&EPA
United States
Environmental Protection
Agency
2012 GREEN INFRASTRUCTURE TECHNICAL ASSISTANCE PROGRAM
Pittsburgh UNITED
Pittsburgh, Pennsylvania
Addressing Green Infrastructure Design Challenges
in the Pittsburgh Region
Clay Soils
Photo: Rain Garden at Edgewood Train Station
Source: Nine Mile Run Watershed Association
January 2014
EPA 800-R-14-004
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About the Green Infrastructure Technical Assistance Program
Stormwater runoff is a major cause of water pollution in urban areas. When rain falls in undeveloped
areas, the water is absorbed and filtered by soil and plants. When rain falls on our roofs, streets, and
parking lots, however, the water cannot soak into the ground. In most urban areas, stormwater is
drained through engineered collection systems and discharged into nearby waterbodies. The
stormwater carries trash, bacteria, heavy metals, and other pollutants from the urban landscape,
polluting the receiving waters. Higher flows also can cause erosion and flooding in urban streams,
damaging habitat, property, and infrastructure.
Green infrastructure uses vegetation, soils, and natural processes to manage water and create healthier
urban environments. At the scale of a city or county, green infrastructure refers to the patchwork of
natural areas that provides habitat, flood protection, cleaner air, and cleaner water. At the scale of a
neighborhood or site, green infrastructure refers to stormwater management systems that mimic
nature by soaking up and storing water. These neighborhood or site-scale green infrastructure
approaches are often referred to as low impact development.
EPA encourages the use of green infrastructure to help manage stormwater runoff. In April 2011, EPA
renewed its commitment to green infrastructure with the release of the Strategic Agenda to Protect
Waters and Build More Livable Communities through Green Infrastructure. The agenda identifies
technical assistance as a key activity that EPA will pursue to accelerate the implementation of green
infrastructure.
In February 2012, EPA announced the availability of $950,000 in technical assistance to communities
working to overcome common barriers to green infrastructure. EPA received letters of interest from
over 150 communities across the country, and selected 17 of these communities to receive technical
assistance. Selected communities received assistance with a range of projects aimed at addressing
common barriers to green infrastructure, including code review, green infrastructure design, and cost-
benefit assessments. Pittsburgh UNITED was selected to receive assistance developing fact sheets and
technical papers to provide solutions for site conditions that are perceived to limit green infrastructure
applicability.
For more information, visit http://water.epa.gov/infrastructure/greeninfrastructure/gi support.cfm.
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Acknowledgements
Principal EPA Staff
Kenneth Hendrickson, USEPA Region 3
Dominique Lueckenhoff, USEPA Region 3
Christopher Kloss, USEPA
Tamara Mittman, USEPA
Community Team
Jennifer Rafanan Kennedy, Clean Rivers Campaign
Sara Powell, Nine Mile Run Watershed Association
Consultant Team
Dan Christian, Tetra Tech
Valerie Novaes, Tetra Tech
Anne Thomas, Tetra Tech
Technical Review Team
Beth Dutton, 3 Rivers Wet Weather
Kari Mackenbach, URS Corporation
Jim Pillsbury, Westmoreland Conservation District
This report was developed under EPA Contract No. EP-C-11-009 as part of the 2012 EPA Green
Infrastructure Technical Assistance Program.
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Contents
Introduction 1
Clay Soil and Stormwater Management Overview 1
Soil Textural Classes 1
Soil Definitions 2
Soils in the Greater Pittsburgh Area 4
Unaltered Soils 6
Swelling Soils 6
Altered Soils 6
Methods to Address Clay Soils 7
Site Evaluation and Soil Infiltration Testing 7
Selecting Green Infrastructure Practices 8
Noninfiltration-Based Practices 8
Infiltration-Based Practices 9
1. Soil Amendment 9
2. Protecting the Existing Soils 9
3. Design Components 10
Examples of Implemented Projects 12
Evaluation of Turf-Grass and Prairie-Vegetated Rain Gardens in a Clay and Sand Soil, Madison, Wl,
Water years 2004-2008 (Selbig and Balster, 2010) 12
1. Design Summary 12
2. Results Summary 12
Maywood Avenue Combined Sewer Overflow (CSO) Bioswales Project, Toledo, OH 14
1. Design Summary 15
2. Results Summary 15
3. Lessons Learned 15
References 18
Manuals, Articles, and Books 18
Websites 19
IV
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Tables
Table 1. Soil Properties 3
Table 2. Summary of Influent and Effluent Volumes over the Period of the Study (Selbig and Balster,
2010) 13
Figures
Figure 1. Soil Textural Classes 2
Figure 2. Water-Holding Properties of Various Soils 4
Figure 3. General Soil Map of Allegheny County, PA 5
Figure 4. Three-Dimensional Plot of Infiltration Rates for Clayey Soil Conditions 7
Figure 5. Off-Line and On-Line Bioretention Systems 11
Figure 6. Example of an Upturned Elbow Outlet Configuration 12
Figure 7. Side-by-Side Rain Garden Configuration 14
Figure 8. Planting Native Prairie Plugs 14
Figure 9. Before and After Bioswales on Maywood Avenue 15
Figure 10. Maywood Avenue Bioswale and Pervious Concrete Section 16
Figure 11. Stormwater Peak Flow Attenuation 16
Figure 12. Stormwater Runoff Volume Reduction 17
Figure 13. Construction in Clay Soil 17
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Introduction
Green infrastructure is often entirely compatible with clay or slowly infiltrating soils for managing
stormwater in urban areas. Although the design of green infrastructure practices in clay or low
permeability soils must be considered early in the planning and design process, many effective design
practices are available, both nonstructural and structural, for this soil type. Many cities throughout the
United States have demonstrated the ability of green infrastructure to help treat, slow, and reduce
stormwater even in low permeability soils.
Green infrastructure is an important design strategy for protecting water quality while also providing
multiple community benefits. EPA defines green infrastructure as structural or nonstructural practices
that mimic or restore natural hydrologic processes within the built environment. Common green
infrastructure practices include permeable pavement, bioretention facilities, and vegetated roofs. These
practices complement conventional stormwater management practices by enhancing infiltration,
storage, and evapotranspiration throughout the built environment and managing runoff at its source.
This paper examines the applicability of green infrastructure practices on clay soils in the Pittsburgh
area. The first section discusses the challenges to stormwater management posed by clay soils; the
second section defines the extent and nature of clay soils in and around Pittsburgh; the third section
describes methods for selecting and designing green infrastructure for sites with clayey or compacted
soils including infiltration-based practices and noninfiltration-based practices; and the fourth section
provides examples of monitored projects on clay soils. The goal of this paper is to provide
recommendations for design that are based on observation, research, and engineering in order to help
practitioners make informed decisions regarding the use of green infrastructure on sites with clay soils.
Clay Soil and Stormwater Management Overview
Clay soil is often thought of as a challenge to green infrastructure in that infiltration rates are minimal
and therefore complete on-site retention is not likely. On the contrary, clay soil has been shown to
provide almost as much runoff retention as sandy soil (see Section 'Examples of Implemented Projects').
Green infrastructure can enhance evapotranspiration, attenuate peak flows, and enhance infiltration,
depending on the system design (see Section 'Methods to Address Clay Soils'). Note that in some cases,
compacted soil is misconstrued as clay soil because of observed surface ponding and low infiltration
rates. Compacted soil and clay soil are not the same and must be handled differently, as described in
Section 'Methods to Address Clay Soils'. Often times, compacted soil can be restored through subsoiling
and soil amendment.
To better understand clay soil, the remainder of this section presents hydrologic characteristics of clay
as well as characteristics of silt and sand.
Soil Textural Classes
A normal uncompacted unit of soil is made up of about 45 percent sand, silt, or clay; 5 percent organic
matter; 25 percent air; and 25 percent water. Different mixtures of sand, silt, and clay produce different
soil textural classes with different material properties (Figure 1). Sand increases the permeability of the
soil, silt increases the capillarity of the soil to help pull water upward toward plant roots, and clay
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further increases soil water-holding capacity as well as cation exchange capacity. Cation exchange
capacity governs the ability of the soil to hold nutrients that are crucial to plant health. Organic matter
is also essential to soil. Organic matter provides nitrogen; pH buffering; air space; food for worms,
insects, and other life; and rainfall absorption (USGS, 2011). Grain size for soil particles decreases from
sand to silt to clay, with sand having the largest grain size and clay the smallest (Table 1).
30 A
NDY CLAY
20 / LOAM /
100 90 BO 70 ^60 SO 40 30 20 10
Percent Sand
Figure 1. Soil Textural Classes
Source: http://www.stevenswater.com/articles/soiltypes.aspx
Soil Definitions
The following are important definitions related to soil hydrology. Table 1 includes values of these
parameters for sand, silt, and clay.
Total Porosity - The total porosity of a porous medium, such as clay or sand, describes the ratio of pore
volume to the total volume of the medium. This pore volume includes both the volume of 1) immobile
pores containing adsorbed water and 2) mobile pores containing water that is free to move through the
saturated system. Coarse-textured soils such as sand or gravel tend to have a lower total porosity than
fine-textured soils such as clay. Particularly in clay soils, the total porosity is not constant because the
soil swells, shrinks, compacts, and cracks with varying moisture levels.
Effective Porosity - The effective porosity is the ratio of the volume of mobile pores containing water
that is free to move through the saturated system to the total volume of the medium.
Volumetric Water Content - Volumetric water content is the quantity of water contained in a given
volume of soil and will differ at saturation, field capacity, and permanent wilting point.
Saturation - Saturation is when soil is at its maximum retentive capacity, i.e. when all pores are filled
with water.
Field Capacity (F.C.) - Field capacity (a.k.a. specific retention, residual water content) is the ratio of the
volume of water contained in the soil sample after all downward gravity drainage has ceased (the
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volume of immobile pores containing adsorbed water) to the total volume of the sample. Field capacity
is reached about one to two days after a heavy rainfall. Refer to Figure 2 for a depiction of field capacity
for different soil textural classes.
Permanent Wilting Point (P.W.P.) - Permanent wilting point is the minimum soil moisture at which a
plant wilts and can no longer recover its turgidity. Refer to Figure 2 for a depiction of permanent wilting
point for different soil textural classes.
Available Water Content (A.W.C.) - Available water content is the amount of water in the soil that is
available to plants. It is the difference between field capacity and permanent wilting point.
Infiltration Rate - The measure of the rate at which soil is able to absorb rainfall. The rate decreases as
the soil becomes saturated.
Saturated Hydraulic Conductivity-The ease with which pores of a saturated soil permit water
movement. Also the infiltration rate when the soil is saturated.
Permeability-The measure of how well a porous media transmits a fluid.
Table 1. Soil Properties
Soil
Properties
Grain Size
Total Porosity
Effective
Porosity
Volumetric
Water
Content at
F.C.
Volumetric
Water
Content at
P.W.P.
Volumetric
Water
Content at
A.W.C.
Saturated
Hydraulic
Conductivity,
Ksat
Permeability
Clay
<0.002 mm
0.34-0.57
0.01-0.18
0.32-0.40
0.20-0.24
0.12-0.16
0.02 in/hr
ID'10- ID'15
cm2
Silt
0.002-0.05
mm
0.34-0.51
0.01-0.39
0.28-0.36
0.12-0.22
0.14-0.14
0.27 in/hr
10"8-10"1:L
cm2
Sand
0.05-2.0
mm
0.25-0.46
0.01-0.43
0.07-0.17
0.02-0.07
0.05-0.10
8 in/hr
10"5- 10"9
cm2
Reference
USDA sand classifications
http://web.ead.anl.Rov/resrad/datacoll/porositv.htm
http://web.ead.anl.Rov/resrad/datacoll/porositv.htm
http://www.terraRis.bees.unsw.edu.au/terraGIS soil/
sp water-soil moisture classification.html
http://www.terraRis.bees.unsw.edu.au/terraGIS soil/
sp water-soil moisture classification.html
http://www.terraRis.bees.unsw.edu.au/terraGIS soil/
sp water-soil moisture classification.html
http://www.terraRis.bees.unsw.edu.au/terraGIS soil/
sp water-saturated water flow.html
http://en.wikipedia.orR/wiki/Permeabilitv (earth sci
ences)
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0.60
0.50
0.40
0.30
0.20
0.10
porosity
field
capacity
wilting
point
clay
heavy
clay loam
clay loam
light clay loam
silt loam
sandy loam
fine sand
i
sand
Source: FISRWG, 10/1998, Figure 2.7
Figure 2. Water-Holding Properties of Various Soils
Soils in the Greater Pittsburgh Area
On any given site within the greater Pittsburgh area there is a likelihood that the soil will contain clay,
but this does not necessarily mean the soil drains poorly or is unsuitable for green infrastructure. This is
why infiltration testing in and around the proposed location of a green infrastructure practice is so
important during design (see Section 'Methods to Address Clay Soils'). Even if a particular location is
deemed unsuitable for infiltration, another location on the same site may be suitable. According to the
USDA Soil Survey of Allegheny County, Pennsylvania, the soil associations in this area can be divided into
"Areas dominantly unaltered by urban development and strip mines" and "Areas dominantly altered by
urban development and strip mines" (Figure 3).
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II
|i
I •:
ll ? S
I Jf if Siljt
8 IB SB Igs Js
s= S*
u £ w 2 5
i Si >SS -* e a
ijii
Mtl
lij
i!i
Source: USDA, 1981
Figure 3. General Soil Map of Allegheny County, PA
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Unaltered Soils
For the areas dominantly unaltered, the predominant soil texture is silt loam with some silty clay loam
(Figure 1). The silt loam in the Pittsburgh area is about 25% sand, 50% silt, and 25% clay. These
unaltered areas tend to be on the north side of the Ohio River and Allegheny River and along the creeks
such as Squaw Run near Fox Chapel, Girty Run near Millvale, and Streets Run near Baldwin. Hydrologic
characteristics of the soils in the area typically range from well drained and slowly permeable to poorly
drained, all of which can be accommodated in green infrastructure design. Generally, the soils on
gentler slopes are greater than 5 feet deep.
Swelling Soils
In the greater Pittsburgh area, outcrops of swelling clay (i.e. clay that is susceptible to large volume
changes due to its moisture retaining capability) are generally sparse (USGS, 1989). If swelling clay is
suspected on a site, a geotechnical investigation would be required to verify swelling clay. Where
swelling clay occurs near building foundations or pavements, siting green infrastructure away from
these structures may prevent any damage. Alternatively, the practice could be lined to keep the water
away from foundations. Lining a system with an impermeable high density polyethylene (HOPE)
geomembrane or a concrete box is a common technique used in locations where infiltration would be
detrimental to adjacent structures or to groundwater. Groundwater contamination is a concern in
locations with contaminated soils and in karst topography. Although there is zero infiltration, lined
systems still have many advantages including pollutant removal through an engineered soil, peak flow
attenuation, and evapotranspiration.
The City of Pittsburgh and much of the area south of the Ohio and Allegheny rivers have soils which are
considered altered. These are mostly urban soils underlain by the in situ silt loam. Typically these soils
are compacted and it is difficult to predict what levels of infiltration can be expected. This unknown
supports conducting infiltration tests at the proposed green infrastructure locations during design.
Ideally infiltration tests should be conducted under saturated conditions. This is because infiltration
rates for clay soils can decline as much with soil saturation as with compaction (Figure 4; Pitt et al.,
1999).
Based on studies of compacted sandy and clayey urban soils (Pitt et al., 1999), average infiltration rates
for urban soils in the Pittsburgh area may range from 0.7 inches per hour to 2.5 inches per hour. Other
published infiltration rate data indicate saturated hydraulic conductivity values of 0.27 inches per hour
for silt loam and 0.06 inches per hour for silty clay loam (Ferguson and Debo, 1990). Because of the
wide range of reported values, these numbers can only be used as an initial estimate until site-specific
infiltration testing is conducted.
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10
Source: Pitt et al., 1999, Figure 3-2
Figure 4. Three-Dimensional Plot of Infiltration Rates for Clayey Soil Conditions
Methods to Address Clay Soils
Soils need to be evaluated early in the design process. Only after addressing the question "What type of
infiltration rate can be expected through the site's soils?" can practices be selected and sized to meet
design storm criteria. Practices include infiltration- and noninfiltration-based practices.
The remainder of this section provides potential procedures for evaluating soils on a site and selecting
and designing green infrastructure practices on sites with low permeability soils.
Site Evaluation and Soil Infiltration Testing
Site evaluation and soil infiltration testing should be completed early in the site planning and design
process. Prescreening may be conducted to identify preliminary sites for green infrastructure practices.
Once preliminary sites are proposed, further investigation at the location of each proposed practice is
recommended. Even if the soil is expected to have a low capacity for infiltration, accounting for the
removal of runoff through infiltration may decrease the required size of the practice.
In evaluating site soils, it is important to differentiate between compacted soil and clay soil. Soils that
have previously been disturbed by development should be considered compacted. Note that for
disturbed compacted soils, typically the compaction only persists about 18 inches below the surface
(PADEP, 2006). Infiltration testing below this depth is important in understanding the true infiltration
rate of the soil.
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During site evaluation, the depths to the seasonal high groundwater table and to bedrock should be
measured. These depths will also affect the design and siting of the practice. The PA BMP Manual
recommends at least 2 feet of separation to bedrock and to seasonal high groundwater. In the greater
Pittsburgh area it is also essential to prevent infiltration altogether in landslide-prone areas. Refer to
the Allegheny County Comprehensive Plan maps for locations of landslide-prone areas
(http://www.alleghenyplaces.com/comprehensive plan/maps.aspx). Refer to Appendix C, Site
Evaluation and Soil Testing, of the Pennsylvania Best Management Practices Manual for detailed
procedures for site background evaluation, test pit observation, infiltration and permeability tests.
Selecting Green Infrastructure Practices
Once site soils are characterized and infiltration rates for the surface and subsurface are known,
appropriate green infrastructure practices may be selected and sized. There are many green
infrastructure practices that are appropriate on sites with low permeability soils including
noninfiltration-based practices and infiltration-based practices. The final selection of practices will
depend on many other factors including space availability, site topography, aesthetics, cost,
maintenance, pollutant removal goals, and stormwater design criteria.
Noninfiltration-based practices include vegetated roofs, water harvesting (runoff capture and reuse),
vegetated filter strips, contained sand/media filters, and constructed wetlands. Infiltration-based
practices include bioretention systems, infiltration basins, permeable pavement, and vegetated swales.
In addition, a designer should consider the importance of detaining the water where it may be difficult
to retain the water due to low permeability soils. This consideration may be important if a greater goal
is to lessen the peak flow burden on the combined sewer system.
Noninfiltration-Based Practices
Noninfiltration-based practices include vegetated roofs, water harvesting (runoff capture and reuse),
vegetated filter strips, contained sand/media filters, and constructed wetlands. Vegetated roofs do well
at removing stormwater through evapotranspiration for small rain events. Water harvesting systems
include rain barrels and cisterns, which are used for water reuse in addition to runoff reduction.
Vegetated filter strips are typically used as a pretreatment mechanism taking on sheet flow from a
paved surface. Contained sand and media filters are used as
flow-through treatment practices that are contained within a
lined system. More information about these practices can be Detention - The stormwater
found in the Pennsylvania Best Management Practices Manual. management practice of
temporarily detaining runoff before
A familiar noninfiltration-based practice in the Pittsburgh area releasing it downstream at a
is a constructed wetland system. Constructed wetlands are controlled rate.
shallow marsh systems that treat stormwater. To support their
wetland vegetation, they require either a high groundwater Retention -The stormwater
table or large drainage area. Pittsburgh's frequent rainfall is management practice of preventing
particularly supportive of wetlands. Wetland systems should stormwater from leaving a
be designed as part of a 'treatment train' to protect them from developed or developing site
sediment and debris. A sediment forebay is commonly used as through interception, infiltration, or
well as a flow splitter to divert heavy flows away so as to not evapotranspiration.
harm the sensitive soil and plants.
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Infiltration-Based Practices
Infiltration-based practices include bioretention systems, infiltration basins, permeable pavement, and
vegetated swales. This section discusses the various techniques available for designing infiltration-based
practices in low permeability soils. Specifically, soil amendments, practices to protect the existing soil
from clogging, and important design components for conveying stormwater are discussed.
General design guidance on each infiltration-based practice can be found in the Pennsylvania Best
Management Practices Manual. The Westmoreland Conservation District also provides design guidance
for bioretention in clay soils (Westmoreland Conservation District, 2013).
I. Soil Amendment
The soil discussion in this section is divided into the near surface soil and the subsurface soil. The near
surface soil is what is termed the planting soil or growing layer. For clayey or compacted soil, it is typical
to either excavate to the depth of the planting soil and replace with an engineered soil, having suitable
properties for drainage and plant growth, or amend the native soil with 2.5 inches of compost over the
surface of the site (King County, 2005). When amending the native soil, the soil and compost are tilled
with a subsoiler or ripper attached to a tow vehicle (Kees, 2008). The engineered soil is typically a
mixture of loamy soil, sand, and compost, the details of which depend on the needs of the plants
selected and the hydraulic properties desired. It is helpful for a professional with knowledge in plants
and soils to formulate the soil mix.
Note that for disturbed or compacted soils, typically the compaction only goes about 18 inches below
the surface (PADEP, 2006). Infiltration testing below this depth is important in understanding the true
infiltration rate of the soil.
For clayey or compacted subsurface soil, it may also be beneficial to amend the existing subsurface soil
with compost to enhance the infiltration rate. This practice increases infiltration rates and also helps
reduce cations and toxicants in the water. The disadvantage is that nutrient leaching occurs for a period
of time (Pitt et al., 1999). Establishing native plants with extensive root systems will also help provide
channels to promote infiltration in the subsurface soil.
2. Protecting the Existing Soils
Regardless of the particular practice selected, underlying soils should be protected for all practices
relying on the infiltration rate of existing soils. Efforts to protect the soils from clogging and compaction
should occur during design, construction, and post-construction.
Design: Pretreatment to provide removal of sediment from runoff should be considered during design.
Pretreatment designs vary depending on the siting and properties of the green infrastructure practice,
but common options include vegetated swales, vegetated filter strips, catch basin sumps, and water
quality inlets. The Pennsylvania Best Management Practices Manual includes design guidance on
vegetated filter strips and vegetated swales.
In the Pennsylvania Best Management Practices Manual, the inclusion of catch basin sumps and water
quality inlets is a design recommendation for roof runoff draining to subsurface infiltration practices
such as a dry well or seepage pit. These pretreatment designs are also recommended for any surface
drains. Water quality inlets consist of one or more chambers that promote sedimentation of coarse
materials and separation of free oil. Some are designed to drop directly into existing catch basins, while
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others may require retrofit construction. Their primary function is to remove sediment, oils and grease,
floatables, and pollutants, which are common constituents of parking lot and road runoff (PADEP, 2006).
Note that for permeable pavement, which typically does not have an upgradient drainage area, the
primary design consideration is ensuring that there is no potential for sediment-laden stormwater to
drain onto the permeable pavement.
The design phase is also the time to address soil erosion and sedimentation control (SESC). SESC
practices should be located on the construction drawings to protect the green infrastructure practices. If
sediment-laden runoff is allowed to drain to a green infrastructure practice, the integrity of the practice
is diminished. For more information on erosion and sediment control, refer to the PA DEP Erosion and
Sediment Pollution Control Program Manual.
Construction: Construction documents must address protection of the green infrastructure practices
during construction. Specifications must include language prohibiting all heavy equipment and
minimizing all other traffic, including foot traffic, from entering the sites for the green infrastructure
practices. If planting soil is used for a practice, it should only be compacted by water droplets.
Construction documents should also include language instructing the contactor to install a temporary
construction fence around the protected areas.
During construction of the green infrastructure practices, careful adherence to the construction
documents related to exclusion of traffic and sediment from the green infrastructure practice sites is
imperative.
Post-Construction: Maintenance of the green infrastructure practice will help sustain the existing soil
infiltration rate. In particular, maintenance of the pretreatment practices is necessary for periodic
removal of sediment. Maintenance intervals vary depending on typical sediment concentrations in the
drainage area runoff, and frequent inspections initially should help determine a proper maintenance
schedule. In many situations, annual sediment removal from forebays and sumps is sufficient.
Sediment transport through filter strips and swales may be more difficult to track.
3. Design Components
This section describes the essential overflow and underdrain system for low permeability soils.
Overflow systems convey excess water safely away from buildings and areas where it could cause a
hazard. Underdrains convey the subsurface water in a green infrastructure practice to help meet
required dewatering times when infiltration rates are too slow. Two common formulas used to account
for the volume of water lost to infiltration include the Morton and Green-Ampt equations.
Overflow Systems: Green infrastructure practices should always be designed with an overflow system
regardless of the existing soil properties. The overflow system is designed to convey the peak flow from
storm events with a greater recurrence interval than the design storm used for sizing the green
infrastructure practice. For practices with an upgradient drainage area (e.g. bioretention, vegetated
swales, wetlands) there are essentially two types of overflow systems: one for on-line systems, and one
for off-line systems (Figure 5). When the green infrastructure practice is an on-line system, an overflow
catch basin or weir is used to handle larger flows. When the green infrastructure practice is an off-line
system, stormwater from the larger storm events bypasses the practice and continues down the
conveyance network, e.g. curb and gutter, storm pipe, or swale. An off-line system is preferred as it has
less exposure to the large storm events. For practices with no upgradient drainage area, such as
10
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permeable pavement or a vegetated roof, the overflow system is typically a downstream catch basin
and conveyance network.
Underdrains: In addition to an overflow system, installation of an underdrain may be important to
meeting acceptable dewatering times, particularly for slowly permeable soils. A perforated underdrain
is placed at the bottom of the practice for lined practices and essentially at a higher elevation within the
soil/aggregate matrix for un-lined practices to promote infiltration. The higher elevation can be
governed by an upturned elbow configuration as shown in Figure 6. The elevation of the outlet is
governed by the required dewatering time of the practice. According to the Pennsylvania Best
Management Practices Manual, a maximum 72-hour dewatering time is recommended for surface
ponding. The outlet configuration would be placed such that the water stored beneath it could infiltrate
within 72 hours.
Source: Tetra Tech
Off-Line System
Water enters the bioretention area from a curb cut.
Once the ponding area is full to the level of the gutter,
stormwater will not enter the area but will be conveyed
down the gutter to a catch basin.
Source: Tetra Tech
On-Line System
Water enters the bioretention area from a curb cut. An
overflow structure is placed within the bioretention
area to convey flows in excess of the design flow.
Figure 5. Off-line and On-Line Bioretention Systems
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Source: Brown, et al., 2009
Figure 6. Example of an Upturned Elbow Outlet Configuration
Examples of Implemented Projects
Evaluation of Turf-Grass and Prairie-Vegetated Rain Gardens in a Clay and Sand
Soil, Madison, Wl, Water years 2004-2008 (Selbig and Balster, 2010)
A study was conducted in Madison, Wisconsin from 2004 to 2008 to compare the capability of rain
gardens with different soil and vegetation types to infiltrate stormwater runoff. Two side-by-side rain
gardens were installed on sandy soil, and two additional side-by-side rain gardens were installed on clay
soil (Figure 7). For each soil type, one of the rain gardens was planted with turf grass and the other with
a native prairie species (Figure 8). Results showed that the rain gardens with clay soils performed such
that at least 99 percent of the inflow was able to be infiltrated after four years of operation.
Underdrains were not used in this study.
I. Design Summary
Each side-by-side rain garden received approximately equal amounts of roof runoff and was sized to a
ratio of approximately 5:1 contributing drainage area to receiving area. The parent soil was excavated to
form berms around the rain gardens to exclude drainage from areas other than the roof. Approximately
4 to 6 inches of screened compost was then worked into the remaining parent soil with a rototiller. The
surface was then leveled and planted leaving approximately 6 inches of ponding depth.
2. Results Summary
The results showed that regardless of soil type or vegetation, the rain gardens were capable of storing
and infiltrating most of the runoff over the 4-year study period. Refer to Table 2 for influent and
effluent data for each rain garden. Other significant observations included the following:
12
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• Median infiltration rates for rain gardens in sand were greater than those in clay.
• Rain gardens with prairie vegetation had greater median infiltration rates than those with turf
grass for each soil type.
• Infiltration was highest during spring and summer.
• Although infiltration rates were reduced during winter months, the hydraulic function of the
rain gardens did not appear to be appreciably altered.
• Based on storage capacity alone, approximately 90 percent of all precipitation measured over
the 4-year study was stored in the gardens. Taking into account infiltration rate and specific
yield of subsurface soils, nearly 100 percent of precipitation was retained.
• Roots in the prairie-clay rain garden extended 4.7 feet deep compared with 0.46 feet in the
turf-clay rain garden. This and greater earthworm activity in the prairie-clay garden may result
in greater capacity of the prairie-clay garden to store and infiltrate stormwater than the turf-
clay garden.
Table 2. Summary of Influent and Effluent Volumes over the Period of the Study (Selbig and Balster, 2010)
[—, data not available; values represent volumes into and out of rain garden from roof and direct precipitation; they include snowmeltfor
runoff but do not include water equivalent for snow falling directly on rain garden. Therefore, the volumes in this table and those presented in
table 4 will be different because table 4 includes estimates of water equivalent for snow using available NOAA data.]
Rain Garden
Turf-sand
Prairie-sand
Percent difference
Turf-clay
Prairie-clay
Percent difference
Volume (cubic feet)
Influent
2004
1,279
1,275
0
5,436
5,859
-7%
2005
749
764
-2
2,923
2,423
21%
2006
1,142
1,206
-5
4,247
3,608
18%
2007
1,341
1,354
-1
5,198
4,437
17%
2008
2,1571
—
—
—
S^Sl1
—
Effluent
2004
0
0
0
191
0
100%
2005
0
0
0
35
0
100%
2006
0
0
0
10
0
100%
2007
0
0
0
12
0
100%
2008
II1
—
—
—
1381
—
1 In water year 2008, all roof runoff was directed to the turf-sand and prairie-clay rain gardens. This doubled the ratio of contributing to
receiving area to 10 to 1 and 8 to 1, respectively.
13
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Figure 7. Side-by-Side Rain Garden Configuration
Figure 8. Planting Native Prairie Plugs
Maywood Avenue Combined Sewer Overflow (CSO) Bioswales Project, Toledo,
OH
The Maywood CSO project in Toledo, Ohio is an example of a neighborhood-scale green infrastructure
project constructed on clay soils (Figure 9). Maywood Avenue is a single 1,300-foot long street in a
neighborhood located on the north side of Toledo. The neighborhood demographics and physical
components are typical of other well-established, older urban neighborhoods in the city. Results
showed that despite the clay soils, the system was able to retain about 64 percent of annual runoff
volume. Peak flows were reduced by 60-70 percent.
14
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Figure 9. Before and After Bioswales on Maywood Avenue
I. Design Summary
The design accounted for a capture of approximately 0.35 inches of runoff primarily through the use of
bioswales and pervious pavement sidewalks and driveway approaches, but based on subsequent flow
monitoring and modeling, the actual capture was greater than this. The post-construction analysis
showed a significant amount of water being retained through infiltration, which was not anticipated in
the design. An underdrain was installed due to the existence of clay soils, but was later closed with a
valve to promote infiltration. The project goal was to determine the effectiveness of using green
infrastructure to reduce stormwater runoff, improve water quality, and assess impacts on stormwater
and CSO management. Refer to Figure 10 for a design detail of the bioswale.
2. Results Summary
Flow monitoring was conducted before and after construction of the green infrastructure practices to
assess effectiveness. Despite being constructed on clayey soils, the new system has shown a decrease in
peak flows and runoff volumes as indicated in Figure 11 and Figure 12. The figures show pre-
construction monitoring in 2010, post-construction monitoring with the valve at the underdrain outlet
open in 2011, and post-construction monitoring with the valve at the underdrain outlet closed in 2011.
With the valve closed, which is normal operation, the water is left to infiltrate below the bioswales.
Long-term simulations using US-EPA SWMM indicate an annual average reduction of runoff volume of
approximately 64 percent. Peak flows are reduced by 60 percent to 70 percent at equivalent rainfall
intensities.
3. Lessons Learned
Construction of the Maywood Avenue bioswale was simplified by the presence of clay soils, which
eliminated the need for trench shoring (Figure 13). Overall the bioswale provided much more
stormwater volume reduction than expected. These results indicate that the infiltration rate of clay soils
can contribute significantly to green infrastructure performance.
15
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TYPE 6
CURBS BUTTER
A VALVE IS INSTALLED
DOWNSTREAM OF THE
DOWNSTREAM CATCH BASIN AND IN
NORMAL OPERATION IS CLOSED TO
PROMOTE INFILTRATION.
NOTE:
POSITION OF 6" PERFORATED SCHEDULE 40
PVC CONDUIT WITHIN THE OOOT#57
AGGREGATE VARIES. CONDUIT SHALL BE
POSITIONED TO DRAIN INTO THE NEAREST
CATCH BASIN PER PLAN, FIELD VERIFIED AND
APPROVED BY THE ENGINEER
PLANTING MIX
3/4" DOUBLE WASHED
ODOT 87 AGGREGATE
L DOUBLE WASHED ODO1
#57 AGGREGATE
6" PERFORATED SCHEDULE 40 PVC TO BE
FIELD VERIFIED BY ENGINEER TO ENSURE
PROPER DRAINAGE TO STORM CB'S
BIOSWALE CELL SECTION (TYP.)
SCALE 1" = 2'
Figure 10. Maywood Avenue Bioswale and Pervious Concrete Section
1200
1000
Q.
•S
v>
d>
0.00
Post-Construction
0.20
0.40 0.60 0.80
Peak Hour Intensity Rain (in/hr)
1.00
42010
42011 (valve open)
42011 (valve closed)
1.20
Figure 11. Stormwater Peak Flow Attenuation
16
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0.35
|>0.30
:> 0.25
0.20
CD
0.15
0.10
0.00
0.00
Post Construction
.•--Valve Open
Pre-Construction
0.50
Post-Construction
Valve Closed
1.50 2.00
Total Rainfall (in)
2.50
3.00
3.50
42010
42011 (valve open)
42011 (valve closed)
Figure 12. Stormwater Runoff Volume Reduction
Figure 13. Construction in Clay Soil
17
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