In addition, these surfaces provide a place for pollutants to
accumulate between rainfall events, and are later washed off into receiving waters.
Keeping runoff on-site to allow for infiltration as well as chemical, physical and
biological processes to take place is the most effective means of reducing pollutant
loadings. This study quantifies how much runoff and pollutant loadings can be reduced
by using swales and landscaped depressions in parking lots. In additional to investigating
basins with and without swales, three paving surfaces were compared. The research is
designed to determine pollutant load reductions measured from three different treatments
within the parking lot; different paving materials in the parking lot, a planted strand with
native trees and a small pond used for final treatment. Pollutant concentrations and
infiltration were measured and analyzed for the various control methods. First year data
collected in the parking lot between August 1998 and August 1999 were evaluated for
this study. Also, sediment samples were collected from each of the swales, two locations
in the strand and two locations in the pond.
Project Area
The study site is a parking lot at the Florida Aquarium in Tampa, Florida. The study uses
the entire parking area, 4.65 ha, to define the drainage basin. The parking lot was
modified for the study by reducing the length of each parking space by 61 centimeters,
which allows for a 122-cm wide grass swale between rows. The vehicle front end now
hangs over a grass swale instead of pavement, which prevented any reduction in the
number of parking spaces within the parking area. Four different scenarios were
investigated to determine the most efficient method of runoff reduction and pollutant
removal. Eight basins, two of each type, were constructed and fitted with instrumentation
to collect flow weighted water quality samples and measure discharge amounts during
storm events (Figure 5). The four treatment types are:
- Asphalt paving with no swale
- Asphalt paving with a swale
- Cement paving with a swale
- Permeable pavement with a swale
Rainfall quality and volume were compared to runoff quality and volume to determine
the effectiveness of each treatment type.
18
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THE FLORIDA AQUARIUM SITE PLAN
LEGEND
• Outfall
Into Underground Pipe
^ Out of Underground Pipe
O Sample Location
Figure 5: Florida Parking Lot Study Site (Rushton, 1999)
Project Results Summary
The larger garden areas (approximately the size of one parking space) account for a
runoff coefficient calculation reduction of 40-50 percent for the smaller basins. The
runoff coefficient is a value that ranges from zero to one and expresses the fraction of
rainfall volume that is actually converted into storm runoff volume. The runoff
coefficient closely tracks percent impervious cover. For rainfall events less than 2 cm,
basins with swales and permeable pavement have 80-90% less runoff than basins without
swales, and 60-80% less runoff than basins with the other pavement types and swales.
The percent of rainfall converted to runoff for each treatment type is shown in Figure 6.
Larger rainfall amounts show fewer differences in runoff amounts between the different
pavement types, but basins with swales have approximately 40% less runoff than the
basins without swales. Soil analysis at the site shows a higher than average gravel content
(8.9%) which may account for the good infiltration rates. Comparisons of rainfall with
storm runoff amounts showed that swales reduced runoff for all rainfall events and
paving types.
Water quality analysis shows that average concentrations varied by paving and
depression storage types. Rainfall has been identified in other studies as a significant
source of nitrogen in runoff. This site displayed the same correlation between
19
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concentrations of ammonia and nitrate in rainfall and their concentrations in runoff.
Phosphorous concentrations displayed the inverse, since concentrations were higher in
effluent samples than in the initial rainfall. The levels were somewhat higher in the runoff
of basins without planted swales and the highest concentrations of phosphorous were
noted in basins where runoff traveled through grassed swales.
Asphalt no swale
o; Asphalt w/swale
Q.
£
O)
4-1
(D
c
<~ Cement w/swale
Permeable w/swale
0% 10% 20% 30% 40% 50% 60%
Percent of Rainfall Volume Converted to Runoff Volume
Figure 6: Percent of Rainfall Volume Converted to Runoff Volume for Events Less Than 2cm
Paving material showed an effect on the concentration of metals in runoff. Basins paved
with asphalt showed higher concentrations of iron, manganese, lead, copper and zinc than
those paved with cement or permeable paving. Many of the major ions also showed a
correlation with the paving material. Potassium, sodium, sulfate and calcium
concentration were much higher in the basins paved with cement, which is made from
limestone, although these levels were still well below levels considered detrimental to the
environment. No consistent pattern was discernable for suspended solids, but generally
measurements were low when compared to similar stormwater studies.
Water quality loads were examined because they provide a more realistic measure for
understanding the impacts of stormwater on receiving waters. Pollutant loads include
both the volume of water discharged and the concentration of pollutants measured.
Higher loads for all constituents, except phosphorous, were noted for basins without
swales, since more water was discharged from these basins. Although phosphorous
concentrations were much lower in basins without swales, loads were about the same.
20
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Removal for Ammonia was 45% for asphalt with swale, 73% for cement with swale and
85% for permeable pavement with swale. Total nitrogen removal was 42% for permeable
pavement with swale, 16% for cement with swale and 9% for asphalt with swale. TSS
removal varied from 91% for permeable pavement with swales to 46% for asphalt with
swales.
Table 7 summarizes the constituent load efficiency of the various treatments. The
concentrations and loads measured during this study were compared to other stormwater
studies conducted in Florida, and the values were much lower than measured values at
other sites. Metal removal was good for the permeable pavement with swale treatment,
with copper at 81%, iron 92%, lead 85%, manganese 92% and zinc 75%. The removals
for the cement with swale treatment were somewhat lower, with the asphalt with swale
treatment showing the poorest performance of the three treatments with swales.
Table 7: Summary of Pollutant Removal Efficiency for the Various Treatment Types
Constituent
Ammonia
Nitrate
Total Nitrogen
Ortho Phosphorus
Total Phosphorus
Suspended Solids
Copper
Iron
Lead
Manganese
Zinc
Asphalt with swale
45%
44%
9%
-180%
-94%
46%
23%
52%
59%
40%
46%
Cement with swale
73%
41%
16%
-180%
-62%
78%
72%
84%
78%
68%
62%
Permeable with swale
85%
66%
42%
-74%
3%
91%
81%
92%
85%
92%
75%
The concentrations of metals in sediment samples collected in swales were consistent
with concentrations measured in stormwater runoff. Higher concentrations of metals were
found in swales paved with asphalt than those of grass. None of the metals measured in
the sediments exceed the level where toxicity to organisms is probable when compared to
the Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric
Administration (NOAA) chemical toxicity guidelines for marine environments. However,
copper and zinc concentrations were above the level where toxicity is possible.
Nutrient concentrations measured in sediment samples for TKN and total phosphorus
were lower in the basins without grassed swales. Sediment samples taken from locations
in the strand and the wet-detention pond were compared to swale samples. The
comparison showed that most of the metals are being settled out in the swales or
21
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deposited in the drop boxes. Sediment samples at the site were tested for 100 organic
pollutants, but only 16 were detected at the site. The high concentrations found in this
and similar studies indicate that atmospheric deposition is the source for most of the 16
detected organic pollutants.
22
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4.4 Vegetated Roof Covers
Field Study
Green Rooftop, Philadelphia, PA
Introduction
Many older American cities are plagued with nuisance flooding on roads and walkways
and chronic overflows of combined sewer systems. In highly impervious cities, vegetated
rooftops offer a practical solution for controlling runoff at the source. A vegetated roof
cover is a veneer of living vegetation installed on top of a conventional roof. By
mimicking natural hydrologic processes, they can achieve runoff characteristics similar to
open space conditions. Green roofs are comprised of three components; subsurface
drainage, growth media and vegetation. Specific hydraulic performance objectives are
achieved through the appropriate selection of these components. Vegetated roof covers
have been used extensively in Germany for 25 years.
Project Area
A 3,000-ft2 rooftop in Philadelphia was fitted with a demonstration vegetated rooftop.
The performance objective was the restoration of predevelopment runoff peak rates for a
24-hour, 2-year return-frequency storm. Although in the Philadelphia area, 90% of all
rainfall is contributed by storms with volumes of 2 inches or less over a 24-hour period.
The "green roof used is only 3.4 inches (8.6cm) thick, including the drain layer
(Figure 7). Its maximum saturated weight is less than 17 lb/ft2 and it weighs less than
51b/ft2 when dry. No additional structural support was necessary for installation. The
saturated infiltration capacity is 3.5 inches per hour. The key features of this system are a
synthetic under drain layer which promotes rapid water drainage from the roof surface,
thin, lightweight growth media suitable for installation on existing roof surfaces and a
meadow-like setting of perennial Sedum varieties selected for hardiness and the ability to
withstand seasonal conditions typical of the area.
Vegetative Layer
Media
Geotextile
Synthetic Drain Layer
Figure 7: Structure of the Philadelphia Vegetated Roof Cover (Miller, 1998)
23
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Project Results Summary
Currently too few storms have been observed to permit quantitative assessment of the
vegetative covered roof. Data are available from one intense storm monitored during a
0.4 inch, 20-minute rainfall event (Figure 8). Supplemental data from a pilot-scale
experimental station were used in this study. Test data show that for storms with less than
0.6 inches, runoff is negligible. During a 9-month period, 44 inches of rainfall was
recorded at the pilot-scale test station, with only 15.5 inches of runoff generated. Runoff
occurred for precipitation events between 0.6 and 1.0 inches, but lagged rainfall
significantly. Attenuation was lower for the pilot-scale experiment than the anticipated
modeled value (40% vs. 48%), which has been attributed to differing drain conditions
and a steeper slope at the test site. Additional benefits of this project include extended life
of the underlying roof materials, reduction of energy costs by improving effectiveness of
insulation and restoration of ecological aesthetic value of open space in densely
populated areas.
10
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0.20 n
0.18 -
n 1 fi
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01 A.
.14
0.12 -
01 n
.1U
0.08 -
Or\£\
Or\A
0.02 -
n nn
D Rainfall
D Runoff
ru ,_,_, m rh r-n
-
-
-i
-
n n n n
5.0 mm
2.5 mm
Time (5-minute intervals)
Figure 8: A Rainfall Event of 0.4 inches with Media Completely Saturated (Miller, 1998)
24
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4.5 Permeable Pavements
Field Study
Permeable Pavements for Stormwater Management, Olympia, WA
Introduction
This study demonstrates the use of permeable surfaces for reducing runoff volume,
improving infiltration and reducing pollutant loadings in an urban parking area.
Numerous problems associated with urbanization, such as flooding, channel erosion and
destruction of aquatic habitats are directly linked to the loss of water-retaining function of
soil in urban landscape. As imperviousness increases, a Stormwater runoff reservoir of
tremendous volume is removed. Water that may have lingered in this reservoir for
anywhere from a few hours to many weeks now flows rapidly across land surfaces and
arrives at stream channels in short, concentrated bursts. The scope of this project was to
review existing information on types and characteristics of permeable pavements,
construct and monitor a full-scale test site and evaluate long-term performance of these
systems. This study of permeable pavements evolved from a growing recognition of the
limitations of traditional Stormwater management in keeping water in the soil by allowing
excess of water to the soil over large areas of landscape.
Study Site
The study site is an employee parking lot on the southeast corner of the King County
Public Works facility in Renton, Washington. The permeable pavement sections of the lot
were constructed for the purpose of this study. A total of eight stalls using four different
pavement types were constructed. In addition a ninth stall of traditional asphalt was used
as a control. The parking stalls are fitted with pipe, gutters and gauges to collect and
measure the quantity and quality of storm runoff from each pavement type. Subsurface
troughs were constructed down the middle of each stall and imbedded into the subgrade
six to 8 inches below the surface. This allows for the collection of only a fraction of the
infiltrated water (about 1.8%). The permeable pavement types studied were:
- A plastic network with grass infilling (<5% impervious)
- An equivalent plastic network with gravel infilling (<5% impervious)
- Impervious blocks with grass infilling (-60% impervious)
- Impervious blocks with gravel infilling (-90% impervious)
Project Results Summary
Data used to monitor the various permeable pavements were from three different storm
events during the autumn of 1996. The volume of runoff generated from cement blocks
with 60% impervious surface stalls and runoff from traditional asphalt are compared
(Figure 9). The storm had a fairly uniform distribution of rainfall (4mm per hour)
throughout the duration of the event. Rain falling on the asphalt yielded sharp hydrograph
25
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peaks and a high total volume of runoff water. Only about one peak per hour (0.03mm
per hour of runoff) was recorded for the cement blocks with 60% impervious surface.
These data are representative of data gathered at the other stalls and reflect little or no
runoff from the permeable pavement stalls.
1.00
0.80
c
o
'f
~
Q.
0.60
C
Q.
E
•=• 0.40
fc
o
c
D
cc
0.20
0.00
Asphalt
60% impervious concrete
block w/grass
' 1 I
' I /
' \ I
/A A A
0.00
200.00
400.00
600.00
Time (minutes)
Figure 9: Surface Runoff from 60% Impervious Pavement vs. Asphalt (Booth, 1996)
In contrast to surface runoff, subsurface flow generally responds more slowly and more
uniformly. The data for a storm of short duration and moderate intensity are represented
in the following graph (Figure 10). Individual peaks on the bar graph indicate rainfall
rates as high as 14mm per hour, lasting for short durations (15-minute intervals). Runoff
gauges on all four systems showed virtually no surface runoff (on average 0.03 mm). It
displays a characteristically attenuated discharge peak and lagged response to the rainfall
inputs. All pervious surfaces responded similarly. For the asphalt surface, the volume of
water running off the asphalt responded quickly to changes in the rate of rainfall. This is
indicated by high peak flows corresponding with precipitation amounts, with little lag
time noted (Figure 11).
26
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4.00-j
3.00 —
0)
Q.
E
.g
8=
O
2.00-
1.00—
Subsurface Runoff
Precipitation
i— 0.16
— 0.12
O
=s
- 0.08
— 0.04
0.00 -| \_v_l_l ! 1 I I I 1—0.00
0 200 400 600
Time in minutes
Figure 10: Subsurface Runoff From Pavement Less Than 5% Impervious Compared to
Precipitation (Booth, 1996)
1.60 1
1.20
0.80
0.40 —
0.00-
l— 0.06
Asphalt
Precipitation
10
20
Time in hours
40
Figure 11: Surface Runoff From Asphalt Compared to Precipitation (Booth, 1996)
27
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Water quality results were obtained from samples collected directly from tipping bucket
gauges. Only five samples from the four subsurface collection troughs and the asphalt
surface runoff were analyzed. Chemical analysis of the subsurface samples showed sub-
detection levels for many of the constituents and relatively low levels for all tested
compounds. Measured concentrations of common metals (copper, lead, zinc aluminum
and iron) were substantially below the reported national averages. Subsurface samples
did show slightly higher concentrations than runoff, which can be attributed to the
troughs collecting the "dirtiest" 2 percent of runoff, from directly under where vehicles
park. Still, these concentrations were below typical values seen in urban runoff.
28
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4.6 Grass Swales
Field Study
Highway Grass Channels, Northern Virginia, Maryland, and Florida
Introduction
The U.S. Department of Transportation, Federal Highway Administration conducted a
field study to determine the pollutant removal efficiencies of grassed channels and swales
along highways in Northern Virginia, Maryland and Florida. Sampling was conducted at
the inflow and outflow areas of the channels, which provided data for quantity and
quality of waters entering and leaving the channels. The samples were analyzed for the
following pollutants:
- Total Suspended Solids (TSS)
- Heavy Metals (cadmium, copper, lead and zinc)
- Nitrogen (Total Kjeldahl Nitrogen and nitrite/nitrate)
- Total Phosphorus
- Total Organic Carbon
Twelve rainfall events were monitored, including both frequent and infrequent rainfall
periods, most involving discrete stormwater runoff events following a minimum of two
days of dry weather. In addition continuous rainfall periods of seven to 14 days were
included to determine overall removal efficiencies.
Project Area
The test area in northern Virginia is located along 1-66. The channel has an average slope
of 4.7% with a total drainage area of 1.27 acres (0.51 ha). Stormwater enters the channel
indirectly, by means of overland flow. Stormwater data were collected from June 13,
1987 through November 12, 1987. The test site in Maryland is a grass channel located
alongside 1-270. This channel has a slope of 3.2% and a total drainage area of 1 acre
(0.40 ha) with stormwater entering by means of overland flow. Data were collected for
the period beginning June 18, 1987 and ending mid-September 1987. The Florida test site
is a grass channel median located between the East and West lanes of 1-4. The Florida
grass channel has a lower slope than the other two test sites with a drainage area of 0.56
acres (0.23 ha). Data collection began at this site on February 25, 1988 and ended on
October 31, 1988.
Project Results Summary
All three locations showed some effectiveness with regard to pollutant removals,
although results varied depending on the method of analysis and the location. The results
for all three locations are represented in Table 8. Sediment core samples were obtained
from the channels and compared to samples from adjacent, upland areas, to determine
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pollutant removal effectiveness of the grass channels. Based on the data from the analysis
the following conclusions were made. Removal of metals appears to be directly related to
the removal of TSS, whereas nutrient removal is not. Removal of TSS can be estimated
using flow depth and travel time relationships. Relatively low nutrient removal may be
observed in channels that are effective in removing other pollutants. The controlling
factors in pollutants removal of grass channels are length, channel geometry, channel
slope and average flow. Both metals and nutrients are removed in grass channels, but
metal removal is more reliable.
Table 8: Long Term Pollutant Removal Estimates for Grassed Swales
VA
MD
FL
TSS
65%
-85%
98%
TOC
76%
23%
64%
TKN
17%
9%
48%
N02/N03
11%
-143%
45%
TP
41%
40%
18%
Cd
12-98%
85-91%
29-45%
Cr
12-16%
22-72%
51-61%
Cu
28%
14%
62-67%
Pb
41-55%
18-92%
67-94%
Zn
49%
47%
81%
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5 CONCLUSION
Pollutant loading reduction data for bioretention systems are promising in that removal
percentages for heavy metals and nutrients seem quite high. Generally, the experimental
data show a fairly consistent removal rate for all of the tested bioretention systems for
heavy metals and most nutrients (Table 9). Field study results support the laboratory
baseline data collected by the University of Maryland, College Park. However, the field
studies provide data for single, simulated rainfall events using synthetic rainfall. A larger
number of sampled events would be required for statistical validity of the results.
Table 9: Pollutant Removal Efficiencies for Laboratory and Field Bioretention Studies
Pollutant
Pb
Cu
Zn
P
TKN
NH4+
NCV
TN
Laboratory
(small)
93-97%
91-97%
93-98%
16-83%
55-80%
<0 -83%
11-26%
60-75%
Laboratory
(large)
93-97%
90-93%
87-96%
0-81%
37-68%
54 -86%
<0-23%
<0^3%
Beltway Plaza
>95%
97%
>95%
65%
52%
92%
16%
49%
Inglewood Plaza
70%
43%
64%
87%
67%
N/A
15%
N/A
The use of synthetic runoff during the bioretention experiments, both in the lab and field,
allowed the concentrations of applied pollutants to be controlled and accurately
measured, so that influent and effluent levels could be compared. In addition, infiltration
could be determined based on the volume of runoff verses volume input. The statistical
analysis applied for the mass loadings was sound. However, testing for these studies has
not been conducted for any actual rainfall events to determine effectiveness of the system
for reducing runoff volume and pollutant loads. A comparison of average pollutant
removal efficiencies is shown in Figure 12.
The grass swale data from the Federal Highway study show trends in removal of metals
as they relate to TSS removal for three different areas in the United States. However a
short study period, using data from only a few storm events, is used to quantify the
results. Additional data from numerous storm events would be required to provide
statistical validity to the analysis. The data from additional, less extensive studies
conducted by the University of Virginia help to validate the highway data, as pollutant
loading removal rates and runoff volume reduction rates were fairly consistent between
the two studies. Conclusions drawn from both studies indicate that not only length, but
also longitudinal slope and the presence of check dams increase the pollutant removal
capabilities (Kuo, 1999).
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Laboratory (small)
Laboratory (large)
Beltway Plaza
Inglewood Plaza
& V x ^ oK tf ^
^ ^ ^
Constituent
Figure 12: Pollutant Removal Rates for Laboratory and Field Experiments of Bioretention Systems
In addition, a study conducted in Ontario, Canada concluded that no evidence existed to
show that nutrient or metal concentrations in soils increased with age in grass swales, as
concentrations varied regardless of age. Also, the Canadian study determined that no
degradation in vegetative quality resulted from continuous exposure to stormwater
runoff. It was shown that vegetation quality was similar to what would be found along
conventional systems (Sabourin, 1999). The Canadian study also showed that total runoff
volumes from grassed swales were 6-30% less than conventional systems and that a
loading comparison revealed that the system released significantly less pollutants than
conventional systems.
Permeable pavements can reduce the percent imperviousness for urban areas, which
allows for greater infiltration rates and reduced runoff volumes. In addition these
alternate pavement types function as stormwater pollutant removal mechanisms.
Preliminary data from the Washington project show effectiveness, but too few storms
have been analyzed. Only the Florida Aquarium parking lot data represent an analysis of
a significant number of actual storm events. As the study continues, and second year data
become available, more compelling proof of the pollutant removal effectiveness and
runoff volume reduction can be realized. The methodology for testing runoff volume
reduction and mass pollutant loadings in the Florida study provided reliable data.
Extensive data exist that show runoff volume reduction using vegetated roof covers in
Europe, especially Germany. The data are specific to temperate climates and results may
vary considerably for other areas in the United States. However, the Philadelphia project
shows the benefits of this application in reducing runoff volume by reducing the level of
imperviousness in urbanized areas. Further, it demonstrates the capacity for retrofit of
green roofs in highly impervious, older, urbanized U.S. cities experiencing chronic CSO
problems. Little data are available from this demonstration project. However, with
continued monitoring, evidence of the suitability of green roofs in the United States may
become more apparent.
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6 RECOMMENDATIONS
A detailed comparison of pre- and post-development conditions and an analysis of
adjacent areas using traditional stormwater controls and LID practices side-by-side,
would provide the best possible assessment of LID effectiveness hydrologically and as a
mechanism for reducing pollutant loadings. The Jordan Cove Urban Watershed project in
Waterford, Connecticut, is currently under construction for a side-by-side analysis,
however, no data are available at this time. Baseline predevelopment hydrological data
are currently being collected for comparison once the development is completed and
monitoring begins.
Most of the current field data available for bioretention facilities are for single, simulated
rainfall events. Fitting the existing, tested bioretention areas in Prince George's County
with monitoring equipment and running a significant number of tests on actual rainfall
events over 9 months to 1 year, would provide higher quality data. Long term studies
would prove or disprove the long-term effectiveness of bioretention systems, as well as
provide information on trends in soil fertility lifetimes and trends in reduced capabilities
over time. The two-year Florida Aquarium study is currently the best possible source for
these data.
The majority of case studies cited above are ongoing investigations, and reported data
represent preliminary findings. Follow-up on these studies will provide better support for
proof of effectiveness of LID practices. Additional studies testing LID practices should
be identified as the use of these practices grows. Preliminary findings should be viewed
as a starting point, and not the empirical proof of effectiveness for the various LID
practices studied. The development of a database for entry and storage of LID study data
could provide a useful tool for future investigation of LID effectiveness.
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7 REFERENCES
Blue Land, Water and Infrastructure. 2000. Website: www.blwi.com/n_fall99.htm
Booth, Derek B., Leavitt, Jennifer and Peterson, Kim. 1996. The University of
Washington Permeable Pavement Demonstration Project, Background and First-
Year Field Results. Center for Urban Water Resources Management, Department
of Civil Engineering.
Center For Watershed Protection. 1998. Better Site Design: A Handbook for Changing
Development rules in Your Community.
Coffman, Larry. 2000. Low-Impact Development Design Strategies, An Integrated
Design Approach. EPA 841-B-00-003. Prince George's County, Maryland.
Department of Environmental Resources, Programs and Planning Division.
Davis, Allen P., Shokouhian, Mohammad, Sharma, Himanshu and Minami, Christie.
1998. Optimization of Bioretention Design For Water Quality and Hydrologic
Characteristics.
Project No. 01-4-31032. University of Maryland, College Park, Department of Civil
Engineering.
Davis, Allen P. and Minami, Christie. 1999. Evaluation of Pollution Removal
Characteristics at Bioretention Facilities At Peppercorn Place. Project No. 01-4-
33173. University of Maryland, College Park, Department of Civil Engineering.
Hawkins, Richard H. 1998. Local Sources for Runoff Curve Numbers. Paper Presented at
the 11th Annual Symposium for the Arizona Hydrological Society, Tucson, AZ.
Kuo, Jan-Tai, Yu, Shaw L., Passman, Elizabeth and Pan, Henry. 1999. Field Test of
Grassed Swale Performance in Removing Runoff Pollution. Paper Presented t the
19999 26th Annual Water Resources Planning and Management Conference
(ASCE), Published in Conference Proceedings.
Miller, Charlie. 1998. Vegetated Roof Covers, A New Method for Controlling runoff in
Urbanized areas. Proceedings from the 1998 Pennsylvania Stormwater
Management Symposium, Villanova University.
Miller, Charlie. 2000. Personal Communication. Roofscapes, Inc.
Prince George's County, Department of Environmental Resources. 1993. Design Manual
For Use of Bioretention in Stormwater Management, Prince George's County,
Maryland. Department of Environmental Resources, Division of Environmental
Management, Watershed Protection Branch.
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Rushton, Betty. 1999. Low Impact Parking Lot Design Reduces Runoff and Pollutant
Loads. Annual Report # 1. Aquarium Parking Lot, Tampa, Florida.
Sabourin, J. F. and Associates Inc. 1999. Performance Evaluation of Grass Swales and
Perforated Pipe Drainage Systems. Research Project, Executive Summary.
Ontario Ministry of the Environment.
United States Environmental Protection Agency. 1997. Urbanization and Streams:
Studies of Hydrologic Impacts. 841-R-97-009
USDOT. 1996. Retention, Detention, and Overland Flow for Pollutant Removal from
Highway Storm water Runoff, volume I: Research Report. U.S. Department of
Transportation, Federal Highway Administration.
Weinstein, Neil. 2000. Personal Communication, Director: Low Impact Development
Center.
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