Green
EPA 841-B-18-001 | March 2021
United States
Environmental Protection
Agency
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Acknowledgments
Credit should be given to the many local government officials and
practicing professionals who offered their time and provided tours and
examples of their work and designs to bring this document to press.
Special acknowledgment goes to Alisha Goldstein who was the primary
author of the Green Streets Handbook. The document was researched,
written and designed during her tenure as an Oak Ridge Institute for
Science and Education (ORISE) participant at the U.S. Environmental
Protection Agency (USEPA).
The Low Impact Development Center, Horsley Witten Group, RBF
Consulting and Tetra Tech, Inc., assisted with document development,
content review, development of original graphics and images, and editing
and formatting the document. Images were provided by Credit Valley
Conservation, the New Hampshire Stormwater Center, Hazen and Sawyer,
Minnesota Pollution Control Agency and Clean Water Services.
Reviewers included staff from USEPA's Office of Water, Office of
Sustainable Communities, Office of Research and Development, and
USEPA regional offices. This document was prepared for USEPA Office
of Water/Office of Wetlands, Oceans and Watersheds/Nonpoint Source
Management Branch.
Disclaimers
This document serves as a guide to green infrastructure best
management practices; selection of and specifications for individual
project designs should be based on a thorough analysis of site
conditions and awareness of local regulations.
Mention of, or referral to, non-EPA programs, products or services, and/
or links to non-EPA sites, does not imply official EPA endorsement of,
or responsibility for, the opinions, ideas, data or products presented
therein, or guarantee the validity of the information provided. Mention of
programs, products or services on non-EPA websites is provided solely
as a pointer to information on topics related to environmental protection
that may be useful to the intended audience.
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Contents
Preface ii
1 Addressing Stormwater Runoff 1-1
1.1 Road-Related Networks and Stormwater Runoff 1-2
1.2 Stormwater Solutions: Green Streets 1-4
1.3 Benefits of Green Streets 1-7
1.4 Additional Resources: Green Infrastructure 1-12
2 Transportation Typologies and Green Infrastructure Practices 2-1
2.1 Transportation Typologies 2-2
2.2 Arterials 2-5
2.3 Collector Roads 2-7
2.4 Local Roads 2-9
2.5 Alleys 2-11
2.6 Parking Lots 2-12
2.7 Identifying Opportunities for Green Infrastructure Placement...2-14
2.8 Reconfiguring Designs to Create Space for
Green Infrastructure 2-17
3 Developing a Green Streets Program: A Process Overview 3-1
3.1 Programmatic Process Overview 3-2
3.2 Establish Objectives 3-3
3.3 Identify Priority Area(s) 3-4
3.4 Characterize Sites 3-5
3.5 Develop Site-Specific Stormwater Plan 3-6
3.6 Engage Community Partners 3-7
Green Streets Handbook
4 Design Considerations 4-1
4.1 Design Checklist 4-2
4.2 Selecting Appropriate Practices 4-3
4.3 Accommodating Utilities 4-5
4.4 Capturing Stormwater Runoff Types 4-6
4.5 Managing Stormwater Flow 4-9
4.6 Planning for Maintenance 4-14
4.7 Selecting Soil Media and Vegetation 4-15
4.8 Providing Pedestrian Access 4-17
4.9 Ensuring Pedestrian Safety 4-18
4.10 Enhancing Street Design 4-19
4.11 Accounting for Extreme Weather 4-21
4.12 Avoiding Design Flaws 4-22
5 Pretreatment Practice Options 5-1
5.1 Pretreatment: Sediment Forebays 5-2
5.2 Pretreatment: Vegetated Filter Strips 5-4
5.3 Pretreatment: Swales 5-6
5.4 Pretreatment: Modified Catch Basins 5-8
5.5 Pretreatment: Flow-Through Structures 5-10
6 Green Street Stormwater Practices 6-1
6.1 Bioretention (Rain Gardens) 6-2
6.2 Bioswales 6-7
6.3 Stormwater Curb Extensions 6-11
6.4 Stormwater Planters 6-16
6.5 Stormwater Tree Systems 6-20
6.6 Infiltration Trenches 6-27
6.7 Subsurface Infiltration and Detention 6-31
6.8 Permeable Pavement 6-34
7 References 7-1
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Preface
In large U.S. cities, 25 percent to more than 60 percent of the land area is
covered by impervious roadways, alleys, driveways, sidewalks and surface
parking lots. Stormwater runoff from these areas can produce significant
runoff volumes and carry pollutant loads that negatively impact the water
quality of surface waterbodies and reduce groundwater recharge because
of the loss of soil infiltrative capacity. This handbook is intended to provide
the reader with a systematic process to begin reducing the impervious
surface footprint of the public right-of-ways and associated off-street
surface parking areas.
Green streets can provide many environmental, social and economic
benefits. In addition to the stormwater runoff reduction and water quality
improvement benefits, green streets can be designed to calm traffic,
provide safer pedestrian and bicycle paths, mitigate urban heat island
effects, improve community aesthetics, promote a sense of place and
stimulate community investments. These enhancements can help to make
a "green and complete street" that is safe and accessible for all users while
also being friendlier to the environment and beneficial for the community
at large.
This handbook is intended to help state and local transportation agencies,
municipal officials, designers, stakeholders and others to select, design
and implement site design strategies and green infrastructure practices for
roads, alleys and parking lots. Green infrastructure practices are designed
to mimic natural systems by intercepting, infiltrating and evapotranspiring
stormwater to reduce runoff and protect or restore site and watershed
hydrology.
The document provides background information on street and road typol-
ogies and offers a programmatic framework to use when identifying areas
Green Streets Handbook
that can be initially designed or later retrofitted with green infrastructure
practices or systems. The handbook also contains information about green
street design considerations, pretreatment and stormwater management
practices, and external resources with additional detail for readers who
wish to go deeper into a specific topic.
Stormwater tree pits in a parking lot, Reston, VA.
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Addressing
Stormwater Runoff
In This Chapter
1.1 Road-Related Networks and Stormwater
Runoff
1.2 Stormwater Solutions: Green Streets
1.3 Benefits of Green Streets (Environmental,
Social, Economic)
1.4 Additional Resources: Green Infrastructure
This chapter provides an overview of stormwater runoff
from transportation infrastructure, including typical
pollutant concentrations and common transporta-
tion-related sources of those pollutants. Green streets
can be designed to incorporate a variety of green
infrastructure practices to manage stormwater onsite,
where precipitation falls. Green streets, which can also
be part of "complete street" solutions, can provide many
benefits including environmental, social and economic
benefits. Many states and local governments across
the country have also developed green street and green
infrastructure design manuals that transportation
designers can use.
Green Streets Handbook
Runoff from urbanized areas contributes to pollution and flooding.
Clean water is essential for protecting swimmers' health.
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1.1 Road-Related Networks and Stormwater Runoff
Transportation Infrastructure Affects Stormwater
Runoff Volume and Pollutant Load
Roads and parking lots are a highly visible part of the landscape. Counties,
cities and towns control 76 percent of the more than 4 million U.S. roads.
The remaining road miles are managed by state highway agencies (19
percent) and federal and other jurisdictions (4 percent) (FH WA 2016).
Roadways are a critical component of the nation's infrastructure, but
because of their imperviousness and associated pollutant loadings they
can also significantly impact water resources.
Transportation-related land uses represent an especially high percentage
of overall impervious surface area within urban and suburban areas.
Within the urban environment, roads, driveways, sidewalks and parking
lots can constitute up to 70 percent of the impervious surface area (Tilley
2006). When it rains or snows, the roadway networks can collect and
convey large volumes of stormwater runoff, facilitating the transport of
the pollutants deposited on the roadways from vehicles, the atmosphere,
road construction or adjacent land uses. As shown in Table 1-1, the types
of pollutant loadings depend on a variety of factors, including traffic
volume, land use, total impervious surface area, storm events (intensity and
duration), and accidental spills.
Table 1-1. Summary of the pollutant types found in road runoff (FHWA19S4)
Pollutant
Particulates
Sources
Nitrogen and phosphorus
Metals (e.g., zinc, iron, copper,
cadmium, chromium, nickel,
manganese)
Sodium, calcium, chloride
Bacteria
Pavement wear
Vehicles
Atmospheric deposition
Rubber tire wear
Winter sanding
Atmospheric deposition
Fertilizer
Sediment
Grease
Tire wear
Motor oil
Brake linings
Vehicle rust
Steel structures
Engine components
Diesel and gasoline
Deicing salts
Animal waste
Transportation network in Chicago, IL.
Land use patterns in a city.
impervious expanse of a parking lot.
Green Streets Handbook -|_1 Road Related Networks and Stormwater Runoff I
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Two of the largest factors that determine pollutant loads are traffic volume
and surrounding land uses. Greater traffic volume, measured in average
daily traffic, results in increased amounts of vehicle-associated pollutants
(Table 1-2). Likewise, areas that have rapid turnover of parked cars (e.g.,
retail parking areas) typically generate higher levels of contamination
because of the vehicle-associated pollutant deposition and surface wear
associated with frequent starting of vehicles (NRC 2008).
Surrounding land uses also affect the volume of runoff on roadways.
Impervious surfaces, especially directly connected areas, convey runoff
that picks up pollutants as it flows. Studies have shown that stream
health (as measured by the concentration of pollutants, habitat quality,
and aquatic species diversity and abundance) decreases as the amount
of impervious area increases in a watershed (Arnold and Gibbons 1996).
Large volumes of runoff entering streams can cause erosion that affects
downstream water quality, destabilizes stream channels and damages
habitat. Runoff can also lead to flooded and closed roadways, creating a
nuisance for users.
Stormwater runoff flowing off impervious surfaces collects and transports
pollutants such as metals, hydrocarbons, bacteria, excess nutrients and
sediments. Under conventional drainage system designs, these pollutants
typically are discharged untreated directly into receiving water bodies such
as streams, lakes and bays.
Fortunately, communities can install practices to help mitigate stormwater-
caused impacts. By replicating a site's original hydrology and encouraging
the capture, infiltration and evapotranspiration of runoff, transportation
network designers and planners can reduce excess stormwater flows while
also managing pollutant loadings. Using these techniques represents a
sound approach to protecting water quality while also meeting a communi-
ty's transportation needs.
Green Streets Handbook
Table 1-2. Summary of pollutant concentrations found in road runoff from highways with
small and large traffic volumes
Pollutant
Event mean concentration
for highways with fewer
than 30,000 vehicles/day
(mg/L)
Event mean concentration
for highways with more
than 30,000 vehicles/day
(mg/L)
Total suspended solids
41
142
Volatile suspended solids
12
39
Total organic carbon
8
25
Chemical oxygen demand
49
114
Nitrite and nitrate
0.46
0.76
Total Kjeldahl nitrogen
0.87
1.83
Phosphate phosphorus
0.16
0.40
Copper
0.02
0.05
Lead
0.08
0.40
Zinc
0.08
0.33
Source: Driscoll et al. 1990
Notes: mg/L = milligrams per liter
USEPA Copper-Free Brake Initiative
The U.S. Environmental Protection Agency (USEPA), states and the
automotive industry are working together to reduce the use of copper and
other materials in motor vehicle brake pads. The wearing of brake pads onto
roadway surfaces contributes excessive levels of copper and other pollut-
ants to waterways. The automotive industry has agreed to reduce copper in
brake pads to less than 5 percent by weight in 2021 and 0.5 percent by 2025.
For more information see USEPA's Copper-Free Brake Initiative website
1.1 Road Related Networks and Stormwater Runoff
1-3
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1.2 Stormwater Solutions: Green Streets
Using Natural Processes to Control Stormwater
Streets and parking lots can be designed using a variety of practices that
mimic or preserve natural drainage processes to manage stormwater.
These practices retain stormwater and snowmelt and promote infiltration
into the ground to reduce runoff volumes that may contribute to flooding
and water quality problems (Figure 1-1). This handbook uses the term
green infrastructure to describe these practices. As defined under Section
502 of the Clean Water Act (CWA): "Green infrastructure means the range
of measures that use plant or soil systems, permeable pavement or other
permeable surfaces or substrates, stormwater harvest and reuse, or
landscaping to store, infiltrate, or evapotranspirate stormwater and reduce
flows to sewer systems or to surface waters."
This handbook is focused on green infrastructure specifically for storm-
water management practices in transportation infrastructure, such as roads
and parking lots, but the term green infrastructure varies in its use in other
Impervious areas Natural areas
30% Evapotranspiration 40% Evapotranspiration
15% Runoff
10% Runoff
10% Shallow
Infiltration
20% Shallow
Infiltration
25% Deep
Infiltration
5% Deep
Infiltration
Figure 1-1. When impervious areas (roads, rooftops, parking lots) cover much of
the land (left image), more than half the rainfall runs off and flows directly into
surface waters, allowing only 15 percent of rain water to soak into the ground. In
contrast, areas that are designed to mimic natural areas (right image) allow only
10 percent of rain to run off and nearly half to soak into the ground.
Green Streets Handbook
contexts. Conservation ecologists use green infrastructure to describe the
creation and networking of natural ecosystems and greenway corridors
(e.g., forests, floodplains) that provide ecological services and benefits.
In the context of stormwater, USEPA uses green infrastructure to refer to
practices such as green roofs, porous pavement, swales and rain gardens
that largely rely on using soil and vegetation to infiltrate, evapotranspirate,
and/or harvest stormwater runoff and reduce flows entering drainage
collection systems.
Some use other terms to reference the same practices as green infrastruc-
ture for stormwater management. For example, low impact development
(LID) is a management approach and a set of practices that can reduce
runoff and pollutant loadings by managing runoff as close to its source
as possible. Other terms include low impact design, sustainable urban
drainage systems, water-sensitive urban design and green stormwater
infrastructure. The definitions of these terms may vary slightly among
organizations and industry professionals; however, these concepts are
generally captured in the CWA definition of green infrastructure. Therefore,
this handbook will use the term green infrastructure from here forward.
Green Infrastructure in Transportation Networks
Traditional stormwater management systems along roads typically direct
runoff into pipes or channels that often carry runoff great distances from
where precipitation falls. In contrast, a green street incorporates a variety
of green infrastructure practices that manage stormwater onsite, where
(or very near to where) the precipitation falls. Because green infrastructure
techniques are location-independent and can be applied across different
regions and climatic zones, designers can adjust the basic forms and
processes of practices to best suit local physical, social, and climatic
conditions and goals. As discussed in Chapter 2, green infrastructure
elements that re-create natural areas can be incorporated into almost all
transportation projects.
1.2 Stormwater Solutions: Green Streets
1-4
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Green Infrastructure Practices Rely on Natural
Processes to Capture and Clean Stormwater
Strategies for green infrastructure design rely on naturally occurring
hydrological and biophysical processes to manage the quantity of flow and
improve water quality (Figures 1-2 and 1-3).
Hydrologic processes:
Infiltration. Water moves from the ground surface into the soil.
Detention. Water is stored temporarily, thus delaying conveyance
downstream.
Retention. Instead of flowing downstream, water is captured and
stored onsite for later evapotranspiration or infiltration.
Interception. Vegetation or buildings capture precipitation.
Evapotranspiration. The leaves of plants release water into the
atmosphere.
Biophysical processes:
Filtration. Vegetation, soil and plant roots strain organic matter,
phosphorus and suspended solids out of stormwater.
Sedimentation. Sediment drops out of suspension and accumulates
as stormwater slows and pools in the practice.
Adsorption. Pollutants and excess nutrients carried in stormwater
attach to clay particles in the soil and remain in place.
Microbial action. Bacteria in the soil and plant roots break down the
pollutants and nutrients.
Uptake. Plants and soil organisms absorb metals and use nutrients
such as nitrogen and phosphorus for their growth.
Green Streets Handbook
Figure 1-2. Modifying or designing parking lot islands
as bioretention areas can capture and temporarily store
runoff, allowing the water time to infiltrate the soil or be
evapotranspired.
Figure 1-3. Soil and plants absorb and filter out excess nutrients
and other pollutants from runoff, while microbes in the soil help
break down the chemical compounds.
1.2 Stormwater Solutions: Green Streets
1-5
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Elements Support Complete Street Initiatives
Developing a green streets program complements the nationally recog -
nized Complete Streets policy initiative supported by the Federal Highway
Administration (FHWA) and USEPA. This initiative promotes street designs
that promote neighborhood character, stimulate economic development,
and serve the mobility and access needs of all users—motorists, transit
riders, bicyclists and pedestrians. As seen in Figure 1-4, Complete Street
objectives are primarily achieved by using measures to calm traffic and
create well-defined barriers between transportation types (e.g., chicanes,
islands, curb extensions, bike lanes).
Fortunately, many communities across the country recognize that a street
is not necessarily "complete" without features that also serve environmen-
tal goals, and they strive to use traffic-calming measures that can double
as stormwater-control features. For example, by placing a vegetated
stormwater curb extension at an intersection or near a crosswalk, commu-
nity transportation designers can encourage reduced traffic speeds and
alert drivers to activity occurring adjacent to the road while also capturing
street runoff. Adding a well-marked pervious pavement bicycle lane
intercepts runoff and protects bicyclists from vehicular traffic. Similarly,
planting street trees helps define road boundaries, protects pedestrians
and motorists, and intercepts and absorbs rainfall.
Rain Garden
Transit
Sidewalk
Bike Lane
Figure 1-4, A green and "complete street" in Seattle, Washington, includes specific
streetcar, vehicle, bike and pedestrian zones and a rain garden and vegetated
stormwater curb extensions to capture and treat runoff.
For More Information-Green Streets and Complete Streets
- Green Streets: A Conceptual Guide to Effective Green Streets Design
Solutions USEPA (2000)
- Managing Wet Weather with Green Infrastructure Municipal Handbook:
Green Streets. USEPA (2015)
- G3 Partnership: Green Streets. Green Towns. Green Jobs. USEPA
- Urban Street Stormwater Guide (2017) and Urban Street Design Guide
(2013). ($) National Association of City Transportation Officials
- Complete Streets. Smart Growth America/Complete Streets Coalition
- Boston Complete Streets. Boston Transportation Department, MA (2013)
- Complete Streets. U.S. Department of Transportation, FHWA
- Toronto Complete Streets Guidelines. City of Toronto. Canada
Green Streets Handbook
1.2 Stormwater Solutions: Green Streets
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1.3 Benefits of Green Streets
Green Streets Provide Environmental, Social and
Economic Benefits
Green streets are an investment in your community because good designs
can provide many additional benefits beyond stormwater management.
The design of streets and public rights-of-way can affect the public's
perception of a community, influence the behavior of residents and visitors,
and shape development decisions, while also helping to create a sense of
place. The use of green streets can provide numerous benefits, such as:
- Improved water quality
- Enhanced community resilience
- Increased groundwater recharge
- Enhanced wildlife habitat
- Improved air quality
- Reduced urban heat island effects
- Increased pedestrian safety and traffic calming
- Enhanced well-being of individuals
- Increased sense of community
- Increased property values
- Reduced water treatment costs
- Reduced infrastructure costs
- Reduced property damage due to flooding
These benefits are grouped and described in further detail on the following
pages.
Sketch of green street components such as a permeable pavement crosswalk, curb bump-outs and bioretention applied to a local road
Green Streets Handbook
1.3 Benefits of Green Streets
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Environmental Benefits of Green Streets
Improves Water Quality
The green infrastructure elements incorporated into green
streets help decrease the volume of stormwater runoff and
pollutants entering water bodies by:
- Capturing the small, frequently occurring storm events.
- Filtering the first flush of runoff that can contain high
concentrations of pollutants.
- Slowing down and temporarily storing runoff.
- Reducing erosion and sedimentation that can negatively
impact aquatic habitat and destabilize stream channels.
Green streets can be designed to use the processes of filtration or infiltra-
tion to reduce the pollutant loadings that are discharged into waterways.
The most cost-effective systems are typically soil-based vegetated
designs, although permeable pavements, filtration and infiltration systems
can also be used to mitigate the effects of stormwater runoff volumes and
pollutant loadings from roads, rights-of-way and parking lots.
Enhances Community Resilience
The use of green streets can increase resilience to chang-
ing weather patterns and can help save energy.
Incorporating street trees and green infrastructure prac-
tices that include vegetation (e.g., bioretention cells,
bioswales) in the right-of-way can provide cooling and wind break effects
that reduce energy use by nearby homes and businesses and, as a result,
reduce emissions at nearby power plants. Green streets can also be
designed to promote alternative modes of transportation such as walking
and biking to reduce vehicle use and associated emissions (NCSC, n.d).
~
Green Streets Handbook
Increases Groundwater Recharge
Green street practices that infiltrate runoff, such as bioret-
ention cells, bioswales, infiltration planters and permeable
pavement, are designed to allow runoff to drain into
subsurface soils and recharge groundwater supplies.
Recharging aquifers can be particularly important in areas of the country
that have limited groundwater supplies and are challenged to meet their
water supply needs.
Stormwater runoff from impervious areas like streets can be directed to
infiltration practices that help recharge groundwater resources. An April
2016 USEPA study of stormwater retention practices used to recharge
groundwater found that the monetary value of this recharged water can be
worth millions of dollars in some states.
Enhances Wildlife Habitat
Vegetated landscape areas can provide habitat for wildlife.
Green infrastructure can be used to mitigate the effect of
habitat loss that is typically a result of urbanization.
Patches of vegetation and/or trees incorporated into a
community's green infrastructure can serve as a nesting location for birds,
temporary resting places for migrating wildlife, or sources of food for
pollinators. In rural settings, larger areas of green infrastructure can serve
both as habitat and wildlife corridors that enable animals to migrate.
1.3 Benefits of Green Streets
1-8
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Environmental Benefits of Green Streets, continued
H Improves Air Quality
Trees and other vegetation on green streets can improve air
quality by directly removing air pollution and slowing
temperature-dependent reactions that form particulate
matter that is hazardous to human health (MWCOG 2007;
Vingarzan and Taylor 2003). The increased shade and evapotranspiration
provided by trees lowers air and surface temperature of impervious areas,
which can reduce the amount of electricity needed for cooling and thus
reduce power plant emissions of pollutants. These benefits are of special
importance to communities designated by the USEPA as nonattainment
areas for the 8-hour ozone standard due to ground-level ozone and fine
particulates in the ambient air.
The monetary and quantitative value of the air quality benefits that can
accrue from trees can be calculated by using standard software models
such as i-Tree. which is a suite of applications developed by the U.S.
Department of Agriculture (USDA) Forest Service to design and evaluate
urban forestry efforts. The i-Tree family of applications (USFS 2014)
includes:
1. i-Tree Streets, which helps quantify the dollar value of
environmental and aesthetic benefits.
2. i-Tree Hydro, which provides watershed scale analyses of
vegetation and impervious cover effects on hydrology.
3. i-Tree Eco, which documents a range of ecosystem
benefits, such as carbon storage and sequestration, oxygen
production, avoided runoff and energy savings.
4. i-Tree Design, which can help designers determine the
benefits of specific trees in a landscape design.
Green Streets Handbook
H Reduces Urban Heat Island Effect
Green streets also can be used to reduce urban heat island
impacts that result from solar radiation absorbed by
pavement, buildings and other hard surfaces and reflected
as heat (USEPA 2008). Temperatures in urban areas can
average 5 to 10 degrees Fahrenheit higher than those in suburban areas.
Using reflective surfaces (e.g., light-colored pavements, sidewalks) and
incorporating vegetation can reduce these temperature impacts. Heat can
be reflected back into the atmosphere by using reflective or light-colored
surfaces, and vegetation can be planted that evapotranspires water and
thereby cools the ambient air temperatures (USEPA 2008). Table 1-3
compares albedos (how reflective or bright an object is) of different
materials. A higher albedo reflects more light and helps with cooling.
Table 1-3. Albedos for various reference materials
Material
Albedo
Concrete (new to aged)
0.2-0.35
Asphalt (new to aged)
0.05-0.2
Deciduous plants
0.20-0.30
Dry grass
0.30
Deciduous woodland
0.15-0.20
Coniferous woodland
0.10-0.15
Artificial turf
0.05-0.10
Grass and leaf mulch
0.05
Source: Santamouris 2001; Pomerantz 2003.
1.3 Benefits of Green Streets
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Social Benefits of Green Streets
Offers Pedestrian Safety and Traffic Calming
Green infrastructure features, such as stormwater curb
extensions, bump-outs and porous/vegetated islands, can
be incorporated into street designs (e.g., placed in intersec-
tions or in the middle of cul-desacs) to help slow traffic,
reduce crossing distances and increase awareness of crosswalk locations.
Adding or enhancing sidewalks, crosswalks and bike lanes can contribute
to greater public safety for all users. Pedestrian deaths account for 12
percent of total traffic deaths in the United States; these typically result
from inadequate or nonexistent pedestrian safeguards such as crosswalks,
pedestrian refuge islands (i.e., safe locations, such as a section of pave-
ment or sidewalk within the roadway, where pedestrians can stop), and
school and public bus shelters (TFA 2011).
Enhances Weil-Being of Individuals
Green street practices can be placed in or along roadways
and sidewalks to create safe and aesthetically pleasing
pathways that encourage active transportation such as
walking or biking. Planting trees creates shade and cools
the air temperature so people are more likely to walk or bike. Green spaces
have been shown to enhance the strength of social ties between neighbors
(Holtan 2014). Neighborhoods with social cohesion have lower rates of
social disorder, anxiety and depression. Green spaces enhance well-being
and help the mind recover from mental fatigue or stress (Kaplan 1995). In
densely developed urban areas, adding green infrastructure provides some
relief in areas otherwise devoid of green infrastructure such as parks.
Green Streets Handbook
Increases Sense of Community
Although this benefit is often qualitative in nature, it reflects
the ability of a feature such as a green street to positively
serve as a signature place or a destination for community
residents or visitors and/or a model for development or
redevelopment (DC OP 2011). In stressed or underserved communities,
greening efforts can serve to help brand or rebrand a community to attract
investments and provide residents and visitors a new perspective about
their community. Green street projects can also serve to help educate the
community about environmental issues such as protecting watershed
health, building neighborhoods' weather resilience and caring for nature.
Potential measures for evaluating this benefit include:
- Anticipated increase in sales by nearby merchants
- The number of events held in the project area
- Number of tourists and visitors anticipated to visit the project
location
- Increases in community investments
- Improved environmental awareness in local schools
For More Information-Social Benefits of Green Streets
Cities Safer by Design: Guidance and Examples to Promote
Traffic Safety through Urban and Street Design World Resources
Institute (2015)
Imaging Livabilitv Design Collection: A visual portfolio of
tools and transformations AARP Livable Communities and the
Walkable and Livable Communities Institute (2015)
Green Values Strategy Guide: Linking Green Infrastructure
Benefits to Community Priorities Center for Neighborhood
Technology (2020)
1.3 Benefits of Green Streets
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Economic Benefits of Green Streets
Increases Property Values
Adding plants and trees to green streets creates attractive
neighborhoods, which in turn can increase nearby property
values by two to five percent (NRDC 2013). A research
study evaluating street trees in Portland, Oregon, found that
street trees added $8,870 to a house's sale price—equivalent to adding 129
finished square feet (sq ft). By extrapolating street tree benefits across the
entire city, the study calculated that the increased property value translated
into an increased annual property tax revenue of $13 million. Additionally,
the benefits were found to outweigh the costs by almost 12 to 1. One study
estimated the benefits created by green streets to be $54 million annually,
compared to the annual cost of $4.61 million required to maintain the green
street elements (Donovan and Butry 2010).
Reduces Water Treatment Costs
Green infrastructure practices that increase infiltration or
use water on-site (e.g., bioretention systems, permeable
surfaces) can reduce the amount of water being conveyed
to wastewater treatment facilities and reduce combined
sewer overflows (CSOs). Reducing the volume of water discharged to
combined stormwater and sewer systems can reduce the need to treat
significant volumes of runoff. Reducing intake volumes can also reduce the
stormwater infrastructure needed to convey this volume of runoff. The
avoided costs and resulting benefits of green infrastructure can be evalu-
ated by determining the amount of stormwater that will be infiltrated or
evapotranspired versus the costs of treatment and ongoing maintenance
and management of the system. A study completed for the City of Lan-
caster, Pennsylvania, found that implementing their Green Infrastructure
Plan could reduce wastewater pumping and treatment costs by approxi-
mately $661,000 per year using the Center for Neighborhood Technology's
methods for evaluating benefits of green infrastructure (USEPA 2014; CNT
2010).
H
Green Streets Handbook
Reduces Infrastructure Costs
In addition to avoided treatment costs, green infrastruc-
ture practices can also reduce gray infrastructure costs by
reducing the need for infrastructure expansion, extending
infrastructure life expectancies and decreasing overall
life-cycle costs.
For example, the City of Lancaster study found that their Green Infrastruc-
ture Plan could cut capital costs for gray infrastructure by $120 million—the
estimated cost for reducing CSOs via gray infrastructure storage, such as
a tunnel (USEPA 2014). In another study in West Union, Iowa, the life-cycle
costs of a permeable paver system and a traditional concrete pavement in
a parking lot were compared; the analysis showed that over the life of the
project, savings could be close to $2.5 million by selecting the permeable
pavement (NRDC 2013). Although green infrastructure could have greater
capital costs, the potential extended life of the system and avoided costs
can provide significant savings when analyzed over a long life cycle.
Reduces Property Damage Due to Flooding
J Lastly, green infrastructure practices can lessen the level
of damage from flooding. Among the types of flooding
that could become more frequent are localized floods
and riverine floods. Localized flooding happens when rainfall overwhelms
the capacity of urban drainage systems, while riverine flooding happens
when river flows exceed the capacity of the river channel.
In areas impacted by localized flooding, green infrastructure practices can
be used to absorb rainfall and reduce the amount of water that is dis-
charged in stormwater systems, pools in streets, or seeps into basements
(Qin 2013). In areas impacted by riverine flooding, green infrastructure,
open space preservation, and floodplain management can all complement
gray infrastructure approaches and reduce the extent of flood damage.
1.3 Benefits of Green Streets
-------
1.4 Additional Resources: Green Infrastructure
Numerous green infrastructure guidance and design manuals are available from online sources. As noted below, many have been tailored to represent the
needs of particular regions of the country*
West
California (Los Angeles). Development Best Management Practices Handbook
California (San Francisco). Green Stormwater Infrastructure Typical Details.
Appendix B of Stormwater Management Requirements and Design Guidelines
California (San Mateo County). Green Infrastructure Design Guide
California. San Francisco Stormwater Management Reguirements and Design
Guidelines
Colorado (Denver). Ultra-Urban Green Infrastructure Guidelines
Oregon. Low Impact Development Approaches Handbook
Oregon (Portland). Stormwater
Management Manual includes Green
Street Typical Details
Washington (Puget Sound).
Integrating LID into Local Codes: A
Guidebook for Local Governments
Washington (Seattle) Streets
Illustrated: Right-of-Wav
Improvements Manual
West
(also includes
Hawaii & Alaska)
Midwest
- Illinois (Chicago"). Green Alley Handbook
- Michigan. Great Lakes Green Streets Guidebook
- Michigan. Low Impact Development Manual for Michigan
- Minnesota (North St. Paull Living Streets Plan
- Minnesota Stormwater Manual
- Missouri (Kansas City). Green Stormwater Infrastructure Manual
- Nebraska (Omaha). Green Streets Plan for Omaha
Southwest
Arizona. Green Infrastructure for
Southwestern Neighborhoods (Spanish
version)
Arizona (Mesa). Low Impact Development Toolkit
Arizona (Pima County). Low Impact Development and Green
Infrastructure Guidance Manual
Texas. San Antonio River Basin Low Impact Development Technical
Guidance Manual
Northeast
Midwest
NM
Southwest
Southeast
Northeast
- District of Columbia. Greening DC Streets: A
Guide to Green Infrastructure in DC
- Maryland Stormwater Design Manual
- Massachusetts (Holvokel. Green Streets
Guidebook
- Pennsylvania. Philadelphia Green Streets
Design Manual
- Rhode Island Low Impact Development Site
Planning and Design Guidance Manual
Southeast
Kentucky (Louisville). MSP Design Manual. Ch. 18 Green Infrastructure.
North Carolina. Stormwater Design Manual
Tennessee (Nashville). Low Impact Development Stormwater
Management Manual
t The map includes a sample of resources available; it does not represent all potential references that might be available from states and territories across the nation.
Green Streets Handbook q 3 Benefits of Green Streets 1"12
-------
Transportation Typologres and Green
Infrastructure Practices
In This Chapter
2.1 Transportation Typologies
2.2 Arterials
2.3 Collector Roads
2.4 Local Roads
2.5 Alleys
2.6 Parking Lots
2.7 Identifying Opportunities for Green
Infrastructure Placement
2.8 Reconfiguring Designs to Create Space for
Green Infrastructure Practices
This chapter covers how green street concepts can
be applied to different road classification systems,
or transportation typologies, including arterial roads,
collector roads, local roads, alleys and parking lots.
Each typology is suitable for many different types
of green infrastructure practices, from bioretention
to bioswales to permeable pavements. Existing
roadways also provide many opportunities for green
infrastructure, including in verge zones along highways,
in parking lanes, and in median spaces or planting areas
of parking lots.
Sidewalk planters capture runoff from a local road in Emeryville, CA.
Green Streets Handbook
-------
2.1 Transportation Typologies
This handbook addresses typical low impact development and green
infrastructure strategies that can be incorporated into public and private
projects within rights-of-way that are part of a private development or
are owned or maintained by a state, county, or municipal department of
transportation (DOT).
The Federal Highway Administration's (FHWA's) road classification system,
or transportation typology, defines roads based on specific function or
purpose: arterial, collector and local. At the local level, additional sub-
classes often include alleys and parking lots (Table 2-1).
Many cities further categorize streets according to land use context,
neighborhood characteristics and other special considerations to recognize
the scope of activities that occur along the street, such as:
- Parkway
- Main street
- Industrial thoroughfare
- Commercial (small, medium, large)
- Downtown historic corridor
- Shopping district
- Transitway
- Neighborhood/residential street
Table 2-1. Transportation category descriptions1
Transportation
category
Description
Examples
Users
Arterial roads
Fast-moving, high-traffic roads for vehicular travel between and around urban
Interstates and highways
areas. These roads typically have several travel lanes (two to four).
Collector roads
Moderate-traffic roads that serve high-density areas, including residential,
mixed use and neighborhood business districts. Speed limits and traffic
volumes depend on adjacent land use. These roads offer some connections
to individual parcels and driveways.
Avenues, boulevards and
parkways
c$o ft 0 ®
Local roads
Low-traffic roads with slow speeds that serve residential areas. Many
Road and streets
&o
-------
Road Usage Influences Management Approach
To avoid compromising safety and disrupting access and mobility, a road's
classifications and the context of the road project should be considered
when determining where to site practices (Figure 2-1). The specific strat-
egies and technologies implemented will vary depending on the following
transportation system characteristics:
- Road usage types
- Traffic volumes
- Specific project conditions
- Adjacent land uses
- Contributing drainage area
- Available space
- Site characteristics (e.g., slope, soils, infiltration capacity)
Sections 2.2—2.6 discuss the type of practices that are typically appropri-
ate for the various road classifications.
Figure 2-1. Numerous factors must be considered when choosing and siting green
For Mor6 Information-Road Classification infrastructure practices as part of a green street design.
Highway Functional Classification Concepts. Criteria and
Procedures, Section 3. U.S. Department of Transportation (2013)
Neighborhood
Characteristics
and Density
Context
Special Considerations
•Jurisdiction
•Utilities
•fnnsprvation Areas 1
Streets
Highway.
Downtown business area.
Neighborhood/residential street.
Green Streets Handbook
2.1 Transportation Typologies
-------
Road Usage Influences Choice of Projects
A variety of site design strategies and green infrastructure practices
are appropriate for developing green streets. Table 2-2 provides a quick
reference for screening practices that could be appropriate for the trans-
portation typology or application being considered.
More detailed descriptions of practices appropriate for each of these road
typologies are outlined in the following sections. Key design features for
each of these practices are discussed in Chapter 4. Specific technical
information for each practice type is provided in Chapter 6.
It should be noted that, in general, most of the green infrastructure
practices in this handbook provide the same basic stormwater functions,
but the shape of the practice (depth, width, geometry) will differ based
on the site and geotechnical factors. For example, bioretention cells and
stormwater curb extensions manage stormwater in a similar manner, but
their construction and optimal site locations are different.
The practices in this handbook were chosen because they can be imple-
mented in a variety of projects, ranging from narrow rights-of-way to urban
sidewalks to highway shoulders. Additional practices not included in this
handbook might also be appropriate in certain applications. Some of the
resources listed within the chapters and in the reference section cover
these practices.
For More Information-Roadway Rating Systems
Incorporating green infrastructure is just one element to consider
when developing sustainable roadways. Other important factors
include the types of materials and resources used, the operation and
maintenance needs, and energy and atmosphere impacts. Several
states and other third parties have developed scorecards to encour-
age transportation departments to address these topics. Some of
these certification and rating systems include:
- Federal Highway Administration INVEST tool
- Illinois - Livable and Sustainable Transportation Rating System
and Guide
- New York State Department of Transportation GreenLITES (Green
Leadership in Transportation Environmental Sustainabilitv^
- Greenroads Rating System ($)
- Institute for Sustainable Infrastructure (Envision rating system!
- EPA Guide to Sustainable Transportation Performance Measures
(2011)
| Green Infrastructure Practices for Roadways and Parking Lots
• Most appropriate
O Depends on site context
O Least appropriate
Bioretention
Bioswale
Stormwater
curb extension
Stormwater
planter
Street trees
Infiltration
trench
Subsurface
infiltration and
detention
Permeable
pavement
Arterial
o
•
o
o
•
o
o
o
Collector
•
o
•
•
•
o
o
o
Local roads
•
o
•
o
•
o
o
o
Alleys
o
o
o
o
o
o
o
o
Parking lots
•
•
o
•
•
o
•
o
Table 2-2. Guide for screening green infrastructure practices for different transportation typologies
Green Streets Handbook
2.1 Transportation Typologies
2-
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2.2 Arterials
Arterials are roads that carry through-traffic between major urban areas or
between the central business district and outlying residential areas. These
roads generally have higher speeds and more traffic lanes than most other
street types. Arterial roads are primarily designed for vehicular transit and
are heavily used by trucks; however, some accommodations are made to
improve accessibility when the road passes through urban areas.
Subcategories for arterials are called major and minor. Minor arterials serve
smaller geographic areas, provide service for trips of moderate length
and might have minimal connection to adjacent parcels as compared to a
major arterial. In urban areas, minor arterials may carry local bus routes.
These distinctions are helpful in identifying the types of users from which
design decisions regarding lane widths can be determined. The minimum
Green Streets Handbook
A bioretention area is located adjacent to an arterial road along the Schuylkill River
in Philadelphia, PA.
desired lane width determines the amount of right-of-way potentially
available for other uses such as stormwater management or bicycle lanes.
The linear stretches of land alongside an arterial road provide opportunities
for siting green infrastructure practices and treatment trains. The selection
of practices is limited by the amount of available area, soil characteristics,
existing topography and roadway safety requirements. A common chal-
lenge is the presence of compacted soils, which is typically the result of
construction-related grading activities. Because of potential compaction
issues, infiltration rates should be tested beforehand. If necessary, soil
should be modified (i.e., by adding soil amendments) to meet design
standards. Using pretreatment devices such as swales and buffer strips
is highly recommended to reduce sediment loads and runoff volumes and
A bioretention area located in the median of an arterial road captures runoff in the
Great Lakes region.
-------
maintain long-term infiltration rates. Green infrastructure practices are
typically suitable in three main arterial road zones (Table 2-3):
- When present, medians are an ideal location for linear practices
such as bioswales and infiltration trenches. Bioretention cells
might be applicable depending on the amount of available area.
Reforestation is an option if the median is large enough and the
trees do not obstruct drivers' lines of sight or interfere with utilities.
- Shoulders and breakdown lanes of a road can be good locations
for permeable pavement or open-graded friction course overlays
(see Chapter 4.12) because traffic is slow and use is low. An
open-graded friction course spreads flow, reduces splashing
and maximizes infiltration. It also improves safety by reducing
hydroplaning and light reflectivity off the road surface.
- The verge, the area adjacent to a roadway, can be ideal for linear
practices such as bioswales, infiltration trenches and tree canopy
enhancements. Trees require ample open space and should not
obstruct drivers' lines of sight or be a collision safety hazard. Low-
growing vegetation might be the best choice for curving roadways.
Medians with rain gardens manage stormwater runoff from the street collected via
stormwater inlets connected to subsurface pipes in Arlington, VA.
Green Streets Handbook
Table 2-3. Suitability of green infrastructure practices for arterial road zones
• Most appropriate
O Depends on site context
O Least appropriate
Medians
Shoulder
and/or
breakdown
lanes
Verge
Bioretention
•
o
•
Bioswale
•
o
•
Stormwater curb extension
o
o
o
Stormwater planter
o
o
o
Street trees
•
o
•
Infiltration trench
•
o
•
Subsurface infiltration and detention
o
o
o
Permeable pavement/open graded friction course
o
o
o
Road runoff will be treated by this bioswale in the median of Adelphi Road, an
arterial road in Maryland.
2.2 Arterials
2-6
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2.3 Collector Roads
Collector roads serve to funnel traffic from local roads to other local roads
or arterials. They have high traffic volumes and multiple travel lanes (two
or three). These roads often serve as routes for public transit and must
provide adequate pedestrian facilities to allow safe and comfortable access
and waiting areas. They offer some connections to individual parcels and
driveways, and they can include on-street parking and shared bike lanes.
Collectors in mixed-use or neighborhood business districts tend to have
slower speed limits to accommodate pedestrians. The addition of green
infrastructure practices can also enhance pedestrian safety. For example,
placing stormwater curb extensions at intersections or near crosswalks
can calm traffic and alert drivers to pedestrian activity. Additionally, exten-
sions can decrease the crossing distance, enabling pedestrians to safely
cross streets.
Figure 2-2 illustrates a collector road through a neighborhood business
district. The placement and types of green infrastructure practices that
are feasible along collectors are denoted in the legend. As shown on the
next page, a street's configuration might also influence the selection of
particular practices.
rrrrttl
Permeable pavement parking lane
• Stormwater planter
Stormwater tree trench
Stormwater curb extension
Bicycle lane
Figure 2-2. A collector road with green infrastructure features in a neighborhood business district.
Green Streets Handbook 2.3 Collector Roads
-------
Implementing green infrastructure practices in urban areas—especially
in the right-of-ways on collector roads—is often challenging because less
space is available and a utility conflict is more likely. In areas with high
pedestrian traffic, practices with a smaller footprint or designs that pre-
serve walkway width are more desirable. Green infrastructure practices are
typically suitable in three main collector road zones (Table 2-4):
- Medians and rights-of-way are ideal for linear practices like
bioswales and infiltration trenches. Collector roads without high
pedestrian traffic might be better suited for bioswales, which often
require more surface area and can handle large runoff volumes.
Wide medians might also be appropriate for bioretention cells.
- On-street parking areas, bike lanes or sidewalks are best suited
for permeable pavement, especially in dense urban areas where
space for multimodal uses is at a premium. If space allows,
stormwater planters can be used to separate a bike lane from a
driving lane. Stormwater curb extensions can be placed mid-block
or at the intersection of a parking lane. Maintenance needs should
be planned and budgeted in advance.
support pedestrian traffic and horizontal and vertical space is
available to accommodate tree growth. Suspended pavement
designs that support the weight of paving and allow soil beneath
to remain uncompacted can help provide sufficient soil volume for
trees. Street trees help define the road boundary, protecting both
pedestrians and motorists.
Table 2-4. Suitability of green infrastructure practices for collector road zones
• Most appropriate
O Depends on site context
O Least appropriate
Bioretention
•
o
•
Bioswale
•
o
•
Stormwater curb extension
O
o
•
Stormwater planter
O
o
•
Street trees
•
o
•
infiltration trench
•
o
o
Subsurface infiltration and detention
o
o
o
Permeable pavement
o
o
o
- Collectors with curbs and sidewalks are appropriate locations for
stormwater curb extensions, stormwater planters and street trees.
These practices should only be installed where sidewalk width will
Bioswale separates sidewalks from bike lanes and
vehicular traffic in Indianapolis, IN.
Permeable pavement parking lane in downtown
Syracuse, NY:
Bioretention cell in sidewalk with seating along a
commercial corridor in Washington, DC.
Green Streets Handbook
2.3 Collector Roads
2-8
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2.4 Local Roads
Local roads are low-traffic roads predominant in neighborhood areas.
Because they serve residences, local roads could have a high pedestrian
presence, sidewalks and shared bike lanes. There will be significant
on-street parking for residents. Local roads account for the largest percent-
age of roadways in terms of total road miles (USDOT 2013).
Porous pavement
in parking Janes-
Catch basin
receives overflows
Flow-through or
Infiltration planters
Street trees for shading and
sto nnwater intercept ion
Eioswales, flow-through
planters or infiltration planters
Pedestrian crossing
at comers
Figure 2-3. A local road with green infrastructure features in a neighborhood area.
Figure 2-3 illustrates the placement and types of green infrastructure
practices that are appropriate along local roads. Other opportunities for
siting practices are described in more detail on the following page.
Green Streets Handbook
2.4 Local Roads
2-9
-------
Many of the green infrastructure practices recommended for collector
roads also apply to local roads; however, local neighborhood characteristics
should be considered as part of the decision-making process. Sufficient
sidewalk widths and adequate separation from vehicular traffic should be
maintained to preserve safety and comfort for pedestrians. Depending on
the design, introducing green infrastructure can enhance pedestrian safety.
Green infrastructure practices are typically suitable in the rights-of-way or
bike or parking lanes of local roads (Table 2-5). When choosing specific
practices, consider the site's stormwater management characteristics:
- Practices applicable to roads with curbs include stormwater curb
extensions, stormwater planters, tree pits and tree trenches, and
bioswales. These practices require curb cuts or inlets to direct
stormwater to the practice from the street.
- Roads without curbs are more commonly associated with
bioretention and bioswales when sufficient area exists to locate
these practices without infringing on vehicular or pedestrian traffic.
These practices depend on sheet flow to convey runoff.
Pervious concrete pavement on a low-speed residential roadway in Shoreview, MN.
Green Streets Handbook
Table 2-5. Suitability of green infrastructure practices for local road zones
• Most appropriate
O Depends on site context
O Least appropriate
Bike or
parking lanes
Right of way
Bioretention
o
•
Bioswale
o
•
Stormwater curb extension
o
•
Stormwater planter
o
o
Street trees
o
•
Infiltration trench
o
o
Subsurface infiltration and detention
o
o
Permeable pavement
o
o
Stormwater curb extension installed with a sidewalk project in Maplewood, MN.
2.4 Local Roads
2 10
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2.5 Alleys
Alleys have many connections to individual parcels and driveways, and they
usually provide access for commercial deliveries, waste collection, access
for emergency vehicles and parking. It is important to preserve right-of-way
access for larger vehicles. Permeable pavement is an ideal practice for
alleys because the drainage area is small and amount of sunlight reaching
the ground is often limited (which can be a factor preventing the use
of vegetated practices). Other appropriate practices include infiltration
trenches and subsurface infiltration and detention (Table 2-6).
Table 2-6. Suitability of green infrastructure practices for alleys
• Most appropriate
O Depends on site context
O Least appropriate
Alleys
Bioretention
o
Bioswale
o
Stormwater curb extension
o
Stormwater planter
o
Street trees
o
Infiltration trench
o
Subsurface infiltration and detention
o
Permeable pavement
o
For More Information-Green Alleys
Chicago Green Alley Handbook. City of Chicago, IL (2010)
Green Streets and Green Alleys Design Guidelines Standards.
City of Los Angeles, CA (2009)
Green Alley: Urban Street Design Guide. National Association of
City Transportation Officials.
Green Streets Handbook
Permeable asphalt alley in Chicago, IL.
Permeable paving in an alley in the Avalon neighborhood in Los Angeles, CA.
2.5 Alleys
-------
2.6 Parking Lots
Parking lots represent a good opportunity to incorporate green infra-
structure into the layout, especially for new designs (Figure 2-4). Although
retrofitting of parking lots might be expensive, it is often cost-effective to
include green infrastructure practices when the parking lot is reconfigured
or when the pavement is replaced or rehabilitated. Depending on the size
of the parking lot and its use patterns, various surficial and subsurface
practices can be incorporated into the design.
When designing new projects, site design principles aimed at minimizing
effective impervious surface area should be evaluated before other prac-
tices are considered. Site design considerations include geometric layout,
the number of parking spaces, the required dimensions of parking spaces
and the direction of surface flow.
Locate planters at
end of parkingaiiles
Overflow inlet
Curb tuts
Connect planters for
greater capacity and/or
to convey overflows to
receiving drainage system
Bioswales
Porous paving drains
to planters or bioswales
Porous pavement
Figure 2-4. A parking lot with green infrastructure features (bioretention areas and street trees).
Green Streets Handbook
2.6 Parking Lots
2 12
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Green infrastructure practices should be designed with vehicle and
pedestrian movement and safety in mind. Long linear practices should
include pathways for pedestrians to cross without stepping on the practice.
Practices must allow adequate room for motorists to safely exit their cars.
Safety can be enhanced if practices are configured to serve as a buffer
between vehicle travel lanes and pedestrians.
Stormwater management practices that include trees and large bushes
can shade areas of impervious cover, providing heat mitigation benefits
by reducing the effects of heat reflection and absorption. Shaded parking
lots are also desirable for drivers who want to keep their vehicles cooler.
Incorporating vegetation into practices can improve the visual aesthetic of
a parking lot, making the establishment appear more welcoming.
Green infrastructure practices are typically suitable in parking bays,
traffic islands and along the perimeter of parking lots (Table 2-7). Islands,
parking bays and parking lot perimeters can be designed or retrofitted to
include bioretention, bioswales, trees, infiltration trenches, street trees and
subsurface infiltration/detention. Permeable pavement is most suitable
for low-traffic, low-speed uses such as parking bays. Interlocking concrete
pavers are more often used in high-load commercial and industrial
settings. If cost or use patterns are a concern, consider using permeable
pavement in the stalls and conventional pavement in the travel lanes. For
an overflow parking lot with infrequent use, consider using grass pavers or
concrete-grid gravel pavers instead of pavement.
For More Information-Parking Lot Design
Design Guidelines for 'Greening' Surface Parking Lots. City of
Toronto, Canada (2013; emaii for copy)
Green Parking Lot Resource Guide. USEPA (2008)
LID Parking Lots: Technical Assistance Memo. California Water
Quaiity Regional Control Board
Sustainable Green Parking Lots Guidebook. Montgomery County
Planning Commission, PA (2015)
Green Streets Handbook
Table 2-7. Suitability of green infrastructure practices for parking lots
• Most appropriate
O Depends on site context
O Least appropriate
Medians
Traffic
islands
Perimeter
or parking
bays
Bioretention
•
•
•
Bioswale
•
o
•
Stormwater curb extension
o
o
o
Stormwater planter
•
o
o
Street trees
•
•
•
Infiltration trench
•
o
•
Subsurface infiltration and detention
o
o
o
Permeable pavement
o
o
o
Permeable pavers installed at the downgradient end of parking bays collect
surface runoff and allow it to infiltrate.
2.6 Parking Lots
2 13
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2.7 Identifying Opportunities for Green Infrastructure Placement
Rights-of-way offer many opportunities for siting of green infrastructure
practices, as depicted by the orange shaded areas on the photos on the
next page. As shown in Figure 2-7, the rights-of-way between sidewalks,
bicycle lanes and the vehicle travel lanes can be ideal sites for a storm-
water planter. Similarly, green elements can be incorporated into long
roadside zones (Figure 2-8) or parking areas (Figure 2-9), or in smaller
spaces such as unused triangles at the intersection of diagonal streets
(Figure 2-10).
For More Information-Road Retrofits
Grey to Green Road Retrofits. Credit Valley Conservation, Canada
(2014)
Municipal Handbook: Green Infrastructure Retrofit Policies.
USEPA (2008)
Road Type Influences Rights-of-Way Zone Usage
Depending on their use categories, street and parking lot rights-of-way
can be divided into zones such as travel lanes, parking lanes, curb
zones/shoulders, throughway zones/pedestrian areas and store
frontage zones. The width allotted to each zone is a critical aspect of
street design; width influences traffic speeds, access for multiple users,
and overall user comfort and safety. The road's use classification and
location will influence whether the right-of-way zones are designed to
emphasize benefits for pedestrians or vehicles (Figures 2-5 and 2-6).
Decisions for travel lane widths are based on transportation typology
and context; however, traffic calming goals and desired use also should
be considered. Travel lane width has been shown to impact traffic
speeds: wider travel lanes are correlated with higher vehicle speeds
(Fitzpatrick 2.000). By reducing the street width, traffic speeds decline
and space in the right-of-way becomes available for other purposes,
such as the placement of green infrastructure practices.
Figure 2-5. In this setting, pedestrian-friendly zones have a relatively high amount of
space in the right-of-way relative to the size of the street.
Verge Zone
or Roadside
Zone
Figure 2-6. In this setting, the right-of-way zones are geared toward vehicles.
Green Streets Handbook
2.7 Identifying Opportunities for Green Infrastructure Placement
2 14
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Existing Roadway Rights-of-Way Offer Available Space for Green Infrastructure
U=
Figure 2-7. Adding a buffer, such as a stormwater pianter, between modes
of transportation can control stormwater and improve safety.
Figure 2-8. Green elements such as a swale, permeable pavement or a
permeable friction overlay can be added in the verge area (roadside zone).
Figure 2-9. Alternative surfaces such as permeable pavement can be used
in on-street parking lanes.
Green Streets Handbook
Figure 2-10. Green infrastructure practices can be incorporated into unused
space at the intersection of diagonal streets.
2.7 Identifying Opportunities for Green Infrastructure Placement
-------
Existing Parking Lot Designs Can Accommodate Green
Infrastructure
Parking stall dimensions are typically mandated by local zoning ordinances
and are determined with respect to car size and frequency of vehicle
turnover. Existing space in parking lots can often be filled with green infra-
structure practices while preserving the same number of parking spaces.
For example, an existing parking lot island surrounded by a curb can
be retrofitted to include a bioretention feature (Figures 2-11 and 2-12).
Similarly, by adjusting the length or placement of the parking stall, space
can be made available to add a swale either in a median between facing
stalls or around the perimeter of the lot (Figures 2-13 and 2-14). Stall
widths can also be varied in the same lot to accommodate green features.
High-turnover stalls nearest to the establishment can be built wider than
stalls farther away, creating room for green infrastructure without reducing
the number of available parking spaces.
Figure 2-13. In this parking lot the impervious median space between facing
parking stalls could be retrofitted to infiltrate runoff.
Green Streets Handbook
CD
o
e>
Figure 2-11. This conventional parking
lot island could be retrofitted for green
infrastructure features.
Figure 2-12. A parking lot island
includes a bioretention feature in
Maplewood, MN.
Figure 2-14. In this parking lot the median space between facing parking stalls
includes a bioretention area.
2.7. Identifying Opportunities for Green Infrastructure Placement
2 16
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2.8 Reconfiguring Designs to Create Space for Green Infrastructure
Reconfiguring roadways offers opportunities to create new space for green
infrastructure. FHWA uses the term "Road Diet" to describe this practice, which
is a high-value, low-cost way to improve safety and enhance a street's overall
functionality. Roadway reconfiguration projects typically include removing a
lane and/or reducing lane width, A classic Road Diet involves converting an
existing four-lane, undivided roadway segment to a three-lane segment consist-
ing of two through lanes and a center, two-way left-turn lane (Figure 2-15).
A Road Diet can provide space that can be reclaimed for other uses such as
bus lanes, bike lanes, bus shelters and green infrastructure features. These
stormwater management features can be built in conjunction with pedestrian
refuge islands or as safety/crossing barriers between motorists and pedestri-
ans—achieving multiple benefits.
In 2014 the City of Lancaster, Pennsylvania, completed an award-winning
retrofit of a dangerous intersection (Figure 2-16). The project removed a
designated turn lane and added green elements, including permeable paver
parking areas and patios, curb extensions and rain gardens, and a cistern that
captures stormwater from the roof of a brewery adjacent to the intersection.
The project calmed traffic and increased pedestrian safety by narrowing the
traffic lane, while also offering aesthetic enhancement and patio space for the
brewery. Research indicates that these types of roadway reconfigurations are
likely to reduce accident rates (TRB 1990),
When a Road Diet is planned in conjunction with roadway reconstruction or
simple overlay projects, safety and operational benefits often can be imple-
mented at low cost (i.e., the cost of restriping the road). Incorporating green
street elements should be considered when the overall design of the street is
being changed or utilities are being installed or upgraded. Chapter 3 discusses
how to select appropriate green infrastructure practices.
Before
After
Figure 2-15, This simple road diet shows how two travel lanes
are removed and replaced with one turn iane and two areas that
could support green infrastructure practices.
Before
Narrow sidewalk
Impervious areas
For More Information-Road Diets
Road Diet Informational Guide. Federal Highway Administration (2014)
Road Diets (Roadway Reconfiguration! Federal Highway Administration
After
Permeable parking
Rain garden
Permeable patio
Figure 2-16. A roadway was reconfigured to replace a turn lane with
green infrastructure practices in Lancaster, PA.
Green Streets Handbook 2.8 Reconfiguring Designs to Create Space for Green Infrastructure I 2 17
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Developing a Green Streets Program:
A Process Overview
In This Chapter
3.1 Programmatic Process Overview
3.2 Establish Objectives
3.3 Identify Priority Area(s)
3.4 Characterize Sites
3.5 Develop a Stormwater Plan
3.6 Engage Community Partners
This chapter covers the process to develop a green
streets program, beginning with establishing objectives,
identifying priority areas, characterizing the sites
and developing a stormwater plan. A green street
stormwater plan will help you identify site constraints
and opportunities, calculate impervious areas and
runoff volumes, select appropriate green infrastructure
practices, and consider costs. An effective green street
program will also engage community partners in the
process.
¦W v ''
w>-iiar 1 • i..
%
Traffic calming and stormwater bioretention curb bump-out
project, Cleveland, OH.
Green Streets Handbook I 3 1
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3.1 Programmatic Process Overview
Pursuing a green street program requires consideration of various tasks
as noted in Figure 3-1. The programmatic process is presented in a linear
fashion, but when retrofitting existing transportation networks, steps
may be completed in a different order or concurrently. For example, if a
street repaving project is under way, then the priority area has already
been established and the objective(s) and a site characterization should
be determined. A discussion of each task is provided in other areas of this
handbook as denoted by the referenced section number.
Identify Priority Area(s) (3.3)
Engage
Community
Partners (3.6)
Obtain Feedback
(Ongoing Process}
¦ Maintenance staff
¦ Externa] entities
¦ City departments
¦ Public stakeholders
Internal
¦ Infrastructure upgrades
¦ Capital improvement projects
¦ Local concern (e.g., flooding)
External
• Community requests
¦ City hotline
• Development
Example Criteria
¦ Effective; meets specific needs
¦ Easy to irrplemenr and maintain
' Visibility
Establish
Objective(s) (3.2)
Typical Objectives
¦ Reducing stormwster quantity
and pollutants
¦ mproving motorist, bicyclist
and pedestrian safety
¦ improv:n Select green infrastructure practice(s)
1 Consider costs
Design (4.0)
Plan
' Develop alternatives
• Adress objectives
¦ Evaluate impact
¦ Consider maintenance
Y
Characterize Site(s)
(3.4)
v /
Existing Conditions
* Physical components
* Modal uses
* Geotechnical site factors
* Context
Figure 3-1. Recommended programmatic process for pursuing a green streets program.
Green Streets Handbook
3.1 Programmatic Process Overview
3-2
-------
3.2 Establish Objectives
Designing green streets requires a multifaceted approach to creating livable
and aesthetically pleasing spaces. The following program objectives are
commonly used to help justify a green streets program:
- Stormwater control. DOTs must often address regulatory
requirements for stormwater runoff quantity and quality from
streets (including MS4 permits, flooding, impaired waters, replacing
aging infrastructure, etc.). Green streets can address multiple
regulatory requirements in a single design.
- Safety. Green street designs can improve motorist, bicyclist and
pedestrian safety by adding practices that slow traffic (curb bump-
outs), adding separate bike lanes, and providing clear and separate
areas for pedestrians and pedestrian crossings.
- Access and mobility. Green streets can be designed to offer
multiple transit options or designed to improve access for bus, bikes
and pedestrians. For example, dedicated bike and bus lanes can be
integrated into a green street design to ensure dedicated access.
- Context. Context refers to the project's physical, economic
and social setting. Green streets can help improve community
cohesiveness, ecological function, aesthetics and transportation
system efficiency.
- Livability. Green streets can improve community livability by
increasing tree canopy cover and vegetated practices. Livability can
also be improved by increasing walkability and access for bikes.
- Cost-effectiveness. Adding green infrastructure can reduce overall
costs when compared to the construction and maintenance of
traditional stormwater infrastructure.
Before embarking on a project, it is advisable to establish goals and
objectives that can be easily communicated to the public and be used to
measure success (examples are in Table 3-1). Early engagement of stake-
holders (see section 3.6) is critical to securing participation and buy-in from
the public and other agencies.
Green Streets Handbook
Table 3-1. Example objectives of a green streets program
Focus area
Objective
Stormwater control
- Identify priority watersheds and project
opportunities
Safety
- Improve pedestrian safety at crosswalks
Access and mobility
- Balance multiple modes of transport
Context
- Create linkages between community destinations
Livability
- Explore opportunities to promote streets for
additional uses (e.g., adding bike lanes)
Cost-effectiveness
- Reduce construction and maintenance costs
Stormwater control, safety and livability are among the objectives fulfilled by these
green infrastructure practices in Greensboro, NC.
3.2 Establish Objectives
-------
3.3 Identify Priority Area(s)
Priority areas can be selected on the basis of a site-specific need or by
using established objectives (see section 3.2) to screen potential project
sites. Priority area selection can be influenced by the municipality's internal
priorities (e.g., needed infrastructure upgrades, upcoming capital improve-
ment projects, existing localized problems such as flooding) or requests
from external sources (request submitted by communities or through
a hotline, planned development). For example, repeated traffic accident
reports (internal) or a request from a community member (externa!) could
influence a decision to retrofit an intersection for safety reasons. Similarly,
redevelopment projects that impact rights-of-way could be routinely
evaluated as part of the review process to determine opportunities to add
green infrastructure practices.
Existing municipal stormwater management plans, capital improvement
projects, weather resiliency plans, or citywide initiatives can be used to help
identify potential green infrastructure sites. A stormwater plan can identify
neighborhoods that have flooding issues that could benefit from wide-
spread implementation of green infrastructure practices. The development
of a new stadium, a commercial development, or a street expansion project
represent opportunities to "green" public rights-of-way and more effectively
manage runoff.
Once a list of projects has been compiled, the projects should be sched-
uled for implementation based on criteria selected for prioritizing projects,
such as need, cost, public demand, etc. When a community is initiating
the use of green infrastructure practices, selecting highly visible projects
with a high probability of success often helps to garner public acceptance
of green infrastructure because successful projects can create support or
demand for similar projects within the jurisdiction.
Green Streets Handbook
To improve safety, curb bump-outs were added to the corners to decrease the
crosswalk distance and make pedestrians more visible to motorists.
oln QQrder
Signs help raise the visibility of a project by communicating why the stormwater
feature was built and the benefits it provides.
3.3 Identify Priority Area(s)
I
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3.4 Characterize Sites
Once goals and priority areas have been identified, a designer must assess
the site to determine which green infrastructure practices are appropriate for
the site conditions. A base map can be a useful tool for determining site con-
straints and other factors that might influence the choice of certain green
elements (Figure 3-2). The site assessment should include physical, modal,
geotechnical and contextual analyses (Figure 3-3). Conducting site visits is
recommended to ensure the accuracy of the existing data, especially if time
has lapsed since the information was surveyed.
The results of the site characterization can help identify factors (e.g., the
presence of underground utilities, high or low soil infiltration rates, or land
use patterns and citizen behaviors) that might influence whether a given
practice is appropriate for the site, given programmatic objectives, perfor-
mance requirements, available funding or maintenance concerns.
For example, infiltrative capacity can determine whether a curb bump-out:
must have an underdrain or be designed as a flow-through planter. The
size of the available area and its contributing drainage area also will
determine what practices are appropriate. Foot traffic, sightlines, overhead
utilities and maintenance requirements should also be considered. Design
alternatives, however, can be used to compensate for some site factors as
presented in Chapter 4.
it«*p slope
setback
building
setback
wetland
setback
wetland
c»ns«fvalJ
-------
3.5 Develop Site-Specific Stormwater Plan
When developing a stormwater plan for a green street, several steps are
necessary: (1) identify site constraints and opportunities, (2) calculate
impervious areas and runoff volumes, (3) select green infrastructure
practices, and (4) consider costs. (Note: a site-specific stormwater
management plan is generally part of watershed plan, master plan or
citywide stormwater plan that addresses larger management areas.)
Step 1. Identify site constraints and opportunities
First, identify opportunities in the rights-of-way, which might include
medians, travel lanes, road shoulders, sidewalks and pathways, and slopes
and drainage easements. Not all rights-of-way are appropriate for green
infrastructure practices, however. Possible constraints should be assessed,
which could include the width of the right-of-way, presence of utilities
(above or below ground), roadway geometry and slope, proximity to storm
drains, run-on stormwater flows, contributing drainage area, type of vehicu-
lar use, potential for pollution spills and high pollutant loads, ease of access
for maintenance, reduced safety for pedestrians or vehicles, presence of
bike and parking lanes, and cultural factors associated with the site.
Step 2. Calculate impervious areas and runoff volumes
Impervious areas associated with roads should be measured to calculate
the volume of stormwater that runs off. Most state and local governments
have specific requirements on how to calculate the stormwater design
volume from impervious areas or the contributing drainage area(s).
Step 3. Select practices
Once the design volume is calculated, potential green infrastructure
practices can be identified for specific locations. Chapter 2 includes
examples of green streets for different street typologies. Chapter 6
provides information on the types of practices that are commonly used on
green street projects.
Green Streets Handbook
Table 3-2. Relative costs for green infrastructure practices (per cubic foot of water)
• High O Medium O Low
Capital
Operations and
maintenance
Bioretention
o o
o o
Bioswale
o o
o o
Stormwater curb extension
o
o o
Stormwater planter
o
o o
Street trees
o o •
o
Infiltration trench
o
o
Subsurface infiltration and detention
•
o
Permeable pavement
o
o
Sources: Clary et al., 2017; RTI and Geosyntec 2014
Step 4. Consider costs
Capital and operations and maintenance costs should be considered when
selecting green infrastructure practices (Table 3-2). Costs will vary by
location (i.e., site conditions or distance to material supplier), type of project
(i.e., retrofit or new construction), and particular application and design
specifications (i.e., required retention volume or depth of practice). Regional
availability of expertise and supplies can also play a significant role in
overall costs. Demand for green infrastructure can also create economies
of scale that reduce material costs (e.g., in Chicago the cost of permeable
pavement for alleys dropped significantly over the project period).
The costs for green infrastructure practices should be considered with
respect to their ability to serve multiple functions, the benefits they provide
and their anticipated life cycle. For example, practices such as permeable
pavement, which serves both as a surface and a stormwater management
practice, can save costs in a jurisdiction where stormwater management is
required. By adding permeable pavement, the need for subsurface detention
facilities, underdrains and related conveyance pipes can be reduced or
avoided. Cost-benefit analyses and life-cycle assessments are useful meth-
odologies for determining the costs of practices within a broader framework.
3.5 Develop Site Specific Stormwater Plan
3-6
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3.6 Engage Community Partners
Communication between all stakeholders should occur throughout the
entire green street planning, design and implementation process. A dialog
should be established with community residents, local business owners,
and staff from public agencies or departments—especially agencies that
need to maintain the green infrastructure or meet their own programmatic
goals and objectives (e.g., landscaping or maintenance staff, fire and
rescue services, planning and zoning departments).
Implementing a diverse outreach plan can ensure that stakeholders are
made aware of projects, educated about the objectives and empowered
to influence the outcomes. With the advent of social media, stakeholders
can be engaged online through participatory surveys, interactive design
tools, websites and other platforms. These methods could also be coupled
with neighborhood open houses, door-to-door outreach and direct-mail
marketing. To encourage discussion, some municipalities have developed
planning scenarios for stakeholders to help them understand the potential
impacts of such decisions. Outreach strategies should be ongoing
throughout the process to give ample opportunity for feedback and to keep
stakeholders up-to-date.
Guidelines to consider for community engagement, as adopted from
The Sustainable Communities Initiative (Bergstrom et al. 2013), include:
- Be proactive and targeted in engagement strategies.
- Build clear opportunities for decision making and
partnerships among community organizations.
- Prioritize community knowledge and concerns.
- Develop cultural competency skills and cultivate humility.
- Support capacity building to engage meaningfully.
- Engagement processes should include space to be iterative
and reflective.
- Target resources to support ongoing engagement.
Green Streets Handbook
Conferring and coordinating with other entities early in the process helps
to secure buy-in, increasing support for the project and possibly helping to
procure matching funds and other financial resources for ongoing main-
tenance and rehabilitation of the practices. Identifying and coordinating
green street implementation with other community improvement projects
(see box) can reduce costs, improve functionality, and increase overall
benefits and acceptance of green infrastructure.
Example Community Improvement and
Green Infrastructure Collaboration Opportunities
- Bicycle, pedestrian, transit or - Open space planning
greenway planning . . .
a 3 ^ a - Street repaving projects
- Urban forestry stewardship ....... . f
3 ^ - Utility infrastructure improvements
initiatives
Safe Routes to School initiatives
Emergency vehicles and routes
Capital improvement projects
Community/private connections
- Climate change resiliency or
Stormwater master planning . . , , .
K a sustainability designs
For More Information-Programmatic Process Elements
Green Values National Stormwater Management Calculator
(Costs). Center for Neighborhood Technology (2009)
Getting to Green: Paving for Green Infrastructure-Financing
Options and Resources for Local Decision-Makers USEPA (2014)
Community Solutions for Stormwater Management: A Guide for
Voluntary Long-Term Planning USEPA (2016)
Green Infrastructure in Parks: A Guide to Collaboration. Funding,
and Community Engagement USEPA (2017)
Nonpoint Source Outreach Toolbox USEPA
Increasing Funding and Financing Options for Sustainable Storm-
water Management. Center for Neighborhood Technology (2020)
3.6 Engage Community Partners
-------
Design
Considerations
In This Chapter
4.1 Design Checklist
4.2 Selecting Appropriate Practices
4.3 Accommodating Utilities
4.4 Capturing Stormwater Runoff Types
4.5 Managing Stormwater Flow
4.6 Planning for Maintenance
4.7 Selecting Soil Media and Vegetation
4.8 Providing Pedestrian Access
4.9 Ensuring Pedestrian Safety
4.10 Enhancing Street Design
4.11 Accounting for Extreme Weather
4.12 Avoiding Design Flaws
This chapter covers design considerations for green infrastructure prac-
tices, including a planning checklist and how to select the most appropriate
practice based on the pollutant of concern. Designs need to accommodate
underground utilities, address stormwater runoff rate and volume, plan
for eventual maintenance, and identify appropriate soil media and plants.
Green infrastructure designs can include artistic elements to enhance
aesthetics and better blend into the community, while also providing for
pedestrian access and safety.
Note: The design details described in this handbook are meant to be
conceptual and not final design specifications. Designers should refer to
state or local requirements and recommendations to inform their designs.
Green Streets Handbook
Green street with streetcar, vehicle, pedestrian zones, rain gardens and trees.
Trench drain conveys street runoff into bioretention cells in Washington, DC.
-------
4.1 Design Checklist
Designing Green Infrastructure
Design of the green infrastructure practice(s) should not
proceed until after a field visit has confirmed that a site
is suitable. This chapter provides information on design
elements that should be considered when developing
detailed design plans to achieve one or more objectives
that pertain to the use of green infrastructure.
The design checklist shown in Table 4-1 summarizes
key questions that designers should answer when
developing the site design plan for a green infrastruc-
ture practice in a street or parking lot. As noted in
the table, further discussion about each question is
provided elsewhere in this document.
Designers should also consider applying the following
practices when initiating a project:
- Conduct a geotechnical study for the site
itself. Do not substitute a report from a
nearby project.
- Be mindful of all uses on the site (e.g.,
carts in a shopping mall, informal
pedestrian pathways) to protect soils and
vegetation from encroachment.
- Design a stormwater control practice
that you would want in front of your own
house or business. The aesthetic appeal
of the practice is important.
- Engage community participants early and
throughout design process.
Table 4-1. Site design green infrastructure planning checklist (after site selection is complete)
Yes/No
Checklist for green infrastructure design
Does your design include green infrastructure practices best suited to remove pollutants of concern?
(See section 4.2)
Has the design taken into account the presence of underground utilities on the site? (See section 4.3)
Does the curb cut design (i.e., size and angle of opening, placement, grading) effectively capture the
stormwater? (See section 4.4)
If needed, is there an appropriate pretreatment device to capture sediment? (See section 4.5)
Is there sufficient space available to treat and/or retain the runoff volume from the contributing
drainage area? (See sections 4.4 and 4.11)
Is there a structural feature at the inlet and along the flow path to dissipate energy, slow the velocity
and prevent erosion? (See section 4.5)
Is there ample volume for retention, correct placement and grade of outflow structures to control
ponding and adequate structures to manage overflow? (See section 4.5)
Is there access for maintenance equipment and space for cleanouts and observation wells?
(See section 4.6)
Does vegetation have sufficient soil volume of the appropriate composition type to thrive?
(See section 4.7)
Has the selection of vegetation accounted for local availability, water requirements, ponding and
salt tolerance, maturity rate, sightlines, propensity for seed dispersal and maintenance needs?
(See section 4.7)
Does the layout of the green infrastructure practice allow movement through the site, especially by
pedestrians (i.e., pathways to allow access between sidewalks and parking lanes across stormwater
feature)? (See section 4.8)
Are there visual or physical barriers around the green infrastructure practice to serve as a safety
marker and protect the vegetation? (See section 4.9)
Does the design support your community's livability objectives? (See section 4.10)
Green Streets Handbook
4.1 Design Checklist
I
-2
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4.2 Selecting Appropriate Practices
The types of green infrastructure practices selected for your design will
depend somewhat on the types of pollutants of concern in your stormwater
and your water quality objectives. Table 4-2 provides an overview of the
potential pollutant removal capability of common green infrastructure
practices, which will help designers choose the practices best suited for
their community's needs.
Various factors will influence the performance of green infrastructure
practices, including site characteristics, design specifications, and oper-
ation and maintenance practices. The use of sequential practices (e.g., a
treatment train approach) in a system also will affect overall performance.
Refer to the additional resources listed (see box, next page) to understand
how site and design factors influence performance.
Stormwater curb extensions, such as this one in Portland, OR, capture
pollutants such as total suspended solids, total phosphorus, zinc and lead.
Table 4-2. Relative effectiveness of green infrastructure practices for various constituents based on pollutant-removal efficiencies when practices are properly maintained
Total
Total
Total
Fecal
Total
Total
Total
Suspended Solids
Nitrogen
Phosphorus
Coliform
Zinc
Copper
Lead
Bioretention
•
o
•
-
•
-
o
Bioswale
o
o
o
o
-
-
-
Stormwater curb extension
•
o
•
-
•
o
Stormwater planter
•
o
•
-
•
-
o
Street trees
•
o
o
•
o
o
o
Infiltration trench
•
o
•
•
•
-
-
Subsurface infiltration and detention
•
o
o
•
•
•
•
Permeable pavement
•
-
o
-
•
o
o
Permeable Friction Course
•
-
-
-
•
o
•
Note: The values for subsurface infiltration and detention were considered equivalent to those for sand filters. Stormwater curb extension and stormwater planters were considered bioretention devices.
For all constituents, O = 0-30%, © = 31-65%, • = >65%, - = no data
Green Streets Handbook
4.2 Selecting Appropriate Practices
I
3
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For More information-Green Infrastructure Practice Performance
Significant research data is available about the performance of green infrastruc-
ture for road and parking lot runoff Monitoring guidance and information on the
pollutant removal effectiveness of green infrastructure and conventional best
management practices (BMPs) can be found in the International BMP Database,
which is managed by the Water Environment Research Foundation WERF). It is
important to note that performance and cost-effectiveness of practices depend
on site conditions and design considerations.
The Transportation Research Board, through its National Cooperative Highway
Research Program, provides funding to review the water quality benefits and
construction and maintenance needs of stormwater BMPs used on roads. Their
reports include:
- Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual
(2015)
- Long-Term Performance and Life-Cvcle Costs of Stormwater Best
Management Practices (2014)
- Measuring and Removing Dissolved Metals from Stormwater in Highly
Urbanized Areas (2014)
- Pollutant Load Reductions for Total Maximum Daily Loads for Highways
(2013)
- Guidelines for Evaluating and Selecting Modifications to Existing Roadway
Drainage Infrastructure to Improve Water Quality in Ultra-Urban Areas (2012)
- Evaluation of Best Management Practices for Highway Runoff Control (2006)
Trees planted in a bioswale between parking stalls.
Green Streets Handbook
The Federal Highway Administration (FFIWA) has developed several resources
to assist communities in modeling, monitoring and managing water quality
impairments from highway stormwater runoff including:
- Stochastic Empirical Loading Dilution Model (SELDM) (2013) A joint project
between U.S. Geological Survey and FHWA, this model helps develop planning-
level estimates of event mean concentrations, flows, and loads from a highway
site and an upstream or iake basin.
- Determining the State of the Practice in Data Collection and Performance
Measurement of Stormwater Best Management Practices (2014) This report
assesses data collection and performance measurement in stormwater
programs at state departments of transportation.
- National Highway Runoff Water-Quality Data and Methodology Synthesis
(2003)
• Volume 1: Technical issues for monitoring highway runoff and urban
stormwater, FHWA-EP-03-054
• Volume 2: Project Documentation, FHWA-EP-03-055
• Volume 3: Availability and documentation of published information
for synthesis of regional or national highway runoff quality data,
FHWA-EP-03-056
- Remotely Monitoring Water Quality Near Highways - A Sustainable Solution
(2015) This document explores selecting and using a renewable and self-
sustaining onsite monitoring system for highway runoff.
Permeable concrete installed in a Washington, DC, alley.
4.2 Selecting Appropriate Practices
-------
4.3 Accommodating Utilities
Although underground utilities are often cited as a challenge
to green infrastructure implementation, their presence on
a site does not need to prevent green project development.
Depending on the site, planners have the option to avoid,
coexist with, modify, or replace utilities when installing green
elements (Figure 4-1). Obstacles arising during project design
can include requirements for:
- Allowing access to utility lines or pipe galleries for
repair or replacement.
- Providing adequate protection around utility lines and
gravel envelopes.
- Eliminating potential for infiltrated stormwater to
migrate into conduits and pipes.
- Leaving space available to accommodate
vaults and valve boxes.
Depending on the site, these obstacles could be too costly
or difficult to overcome. In other cases, workarounds are
available to handle these utility challenges and enable
construction of green infrastructure within the right-of-way.
Key steps to eliminate problems include:
- Placing all utility vaults outside the "wet" zone of the
stormwater feature when possible.
- Lining the practice along curbs or next to utility
trenches with a thin, impermeable geotextile or liner
to prevent migration of infiltrated stormwater.
- Constructing a deeper-than-conventional curb profile
to physically separate roadbed subgrade or utility
lines from the stormwater feature.
- Installing a clay or other impermeable plug
within the utility trench to inhibit movement of
stormwater within the trench line.
Green Streets Handbook
The easiest and
most cost-effective
option is to site the
stormwater feature
clear of any utility
conflict or reduce
the feature size to
provide sufficient
setback from the
utility.
Utility companies
accept that the
practice will coexist
with the utility.
Sufficient protection
and/or clearance
exists on the site.
If the utility must
be accessed,any
damage to the
stormwater practice
will be repaired.
Modify
The entities agree
that the feature and
the utility can coexist,
but alterations to the
design of either could
occur (e.g., planned
elements of the
stormwater feature
such as inlets and
outlets might need
to be moved to avoid
conflict).
Replace
~
To avoid conflicts,
the utility is replaced
or relocated. This
process would
incur the highest
cost unless the
entire project was
planned as part of
an infrastructure
enhancement project.
Source: Adapted from the San Mateo Green Infrastructure Design Guide (San Mateo 2020).
Figure 4-1. Options for accommodating utilities during design and planning of green infrastructure.
Underground utilities in New York City, NY,
I
4.3 Accommodating Utilities
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4.4 Capturing Stormwater Runoff Types
An essential element of green infrastructure project design is ensuring
the stormwater enters the system and is captured. In urban environments
where curbs are prevalent, stormwater flow accumulates as it moves along
the curbed edges of roadways. Adding curb cuts allows this concentrated
flow to spill into green infrastructure practices. In contrast, stormwater
drains off curbless roadways under sheetflow conditions to the lowest
area.
For both concentrated flow entering a practice through curb cuts and
sheet flow conditions, a minimum 2-inch elevation drop is recommended
between the surface drainage and finish grade at the entrance to the
stormwater feature to ensure that stormwater freely moves into the
practice even with some sediment accumulation. To prevent erosion, an
inlet should be designed with a dry sump, splash pad or other element
that dissipates energy and spreads the flow. Riprap, stone and gravel are
typically used, but some communities are moving away from these materi-
als because they are difficult to maintain cost-effectively.
Capturing Concentrated Flow: Curb Cuts
To capture stormwater runoff from curbed roads, curb cuts are added at
intervals along a raised curb, resulting in areas of concentrated flow. This
practice is commonly used in urban bioretention cells, stormwater curb
extensions, stormwater planters and urban tree trenches. Three
key criteria should be considered when designing curb cuts:
- Placement. The curb cut should be placed in the pathway
of stormwater flow alongside the gutter line. During the
low levels of flow, water is directed into the feature; during
high flow volumes when the feature is at capacity, the
flow bypasses the curb cut and is directed downgradient
along the curb.
- Grading. Slope the bottom of the concrete curb cut
toward the practice (Figure 4-2). If the flow lines along the
gutter are on a steep slope, developers can add a small,
low-profile asphalt/concrete berm or other pavement
modifications such as a runnel to direct stormwater flow into the
practice (Figure 4-3).
- Size and angle of opening. The inlet opening can be sized for the
storm event using standard FHWA software (Hydraulic Toolbox)
or other design procedures that account for ponding, spreading
of flow, slope and other conditions that affect the efficiency of the
inlet. The curb cut opening should be as wide as possible to avoid
restricting flow or becoming blocked by debris (Figure 4-4). The
recommended minimum width is 18 inches or 3 feet in between
wheelstops in a parking lot (Figure 4-5). The sides of the opening
should have either vertical or chamfered (i.e., cut) sides with
45-degree angles (Figure 4-6). Side wings work weil for practices
that have steeper side slope conditions to retain the side-slope
grade (See Figure 4-7).
Curb cuts can be modified based on site-specific conditions. Grated curb
cuts prevent trash and other floatables from entering the practice (Figure
4-8). A trench drain (a shallow concrete trench with a grate or solid cover)
can convey runoff to the practice where pedestrians or vehicles must cross
the drain area (Figure 4-9). These drain systems help to provide egress space
for on-street parking and to maintain grade and access for pedestrians.
Figure 4-2. An angled curb cut with a graded
gutter, Seattle, WA.
Figure 4-3. A runnel directs stormwater flow,
San Juan Island, WA.
Green Streets Handbook
4.4 Capturing Stormwater Runoff Types
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Figure 4-4. Metal extension inlet structure provides
a wide opening for stormwater flow to enter the
stormwater feature.
Figure 4-5. The space between adjacent wheel stops
allows stormwater runoff to enter a vegetated swale in
a parking lot in Cleveland, OH.
Figure 4-6. A curb cut with 45-degree chamfered
edges conveys stormwater into a roadside rain
garden in Friday Harbor, San Juan Island, WA.
Figure 4-7. A curb cut with wings retains the side slope
grade and directs street runoff into a bioretention
feature in Portland, OR.
Figure 4-8. A grated inlet prevents large floatable
trash from entering practice along Deaderick Street
in Nashville, TN.
Figure 4-9. Trench-grated drain conveys stormwater
between swales while also capturing runoff in
Seattle, WA.
Green Streets Handbook
4.4 Capturing Stormwater Runoff Types
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Capturing Sheetflow
In areas without curbs and gutters, practices are designed to capture runoff
via sheetflow across pavement and other surfaces. [Establishing sheet
flow conditions allows for an even distribution of runoff into the feature
(Figures 4-10 and 4-11). Moreover, in conditions of low-velocity sheetflow,
pretreatment such as a pea gravel apron installed between the impervious
area and the practice can help capture suspended sediment
Green infrastructure practices that capture sheet flow from curbless
streets and parking lots often include a band of concrete edging that lies
flush with the stormwater feature and the street/parking lot surface (Figure
4-12). Because of concrete's fine-grain composition, it is easier to use
concrete than asphalt to achieve the necessary flat slope that will direct
sheetflow into the stormwater feature.
Sidewalks can be designed with slight inslopes or outslopes to direct
sheetflow into green infrastructure practices, but the sidewalks must also
comply with local codes and ordinances and meet the slope requirements
outlined in the Americans with Disabilities Act.
Figure 4-11. A curbless grassed and gravel parking lot allows sheetflow
stormwater runoff to enter a vegetated swale in Staunton, VA.
Green Streets Handbook
Figure 4-10. A curbless street allows sheetflow stormwater runoff to enter the
vegetated swale in Lansing, Ml.
Figure 4-12. A sloped concrete band along a road evenly distributes
stormwater to an adjacent vegetative swale in Seattle, WA.
4.4 Capturing Stormwater Runoff Types
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4.5 Managing Stormwater Flow
After a site-appropriate practice is selected to capture the stormwater
flow, several techniques should be considered to manage the flow as it
enters and exits the practice. Correct design elements can prevent erosion,
enhance treatment capabilities and maintain the stormwater feature's
function:
- Pretreatment practices can trap sediment or debris suspended in
the runoff before it enters the practice.
- Energy dissipation elements help prevent scouring and erosion of
the media around the inlet
- Overflow structures allow excess flows to exit the system to
prevent scouring or other damage.
- Bypass structures permit excess flow to bypass the practice
completely.
- Back-up infiltration practices catch flows that exceed the design
capacity of the practice.
- Underdrains remove excess volume to protect the system and also
to reduce ponding or improve infiltration in iow-permeability areas.
Pretreatment Practices
Pretreatment is often recommended to trap sediment or debris before
it moves through the stormwater management practice because the
sediment could clog the practice, reducing infiltration. Commonly used
sediment pretreatment devices include forebays, swales/channels, catch
basin sumps, grit chambers and filter strips (Figure 4-13). For details about
specific pretreatment practices, refer to Chapter 5.
Depending on the volume of flow and available space, pretreatment
measures are often designed at the entrance to the practice using a
forebay with a overflow structure such as a weir (Figure 4-14). Pretreatment
measures should be sized according to the expectant loads and type of
debris (e.g., floatables, leaves, sediment). The area downstream of the
forebay commonly has high-density planting of vegetation that acts as a
containment dam. To ensure the functionality of any pretreatment mea-
sure, accumulated sediment should be periodically removed.
Green Streets Handbook
Figure 4-13. A sediment forebay slows the concentrated flow to ailow
sediment to drop out of suspension in Tucson, AZ.
Figure 4-14. A sediment forebay with weir helps trap sediment and control
flow volume in an alleyway bioswale in Los Angeles, CA.
4.5 Managing Stormwater Flow
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Energy Dissipation
Adding energy-dissipating elements at both the inlet and
along the length of the green infrastructure practice will
help manage fast-moving stormwater flows. A concrete
splash pad (Figure 4-15), riprap or landscape stone
should be installed just inside the inlet to dissipate the
flow as it enters, which will help prevent scouring and
erosion of the soil media around the inlet.
Throughout a linear practice, especially those with a
steep grade, check dams and weirs should be built at
intervals to reduce the velocity, thereby avoiding wash-out
and increasing storage (Figures 4-16, 4-17 and 4-18).
Check dams are stone, concrete, wood or soil berms
that are perpendicular to the flow and span the width of
the treatment cell. Check dams help pond water, which
increases infiltration by slowing water flow velocity in high
slope conditions (BES 2008) and reducing erosion. Scour
protection, which can be provided by placing a strip of
gravel at the downstream side of the check dam, can also
control erosion. Check dam height should be less than
the top elevation of the curb. The placement of check
dams is dictated by flow rates and velocities.
Weirs can be designed with adjustable heights to provide
flexibility on sites that have variable soil conditions.
These practices also help control the ponding of water,
which influences the hydraulic residence time and
effective treatment. A longer retention time helps to settle
sediment out of suspension and filter pollutants. As a
result, check dams are also applied on sites with minimal
longitudinal slopes to promote infiltration where the soils
are suitable, or to promote filtering to an underdrain in
areas with poorly draining soils.
Figure 4-17. Concrete check dams slow flow in
a stormwater curb extension with a 4.2% slope
in Portland, OR.
Figure 4-18. Concrete check dams with
splashpads slow flow velocities along a steep
slope in Seattle, WA.
Figure 4-15. Concrete paver splashpad
dissipates energy from stormwater entering
from a trench in Washington, DC.
Figure 4-16. A piled stone weir/gravel filter
combination slows the water flowing through
this bioretention feature in Gainesville, FL.
Green Streets Handbook 4.5 Managing Stormwater Flow I 4 10
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Overflow Structures
Overflow structures are designed to discharge excess stormwater flow from the
feature to prevent flooding or damage to it. Practices can be designed as offline
or on-line practices. An offline practice is sited outside of the normal runoff
flowpath and is designed to receive and treat a specified water quality volume.
Offline practices must infiltrate the required design storm amount and will have
an emergency overflow path or a bypass/flow-splitter device (see next page) to
convey excess flows to an alternative practice or storm drain system. On-line
systems are placed within the normal runoff flow path and always require an
outlet to allow excess flow to move through or around the practice.
A system should be designed to dewater within 24 to 72 hours after saturation
(refer to your local jurisdiction for specific time requirements for dewatering). This
design feature will help prevent long-term saturation and ensure the system has
storage available for the next storm event. Dewatering also reduces the likelihood
that mosquito breeding can occur within the practice.
Key design considerations for overflow systems include:
- The overflow inlet should be sized to pass flows that exceed the design
storm event. The inlet structure should be wide enough to allow access
for cleaning the outflow pipe or the underdrain. The top of the inlet should
be set at the ponding depth, approximately 6 to 12 inches (depending on
local regulations and site conditions) above the top of the mulch layer
(Figures 4-19,4-20 and 4-21). Using a domed grate on the top will prevent
debris from entering the overflow structure and will be less likely to
become clogged than a flat grate (Figure 4-22).
- An overflow weir should be included in on-line facilities. The weir should
safely convey overflow from a larger-scale storm event to an adequate
outfall. For small-sized practices receiving low flows, a stabilized
reinforced grass outfall might be sufficient.
- The overflow outlet should drain to a stabilized outfall and be connected
to a manhole, inlet or other structure. Carefully consider maintenance
requirements because of the potential for clogging of the inlets and the
consequence of the underdrain becoming blocked. Calculate hydraulic
grade lines to ensure the outfall pipes are adequately sized.
Green Streets Handbook
Figure 4-21. Raised overflow
drain allows a design volume
of stormwater to collect in a
bioretention area in Portland, OR.
Figure 4-22. Beehive overflow grate
prevents debris from entering the
overflow structure in a roadside
bioswale in Arlington, VA.
Figure 4-19. Raised overflow
structure in a bioretention feature in
Houston, TX.
Figure 4-20. Concrete band
constructed around the outflow
allows ponding in Nashville, TN.
4.5 Managing Stormwater Flow
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Bypass Devices
Bypass devices such as diverters and splitters can be used to prevent high
water flows from causing damage to a stormwater feature. Bypass devices
are typically incorporated into off-line green infrastructure practices (i.e.,
outside of the normal runoff flow path). Off-line practices are designed to
receive and treat a specified water quality volume (e.g., the runoff gener-
ated from a " inch, 24-hour storm). In the case of roadside practices, the
size of the opening and depth of the feature controls the amount of runoff
allowed to enter the practice (e.g., planter, bioretention cell)—allowing flow
to be bypassed in two ways:
1. A practice is designed with an entrance that restricts the
amount of water able to enter the practice (e.g., curb cuts,
weirs); therefore, high-volume flows are split so only a
controlled amount of runoff enters the practice while the rest
continues on its normal flow path.
2. A practice is designed to collect a controlled amount of
runoff until reaching its water quality treatment design. At
that time, the system will redirect all excess stormwater
back into the normal runoff flow path, which is often a
conventional curb-and-gutter stormwater conveyance
system (Figure 4-23).
Back-up Infiltration Practices
Backup infiltration approaches can be used when adjacent surface areas
are available to provide additional infiltration capacity. For example,
overflows from permeable pavements can be managed by placing a strip
of exposed gravel downslope of the pavement that will direct excess runoff
to a nearby grassed area, or by incorporating vegetated swales that can
collect and infiltrate excess volume (Figure 4-24).
Green Streets Handbook
Figure 4-23. In this tree pit bypass system in Washington, DC, curb cuts allow
stormwater to enter until the practice is filled, at which point additional flow
bypasses the system and continues down the street to the storm drain.
Figure 4-24. Vegetated swales were installed adjacent to a permeable parking lot
in Chicago, IL, to provide overflow control and back-up infiltration as needed.
4.5 Managing Stormwater Flow
4 12
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Underdrains
Underdrains can also be used to manage excess volumes of stormwater
flow, depending on the suitability of the underlying soil structure, soil
condition, depth to seasonal mean high water table and the capacity of the
system relative to volume. Overflow systems are generally preferred over
underdrains because they are easier to maintain and not as likely to clog.
Overflow devices also allow the feature to be used to retain and infiltrate the
desired water quality volume. In contrast, systems with underdrains often
serve primarily as filtration systems. Underdrains are also used to reduce
excessive ponding or improve infiltration in areas of lower permeability (i.e.,
where native soils have infiltration rates of less than 0.5 inches per hour).
If an underdrain is included, it should be designed appropriately to convey
flows to existing inlets or manholes.
An underdrain consists of a perforated pipe set in a drainage gravel bed
(Figure 4-25). The underdrain pipe is typically a 4- to 6-inch polyvinyl
chloride (PVC) or high-density polyethylene (HDPE) perforated pipe with
equally spaced holes. The upstream end of the underdrain is fitted with a
cleanout to allow the underdrain to be inspected and cleaned if necessary.
A cleanout consists of a pipe that is accessible from the surface of the
practice. The pipe is connected to the underdrain at a 45-degree angle in
the direction of flow via an elbow or wye (y-shaped plumbing fitting). A
cleanout typically extends vertically 6 to 12 inches beyond the top of the
mulch layer, set flush with the designed ponding depth.
The top end of the cleanout is fitted with a locking cap. The exact size of
the underdrain opening should be selected based on the drainage area of
runoff entering the practice and the time needed to dewater the system.
The system should be dewatered within 24 to 72 hours after saturation
(refer to local jurisdiction for specific time requirements for dewatering).
The upstream end of the underdrain is also capped. The downstream end
of the underdrain is connected to an overflow inlet or curb cut. The under-
drain may be level, but it is recommended to have a minimal slope, such
Green Streets Handbook
as 0.5 percent, so that any accumulated debris or sediment can be flushed
through the system as it drains.
If water retention is a performance requirement, underdrains can be
installed above the bottom extent of the practice or designed with a
90-degree upturned pipe so that the system begins to drain only after the
required water volume is retained. The water percolates down through the
soil into the internal water storage (IWS) layer and is slowly released into
the soil underneath the practice.
Figure 4-25. In this underdrain design cross-section image, an upturned pipe
connected to a slotted underdrain ensures that a permanent internal water storage
layer is maintained within the practice before the excess infiltrated water spills into
a secondary drainage network. In this design, a surface overflow drain is included
to provide added protection against high volume flows.
Slotted underdrain
Outlet structure
Temporary
wat<>r storage
Internal water
storage (IWS)
elevation
Gravel layer
45-degree elbow
Capped cleanoui/
observation well
Capped "tee"
connector for
maintenance
Outlet to
drainage
network
4.5 Managing Stormwater Flow
4 13
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4.6 Planning for Maintenance
Structural Practices
Nonstructural Practices
Maintenance should be considered as part of
any green infrastructure design. To perform
recommended tasks, the design plan must allow
for access into the practice by personnel and
maintenance equipment and must provide space
for pipe drain cleanouts and possibly observation
wells (Figures 4-26 and 4-27).
Certain design practices can influence the type
of maintenance needed. For example, the size of
openings on a grated trench drain could limit the
type of trash that enters the practice, reducing
the amount of clean-out needed. In some cases,
however, small grate openings can clog easily,
needing more frequent maintenance in areas
with abundant trash (Figure 4-28).
Site conditions can also influence selection of the
practice and requisite maintenance. For example,
a curbless neighborhood might not be suitable
for permeable pavement without the construc-
tion of sediment traps because pavers can easily
become clogged.
Specific maintenance for each stormwater
management practice is discussed in Chapter 6.
At a minimum, practices should be inspected
annually to remove trash, clean inlets/outlets,
remove invasive species, prune vegetation and
replace mulch. Maintenance should be con-
ducted after large storms and more frequently
while vegetation becomes established.
In addition to the specific maintenance practices
required for each green infrastructure practice,
communities can identify and implement non-
structural practices that help prevent pollution
from entering the watershed drainage system
(see box at right). These practices in turn reduce
the maintenance needed on structural practices.
Nonstructural practices require programmatic
management to develop implementation plans,
select appropriate technology and budget the
resources for these ongoing tasks. Quantification
of performance for nonstructural practices varies
widely because it depends on the frequency and
type of application, site-specific characteristics
and climate.
Key Nonstructural Practices
- Street sweeping
- Catch basin and storm drain cleaning
- Irrigation runoff reduction practices
- Slope and channel stabilization
- Trash management
- Anti-icing management
- Water-smart landscaping
- Erosion control on construction sites
- Spill prevention and response plans
- Education/awareness for the public
and employees
Figure 4-26. A wide-angled curb cut with an energy-
dissipating splashpad also serves as access steps
for maintenance in Maplewood, MN.
Figure 4-27. To facilitate
maintenance, an
observation well is installed
next to a bioretention area
in Houston, TX,
Figure 4-28. The small
spaces in this grate are
clogged with cigarette butts,
which block drainage and
are difficult to remove.
Green Streets Handbook
4.6 Planning for Maintenance
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4.7 Selecting Soil Media and Vegetation
Soil Media Selection
The specifications for filter media mixes will vary by availability of local
materials, local climatic conditions and stormwater requirements for the
specific placement of the green infrastructure practice within the trans-
portation corridor. A typical filter media mix will include a well-blended,
homogenous combination of the following soil types:
- Sand. Must be cleaned and washed to be free of deleterious
materials. A medium "concrete" sand such as ASTM C33 or an
equivalent is often used (average particle diameter <2.0 millimeters
is recommended).
- Silt and clay. Includes fines with a texture of sandy loam, loamy
sand or loam mixture to encourage nitrogen, phosphorus, metal
and other pollutant removal. (Note: a low-clay content, less than 2
percent, is necessary to avoid clogging.)
- Organic matter. Commonly includes a compost or mulch
amendment.
To support plant growth while removing phosphorus from runoff, the filter
media mix must have a low phosphorus index (P Index). The P Index is a
management tool that estimates the relative risk of phosphorus leaching.
Recommended levels are between 10 and 30 milligrams per kilogram when
using the Mehlich-3 test (MPCA 2013). Organic matter can be a source
of phosphorus loading and must be carefully managed where elevated
phosphorus concentration is a concern.
Geotextile fabrics are often used in green infrastructure infiltration prac-
tices to protect the filter media from becoming clogged by the sediments
and clays contained within in-situ soils. The liners typically extend along
the side slopes. The liner should have sufficient openings that are properly
sized for the existing soil conditions to prevent clogging. Impermeable
liners can be used to prevent infiltration into sensitive sites. The material
should be durable and flexible. Composite systems of nonwoven geotex-
iiles are used to prevent puncture during construction.
Green Streets Handbook
In preparation for planting local native vegetation, a soil media mix was chosen
and backfilled into this roadside bioretention area in San Diego, CA.
4.7 Selecting Soil Media and Vegetation
4 15
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Vegetation Selection
Planting schemes will vary depending on the site location and design specifications; however, soil
type and moisture conditions will usually determine the types of species selected. For example,
facultative wetland plants are typically used on the bottom of a bioretention cell, while facultative
upland species are frequently chosen for areas around the perimeter of a bioretention cell or in
mounded areas. Numerous factors should be considered when selecting plants:
- Soil moisture conditions. Choose plants that can tolerate summer drought, ponding
fluctuations and saturated soil conditions for the design drawdown period.
- Sunlight. Assess existing and anticipated exposure (e.g., when vegetation is mature).
- Expected pollutant loadings. Select plants that tolerate pollutants from contributing land
uses (e.g., choose salt-tolerant plants in cold climates where road salt use is common).
- Adjacent plant communities and habitats. Select native plants and hardy cultivars; this is
particularly important in areas with significant invasive species.
- Location aesthetics. Consider the type of neighborhood, adjacent land uses, and expected
pedestrian and roadway traffic (providing pathways and maintaining sight distances).
- Maintenance needs. Assess a plant's growth rate and its propensity for seed dispersal.
Native plants are usually adapted to the local climate and provide habitat for wildlife. Selected vege-
tation should grow tall enough to exceed the desired design flow depth. Additionally, the vegetation
should be moderately stiff and non-clumping to provide sufficient surface contact for water quality
treatment and to avoid formation of concentrated flow conditions. A mix of fibrous and deeply rooted
small trees, shrubs, and perennials will help maintain soil permeability.
Native plants are adapted to local climate conditions
and provide valuable wildlife habitat.
Street trees provide water storage, interception and
evapotranspiration.
Anticipate plants' mature conditions to avoid choosing a species that could
interfere with overhead electric lines or with roadway sightlines and or that would
require intensive maintenance because it has a propensity to grow and disperse
seeds. Properly selecting plants and supporting them during establishment should
eliminate the need for fertilizers and pesticides. Initially after planting, frequent
maintenance will be necessary to ensure the vegetation becomes established.
Sufficient soil volumes should be made available to the plant (especially trees) to
ensure proper growth. If the site doesn't provide ample space, construct root paths
to an adjacent open space or structural cells that can support sidewalks or pave-
ment while providing space for unimpacted soil below the ground surface.
Urban Street Trees
including urban trees in green infrastructure designs could pose chal-
lenges that must be considered. These include space requirements
for the tree pit, soil quality and texture, overhead and underground
utilities, pavement, and proximity to structures. A detailed site
evaluation can identify these challenges and options to mitigate any
problems. EPA's Stormwater Trees: Technical Memorandum (2016)
includes information on site evaluation and site constraints, choosing
the right tree, inspection and maintenance.
Green Streets Handbook 4.7 Selecting Soil Media and Vegetation I 4 16
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4.8 Providing Pedestrian Access
Adding Walkways and Bridges
Across or Around Practices
When incorporating green infrastructure into
a street or parking lot design, pedestrian
movement should be carefully considered.
Providing clear paths for pedestrians is crucial to
the design and is a good practice for protecting
green elements from damage.
For on-street parking, adequate space should
be provided to allow people to exit their cars
and access the sidewalk. A minimum 3-foot-
wide egress zone adjacent to the street curb is
suggested.
Figure 4-29. Permeable pavement walkways provide
access to on-street parking in Seattle, WA.
Walkways (Figures 4-29 and 4-30) or bridges
(Figure 4-31) can be provided for people to safely
cross the green infrastructure practice and
access the sidewalk. The use of bridges pre-
serves space, provides continuity of stormwater
flow and prevents soil compaction, erosion and
trampling of vegetation.
For areas with pedestrian traffic and little room
for stormwater planters or tree boxes, porous
surface materials (Figure 4-32) are an option
to consider. Using these materials allows water
to infiltrate and preserves sidewalk width for
pedestrian use.
Figure 4-31. A grated walkway bridge allows
pedestrians to access parked cars on Bagby Street in
Houston, TX.
Figure 4-30. Walkway built across vegetated swale to
allow users to access their cars in Portland, OR.
Figure 4-32. Tumbled green glass fills the spaces
between permeable pavers in a sidewalk area in
Chicago, IL.
Green Streets Handbook 4.8 Providing Pedestrian Access I 4 17
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Figure 4-34. Fence protects pedestrians from the
drop in grade in the adjacent bioretention feature in
Minneapolis, MN.
4.9 Ensuring Pedestrian Safety
Providing Visual and Physical
Barriers Around Practices
These design features help ensure that streets or
parking lots are safe and accessible for all users.
Many green infrastructure practices can be
used to enhance the pedestrian experience and
provide a buffer against vehicular traffic, reduce
pedestrian crossing distances and/or improve
sight angles at intersections.
An important aspect with regard to pedestrian
safety is assuring that people can detect and
are guarded against a sudden drop in grade.
Check your city's guidance to determine (1) the
maximum allowable depth for a stormwater
management practice that is installed adjacent
to a pedestrian area and (2) the appropriate or
required barrier needed to enclose the practice.
A suggested guideline is to install a barrier
when the vertical drop is at minimum 6 inches
immediately adjacent to a sidewalk. Common
techniques to either visually or physically denote
a vertical drop include a raised curb edge
(Figure 4-33), railing (Figure 4-34), fence (Figure
4-35), detectable warning/paving strips, bollards
and/or seating (Figure 4-36).
Figure 4-33. A raised curb with inlets defines
the edges of a sidewalk stormwater planter in
Washington, DC.
Figure 4-35. Short fencing protects pedestrians
from stepping into this stormwater tree box in
Washington, DC.
Figure 4-36. Seating adjacent to a bioretention unit
provides an amenity for passersby and also serves
as a barrier in Washington, DC.
Green Streets Handbook
4.9. Ensuring Pedestrian Safety
4 18
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The Beckoning Cistern serves as an artistic feature
and a stormwater management practice. Designed
to resemble a large upturned hand, the 15-foot-tall
structure adds visual interest while collecting roof
runoff, some of which is directed into a series of
cascading stormwater planters along Vine Street in
Seattle, WA.
Concrete art can highlight the presence of green
infrastructure. The raindrop ripple effect sidewalk
etching allowed the Watershed District's Public Art
Initiative to call attention to the function and benefit of
rain gardens in managing stormwater in the Bartelmy-
Meyer neighborhood in Maplewood, MN.
Artists collaborated on this curving bioretention
design for the Waterloo Parking Lot in a Cleveland,
OH, art district.
A bioretention area artfully designed to resemble
a rocky river wraps around the Oregon Convention
Center in Portland, OR.
4.10 Enhancing Street Design
Adding Artistic Elements
Green street design can incorporate artistic features such as sculptures, murals and concrete imprints.
In many cases, the stormwater management practice itself is designed as an artistic feature. These
elements can enhance community aesthetics and attract visitors.
Green Streets Handbook
4.10 Enhancing Street Design
4 19
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Adding Community Amenities
incorporating user amenities such as benches, bicycle racks and street-
lights into green streets planning and design helps encourage use of the
area by pedestrians and cyclists. By creating an attractive and welcoming
streetscape, community livability improves, which potentially benefits
neighborhoods and businesses.
Decorative stone benches installed at the edge of a bioretention area offers a
resting spot for pedestrians along Sandy Boulevard in Portland, OR.
Green Streets Handbook
Benches installed next to stormwater curb bumpouts provide an area to rest
in the New Columbia neighborhood in Portland, OR,
Incorporating bicycle lanes and bicycle racks into green street design
encourages non-motor vehicle access along city streets in Austin, TX.
4.10 Enhancing Street Design 4-'
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4.11 Accounting for Extreme Weather
Arid Climates
Designing practices for arid regions requires different considerations. The
low amount of annual precipitation in these areas reduces the storage
area needed to treat water quality. Because of high evaporation rates,
any harvested rainwater should be stored in a closed container instead of
stored with a large surface area exposed to the sun. The low frequency of
storm events can lead to a build-up of pollutant concentrations. Therefore,
the capture volume designated for first-flush treatment should be greater
than those designated for humid regions.
The soil and topography in arid regions are conducive to soil erosion and
increased sediment transport due to flashy storm events and wind action.
Particular care should be given to the selection of vegetation according to
these principles:
- The type of plant species and the number of plantings should
be chosen with respect to the available water supply. Native and
drought-tolerant plants are suggested.
- If irrigation is deemed necessary, group plants according to their
water needs and adjust irrigation schedules according to the
season and weather.
- Plants should be able to tolerate inundation.
A resource for determining water needs for specific plants is presented in
Brad Lancaster's Rainwater Harvesting for Drylands and Beyond. Volume 7
and the Arizona Municipal Water Users Association's Landscape Plants for
the Arizona Desert.
Note: Before harvesting rainwater or designing and installing any green
infrastructure, check the regulations pertaining to water rights in your
locale.
Green Streets Handbook
Cold Climates
For a cold-climate environment, the predominant design consideration are
snow and deicing agents. Areas adjacent to roadways or parking surfaces
are commonly used to stockpile snow that has been plowed from surfaces.
These areas accumulate large water volumes and high pollutant loadings
(e.g., sand and gravel, deicing chemicals, hydrocarbons). Infiltration
practices should not be placed in areas that are dedicated as snow storage
areas. Deicing agents and debris from the roadway will negatively impact
vegetative growth and can clog media and permeable surfaces.
Two suggested management strategies can help overcome the challenge
of co-managing snow and stormwater:
- If possible, collect snow on an impervious pad and divert the
meltwater for treatment (e.g., detention and routing to a wastewater
treatment facility).
- Minimize the pollutants associated with meltwater runoff by using
improved application technology with trucks and reducing the use
of deicing chemicals.
- Design pretreatment facilities to remove particulate material
before any snowmelt enters a stormwater infiltration
practice.
Research has shown that green infrastructure, such as permeable
pavement, groundwater recharge by local infiltration, and road drainage
infiltration systems, can be effective under cold-climate conditions as long
as they are adequately maintained to assure their effective performance
(MCPA2013).
4.11 Accounting for Extreme Weather
4-21
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4.12 Avoiding Design Flaws
Improper design and a failure to consider the surrounding site characteristics can lead to diminished function of green infrastructure. The following images
present and explain some design problems that prevent a practice from functioning at full capacity or cause other problems.
These permeable pavers received runoff from a
gravel driveway and became clogged with sediment.
The large-spaced grate on this overflow drain will not
prevent floatables and debris from entering.
I hese unsecured blocks installed next to a
bioretention area pose a safety risk.
These trash cans, installed in front of
stormwater inlets, block flow.
i'his undersized curb cut is easily
clogged by mulch and other debris.
The overflow drain is placed in the flow
path of water entering the practice.
i his stormwater planter does not
provide space for passenger exit.
Green Streets Handbook
4.12 Avoiding Design Flaws
4-22
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Pretreatment Practice
Opttons
In This Chapter
5.1 Sediment Forebays
5.2 Vegetated Filter Strips
5.3 Swales
5.4 Modified Catch Basins
5.5 Flow-Through Structures
This chapter covers information on pretreatment
methods that should be considered when designing
green infrastructure systems. Pretreatment practices
help protect the main treatment systems by dissipating
energy and reducing flow velocity, removing coarse
sediments and large particles from the flow, capturing
trash and other debris, and reducing overall stormwater
flow volume by encouraging infiltration. Successful,
functioning pretreatment practices will help improve
performance, reduce maintenance and increase
lifespan of the overall stormwater management system.
Note: The design details described in this handbook
are meant to be conceptual and not final design
specifications. Designers should refer to state or local
requirements and recommendations to inform their
designs.
Ik
mm
jVTOLjF"*"1 .WWi - ««
- •
> m
A sediment forebay provides pretreatment for parking lot runoff entering
a bioretention cell at Villanova University, PA.
Green Streets Handbook I 5 1
-------
5.1 Pretreatment: Sediment Forebays
Description
A sediment forebay is an excavated pit or basin with a berm or weir
designed to slow and detain incoming runoff. Sediment forebays are placed
before practices such as bioretention systems or bioswales to dissipate
energy from runoff and allow for sedimentation to occur. Sediment fore-
bays serve to minimize, but do not eliminate, the amount of sediment being
transported into downstream practices.
Site Considerations
Sediment forebays provide pretreatment that enhances the performance
and longevity of downstream practices. With proper maintenance,
sediment forebays can have a long life cycle. As a surface practice, they
should be easily accessible for sediment removal and other maintenance.
Sediment forebays provide a greater detention time than proprietary
separators. Although sediment forebays allow sedimentation of some
particulate matter, they primarily remove only coarse pollutants and no
soluble pollutants (MADEP 2008). Frequent maintenance is essential to
ensure proper performance.
Design Considerations
Slopes should be designed for safety and erosion control (maximum 3:1
[horizontal run: vertical rise (H:V)] slope). The forebay volume should be 10
percent of the water quality volume at minimum. The depth should be a
minimum of 2 feet and a maximum of 6 feet.
Energy dissipation methods, such as splash blocks or riprap, should be
included at both the inlet and outlet locations. Exposed earth slopes
and bottom of basins should be stabilized using seed mixes that are
appropriate for the soils, suitable for expected mowing practices, and
drought-tolerant or resilient to inundation periods, depending on the volume
of stormwater expected. To facilitate maintenance, the bottom of the
pretreatment practice may be "hardened" with concrete to allow for easier
collection and removal of sediments. Always design your system to allow
access to the pretreatment practice for maintenance.
Green Streets Handbook
A sediment forebay provides pretreatment for a bioretention cell in Barnstable, MA.
Sediment Forebays
Advantages: Most suitable for:
- Relatively inexpensive - Bioretention
- Long-lasting if - Bioswales
properly maintained - Curb extensions
5.1 Pretreatment: Sediment Forebays
5-2
-------
Maintenance Requirements
Because sediment forebavs help reduce the sediment load entering green
infrastructure practices, it is imperative to remove accumulated sediment
to ensure the system continues to function as designed. The frequency of
cleaning required depends on the contributing sediment loading rate and
the occurrence of storm events. The contributing sediment loading rate
is based on the size and type of drainage area. One suggested practice
is to install a staff gage or other measuring device to indicate the level
of sediment accumulation and to establish a level at which clean-out is
required. Typical maintenance needs required for sediment forebays are
outlined in Table 5-1.
Table 5-1. Recommended maintenance activities for sediment forebays
Activity
Frequency
Additional advice
*o
C/3
Remove sediment
As needed,
but annually
at minimum
If excessive sedimentation is
observed, the site might need
to be regraded and reseeded
to avoid excessive upland
erosion.
Remove any trash on the
surface
Twice per
year
inspect for rutting caused by
concentrated flow
Annually
Eroded areas should be filled
in with soil and the bare areas
should be reseeded.
Vegetation
Mow embankments to control
growth of woody vegetation
Annually (in
spring)
If at least 50% vegetation
coverage is not established
after 2 years, provide
additional plantings.
Remove and replace vegetation
as necessary
As needed
If at least 50% vegetation
coverage is not established
after 2 years, provide
additional plantings.
Weed invasive and exotic
species, preferably using
nonchemical methods such as
hand pulling and hoeing
Annually
Green Streets Handbook
A sediment forebay provides pretreatment for a rain garden in Maplewood, MN.
Sediment Forebays
Key design features and maintenance needs:
- Periodically remove sediment
- Provide a mechanism to dissipate energy from incoming flow
- Avoid compaction during construction and maintenance or by
service vehicles
5.1 Pretreatment: Sediment Forebays
-------
5.2 Pretreatment: Vegetated Filter Strips
Description
Vegetated filter strips are gradually sloped, densely vegetated areas
designed to receive and treat sheet flow. They are designed as flow-
through devices to slow down and infiltrate runoff and to remove sediment
before it reaches a downstream stormwater management practice.
Vegetated filter strips can be distinguished from grassed swales because
the filter strips typically have more surface roughness, energy dissipation
capacity and denser vegetation, while grassed swales serve more as
grassed conveyance systems. Performance is limited by grading, because
little to no treatment is achieved if the filter strips are short-circuited by
concentrated flow paths (MADEP 2008).
Installing a level spreader might be necessary to ensure runoff becomes
sheet flow before it enters the vegetated filter strip.
Filter strips can be amended with compost and subsurface gravel to
increase removal of dissolved metals and increase moisture capacity,
which can improve infiltration rates and reduce flow velocities. An example
of this is the compost-amended vegetated filter strip (CAVFS), currently in
use in Washington State (WSDOT 2016). Designs can also be modified to
provide significant pollutant reduction by incorporating a media filter drain
in areas with minimal slopes.
Site Considerations
Filter strips are best suited to smaller drainage areas, low-velocity
roadways or small parking lots because they do not have the capacity
to reduce peak discharges or handle large velocities (WSDOT 2011). The
maximum impervious contributing length should be 75 feet to 100 feet,
and the maximum pervious contributing length can be up to 150 feet
(SEMCOG 2008; MPCA 2013). Vegetative filters are not suited for areas
with traditional curbs and gutters, for sites with excessive longitudinal
slope (greater than 5 percent), side slopes (greater than 25 percent), or in
areas with unstable slopes or erosive soils (MPCA 2013).
Green Streets Handbook
Advantages:
- Perform better than swales
because the non-concentrated
flow allows for greater
sedimentation and infiltration
- Reduces pollutants associated
with sediments such as
phosphorus, pesticides and
insoluble metallic salts
Vegetated Filter Strips
Most suitable for:
Bioretention
Bioswales
Subsurface infiltration and
detention
Design Considerations
- Slope. To prevent erosion or channelization from developing, design
filter strips with slopes between 2 and 6 percent to ensure sufficient
velocities and level surface with no pits, gullies, or ruts.
- Size. The flow length should be at least 25 feet for sufficient
treatment, but should remain less than 75 feet long for impervious
drainage areas and 150 feet for pervious drainage areas to prevent
channelization from occurring. It is recommended that the filter strip
width be equivalent to the width of the area draining to the strip.
- Border. To ensure even flow, it is often necessary to border the
perimeter of the parking lot or road with a level spreader. Examples
of spreader devices include a strip of pea gravel, slotted sections in
the highway shoulder that are perpendicular to the road direction,
concrete sills or a strip of porous pavement (Young et al. 1996).
Level spreaders help to evenly distribute flows and trap sediments.
- Vegetation. Dense, soil-binding deep-rooted grasses that are water
tolerant should be used in the construction of vegetated filter (Young,
et al. 1996). If the filter will receive runoff from highways that require
heavy application of deicing salts, salt-resistant plants should be
specified.
5.2 Pretreatment: Vegetated Filter Strips
-------
Maintenance Requirements
!t is important to periodically evaluate the condition of the filter strip during
the first two years of construction, particularly after major storm events.
Typical maintenance needs required for vegetated filter strips are outlined in
Table 5-2. The frequencies provided are minimum suggestions; the activities
should occur as needed.
Table 5-2. Recommended maintenance activities for vegetated filter strips
Activity
Frequency
Additional advice
Remove sediment to ensure
sheet flow into the filter area
and to avoid concentrated flow
Annually
If excessive sedimentation is
observed, the site might need
to be regraded and reseeded
to ensure sheet flow can be
maintained.
Remove any trash on the
surface
Twice per
year
CO
Inspect for rutting caused by
concentrated flow
Annually
Eroded areas should be filled
in with soil and the bare areas
should be replanted.
Turn ortill soil, especially if
compaction occurs
As needed
If maintenance efforts are
unsuccessful, the soil media
and underdrain might need to
be removed and replaced.
Mow turf or grass
At least
annually
If at least 50% vegetation
coverage is not established
after 2 years, provide
additional plantings.
Vegetation
Remove and replace vegetation
as necessary
As needed
If at least 50% vegetation
coverage is not established
after 2 years, provide
additional plantings.
Weed invasive and exotic
species, preferably using
nonchemical methods such as
hand pulling and hoeing
Annually
Green Streets Handbook
Vegetated filter strip at the edge of a parking lot intercepts and filters
stormwater runoff before the water reaches the infiltration bed at the center
of the practice.
Vegetated Filter Strips
Key design features and maintenance needs:
- Ensure site is graded accurately to maintain sheet flow along
entire flow length
- Use level spreaders to slow incoming flow velocities
- Avoid compaction during construction and maintenance or by
service vehicles
- Periodically remove sediment
- Maintain a dense vegetative cover
5.2 Pretreatment: Vegetated Filter Strips
5-5
-------
5.3 Pretreatment: Swales
Description
Pretreatment swales are shallow, vegetated channels that capture runoff and
slowly convey it along the swale while infiltrating and filtering coarse sediment.
They are similar to bioswales, except that they are designed primarily for
conveyance without enhanced infiltration/filtration components; therefore,
they provide limited water quality enhancement and reduction of runoff
volume and peak discharge. Pollutant removal rates will vary greatly with the
species of vegetation chosen for the swale. Types of swales include drainage
channels, grass channels and dry swales.
Site Considerations
These practices provide coarse sediment removal and limited infiltration and
detention. They also convey stormwater to downstream practices. They are
applicable in parking lots and roadways as a pretreatment practice. Swales
can be used in treatment trains to provide initial treatment for practices such
as ^pretention, surface and subsurface infiltration practices, and stormwater
basins.
Design Considerations
Swales should be designed for capacity and stability so the design depth can
convey the maximum specified design flow but the channel will not erode
under maximum design flow velocities. To maximize treatment performance,
runoff should flow through the entire swale. Therefore, runoff should be
directed to an inlet and should not enter as sheet flow along the entire length of
the swale (CEEI and NHDES 2008). Depending on the longitudinal slope, check
dams might be necessary to slow down flow and encourage surface contact.
Channel cross-section design should be trapezoidal or parabolic. A study con-
ducted in Texas and California by the University of Texas Center for Research
in Water Resources in Texas determined that the optimum cross-section for
swales in highway medians is a V-shape, rather than the trapezoidal shape
commonly listed in manuals, because most of the treatment occurs along
the slopes (Barrett 2004). The bottom of the swale should not be within the
seasonal high water table.
Green Streets Handbook
Pretreatment Swales
f
Advantages:
- Provide stormwater
conveyance
- The open-drainage systems
provide easy access for
maintenance
- Are a iess-costiy alternative
to curb-and-gutter
stormwater conveyance
systems
Most suitable for:
- Bioretention
- Bioswales
- Subsurface infiltration and
detention
Grass swale serves as pretreatment for a bioretention area in the High Point
neighborhood in Seattle, WA.
5.3 Pretreatment: Swales
5-6
-------
The design should include vegetation types that will maximize treatment.
Vegetation species should reflect the site specific soil, topography, flow
velocities and maintenance needs. If using trees or shrubs in the vegetated
swale design, plants that are resilient to both drought and flooding should
be selected. Trees should not be planted in areas that require enhanced
structural stability (BES 2006). Swales' effectiveness for stormwater
treatment is greater where more surface contact occurs. For this reason, a
fine, close-growing, flood-resistant grass should be selected.
Maintenance Requirements
It is important to periodically evaluate the condition of the swales during
the first year after construction, particularly following major storm events.
Mow vegetation to maintain heights of 4 to 6 inches. After 5 years, scrape
the channel bottom to remove sediment buildup and restore the original
cross-sectional geometry. Typical maintenance needs required for pretreat-
ment swales are outlined in Table 5-3.
A pretreatment bioswale conveys and treats runoff from a parking lot and road in
Stafford, VA.
Green Streets Handbook
Table 5-3. Recommended maintenance activities for pretreatment swales
Activity
Frequency
Additional advice
Remove sediment, especially
if 3 inches accumulate in any
spot or it covers vegetation
Annually
If excessive sedimentation is
observed, the site might need to be
regraded and reseeded to ensure
sheet flow can be maintained.
Remove any trash on the
surface
Twice per
year
o
C/5
inspect for erosion
Annually
Eroded areas should be filled in
with soil and the bare areas should
be reseeded.
Turn or till soil, especially if
compaction occurs
As needed
If maintenance efforts are
unsuccessful, the soil media
and underdrain might need to be
removed and replaced.
Mow turf or grass
Dependent
on grass
type
If at least 50% vegetation coverage
is not established after 2 years,
provide additional plantings.
c
o
?
CO
+-»
0)
CJ>
Qi
Remove and replace
vegetation as necessary
As needed
If at least 50% vegetation coverage
is not established after 2 years,
provide additional plantings.
>
Weed invasive and exotic
species, preferably using
nonchemical methods such
as hand pulling and hoeing
Annually
Pretreatment Swales
Key design features and maintenance needs:
- Ensure accurate grading to maintain sheet flow
- Use level spreaders to slow incoming flow velocities
- Avoid compaction during construction and
maintenance or by service vehicles
- Periodically remove sediment
- Maintain a dense vegetative cover
5.3 Pretreatment: Swales
-------
5.4 Pretreatment: Modified Catch Basins
Description
A catch basin is an inlet device designed to capture sediment, debris and
associated pollutants. Catch basins can be modified with a deep sump to
provide extra storage for the accumulation of sediment (Figure 5-1). They
can include a hood or inverted elbow to minimize the amount of floatables,
oil and grit that can exit the catch basin and enter the downstream
treatment practice (Figure 5-2). Finally, they are considered part of a green
infrastructure approach if they are modified as leaching catch basins that
have perforated sections to allow water to infiltrate surrounding soil.
Site Considerations
Catch basin modifications such as deep sumps and hoods can be used
for water quality improvement, but are not designed to reduce runoff
volume or peak discharge. Leaching catch basins should not be used
where infiltration is not desired (e.g., because of potential groundwater or
soil contamination or presence of high groundwater or bedrock). Modified
catch basins provide pretreatment for downstream practices by removing
Advantages:
- Minimal space
requirement
- Compatible with
subsurface storm drain
systems
- Is long-lasting if properly
maintained
- Design allows easy
access for maintenance
Modified Catch Basins
Most suitable for:
- Bioretention
Bioswale
Curb extension
Stormwater planter
Trees trenches
Infiltration trench
Subsurface infiltration
and detention
coarse sediment, debris, floatables, oil and grit. Modified catchbasins might
be the only applicable practice for sites with constrained spaces, poor
infiltrating soils, or existing subsurface contamination.
Outlet
Deep Sump Catch Basin Operation Steps:
Runoff flows into the deep sump catch basin typically
through an inlet or surface grate on the street (1) and
drops into the sump (2).
The sump provides a deep collection area (2) between
the incoming flow (1) and outgoing flow (3), which
allows coarse sediments and trash to drop out of
suspension. Trash grates, hoods (4), or filter skirts can
enhance performance by preventing floatables from
entering outflow pipes.
Outgoing flows (3) continue to a centralized drainage
network or can be designed to discharge to a surface or
subsurface green infrastructure practice.
Csicn liasm Grate
Hood
i
CO
Figure 5-1. Simple modified catch basin
Figure 5-2. Hooded catch basin
Green Streets Handbook
5.4 Pretreatment: Modified Catch Basins
-------
Modified catch basins are highly applicable in urban and retrofit situations
because they are compatible with subsurface storm drain systems and
require limited space. Constraints include the presence of underground
utilities, shallow bedrock, or a high groundwater table. Catch basins should
be easy to access, and they should not be used unless adequate funding
for regular inspections and maintenance is ensured.
Design Considerations
Inlets must be sized appropriately to capture the design volume. Inlet sizing
is particularly important on steep slopes to ensure that runoff is adequately
captured (RIDEM and CRMC 2010). Grates should be sufficient to keep out
larger debris, typically with holes of 1 inch or less (MADEP 2.008). Recom-
mended maximum drainage area is less than 0.25 acre of impervious areas
(NHDES 2008).
Sump depths should be 4 feet or deeper to allow accumulation of sediment
and to limit resuspension of accumulated sediment. Except for leaching
catch basins that are designed for infiltration, all flow will exit the catch
basin through an outflow pipe. These outflow pipes should include a hood
or elbow to limit the amounts of floatables, oil and grit that are transported
downstream.
To enhance pollutant removal, these systems may be designed off-line to
divert large flows to another practice designed for water quantity (MPCA
2013).
Maintenance Requirements
Maintenance is relatively easy and, if properly maintained, these systems
can be long-lasting (MADEP 2008). Typical maintenance of catch basins
includes trash removal (if a screen or other debris capturing device is used)
and removal of sediment using a vacuum truck or wet-vac. As a rule of
thumb, once the sump is half full of sediment, it cannot provide additional
sedimentation. Depending on location, several cleanings of the sump might
be required per year. At minimum, inspection should occur twice annually,
once after snow melt and once after leaf drop.
Green Streets Handbook
Operators need to be properly trained in catch basin maintenance.
Maintenance should include keeping a log of the amount of sediment
collected and the date of removal. Some cities have incorporated the
use of geographic information systems to track sediment collection and
to optimize future catch basin cleaning efforts. The disposal of trapped
sediment, debris, oil and grit removed during maintenance activities should
be considered during design. Avoid damaging the hood during cleaning
activities.
Modified Catch Basins
Key design features and maintenance needs:
- Ensure adequate size for both the inlet and the catchbasin to
capture and detain the flow
- Requires access for maintenance
- Inspect and maintain practice at least twice annually
(frequency is site-dependent)
Hip
MP
A curb inlet cover allow runoff to enter a catch basin but prevents inflow of trash.
5.4 Pretreatment: Modified Catch Basins
5-9
-------
5.5 Pretreatment: Flow-Through Structures
Description
Flow-through structures are subsurface structures that include a settling or separation
unit that improve water quality by removing coarse sediments, floatables, oil and grit from
runoff. These types of structures include vortex separator systems, oil and grit separators,
and proprietary devices.
The vortex separator systems, also known as swirl separators, hydrodynamic separators
and swirl concentrators, use vortex action to separate coarse sediment and floatables from
stormwater. Although these practices are not designed to reduce runoff volume or peak
discharge, they do provide water quality pretreatment by removing coarse sediment, float-
ables, oil and grit. Like catch basins, pretreatment flow structures are not considered green
infrastructure practices, but they are useful tools that can reduce the negative environmen-
tal impacts of transportation infrastructure on water resources. In highly urbanized areas
with large percentages of impervious surfaces, these practices can be essential elements
of hybrid gray and green infrastructure stormwater management systems.
Site Considerations
These practices are commonly used near the source of runoff and serve as pretreatment
to a number of downstream stormwater management practices. These structures can be
constructed in locations with potentially high pollutant loads where other practices might not
be applicable. Some states and municipalities require oil and grit separators on sites with
higher expected pollutant loads or risk of petroleum spills (i.e., high-turnover parking lots, gas
stations, fleet storage areas, and vehicle and equipment maintenance areas).
Because they are subsurface systems that require a relatively small footprint, these systems
are useful in situations where land availability is limited. The drainage area for such systems
is limited by both the capacity of the chosen system and the available land area.
Vortex separator being installed.
Flow-Through
Advantages:
- Effectively captures
coarse sediments and
floating debris
- Minimal space
requirement
- Can be implemented
In any soil or terrain
V
Structures
Most suitable for:
- Bioretention
- Bioswale
- Curb extension
- Stormwater planter
- Trees trenches
- Infiltration trench
- Subsurface infiltration
and detention
For More Information-Pretreatment
Underground Hvdrodvnamic Separators. Fact sheet. Montgomery County,
MD (2018)
Pretreatment. Philadelphia Water Stormwater Management Guidance
Manual (Chapter 4, Section 10). City of Philadelphia, PA (2018)
Pretreatmsnt Practices. New Hampshire Stormwater Manual, Volume
2: Post-Construction Best Management Practices Selection and Design,
Chapter 4-4. New Hampshire Department of Environmental Services (2008)
Structural Pretreatment BMPs. Massachusetts Stormwater Handbook
(Volume 2, Chapter 2). Commonwealth of Massachusetts (2008)
Green Streets Handbook
5,5 Pretreatment: Flow Through Structures
5 10
-------
Design Considerations
These practices should be designed off-line to handle the first flush (initial
runoff from precipitation event) for water quality improvement; a bypass
line should be provided to handle larger flows. Design options include
multichamber systems and devices that include vortex-induced circulating
flow paths to promote sedimentation and removal of trash, oil and grease.
By attaching the inflow at a tangential angle to the cylindrical system, a
swirling action is induced. Coarse sediment is removed by sliding down a
cone in the center of the system to a settling chamber or by directing runoff
through a screened area that traps and drops sediment into a chamber.
Depending on the manufacturer, these systems can treat flows from 0.75 to
300 cubic feet per second.
In multichamber systems, typically the first chamber provides sedimenta-
tion, the subsequent chamber provides additional sedimentation and oil
and grease removal (with a hood or inverted elbow), and the final chamber
contains the outlet to the downstream practice (Figure 5-3). Devices
should be able to safely pass the desired design storm and should include
an overflow for large storms to limit resuspension of captured particles.
Similar to a deep sump catch basin, the sump in the initial chamber should
be at least 4 feet deep (CEI and NHDES 2008).
Maintenance Requirements
These systems require proper maintenance to limit the potential for
resuspension of captured sediment. Units should be inspected after major
storms and at least one per month (MADEP 2008). Units should be cleaned
of captured sediment and debris twice per year. More frequent cleaning will
provide more available volume for future storms and less resuspension
and associated pollutant transport. The rate of sediment accumulation will
depend on the site characteristics; the maintenance plans should reflect
these characteristics. Because these practices could be expensive to
construct and maintain, costs should be a key consideration when evaluat-
ing and selecting them.
Green Streets Handbook
Off-line Oil and Grit Separator
Top View
Chamber'
Chamber 2
Manhole
cover
1 [J Ch
t g jVir
f Baffle fj ^
I
Manhole
ewer
t—
.Manhole
cover
Inlet] ¦ | || Outlet
Tfighflow-bypass^
Diversion
weir
10 feet
Manhole covers
Side View
CD
CO
cD
•fc
Coarse grit
and debris
~
Diversion
Inlet / weir
Outlet
ighfiow-by'passr)
—
Chamber 1
Baffle
lyJ
Chamber 2
4 feet
-Fine grit
and
debris
Figure 5-3. An off-line oil and grit separator diverts incoming stormwater into two
chambers that slow flow and allow oil and grit to separate from the water stream.
Flow-Through Structures
Key design features and maintenance needs:
- Install as an off-line device to limit potential for resuspension
of captured material
- Inspect units monthly and after major storms
- Clean as needed, but at least twice per year
5.5 Pretreatment: Flow Through Structures
-------
Green Street Stormwater
Practices
In This Chapter
6.1 Bioretention (Rain Gardens)
6.2 Bioswales
6.3 Curb Extensions
6.4 Stormwater Planters
6.5 Stormwater Tree Systems
6.6 infiltration Trenches
6.7 Subsurface Infiltration and Detention
6.8 Permeable Pavement
This chapter covers site design strategies and storm-
water management practices that can be incorporated
into street and parking lot designs for the retention
and treatment of runoff. Information on pretreatment
methods that should be considered and incorporated
as necessary in the design of the practices and systems
is included in Chapter 5. For each practice, information
on siting opportunities, design details, performance and
supplemental resources is provided.
Note: The design details described in this handbook
are meant to be conceptual and not final design
specifications. Designers should refer to state or local
requirements and recommendations to inform their
designs.
Sand-filled permeable pavers allow rainfall to infiltrate instead of
generating erosive runoff in a sensitive coastal area in Virginia Beach, VA.
Green Streets Handbook
-------
6.1 Bioretention (Rain Gardens)
Description
A bioretention area is a shallow surface depression usually planted with
native vegetation to retain, infiltrate and filter both runoff and pollutants.
The volume of runoff is reduced by infiltration and retention in the soils and
through interception, uptake and evapotranspiration by the plants. Peak
discharges are also reduced. Physical, chemical and biological processes
in plants and soils help to absorb and treat pollutants.
The form of bioretention is flexible and can be designed for collection
with (1) filtration and infiltration or (2) filtration and conveyance. Once
established, bioretention typically requires minimal maintenance. In-ground
bioretention is typically in the form of cells, rain gardens or swales.
Stormwater curb extensions, stormwater planters and bioswales use
the principles of bioretention but include unique design features and are
described as different green street practices in this guidebook.
Site Considerations
Bioretention has a significant advantage over other practices because
it can vary in size, shape and placement. Bioretention practices can be
designed to accommodate large volumes of stormwater runoff or designed
to treat small drainage areas. Depending on the source of runoff, they are
placed either directly adjacent to the area generating runoff or offset in
sidewalks, public plazas or street medians. Bioretention can be designed as
a series of multiple cells along the roadways or parking lots.
Bioretention systems can be either infiltration or flow-through systems, but
should be designed with pretreatment to address potential sediment loads
and debris that can be common in roadways. In ultra-urban areas or retrofit
projects, bioretention might be more difficult to site due to the presence of
existing infrastructure such as buildings or utilities. Design alternatives that
can help overcome site constraints are discussed on the next page.
Green Streets Handbook
r
Bioretention
Advantages:
- Can be sized for large
and small drainage
areas.
- Good for highly
impervious areas
- Good retrofit capability
- Modest maintenance
requirements
- Provides aesthetic
enhancement
- Reduces runoff
- Reduces pollutant
load, thus reducing
treatment costs
- Provides wildlife habitat
Road runoff drains through a curb cut and into this bioretention feature on a
residential front yard in Maplewood, MN.
Most suitable for:
- Parking lot perimeters
- Parking lot islands
- Sidewalks
- Street frontage
- Intersections
- Road medians
- Road shoulders
6.1 Bioretention (Rain Gardens)
6-2
-------
Overcoming Site Challenges
Site constraints such as land use and environmental conditions can create
perceived obstacles for implementing bioretention, however, many design
alternatives are available to help overcome these challenges (Table 6-1).
Table 6-1. Bioretention: site constraints and design alternatives
Challenge
Design alternatives and recommendations
High pedestrian activity
Provide pedestrian bridges or walkways across the
practice to allow for uninterrupted movement.
Sites requiring depths between
6 to 12 inches
Install barriers or additional protection around the
practice as a safety provision for pedestrians.
Site slopes that are greater
than 10%
Incorporate diversion berms, check dams, or terracing
with weirs to allow for the bottom to be flat-sloped.
Sites near heavy traffic or high
pollutant areas (i.e., potential
hots pot)
Avoid placing infiltrating systems due to concerns of
groundwater contamination. Recommended practices
include pretreatment and/or impervious liner.
Proximity to water table
Recommended 4-foot separation to water table, with a
minimum separation of 2 feet with impermeable liner
and underdrain or very low-volume roadways.
Sites near sensitive areas
such as building foundations
or road gravel base materials
or above karst topography or
brownfields
Incorporate impermeable liners to direct water
downward to avoid lateral flow or to prevent vertical
flow to underlying sensitive areas depending on what
the site requires. Provide a minimum setback of 10
feet from any foundation.
Areas that have significant salt
usage or storage during winter
months
Avoid using infiltrating bioretention cells in snow
storage areas (especially in areas where salt is
applied) due to the potential for impacting downstream
environmentally sensitive areas.
Poor draining native soils (i.e.,
hydroiogic soil groups C and D)
Amend soils or design practice with an underdrain
to convey excess runoff to a downstream practice or
stormwater conveyance system.
Compacted soils
Either rototill or mix compacted soil with soil
amendments or entirely replace compacted soil with
structural soils or modular structural cells.
Green Streets Handbook
Bioretention in sidewalk with protective stone wall that doubles as a bench in
Washington, DC.
Roadside bioretention area includes a sidewalk bridge over the inlet to avoid
obstructing pedestrian flow.
6.1 Bioretention (Rain Gardens)
-------
Components: Bioretention
A bioretention practice typically includes (Figure 6-1):
- Inlet (or sheet flow)
- Native vegetation, or vegetation that is resilient to both wet and dry conditions
- Bioretention soil media that includes a mixture of sand, soil and organic matter
Practices can be designed with optional features to convey inflow, manage
overflow and provide pretreatment:
- Inflow structure(s) (e.g., flume, inlets, runnels)
- Highly permeable mulch layer
- Vegetated filter strip
- Forebay or ponding areas
- Outflow/overflow inlet
- Underdrain
Tempotaty
Outlet structure
water storage
Bioretention
soil media
Native soil
Outlet to
drainage
network
Figure 6-1. Cross-section of a common bioretention practice design.
Design Considerations
Sizing
Design considerations for bioretention cells are largely influenced by the design objective (e.g., improve
water quality or provide channel protection, increase groundwater recharge, reduce peak flow) and the
geographic/climatic region of the United States in which it is being applied. Bioretention cells can have
many different configurations that are dependent on the land use, climate and pollutant loads. The
bioretention feature should have a 2 percent or less longitudinal slope and recommended side slopes of
4:1. The cross section design can be parabolic, trapezoidal, or flat with a minimum 2-inch freeboard.
Inlet Design
For uncurbed areas, a maximum side slope of 3:1 is recommended to reduce the velocity of runoff from
the paved areas and to filter out some of the sediment and finer particulates that can clog the bioreten-
tion surface. The slope vegetation should include some ground cover plants. For curbed parking lots and
roads, designated inflow points must be provided where the majority of the flow will enter. Inflows should
be designed to be nonerosive; energy dissipaters or diversions may be necessary to direct erosive flows
away from the inlet.
Bioretention
Key Design Features:
- Flexible in size and configuration
- Maximum drainage area: 5:1, not more
than 1 acre to one rain garden
- Ponding depths between 6 and 12
inches, which will allow for drawdown
within 48 hours
- Plant selections that tolerate hydrologic
variability, salts and environmental stress
- Amend soil as needed
- Provide overflow for extreme storm
events
- Stable inflow/outflow conditions
Green Streets Handbook
6.1 Bioretention (Rain Gardens)
6-
-------
Maintenance Requirements
Yearly inspections at a minimum are recommended to monitor infiltration
and drainage. For the first 1 to 2 months of vegetation establishment,
watering is recommended once every 2 to 3 days. If infiltration rates are
lower than expected, it might be necessary to cultivate or replace media
(top 2 to 3 inches) to improve the infiltration rate. The following activities
and minimum frequencies should be determined with regards to the
specific site and as warranted by environmental conditions (Table 6-2). The
maintenance cost is similar to traditional landscaping but initial training for
workers may be necessary.
Table 6-2. Recommended maintenance activities for bioretention practices
Activity
Frequency
Additional advice
Debris
Remove sediment or trash that has accumulated.
Semi-annually
If sediment loads are excessive, observe and add upstream sediment controls to
lessen load.
Inspect underdrains for obstructions.
Yearly
Remove any obstructions.
Vegetation
Cut back grasses and herbaceous vegetation.
Weed invasive and exotic species, preferably using
nonchemical methods such as hand pulling and hoeing.
Prune trees and shrubs.
Bimonthly during establishment;
yearly afterwards (preferably in
early spring)
If at least 50% of vegetation coverage is not established after 2 years, provide
additional plantings. When replacing vegetation, place the new plant in the same
location as the old plant, or as close as possible to the old location. The exception
to this recommendation is if plant mortality is due to:
- Initial improper placement of the plant (i.e., in an area that is too wet or too
dry).
- If diseased/infected plant material was used and there is risk of persistence
of the disease or fungus in the soil.
Separate herbaceous vegetation rootstock when over-
crowding is observed.
Every 3 years
Remove and replace vegetation as necessary.
Yearly (preferably in spring)
'o
CO
Turn ortill soil, especially if compaction occurs.
Yearly
If maintenance efforts are unsuccessful, the soil media and underdrain might
need to be removed and replaced.
Evaluate check dams for undercutting and soil substrate
for channel formation.
Every 2 to 3 years (preferably in
spring)
Remove and properly dispose of the previous mulch layer,
or rototill it into the soil surface and add a new layer of
mulch.
Yearly
Do not exceed 3 inches in depth for mulch layers. Avoid blocking inflow entrance
points with mounded mulch or raised plantings. Once a full groundcover is
established, mulching might not be necessary.
Stabilize any areas where erosion is evident.
As needed
Determine the cause for erosion; this could require adding new features to
dissipate energy or to allow the flow to bypass the practice.
Green Streets Handbook
6.1 Bioretention (Rain Gardens)
6-5
-------
Performance
Bioretention pollutant removal performance data is limited but growing in
availability. Bioretention appears to be one of the most effective water quality
practices given that this practice can remove many pollutants of concern;
however, actual mass loading reductions will vary based on flow attenuation and
influent water quality. Overall, removal of pollutants has been positively linked
to the length of time the stormwater remains in contact with the herbaceous
materials and soils (Colwell et al. 2000).
Data indicate that the ability of bioretention to remove total suspended solids,
metals (dissolved and particulate-bound), and oil and grease is very strong, while
its ability to reduce nitrogen and phosphorus has been mixed (Davis et al. 2009).
Because consistent removal of excess nutrients from the pollutant stream is
important when considering bioretention, more recent studies have evaluated
how amendments to the media can improve adsorption rates.
For More Information-Bioretention
Fact Sheet: Bioretention (Rain Gardens) City of Lancaster, PA (2011)
Minnesota Stormwater Manual: Bioretention: Phosphorus Sorption.
Minnesota Pollution Control Agency (2015)
New Jersey Stormwater Best Management Practices Manual: Bioretention
Systems. New Jersey Department of Environmental Protection (2016)
Stormwater BMP Manual: Bioretention. North Carolina Department of
Environment and Natural Resources (2018)
Technical Guidance Manual for Puget Sound: Chapter 6.1 Bioretention.
Washington State University Extension and Puget Sound Partnership (2012)
Bioretention for Infiltration Conservation Practice Standard 1004.
Wisconsin Department of Natural Resources (2004)
State-of-the-Art Review of Phosphorus Sorption Amendments in Bioret-
ention Media: A Systematic Literature Review. Marvin, J.T., E. Passeport,
and J. Drake (2020) ($)
Green Streets Handbook
Bioretention in a residential neighborhood in Portland, OR.
Bioretention area outside the recreation center at the University of Florida,
Gainesville, FL.
6.1 Bioretention (Rain Gardens)
6-6
-------
6.2 Bioswales
Challenge Design alternatives
High pedestrian activity
Provide pedestrian bridges or walkways across bioswales to
aliow for uninterrupted movement.
Unsafe site depths for
pedestrians
Provide barriers or additional protection around bioswale (in
pedestrian areas, depths should not exceed 6 to 12 inches)
Site slopes that are
greater than 5%
Incorporate terracing, diversion berms or check dams to
accommodate steeper-sloping sites.
Grassed bioswale in New Hampshire.
Description
Bioretention swales, also referred to as bioswales or vegetated swales, are typically
parabolic or trapezoidal depressions that use bioretention soil media and vegetation to
promote infiltration, water retention, sedimentation and pollutant removal. Bioswales
differ from bioretention cells because they are designed to be conveyance treatment
devices. Bioswales are typically dug to a depth of 12 to 24 inches and compost-amended;
in contrast, installing a bioretention cell entails replacing the full volume of soil with
an engineered planting media. Similar to traditional grassed swales that convey flows,
bioswales provide additional water quality benefits because the stormwater interacts
with the plants and bioretention soil Bioswales are typically located in rights-of-way or
parking lots and receive flow from adjacent impervious areas. Bioswales can be used in
conjunction with pretreatment BMPs such as sediment forebays, vegetated filter strips, or
other sediment-capturing devices that prevent sediments from accumulating in the swale
and negatively affecting treatment and retention performance.
Site Considerations
Advantages:
- Combine stormwater
treatment with
conveyance
- Can replace curb and
gutter systems at lower
cost
- Mitigate peak runoff
velocities
- Can be sized for various
iayouts and topography
- Reduce total
suspended solids and
metal concentrations
Bioswales
Most suitable for:
- Parking lots
Sidewalks
Road medians
Road shoulders
Overcoming Site Challenges
Bioswales can be designed to overcome site constraints (Table 6-3).
Table 6-3. Bioswales: site constraints and design alternatives
Rights-of-way are ideal for bioswales, particularly for roads with wide
shoulders or rights-of-way that have long, uninterrupted stretches of land to
convey the necessary design flows (e.g., medians, the planting strip between a
sidewalk and a roadway). Because they are easy to implement and relatively
low cost to construct, bioswales are applicable for both retrofits and new
residential and commercial development.
Green Streets Handbook
6.2 Bioswales
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Components: Bioswale
A bioswale typically consists of (Figure 6-2):
- A trapezoidal or parabolic channel
- Vegetation (dependent on site requirements)
- Bioretention soii media
Bioswales can be designed with optional features such as:
- Check dams or terracing for steeper slopes
- Curb cuts or other inlet configurations (if area is curbed)
Capped
Cleanout
Port
LTD
Check
Dam
Gravel Splash Pad
Underlain by Geotextile
2% Max. Bed Slope
(5% Max. Average Slope)
Outlet to
Drainage
Network
Figure 6-2. Cross-section of a bioswale designed with a check dam to control slope.
Design Considerations
Sizing
The area draining to a specific swale should typically be less than 100 feet in length and no more than 1 acre.
If pretreatment is included, the maximum drainage area should be 5 acres. The bioswale should be designed
to convey applicable storm events without generating erosive velocities.
Channel Geometry
The bioswale channel may be trapezoidal or parabolic in shape, with side slopes of 3:1 or flatter (note: rectan-
gular shapes with stabilized vertical walls are generally referred to as planters; see section 6.4) and optimally
a longitudinal slope with a 1 to 2 percent grade. A maximum 6-inch ponding depth is recommended. The
bioswale media should be located in the center of a level area.
Inlet Design
If the perimeter of the swale is curbed, runoff can enter the swale through a curb cut opening, inlet protection
such as pea gravel or a splash pad should be installed to help dissipate the energy of the concentrated flow,
thereby preventing erosion. In an uncurbed perimeter, flow may enter the bioswale as sheetflow directly or
may be conveyed over a filter strip before entering the swale. If excessive sediment is expected, pretreatment
such as a forebay area can also be included in the design to extend the life of the bioswale.
Bioswales
Key Design Features:
- Maximum drainage area: 5:1
- Bottom width of 2 to 8 feet
- Side slopes from 3:1 (H:V) to 5:1
- Longitudinal slope from 1% to 5%
- Maintain 0.5 to 1 -foot freeboard
without exceeding maximum
permissible velocity
- Runoff from the designated water
quality event should not overtop
vegetated liner (vegetation used for
treatment)
- Ensure vegetative cover is greater
than 80%
- Till soil if compaction is evident
Green Streets Handbook
6.2 Bioswales
-------
Vegetation
Bioswales can be planted with many types of vegetation, including:
- Grasses, such as turf grasses or tall grasses
- Herbaceous plants, such as sedges or rushes
- Shrubs and trees (typically found on the edges or slopes of bioswales)
Climate will affect plant selection. In drier areas, bioswales often use xeri-
scape vegetation. Xeriscaping is a method of landscaping that uses more
drought-tolerant plantings so that minimal or no irrigation is needed in
between rain events. Ideally, these plantings will also have low maintenance
needs (e.g., requires no mowing or pruning). Bioswales that would receive sig-
nificant quantities of salt-laden runoff should be landscaped with salt-tolerant
species. Proper selection of plant species and support during establishment
of vegetation should eliminate the need for fertilizers and pesticides.
Select vegetation that grows high enough to exceed desired design flow depth.
Additionally, the vegetation should be moderately stiff and non-clumping to
provide sufficient surface contact for water quality treatment and to avoid
concentrated flow conditions. Riprap or landscape stone can also be used in
bioswales, particularly at the edges to provide erosion protection.
Soils
Bioswales are usually excavated to a depth of 12 to 24 inches, tilled to improve
infiltration potential, and then backfilled with a filter soil media mix (see
section 4.7).
Maintenance Requirements
Bioswales should be inspected yearly at a minimum to monitor sedimentation
and erosion. Bioswales planted with turf require more regular maintenance
than bioswales planted with perennials and shrubs. Vegetation, including
grasses, should be maintained at heights of approximately 4 to 6 inches. The
maintenance cost is similar to traditional landscaping but may require initial
training for workers. Follow the maintenance activities and minimum frequen-
cies for Bioretention (see section 6.1), while also evaluating check dams for
undercutting and soil substrate for channel formation (yearly).
Green Streets Handbook
32
~o
CD
Bioretention feature with grasses and flowering plants outside a public library in
Cleveland, OH.
Bioswale designed with drought-tolerant plants in arid Tucson, AZ.
6.2 Bioswales
-------
Performance
Bioswales remove pollution through three primary removal mechanisms:
settling, filtering/infiltration and uptake/accumulation in plants. Using
bioswales, it is possible to achieve a 40 percent annual runoff volume
reduction (CWP and CSN, 2008; CWP 2007). Current data suggest that
bioswales are effective in removing suspended solids. In contrast, studies
have shown that bacteria levels are increased in the bioswale effluent. A
possible explanation for the introduction of bacteria is waste from wildlife
and the pets of nearby resident. Performance is improved when bioswales
are built with a pretreatment device such as a filter strips because the
sheet flows from parking lots or roadways are diffused.
For More Information-Bioswales
Biofiltration Swale: Design Guidance. California Department of
Transportation (2012)
Standards for Green Infrastructure City of New York Department
of Environmental Protection (2020)
Biofilters for Storm Water Discharge Pollution Removal. Oregon
Department of Environmental Quality (2003)
Roadside bioswale with curb-cut inlet in Greensboro, NC.
Bioswale next to a permeable pavement sidewalk
in Seattle, WA.
Parking lot bioswale conveys runoff from a parking lot
in Wilsonville, OR.
Bioswale at Los Angeles Zoo parking lot.
Green Streets Handbook
6.2 Bioswales
6 10
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6.3 Stormwater Curb Extensions
Implementing stormwater curb extensions can
meet additional goals such as traffic calming.
The presence of curb extensions narrows
the pedestrian crossing distance, increases
visibility of pedestrians, and has been shown to
reduce vehicular speeds. They are also suitable
in areas with steep-slope conditions because
they can provide a 'backstop' for stormwater
runoff. In addition, they provide landscaping
opportunities to beautify the neighborhood.
FARKWAY
Figure 6-3. Potential locations for curb extension practices. Stormwater curb extension in State College, PA.
Description
Stormwater curb extensions, also called stormwater bump outs, are modified traffic-calming
devices that extend the curb into the roadway to reduce traffic speed and capture stormwater
runoff from roadways and/or sidewalks. The area behind the curb is filled with a bioretention
soil mix and vegetation similar to a bioretention cell or bioswale. The vegetation can be
groundcover, shrubs or trees depending on site conditions, costs and design context.
This green infrastructure practice provides stormwater treatment and retention within the
right-of-way. Curb extensions can be designed in several configurations to provide both
filtration and retention. Pretreatment practices such as vegetated filters and sediment traps
are recommended upstream of this practice.
Site Considerations
Stormwater curb extensions can be incorporated in new development and offer an ideal
retrofit approach for existing streets. They can be installed upstream of storm sewer inlets
and without any modifications to existing catch basins. Overflow from curb extensions can
continue to flow down the street to storm sewer inlets. Their small footprint presents minimal
disturbance to rights-of-way and provides flexibility in siting. Stormwater curb extensions can
be placed in multiple locations along a street section or at intersections to minimize impact
to parking (Figure 6-3). They are relatively
inexpensive and, when sized correctly, are often
capable of treating the entire runoff volume
from the street on which they are located.
Stormwater Curb Extensions
Advantages:
- Provides traffic
calming and
improves pedestrian
safety
- Enhances site
aesthetics
- Offers air quality
and climate benefits
that improve
environmental health
- Reduces total
volumetric runoff
- Provides water
qualitytreatment
- Presents minimal
disturbance to the
area and existing
infrastructure
- Reduces effective
impervious area
Most suitable for:
- Neighborhood
streets and some
collectors
- Intersection
- Midblock
- Any length of
roadway
Green Streets Handbook
6.3 Stormwater Curb Extensions
-------
Overcoming Site Challenges
Stormwater curb extensions can be designed to overcome site constraints
such as sloped landscapes and the presence of underlying utilities, while
also enhancing safety and minimizing the loss of parking spaces. Common
site challenges and design alternatives are described in Table 6-4.
Table 6-4. Stormwater curb extensions: site constraints and design alternatives
Challenge
Design alternatives and recommendations
Removal of on-street parking is
required.
Minimize impact by selectively placing curb
extensions at intersections or mid-biock crossings.
Ensure safety for all modes of
transportation.
Be conscious of street width, turning radii and
sightlines for all users.
Prevent vehicles from driving
onto the sidewalk and harming
pedestrians.
Provide barriers such as bollards, planters or
benches around stormwater curb extension.
Site slopes that are greater
than 5%.
Incorporate terracing, diversion berms, or check
dams to accommodate steeper-sloping sites.
Sites that are not stable or have
high sediment loads.
Plan to include pretreatment practices to avoid high
amounts of maintenance.
Conflict with underlying utility or
fire hydrant.
Reorient the design.
Proximity to water table.
Recommended a 4-foot separation to water table
with a minimum separation of 2 feet.
For More Information-Stormwater Curb Extensions
Northeast Fremont Street Green Street Project. City of Portland
Bureau of Environmental Services (2007)
San Francisco Better Streets: Curb Extensions (Buib-outsY City and
County of San Francisco (2015)
City of Philadelphia Green Streets Design Manual. City of
Philadelphia (2014)
Tennessee Permanent Stormwater Management and Design
Guidance Manual: Urban Bioretention. Tennessee Department of
Environment and Conservation (2015)
Green Streets Handbook
Parking impacts minimized by using a mid-street stormwater curb extension in
the Barton Creek neighborhood, Seattle, WA.
Black and yellow-striped bollards placed around a stormwater curb extension
ensures safety for motorists in Tucson, AZ.
6.3 Stormwater Curb Extensions
6 12
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Components: Stormwater Curb Extension
A stormwater curb extension typically consists of (Figure 6-4):
- Low-profile vegetation
- Curb cuts (berms, inlet deflectors or pavement modifications
are often used to direct flow towards curb-cut inlets)
- Bioretention soil media
Stormwater curb extensions can be designed with optional
features such as:
- Forebays
- Check dams or terracing for steeper slopes
- Underdrains
- Overflow structures
Design Considerations
Inlet Design
Runoff for uncurbed roads and sidewalks is generally conveyed via direct
sheet flow or shallow concentrated flow into stormwater curb extensions;
curbed roads and sidewalks require curb cuts to direct the flow. Alterna-
tively, runoff may enter via an existing or proposed inlet, typically located at
a low point or depression in a road or parking lot.
A curb cut should be made where the majority of the flow will enter; in some
cases, more than one curb cut might be necessary to capture flows from
multiple locations. For more information on curb cuts, see section 4.4.
Berms, inlet deflectors, or pavement modifications (e.g., depressions),
can be used to direct flow to the curb cuts or inlets (particularly those at
a 90-degree angle). The following elements should be evaluated when
determining the dimensions and shape of the curb cut opening: ponding
Green Streets Handbook
1 Outlet/Overflow
2 Plants
6 Cat-cti Basin
3 Chedt Dim
4 Slone SetfJjrg Area
5 Flow Inlet
7 Mukh Ljyw
B Growing Layier
9 Slone R«ervoiir
10 Underdrain
., Untompacted
Subgrade
Figure 6-4. Components of a stormwater curb extension.
depth, spread of flow, slope and design storm event. To protect the media
around the inlet from scouring and erosion, a concrete splash pad or a
course of riprap or gravel should be installed just inside the curb cut to
dissipate the flow as it enters.
A curb opening can be
designed with a forebay
structure to capture sedi-
ment. Concrete pads are
typically used as forebays
to help remove sediments.
Hand removal of sediments
from a small concrete pad is
much easier than removing
sediments from a mulch and
soil layer or a pretreatment
forebay filled with stone or
gravel.
Stormwater Curb Extensions
Key Design Features
- Include low-profile vegetation
- Level storage bed bottoms
- Mark curb cuts to be highly visible
to motorists
- Work around existing utilities
- Refer to bioretention key design
features
6,3 Stormwater Curb Extensions
-------
Sizing
The surface area of the curb extensions is typically 5 to 10 percent of the
drainage area.
Underdrains
Stormwater curb extensions can be designed with or without an under drain.
Systems with poor underlying soil typically include an underdrain to ensure
drainage within a set time period. The underdrain can be placed a few feet
above the bottom of the practice to create internal water storage to promote
infiltration. Even with this storage layer, practices with underdrains provide
less water quantity reduction than practices without them.
Overflows
Overflows are typically conveyed through an overflow curb cut at the down-
stream end of a curb extension. If an overflow structure is incorporated
into the design (typically with an underdrain), it should be sized to pass the
design storm event. Grates on the top of overflow inlets should be sized to
exclude trash and animals while allowing stormwater to drain at a steady
pace. The structure should be large enough to provide access to clean out
the outflow pipe or the underdrain. The top of the overflow structure should
be at the maximum ponding depth.
Vegetation
Vegetation selection for stormwater curb extensions is similar to a
bioretention cell (see section 6.1). Selected vegetation should be low profile
(typically 36 inches or less at maturity) to allow unimpeded sightlines for
pedestrians and motorists.
Soils
Native soils are typically excavated to a depth of 18 to 24 inches and tilled
to improve infiltration potential. The curb extension is then backfilled with a
bioretention filter media mix.
Green Streets Handbook
Runoff enters the upper end of this curb extension, and the overflow volume
exits through an opening on the lower end and drops into a storm drain.
Densely planted low-growing grasses
fill a stormwater curb extension in
Portland, OR.
Mature grasses and a tree pit treat
stormwater in a curb extension in
Gresham, OR.
6.3 Stormwater Curb Extensions
6 14
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Maintenance Requirements
Maintenance of curb extensions is similar to that of a bioretention practice
(see section 6.1) In addition, evaluate the condition of curb extension
perimeter and inflow/outflow points. Repair or replace as needed. Yearly
inspections are recommended at a minimum.
Performance
Similar to bioretention cells, stormwater curb extensions use the physical,
chemical and biological processes in plants and soils to absorb and treat
pollutants and help maintain the hydrologic balance of an area. Research
has shown that stormwater curb extensions are highly efficient at removing
pollutants, with results similar to a bioretention cell. Refer to the perfor-
mance statistics for bioretention in section 6.1 for more information.
Stormwater curb extensions promote stormwater infiltration and retention
in the soils, as well as interception, uptake and evapotranspiration by the
plants. As a result, curb extensions are able to provide significant reduc-
tions in both peak flow rates and annual stormwater volume.
Mid-street stormwater curb extension in a neighborhood in Kansas City, MO.
Green Streets Handbook
Stormwater curb extension decreases crossing distance and improves
intersection safety in the Capitol Hill neighborhood in Seattle, WA.
End-of-street stormwater curb extensions in a neighborhood in Portland, OR.
6.3 Stormwater Curb Extensions
6 15
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6.4 Stormwater Planters
Description
Stormwater planters are becoming common components of municipal stormwater programs.
Planters are narrow, fiat-bottomed landscape areas that are typically rectangular in shape with
vertical walls. Planters usually receive runoff from surrounding impervious areas, including
rooftop areas, sidewalks and roadways. Constructed from a variety of different materials, they can
be configured in different ways to effectively capture and treat incoming flows. The two primary
types of planter boxes are:
- Infiltration planters. These have open bottoms and allow stormwater to infiltrate into
the subsoil beneath. As stormwater percolates through the planter box soil, pollutants
are removed by filtration, absorption and adsorption, and chemical and biological uptake,
infiltration planters are appropriate to use in well-drained soils. Infiltration planters have a
greater potential for runoff reduction than do flow-through planters.
- Flow-through planters. These have impervious bottoms or are placed on impervious
surfaces. Once the soil in flow-through planters is saturated, excess water is collected
in an underdrain to be discharged to the conveyance system or to another green
infrastructure practice. They are appropriate for soils with poor drainage, prior
contamination or high seasonal groundwater table.
Site Considerations
Stormwater planters are ideal for urban or ultra-urban areas where space is limited or in areas
with steep slopes. Planters are also ideal for retrofit projects because they can be built between
driveways, entryways, utilities and trees, adjacent to buildings and parking lots, and in rights-of-way.
They can be used to capture surface runoff from roadways or be connected to a downspout from a
rooftop. They should be placed reasonably close to the source of runoff.
Planters can be situated either aboveground (receiving water via surface flow) or belowground
(receiving water via underdrains). In rights-of-way, aboveground planters can be designed with a
perimeter seating for pedestrians. Belowground planters can be equipped with fences and/or adja-
cent benches to provide a pedestrian-oriented streetscape. They can be built singularly or in series.
Stormwater Planters
Advantages:
- Enhance site
aesthetics
- Reduce total
volumetric runoff
- Provide some
water quality
treatment
- Reduce effective
impervious area
- Widely
applicability in
ultra-urban areas
Most suitable* for:
- Sidewalk areas
- Buffer zone
between sidewalk
and street
- Areas with
expanses of
impervious
surface where
bioretention is not
an option
' Typically applied in urban locations
Curb cuts in the sidewalk and street allow for runoff to flow
into this stormwater planter in Portland, OR.
Stormwater planters are typically not used in low- to medium-density settings because the
hardscape infrastructure required increases the cost of the practice, so it is generally not as cost
effective as bioretention or bioswales. Planters are typically used in areas where site constraints and right-of-way use patterns require confined and protected
practices. Because these practices are normally in urban places where space is a constraint, their performance is limited by the capacity of the planter.
Green Streets Handbook
6,4 Stormwater Planters
-------
Overcoming Site Challenges
Stormwater planters can be designed to overcome site challenges such
as high pedestrian activity, safety concerns, or high-sediment-load runoff
(Table 6-5).
Table 6-5, Stormwater planters: site constraints and design alternatives
Challenge
Design alternatives and
recommendations
High pedestrian activity or
vehicle traffic.
Provide pedestrian bridges to allow for
crossings in the sidewalk. Aboveground
planters can provide a seat wall for
pedestrians.
Belowground planters are
perceived as safety risk for
pedestrians.
Install tree fences, barriers and/or benches
to provide protection around planter.
Sites that are not stable or
have high sediment loads.
Incorporate pretreatment practices to avoid
high amounts of maintenance.
Sidewalk planters are equipped with bridges to provides access to
parking areas in Seattle, WA.
Green Streets Handbook
Interconnected stormwater planters include protective walls alongside the parking
lane in Niagara Falls, NY.
A pedestrian-friendly sidewalk planter includes safety rails and a metal sidewalk
bridge in Baltimore, MD.
6,4 Stormwater Planters
6 17
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Smretention
soil media
Planter wall
tlnderdrain
Curb cut 11
inlet —|
Overflow
pipe —H-0 "
Components: Stormwater Planter
A stormwater planter typically consists of (Figure 6-5):
- Vertical walls, typically made of a durable material that is
context-appropriate
- Access point such as a curb cut or downspout connection
- Vegetation
- Bioretention soil media
Stormwater curb extensions can be designed with optional
features such as:
- Splash pad
- Underdrains
- Overflow structures
- Liners
Clean ttone
storage tayer
Figure 6-5, Stormwater planters, unit plan view (left) and cross section (right).
Vegetation
Design Considerations
Hardscape Materials
Stormwater planters may be constructed of any durable material, such as
stone, concrete, brick, plastic lumber or wood. Stand-alone planter boxes
are typically constructed of pre-cast or cast-in-place concrete or other
materials used in the nearby streetscape.
Sizing
Stormwater planters should be sized appropriately for storage volume
requirements and available space. The space needed for planter boxes
might not be available in all situations within the urban environments.
Minimum sizing requirements will depend on local stormwater regulations.
A typical planter box may have an interior size of 2 feet by 2 feet with a
depth of 12 inches (of which 6 inches is for storage depth) and slope of
less than 0.5 percent. For infiltration planters, at least 2 feet of infiltration
medium should be included between the bottom of the practice and any
underlying constraint (e.g., solid rock, high groundwater table).
Inlet Design
Planters placed in rights-of-
way typically have curb cut
inlets that capture flows from
roadways and/or have notches
in the planter walls to receive
sidewalk runoff. Planters
that are installed adjacent to
buildings receive flows from
Stormwater Planters
Key Design Features
- Infiltration rate of soil will
determine size and site
applicability
- Runoff should drain within 3 to 4
hours after storm event
- Provide a flow bypass for winter
conditions
downspouts; to reduce scour
and erosion, these inlets typically have a splash pad or a course of stone at
the base to dissipate flow energy.
Liners
Flow-through planters typically use an impermeable liner or other impervi-
ous bottom to prevent runoff from infiltrating into native subsoils. Planters
that are adjacent to buildings should also have a waterproofing membrane
on the sides of the planter to protect the building's foundation.
Green Streets Handbook 6.4 Stormwater Planters 618
-------
Belowground stormwater
planters are typically exca-
vated to a depth of 18 to 24
inches and tilled to improve
infiltration potential or back-
filled with a bioretention soil
mix. Use backfill to enhance
infiltration, especially if the native soils do not have a minimum infiltration
rate of 0.5 inches per hour. Aboveground stormwater planters are filled with
18 to 24 inches of a bioretention soil mix.
Stormwater planters in Washington, DC, are
designed in a series to collect and treat road
runoff while allowing adequate pedestrian
access to the street and sidewalk.
Vegetation
Vegetation selection for
stormwater planters is
similar to a bioretention
cell (see section 6.1). They
generally include a variety
of shrubs, small trees and
native herbaceous species
that are appropriate for
the streetscape. Some
designers are using sedum
and other green roof plants
(e.g., the National Institute of
Medicine in Bethesda, MD).
Performance
Stormwater planters exhibit water quality benefits similar to those of
bioretention, which mimic nature by employing a rich diversity of native
plant types and species. In addition to improving water quality and reducing
runoff quantity, the locally adapted vegetation exhibits good tolerance to
pests, diseases and other environmental stressors.
Green Streets Handbook
Maintenance Requirements
For More Information-Stormwater Planters
City of Philadelphia Green Streets Design Manual. City of
Philadelphia (2014)
Stormwater Planters. Oregon State University Extension Service
(2018)
Low impact Development Approaches Handbook: Flow-Throuah
Planter. Oregon Clean Water Services (2009)
A stormwater planter can be designed to Roof downspout is directed into a
capture and treat roof runoff. stormwater planter in Emeryville, CA.
The maintenance requirements for a planter are influenced by site con-
ditions such as frequency of sediment build-up or growth of vegetation.
The maintenance activities and frequencies outlined for bioretention (see
section 6.1) should be followed for stormwater planters. Inspect the planter
box for structural integrity at least yearly.
Downspout
from drainage
area
il media
Underdrain
Gravel splash pad
Overflow/
outlet
6,4 Stormwater Planters
6-19
-------
6.5 Stormwater Tree Systems
Description
Stormwater tree systems (i.e., pits and trenches) consist of a tree or shrub,
bioretention soil media, and a gravel reservoir to intercept and capture
stormwater. The tree pit can be designed as an infiltration practice. If
infiltration is not desirable because of a groundwater contamination threat,
poorly draining native soils, or a high groundwater table, systems can be
designed with an underdrain that directs treated runoff to a downstream
practice or stormwater conveyance system.
Stormwater tree systems typically receive road runoff through a curb cut,
catch basin or stormwater inlet. Captured runoff temporarily ponds on the
surface before infiltrating and filtering through a bioretention soil media
and/or a stone reservoir. These practices improve water quality through
filtration and adsorption, reduce peak discharge through subsurface
storage, and can reduce runoff volume through the uptake and evapo-
transpiration by trees. If designed for infiltration, these practices achieve
additional reductions of runoff and peak flow. Types of stormwater tree
systems include:
- Tree Pits. Stormwater tree pits are typically installed upstream of
existing catch basins to improve water quality through filtration and
adsorption before directing runoff to a downstream stormwater
management practice or conveyance system. Unlike tree trenches,
tree pits only include one tree or shrub. A number of proprietary
tree pit systems on the market include pretreatment sumps and/or
subsurface structural supports. These structural elements preserve
volume for soil media while also providing support for sidewalks.
- Expanded Tree Pit. An expanded tree pit has a contiguous
bioretention cell designed to collect and treat stormwater. It is
also referred to as a tree box filter, tree box, or bioretention tree pit.
Because these systems generally have surface volumes that permit
ponding, they achieve more stormwater reduction and treatment
than tree pits. Tree pits have an average lifespan of 25 years,
although vegetation might need to be replaced more frequently
(LIDC2005).
Green Streets Handbook
Stormwater Tree Systems
Advantages:
- Reduce runoff volume and
delay peak flows
- Enhance site aesthetics
- Shade and shelter individuals
and buildings
- Reduce air temperature
- Reduce cooling and heating
costs
- Capture/reduce air pollutants
- Evapotranspire runoff
- Reduce noise pollution
- Improve psychological health
- Provide a sense of piace
- Simple to install
- Available in multiple sizes
Most suitable* for:
- Sidewalk areas
- Buffer zone between
sidewalk and street
- Medians
- Parking lots
r Typically applied in urban locations
Tree Trench. The stormwater
tree trench is a variation of the
tree pit. Tree trenches include a
stone storage layer, bioretention
soil media and multiple trees
planted in sequence with a
common gravel trench for water
storage. Tree trenches are most
commonly designed as off-line
structures. Multiple design
variations are available, but
typically a catch basin captures
runoff and conveys it through
a perforated pipe in the gravel
trench. Water is stored in the
trench and is taken up by the
trees and the underlying soil, if
designed for infiltration.
A tree pit captures runoff in a parking
lot in Lawrence, KS.
6.5 Stormwater Tree Systems
6-20
-------
Overcoming Site Challenges
Stormwater tree systems can be designed to overcome site challenges
such as a high groundwater table, insufficient soil volume or concerns for
soil upheaval (Table 6-6).
Table 6-6. Stormwater tree systems: site constraints and design alternatives
Challenge Design alternatives and recommendations
High groundwater table or
poor-draining native soils
Design practice with an underdrain to convey excess
runoff to a downstream practice or stormwater
conveyance system.
Compacted soils
Either rototill or mix compacted soil with soil
amendments, or entirely replace soil with structural
soils or modular structural cells.
Tree pit depths great enough
to pose a pedestrian fall risk
Install fences, barrier and/or benches to provide
protection around the tree pit.
Underground or aboveground
utility present
Select trees with mature heights under the average
height of overhead utilities (typically 30 feet). Provide
adequate clearance of underground utilities, which
should be protected from water and root penetration.
Insufficient soil volume to
ensure proper tree growth
Construct root paths to an adjacent open space or
add structural cells that can support sidewalks or
pavement while providing space for soil below ground.
Proximity to buildings
Incorporate an impermeable liner or underdrain into
the design to prevent infiltration into the building
foundation.
Limited sidewalk width
When necessary, place paving stones, cobbles, or
porous rubber as a surface material around the trees
outside the root ball area.
Concern for sidewalk upheaval
Provide areas for unrestricted root growth beneath the
surface using root paths or structural soils below the
sidewalk. Ensure that trees are planted below grade.
Green Streets Handbook
Site Considerations
Tree pits and tree trenches are ideal for urban and ultra-urban environ-
ments because they help to reduce the urban heat island effect, improve air
quality, enhance community aesthetics and create a walkable environment
that is safe, healthy and comfortable. Street trees can induce traffic
calming if planted to create vertical walls that frame the street and guide
motorists along a defined edge, or if they are planted in street medians to
better divide opposing traffic flows (Burden 2006).
Tree pits and tree trenches are widely applied in retrofit situations because
they can be installed within the sidewalk (although the sidewalk must not
be encroached upon to a point that pedestrian traffic is affected). These
practices are most commonly seen on sidewalks of urban or commercial
streets; however, they are also applicable in parking lots.
Expanded tree boxes are another practice worth considering. This practice
involves the use of a vault or other structural device to provide larger
volumes for additional retention and room for the tree roots to expand. The
use of these systems promotes the growth of healthy mature trees and can
provide significant stormwater retention or detention volume.
Because of their relatively rigid shape, these practices are not typically
suitable in residential or rural applications, where more natural-looking
practices such as bioswales or bioretention practices are generally more
appropriate and cost-effective. Tree pits can be part of a treatment train
and can receive inflow from a pretreatment practice to enhance sediment
and trash removal.
6.5 Stormwater Tree Systems
6-21
-------
Design Considerations
Siting
Evaluating existing site conditions, such as soils, hydrology, topography, vegetation
patterns and invasive species, is necessary to determine the proper placement and
design requirements for planting a tree. For example, minimal availability of planting
surface areas would influence species selection and require soil modification to
support plant growth and health. Plants should be located as far from the curb as
possible to prevent injury from salt, sand and snow. Along roadways, it is important
to anticipate activities such as mowing and snow storage when situating trees.
There must be a setback from the road to maintain line-of-sight requirements,
including for street signs, signals and lights—especially at intersections, curb cuts
and medians.
Slope
For longitudinal slopes greater than 5:1 (H:V), consider a terraced approach.
Hardscape Materials
A number of design options are available for the tree pit enclosure. To maximize root
growth, shallow concrete barriers can define the edge of the practices, allowing for
uninhibited root growth in all directions and maximizing infiltration. Enclosed vaults
may be used where infiltration is not desirable, where there is soil or groundwater
contamination, or where a high groundwater table is a concern. Vaults used in tree
pits can be rectangular or cylindrical. Other design variations include bottomless
tree vaults or vaults with some sides left open to encourage root growth.
Stormwater Tree Systems
Key Design Features
- Select appropriate tree species
- Allow sufficient root zone growth area
- Provide mechanism for tunneling stormwater runoff to the tree
- Ensure proper spacing and avoid conflicts with utilities, buildings and pedestrian traffic
- Provide high infiltrative capacity to prevent ponding after 72 hours
Green Streets Handbook
Components: Tree Systems
Although there are many
design variations, a stormwa-
ter tree system (Figure 6-6)
typically consists of:
- Tree boxes
- One or more trees or
shrubs
- Bioretention soil media
- Gravel reservoir
- An underdrain
Optional design components
include:
- Pretreatment sump
- Impermeable liner
- Connection to subsurface
chambers
- Observation well (if
needed)
- Overflow outlet
Im pervious surface
Vegetation
centered in
treatment
Qv Conveyance
protection bypass
Bioretention soil mix
80% sand, 20% compost
:::::
Existing subgrade
Mound 6" berm
around tree filter rim
Cross section of
. 72" diameter
concrete vault
12" Overflow pipe
12" Perforated
subdrain
12" Overflow outlet,
discharges to existing
storm drain or the
surface
Figure 6-6. Stormwater tree pit schematic.
6.5 Stormwater Tree Systems
6-22
-------
Space Requirements
To reach mature growth, trees require sufficient soil volume with
ample void space. The recommended soil volumes depend on
the size and the number of trees sharing the soil bed. Although
no universal standard for soil volume requirements for expected
mature tree size exists in arboriculture, it is generally accepted that
a large-sized tree (16 inches diameter at breast height) needs at
least 1,000 cubic feet of uncompacted soil (USEPA 2013). If soil
volume is insufficient for root establishment, tree growth will be
stunted and roots may be forced to grow upward, causing heaves
in the sidewalk. If retention is an objective, sufficient volume for
tree root growth and continued retention should be included in the
design.
catch basin
inlet
-7.
Cross section of tree box with a structural support system under the sidewalk.
Modular structural cells are typically constructed of plastic or fiberglass and
are designed to support pavement and loading requirements (Figure 6-7).
Soil is added to the cell framework, which provides structural support for
root growth. Structural cells are more commonly used in locations that
have inadequate volumes of soil for tree growth or where highly compacted
soils do not allow for root growth (typically under paved areas).
Optional Design Considerations
Alternative designs can help accommodate site-specific conditions or
goals. In addition to the enclosed vault option cited above, an impermeable
liner around the sides and on the bottom can be combined with an
underdrain system to inhibit infiltration in cases where foundation flooding
problems or the presence of underground utilities or contaminated soils
make infiltration undesirable. Setbacks to existing buildings and foundations
should be considered when determining the desirability of infiltration. Tree
pits and tree trenches can be designed to connect to subsurface infiltration
structures to provide additional storage and groundwater recharge. Addi-
tionally, planting trees in groups can reduce wind impacts and create shade.
uncompacted growing
media
soil support stiucture
perforated pipe
inlet/outlet
Different designs can be used separately or in conjunction with
one another in challenging situations (i.e., utility conflicts or limited
sidewalk area) to provide ample space.
- The tree is surrounded by an open, unpaved soil area that
can be planted or covered with mulch. This method requires
more street space than the other two methods.
- The tree is provided with root paths that use aeration or
drainage strips to guide root growth under the pavement.
Root paths may connect adjacent green spaces.
- The tree is provided with a specially designed soil area to
promote root growth under the pavement. A variety of solid
and permeable pavements can be used to cover the soil. The
underlying soil may consist of structural soils or modular
structural cells.
Structural or soils cells offer void space for root growth while providing
load support to meet pavement design requirements. Structural soils are
composed of crushed stone, clay loam and a hydrogel stabilizing agent,
which can be compacted to meet pavement design requirements. The
stone provides void space for root growth.
catch basin
Green Streets Handbook
6.5 Stormwater Tree Systems
6-23
-------
Inlet
Water typically enters tree trenches through a catch basin, but can also
enter from curb inlet or from permeable paving on the sidewalk above
the storage trench. The stormwater then flows through a perforated
distribution pipe or an underdrain into the filter media.
Underdrains
Tree pits are typically designed with an underdrain to provide filtration
of small volumes of stormwater before discharging to the existing
storm drain system or a downstream practice. Where soils have an
infiltration rate greater than 0.5-inch per hour, tree pits can be designed
to infiltrate (and underdrains are not needed). However, underdrains
may be advisable when there is an underground conflict with utilities or
issues with potential groundwater contamination due to resident soils. If
adjacent land uses have a high potential to discharge soluble pollutants
of concern, infiltration systems might not be appropriate.
Vegetation
Tree pits contain a single tree or shrub; tree trenches could contain
multiple trees or shrubs in series. Native vegetation species should be
Reforestation and Afforestation
Improving the tree canopy on a large scale can be a form of reforestation or
afforestation. Reforestation is the replacement of trees that were previously
lost to construction or deforestation. Afforestation is the planting of a new
tree community in an area where they have been absent for a significant
period of time, such as an old farm field (Prince George's County 2005).
These practices involve planting trees on existing turf or barren ground,
with the goal of establishing a mature forest canopy that will intercept
rainfall, increase evapotranspiration rates and enhance soil infiltration rates.
Reforestation and afforestation require large land areas and are therefore
most suited for sites near existing forests, along waterways or steep slopes,
and aiong existing highways or other roads.
Green Streets Handbook
selected based on soil conditions and the historic plant community in the
area. To provide maximum tree canopy benefits, street trees should be
planted near each other whenever possible while maintaining sufficient
area for each tree's individual root growth. For sites in cold climates near
roadways, it is essential to select trees that have a high tolerance for
pollutants and salt. Salt spray has been shown to affect areas over 30 feet
away from the road (MHD 2006). Potential thermal impacts from adjacent
structures should also be evaluated when selecting tree species and
designing tree box planters.
Ideally for reforestation and afforestation, plantings should provide a multi-
layer canopy structure of about 50 percent large trees and 50 percent small
trees and shrubs (Hinman 2005). Using a diversity of plant types and sizes
(e.g., evergreens, deciduous trees, shrubs) will increase the pest and disease
resistance (MHD 2006). For many sites, a ratio of two evergreens to one
deciduous tree will provide a mix similar to native forests (Hinman 2005).
To foster a forest-type microclimate on altered, disturbed landscapes,
pioneer species that thrive in infertile soils can be planted first. Establishing
these faster-growing varieties of plants before others mimics the natural
succession pattern and will create an environment that will provide shade
cover to enable more difficult-to-establish species to develop (MHD 2006).
Stormwater catch basin tree pit, Charlottesville, VA.
6.5 Stormwater Tree Systems
6-24
-------
Soils
In addition to the space requirements mentioned earlier, soils should remain uncompacted so
water and nutrients can infiltrate into void spaces. It might be necessary to enhance the existing
soil with fertile topsoil, especially for reforestation projects. To increase the permeability of native
soil, a compost-amended soil can be added. Care should be taken to prevent soil compaction
during planting.
Structural soils are engineered soil-on-gravel mixes that are designed to support tree growth and
serve as a sub-base for pavements. They are typically 70% to 80% angular gravel, 20% to 30%
clay loam soil and a small amount of hydrogel (~3%), which provides 20% to 25% void space.
Bioretention soil mixes are commonly used for extended tree pits. The University of New Hamp-
shire Stormwater Center (UNHSC) recommends a bioretention soil mix that is comprised of 80%
sand and 20% compost to maximize permeability while providing minimum organic content.
UNHSC also recommends 3 feet of bioretention soil mix. Supporting material for the Minnesota
Stormwater Manual suggests 50% to 65% coarse sand, 25% to 35% topsoil and 10% to 15%
compost (MPCA 2013).
Performance
Trees retain water, improve water quality and
offer many other community benefits when
properly planted. Trees generally absorb the
first 30% of precipitation events through their
leaf system and release it through evapora-
tion,Up to an additional 30% of precipitation
is absorbed into the ground and is taken in
and held by the root structure before being
absorbed and released to the air as transpira-
tion (Burden 2006). Trees also enhance water
quality by using nutrients for plant processes
at the surface and within the soil media. The
soil matrix removes pollutants as well through
chemical binding of charged particulates,
biological uptake by microbial communities
in the soils and physical removal through
filtration.
Tree pits treat stormwater runoff at a park in
Portland, OR.
For More information-Stormwater Tree
Systems
Urban Street Trees- 22 Benefits Specific Applica-
tions. Dan Burden, Glatting Jackson and Walkable
Communities, Inc. (2006)
Stormwater. Trees, and the Urban Environment.
Charles River Watershed Association (2009)
Minnesota Stormwater Manual; Trees. Minnesota
Pollution Control Agency (2013)
Green Infrastructure Practices: Tree Boxes (Fact
Sheet FS1209Y Rutgers University Cooperative
Extension,. New Jersey Agricultural Experiment Station
(2013)
Regular Inspection and Maintenance Guidance for
Bioretention Svstems/Tree Filters. University of New
Hampshire Stormwater Center (2009)
Stormwater to Street Trees: Engineering Urban
Forests for Stormwater Management. USEPA Office
of Wetlands, Oceans and Watersheds (2013)
Stormwater Trees: Technical Memorandum. USEPA
Great Lakes National Program Office (2016)
i-Tree: Tools for Assessing and Managing Commu-
nity Forests. U.S. Forest Service
Quantifying the Benefits of Urban Forest Systems as
a Component of the Green Infrastructure Stormwater
Treatment Network. Kuehler et al. Ecohydrology (2017)
The Role of Trees in Urban Stormwater Manage-
ment. Berland et al. Landscape and Urban Planning
162:167-177 (2017)
Green Streets Handbook
6.5 Stormwater Tree Systems
6-25
-------
Trees are planted in
groves connected by
trenches in this parking
lot at the Maplewood
Mall, MN. Tree trenches
extend 8 to 12 feet wide
and 4 feet deep for a
total of 1 mile in length.
Angled curbs were
designed to allow snow
plows to roll smoothly
over them.
Maintenance Requirements
QJ
Maintenance of street trees is performed by
arborists, landscape professionals, homeowners ^
or volunteers. For an extended tree pit, refer to the ^
maintenance recommendations for bioretention.
Supplemental irrigation might be required during
initial tree establishment. Table 6-7 outlines long-term
recommended maintenance activities that should be
conducted for stormwater tree systems.
Table 6-7. Recommended maintenance activities for stormwater tree systems
Activity
Frequency
Additional advice
Debris
Inspect planter box structural integrity.
Annually
Any damaged components should be repaired or replaced.
Remove sediment or trash that has accumulated.
Two to four
times per year
Inspect underdrains for obstructions.
Annually
Backflush if obstructions are found.
Vegetation
Weed invasive and exotic species, preferably using nonchemical
methods such as hand pulling and hoeing. For reforestation and
afforestation project, remove ferns and grasses that would compete
with tree seedlings.
Annually
(preferably in
spring)
If the survival rate of planted vegetation falls below 80% during this 3-year period, the
cause of plant mortality should be investigated and corrected. Possible causes could
be poor soils, soil compaction, or improper plant species selection (Hinman 2005).
Check tree system after storm event to ensure stormwater is not
ponding after 24 to 72 hours (check local codes).
As needed
If ponding does occur, either increase the infiltrative capacity of the soils or add an
underdrain.
Prune trees, including the removal of dead and diseased limbs and
clear overgrowth to maintain street sign visibility, pedestrian vertical
clearance, and line of sight on curved roads and intersections.
Annually
Protect tree from deer or other wildlife using tree guards or fencing.
As needed
*o
CO
Turn ortill soil, especially if compaction occurs.
As needed
If maintenance efforts are unsuccessful, the soil media and underdrain might need to
be removed and replaced.
Evaluate soil substrate for channel formation and proper root growth.
Annually
Remove and properly dispose of the previous mulch layer, or rototill
into the soil surface and add new mulch layer.
Every 2 years
(preferably in
spring)
Do not exceed 3 inches in depth for mulch layers. Avoid blocking inflow entrance
points with mounded mulch or raised plantings. Once a full groundcover is established,
mulching might not be necessary.
Green Streets Handbook
6.5 Stormwater Tree Systems
6-26
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6.6 Infiltration Trenches
Description
Infiltration trenches are excavated linear areas that are filled with layers of
stone and sand wrapped in geotextile fabric. The trench is covered with
stone, gabion, sand, or grassy surface with surface inlets. Stormwater
is stored in the stone reservoir and slowly infiltrates through the bottom
and sides of the trench, thereby reducing stormwater volume and peak
discharge. As the water flows into the existing subsurface, pollutants and
sediments are filtered out to improve water quality of the discharge. Underd-
raws can be included if native soil has lower permeability than desired. This
system requires pretreatment to remove suspended solids.
Site Considerations
Infiltration trenches are ideal for linear transportation, linear parking lots
and retrofit applications due to their relatively small foot print compared to
the water storage capabilities. At minimum, they are generally 24 inches
wide and 3 to 12 feet deep. Infiltration trenches are applicable only for small
drainage areas, typically of less than 5 acres (RIDEM and CRMC 2010). They
are typically implemented at the ground surface to intercept overland flows.
Infiltration trenches can also be installed below roadways or impervious
areas with proper design. The design must prevent infiltration into the
subbase of the pavement; therefore, it should slope slightly away from the
subbase or be located at a depth below the subbase. Infiltration trenches
can be used in a site's upland areas to reduce the amount of runoff
downstream.
For More Information-infiltration Trenches
infiltration Trench. City of San Diego (2011)
Infiltration. Stormwater Manual (Chapter 3.8). District of Columbia
(2013)
Best Management Practice Fact Sheet 8: infiltration Practices
(Publication 426-127). Virginia Cooperative Extension (2013)
Green Streets Handbook
r
Infiltration Trenches
Advantages:
- Reduce total volumetric
runoff
- Provide water quality
treatment for fine sediment,
trace metals, nutrients,
bacteria and organics
- Reduce downstream
flooding and localized
flooding
- Reduce the size and cost
of downstream stormwater
control facilities
* Typically used on collector or arterial roads
Infiltration trench with grass cover.
- Provide groundwater
recharge
- Avoid loss of parking
spaces when designed
underground
- Appropriate for small sites
and where space is limited
Most suitable* for:
- Any length of roadway
- Parking lot
- Median
6.6 Infiltration Trenches
6-27
-------
Overcoming Site Challenges
infiltration systems can be designed to overcome multiple site challenges
(Table 6-8).
Table 6-8. Infiltration trenches: site constraints and design alternatives
Challenge
Design alternatives and recommendations
Sites that are not stable or
have high sediment loads
Plan for pretreatment practices to avoid frequent and
intensive maintenance.
Low permeability of native
soils or compacted soils
Consider adding an underdrain that modifies the
practice to be more of a soil filter or sand filter
(i.e., converting to a different BMP).
Cold climates
Design the maximum effective depth for runoff below
the frost line to allow infiltration to occur through the
winter months.
Sites with high pollutant ioads
(i.e., potential hotspots) or
contaminated soil
Avoid placing infiltrating systems due to concerns of
groundwater contamination. Recommend practices
include extensive pretreatment and/or impervious
liner.
Proximity to water table
Maintain a recommended 2-foot separation to water
table (3 feet preferred in some regions) and a minimum
of 2 feet from the bottom of the infiltration trench to
the bedrock (10 feet for fractured bedrock).
Proximity to drinking water
wells
Trenches should be set back a minimum of 150 feet
from public drinking water wells to limit groundwater
contamination.
Proximity to building
foundations
Trenches should be situated 100 feet upgradient or
10 feet downgradient to avoid potential seepage.
Infiltration Trench
Key Design Features
- Permeable filter fabric/material surrounds the stone on
both sides of the trench
- An observation well allows for frequent inspection
Green Streets Handbook
Components: Infiltration Trench
An infiltration trench (Figure 6-8) typically consists of:
- An observation well
- Clean washed stone (typically 0.75 to 1.5-inch in diameter)
- A filter layer using either filter fabric, pea gravel (typically 3/8 inch) or
sand
- Permeable filter fabric or sand filter on sides of trench
Optional design components include:
- Turf or grass cover
- Washed sand filter at bottom of practice for final filtration and even
disbursement
- An elevated underdrain to promote internal storage and detention
- An impermeable liner (only in highly polluted areas)
Infiltration Trench
(Level .SprearteO
Filter Fabric
•Surface Materials
(stowe cr0rfl££)
(Filter strvp)
PomArea
overflow
soil Merita
Figure 6-8. Stormwater infiltration trench schematic.
-------
Design Considerations
Inlet Design
Runoff can enter an infiltration trench through sheet flow or piped inflow.
To prevent clogging from sediment, pretreatment is required. When sheet
flow is draining to the system, pretreatment might include a grass filter strip
or gravel apron. If inflow is piped in, pretreatment might include a sediment
forebay or a flow-through structure that collects sediment before conveying
the water to the system. For areas with high pollutant loads, an oil and grit
separator or similar device may be necessary. See Chapter 5 for descrip-
tions of a number of pretreatment practices. Source control strategies, such
as the elimination of excessive sanding/salting practices, should also be
pursued. To ensure stormwater distribution in the stone trench, a perforated
rigid pipe of at least 8-inch diameter can be connected to the inlet.
Slopes
Infiltration trenches are feasible if adjacent side slopes range from 2 to 15
percent. Slopes must be sufficiently steep to convey runoff to the practice,
but must not cause erosion. To prevent underground infiltration trenches
from draining into the subbase of the adjacent pavement, they should be
sloped slightly away from or be located below the subbase.
Filter Layer
Filter fabric is used around the sides of the trench to define the system
and prevent any potential contamination of runoff that is not completely
treated. A filter layer should be incorporated into the top of the trench (6 to
12 inches below the surface) to prevent clogging from sediment carried
in runoff but not removed by pretreatment and/or soil migration into the
stone layer if turf or grass cover is included. Including filter fabric close to
the surface minimizes maintenance and reconstruction needs if clogging
occurs above the liner, as this portion can easily be removed and replaced.
An alternative to filter fabric is the use of pea gravel or sand in the top 1 foot
of the trench. The pea gravel improves sediment filtering and maximizes
pollutant removal.
Green Streets Handbook
Observation Well
An observation well should be installed at the lower end of the infiltration
trench to monitor how the system drains after large storms and to verify
that the system is not clogged. The well should consist of a perforated PVC
pipe with a 4- to 6-inch diameter that is constructed flush with the ground
elevation and fitted with a lockable well cap. For larger trenches, which
might require pumping to remove sediment, a 12- to 36-inch diameter PVC
pipe is recommended to facilitate maintenance.
Backfill
The aggregate for the trench should consist of a clean aggregate with a
maximum diameter of 3 inches and a minimum diameter of 1.5 inches.
Void space should be in the range of 30 to 40 percent.
Vegetation
Infiltration trenches may be bare gravel or may be covered by turf or grass.
Use a no-mow or low-maintenance seed mix for grass-covered trenches.
: ¦ f ' . ' ¦ -
*¦ \ "
jSr *•
f -VbI
" f * :¦» A
-T
1 '
Pr-M' •'
Infiltration trench (gravel) adjacent to a roadway.
6.6 Infiltration Trenches
6-29
-------
Maintenance Requirements
These activities should be performed every 6 months and after every major
storm (MADEP 2008). Suggested maintenance activities and frequencies
are provided in Table 6-9. Additional maintenance is needed for pretreat-
ment practices.
Performance
Infiltration trenches reduce stormwater volume, reduce peak discharge
and improve water quality. By providing infiltration, these systems can
promote groundwater recharge, contribute to baseflow for streams and
help maintain the natural hydrologic balance that existed on the site before
development. As the water filters through the system and into the existing
subsurface, pollutants and sediments are removed and the water quality
of the discharge improves. The primary pollutant removal mechanisms are
settling, physical straining and filtration.
Infiltration trench adjacent to a Minnesota roadway.
Table 6-9. Recommended maintenance activities for infiltration trenches
Activity
Frequency
Additional advice
Debris
Inspect and remove sediment that has accumulated in the top foot of
stone aggregate.
Two to four
times per year
Inspect underdrains for obstructions.
If obstructions are found, backflush the obstructions.
Vegetation
Mow turf or grass. Remove invasive and exotic species, preferably
using nonchemical methods such as hand pulling and hoeing.
Yearly
(preferably in
spring)
If at least 50% vegetation coverage is not established after 2 years, provide additional
plantings.
Check trench after storm events to ensure stormwater is not ponding
after 72 hours.
After major
storms
If ponding does occur, check for clogging and/or evaluate the infiltrative capacity of the
soils.
Media
Check water levels, drawdown time and water quality using the
observation well.
Two to four
times a year
Ifthe bottom ofthe trench is clogged, all of the stone aggregate and filter fabric must
be removed. If clogging appears only at the surface, remove and replace the first layer
of stone aggregate and filter fabric.
Green Streets Handbook
6.6 Infiltration Trenches
6 30
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6.7 Subsurface Infiltration and Detention
Description
Subsurface infiltration and detention practices are subsurface systems that
capture, temporarily store and slowly release stormwater to reduce runoff
peak discharge. They provide stormwater quality treatment by decreasing
sediment mobilization, transport and deposition, and they encourage
biochemical processes in the underlying soils. Additionally, the water from
these systems can be harvested and treated for other uses such as land-
scape irrigation or as a water source for fountains and ice skating rinks.
Design variations for subsurface infiltration and detention systems vary by
materials, configuration and layouts, which are specified by manufacturers.
Subsurface infiltration systems consist of an infiltrative chamber system
typically made of precast concrete or plastic that includes perforated pipes,
galleys and chambers. The chambers can store large volumes of runoff
which is allowed to slowly infiltrate into the ground. Subsurface detention
practices temporarily store runoff before releasing it to a downstream
practice or conveyance system. Although not designed for water quality
benefits, these systems do provide some water quality improvement
through sedimentation.
The typical elements of a subsurface system include infiltration pits,
chambers, perforated pipes and galleys:
- Infiltration pits. This system consists of a precast barrel with
uniform perforations. The barrel will sit on top of stone and will
be backfilled with stone to promote infiltration. To create a sump
for collection of sediment, the perforations should not extend to
the bottom of the barrel. Pits may be placed in series to allow the
overflow of one to be conveyed to the next pit in sequence.
- Chambers. Chambers consist of prefabricated modular or
cylindrical cells surrounded by crushed, washed stone. If designed
for infiltration, the chambers will have open bottoms or perforations.
If designed solely for retention, the chambers are typically encased
in an impermeable liner or are constructed of nonperforated pipes
and are then discharged to an outlet control structure.
Green Streets Handbook
Subsurface Infiltration and Detention
Advantages:
- Capture and store large
volumes of runoff
- Are suitable for highly
urbanized area with limited
surface space availability
- Reduce downstream flooding
and localized flooding
- Provide groundwater
recharge
- Quick installation process
Most suitable* for:
- Parking lot
- Sidewalks
- Roadways
* As long as maintenance access to these systems is available
- Perforated Pipes. A perforated pipe system acts like a leaching bed
and consists of rows of perforated pipes that dose a leaching bed.
- Galleys. Galleys are concrete rectangular vaults or systems
of interlocking modular units. If designed for infiltration, the
rectangular vaults will have perforations.
Subsurface chambers, during and after (top right) installation.
6.7 Subsurface Infiltration and Detention
6 31
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Design Considerations
Inlet Design
Stormwater typically enters subsurface practices through a catch basin
or curb inlet (USEPA 2001). It can also enter the subsurface pit through
porous pavement. Pretreatment is essential to prevent sediment or debris
from migrating into and clogging the infiltration bed. Filter strips and
modified catch basins (see Chapter 5) are good options for pretreating
runoff entering subsurface infiltration and detention practices.
Materials
Many prefabricated subsurface infiltration or detention products are
available. Systems can be constructed of concrete, steel or plastic (USEPA
2001). When determining the type of material to use for subsurface infiltra-
tion or detention structures, design engineers should consider the loading
requirements and the available area. For example, steel and plastic require
more fill than does concrete to maintain strength under compression.
Large concrete structures provide more storage than pipes, but pipes are
more versatile in their angling and arrangement (USEPA 2001). Enough
stone should be included in the storage areas to prevent subsidence.
Overflows
Subsurface structures are typically designed to drain fully within 72 hours
to provide adequate pollutant removal while also ensuring the system
drains between rain events (MADEP 2008). Water standing for longer than
5 days can lead to potential mosquito breeding (Connecticut Department of
Environmental Protection 2004). Detention systems must have outlet pipes
sized to release stored runoff at the required rates.
Observation Well
An observation well, or manhole access for chamber systems, should be
included to monitor how the system drains after large storms and to verify
that the system is not clogged. The observation well should be placed at
the invert of the stone bed and in the middle of the system.
Green Streets Handbook
Components: Subsurface Systems
A subsurface infiltration system
(Figure 6-9) typically consists of:
- Inlet
- Pretreatment
- Perforated pipe
- Chamber
- Observation well
- Aggregate fill
Optional design component:
- Impermeable liner
Subsurface Infiltration and
Detention System
Mqur'';;.)!!1 ;i I aj
Outflow o>
Depth
M Ok
Infiltration
Figure 6-9. Subsurface infiltration and detention system schematic.
Vegetation
Trees or shrubs with long tap
roots should not be planted
within the immediate vicinity of
subsurface structures.
Soils
The bottom of infiltrating prac-
tices should be level to promote
evenly dispersed infiltration.
During construction, any area intended for infiltration should not be com-
pacted, Erosion and sediment control techniques should be implemented
during construction to prevent any sheet flow or windblown sediment from
entering the infiltration area. Subsurface infiltration rates should typically
be at least 0.5 inch per hour for infiltration practices.
Infiltration Trench
Key Design Features
- Provide an accessible
maintenance entry point
- Include an observation well to
allow for inspection
- Size the chamber according to
the storm design volume
6.7 Subsurface Infiltration and Detention
6 32
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Maintenance Requirements
Because these systems are below ground, they are more difficult to
maintain and clean than aboveground practices (USEPA 2001). These
systems should therefore be located in areas where maintenance vehicles
such as vacuum trucks can easily operate and excavate, if needed (RIDEM
and CRMC 2010). Key maintenance practices needed are presented in
Table 6-10.
Table 6-10. Recommended maintenance activities for subsurface infiltration and
detention systems
Activity
Frequency
Additional advice
Conduct observation well inspection
As needed
Monthly during the first
of system to verify drainage times.
year of infiltration to
tA
ensure functionality
is
JO
Q)
Remove sediment or trash that has
Two to four
If excessive clogging
O
accumulated to prevent clogging of
times per
builds up, the system
pretreatment practices and inlets.
year
should be excavated and
replaced.
Performance
Subsurface infiltration and detention practices reduce both the volume
of runoff and pollutant loads in runoff. Furthermore, the practices help
recharge groundwater and reduce the size of downstream stormwater
management practices. Subsurface infiltration provides water quality
improvement through filtration into underlying soils.
These practices can be included as part of a series of stormwater
management and treatment practices, called a treatment train. Detention
practices can slow runoff volumes and slowly release them to down-
stream practices that will provide additional water quality improvement.
Subsurface infiltration chambers can be used to provide additional storage
volume and groundwater recharge as part of a treatment train (Connecticut
Department of Environmental Protection 2004).
Green Streets Handbook
Perforated pipes in New York, NY.
For More Information-Subsurface Detention
Subsurface Detention. Stormwater Management Practice
Guidance (Chapter 4.8). Philadelphia Water Department (2018)
Infiltration Practices. Stormwater Design Specification No. 8.
Virginia Department of Environmental Quality (2013)
6.7 Subsurface Infiltration and Detention
6 33
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6.8 Permeable Pavement
Description
Permeable pavements are paving systems that allow runoff to infiltrate through void
space instead of becoming surface runoff. Water filters through void spaces within
the paved surface into a stone reservoir and eventually infiltrates into the existing
ground below. Where infiltration is not possible, permeable pavement systems can
be designed with an underdrain that will convey treated runoff to another stormwater
management practice or storm drain system.
Permeable pavement systems reduce runoff volumes and peak discharges by
providing internal storage, and they improve water quality by filtering and infiltrating
stormwater into the ground. Pretreatment is strongly recommended upstream of the
practice to reduce sediment loads and to prevent debris from entering the system and
clogging the drainage spaces between the pavers or the permeable surface. Some
practitioners argue that "runon" from upland sources should be avoided or prohibited.
Recommended pretreatment techniques are filter strips and swales (see Chapter 5).
Types of Permeable Pavement:
Porous Asphalt
Porous asphalt is a hot-mix asphalt with a reduced amount of sand or fines, which
allows for increased interconnected pore space for water to drain through the pave-
ment into a crushed stone reservoir and base. To maintain proper infiltration rates
through the paving layer, the amount of asphalt binder in the mix must be minimized
to prevent clogging of voids.
- Permeable friction course (PFC) is an application of porous asphalt over
standard asphalt. PFC is also known as open-graded friction course on some
highways. A PFC is a thin layer of porous asphalt, typically 1 to 2 inches thick,
which is laid over standard asphalt. The stormwater travels through the voids in
the permeable PFC asphalt until it reaches the impermeable asphalt boundary
below and then flows towards the adjacent road perimeter. The principal
purpose of this layer is to reduce hydroplaning by quickly removing precipitation
from the pavement surface. The application of PFC leads to shorter stopping
distances for cars, quicker surface drying periods, less splash and spray during
precipitation (ASCE 2015). Additionally, PFC reduces the amount of pollutants
discharged, reduces noise and improves safety for motorists.
Green Streets Handbook
Permeable Pavement
Advantages:
- Reduces runoff volume
and peak discharge rates
- Increases groundwater
recharge through
infiltration
- Avoids loss of parking
spaces
- Reduces occurrence of
freezing puddles and
black ice and requires
less applied deicer
Most suitable for:
- Parking lots
- Parking lanes
- Driveways
- Sidewalks
- Walking paths
- Low-traffic roads
- Biking lanes
- Parkways
- Road shoulders on
higher-volume roads
Permeable friction course on the shoulder of I-293 in New Hampshire.
6.8 Permeable Pavement ' !
-------
Pervious Concrete
The design of pervious concrete differs from standard concrete because the fines have been removed
from the concrete mix and different cementitious materials and chemicals have been added, such
as fly ash and air-entraining agents. When installed, pervious concrete looks similar to conventional
concrete except it typically has a rougher surface and allows for infiltration into the ground. Pervi-
ous concrete is also available in precast concrete panels that are placed together on site.
Pavers
Pavers are pre-cast paving units that are arranged to leave void spaces between the pavers. These
voids are filled with sand, fine gravel, or are planted with turf or grass to allow for water to infiltrate
through the pavers into the underlying stone reservoir. Many types of pavers are available, including
the following three:
- Permeable interlocking concrete pavement (PICP). PICP is comprised of a layer of durable
concrete pavers separated by joints that are filled with small stones. The blocks are impervi-
ous, but the joints permit infiltration to the stone reservoir. The joints, or interlocking shapes,
can vary from simple notches to built-in concrete joint spacers. PICPs are highly attractive,
durable, easily repaired, require low maintenance and can withstand heavy vehicle loads
- Concrete grid pavement (CGP). CGP is an extensive concrete grid that uses large spaces
filled with stone aggregate or with sod or turfgrass. The reinforced concrete structure provides
stability for bearing the weight of vehicles; the stone or sod-filled spaces provide permeability.
Unlike PICP, concrete grid pavements are generally not designed with an open-graded, crushed
stone base for water storage and thus have lower infiltrative rates. Moreover, grids are for
intermittently trafficked areas such as overflow parking areas and emergency fire lanes.
- Grass pavers (turf blocks). Grass pavers are a type of open-cell unit paver in which the cells
are filled with soil and planted with turf. The pavers can be made of concrete or synthetic
material. The pavers serve to distribute the weight of traffic evenly and prevent compaction
of the underlying soil.
Porous Recycled Surface Products
These products are generally more attractive than porous asphalt and are suitable for pedestrian
and light vehicular traffic loads. They are typically highly reflective, colorful porous paving systems
that provide greater design flexibility. Constructed of a porous, hard surface paving made from recy-
cled glass, waste granite, rubber, aggregates and/or other recycled material, they are often bound
together with a proprietary pigmented binder. Similar to porous pavement, this design alternative
allows runoff to drain through the paved surface into a crushed stone reservoir.
Permeable paver installation in parking lanes in Louisville, KY.
Permeable interlocking concrete pavement, Chicago, IL
Pervious concrete trench in the center of an alley, Chicago, IL.
Green Streets Handbook g 3 Permeable Pavement I ^
-------
Site Considerations
Generally, permeable pavement is recommended for low-volume and
low-speed applications with limited turning traffic. The use of permeable
paving can potentially reduce the size and extent of downstream
stormwater collection, conveyance and detention. Because permeable
pavement systems provide their own stormwater management, they
can be used to maximize drivable surface area. Permeable pavements
can be designed for only a partial area of the design site and installed in
combination with impermeable pavement such as in the parking lane of
a street or in the parking stalls of a parking lot. It is not recommended to
drain impermeable surfaces onto the permeable areas due to clogging
concerns.
Permeable pavements are generally not appropriate for high-traffic or
high-speed areas because they have lower load-bearing capacity than
conventional pavement; however, interlocking pavers have been used in
high-load installations in cargo ports and airports. Although pavers tend
to be more costly to install than other paving systems, they are easier to
repair because small sections can be removed and replaced (San Mateo
County 2020). In contrast, damaged permeable pavement is difficult to
repair because it is made in large batches. When selecting the type of
material, consider the traffic volume, type of use and expected mainte-
nance frequency.
Care should be taken to not place permeable pavements adjacent to land
uses or areas that could contribute high sediment or organic materia!
loadings (e.g., heavily wooded or landscaped areas where leave, mulch or
soil can wash off and clog the pavement).
Overcoming Site Challenges
Permeable pavement systems can be designed to overcome site
challenges such as a high groundwater table, high-traffic areas, or steep
slopes (Table 6-11).
Green Streets Handbook
Permeable pavement in parking stalls in Concrete grid pavers in Emeryville, CA.
Williamsburg, VA.
Table 6-11. Permeable pavement: site constraints and design alternatives
Challenge
Design alternatives and recommendations
Potential groundwater
contamination or proximity to
water table or bedrock
Line the subsurface reservoir with an impermeable liner.
For areas where there is a potential for hazardous spills
(e.g., gas stations, loading docks), permeable pavement
is not recommended
Cold climate
Avoid applying sand, which can clog the surface of the
material. Do not use areas with permeable pavement as
plowed snow storage areas.
Conflict with underground utilities
Offset infiltration trenches away from utility lines.
High-traffic or high-speed areas
Permeable pavements are not recommended because
they have lower load-bearing capacity than conventional
pavement.
Steep slopes
Construct subgrade check dams, baffles, or terraces to
provide a level area for storage area.
Low permeability of native soils or
compacted soils
Replace or amend soils to improve permeability.
Low structural capacity of clay
soils
Increase the subbase depth and/or add geogrids to
provide additional support.
6.8 Permeable Pavement
6 36
-------
Components: Permeable Pavement
Permeable pavement components
(Figures 6-10 and 6-11) typically consist
of:
- Pavers or pervious pavement. 4 to
6 inches of permeable materia! (e.g.,
asphalt or concrete) with 10 to 25
percent void space. Paver thickness is
determined by loading rates.
- Choker course for porous asphalt, 1
to 2 inches of small-sized, open-graded
aggregate below the paver/pavement
layer. Provide a level bottom to promote
even infiltration through the practice.
- Open-graded base reservoir. 3 to 4
inches of crushed stones (typically
€|-%inch in size) with a high void
content to maximize the storage of
infiltrated water and to create a capillary
barrier to winter freeze/thaw.
- Open-graded subbase reservoir.
Thickness depends on water storage
requirement and traffic loads. Uniformly
graded, clean and washed coarse
aggregate (%-2yz inch in size with 40
percent void space) are used. Might not
be required in pedestrian or residential
driveway applications.
- Subgrade. The infiltrative capacity of
the aggregate determines how much
water exfiltrates from the subgrade to
the surrounding soiis. An uncompacted
subgrade is preferable.
Optional design components include:
- Underdrain
- Impermeable liner for conditions
where infiltration is undesirable.
- Geotextile or other filter material
such as pea gravel placed between the
subbase and the subgrade to prevent
the migration of soil.
- Observation well to enable visual
monitoring and inspection of the
system for maintenance.
Pornwabio Surface Course
Sand F ifter Layer
far Systems *i"
Untefttvint {3rmin,)
Natural Sal for Systems
Without UndQftfratn 117" mm)
Tor:nrTri
4
Structure for Accew
(op6crwij
fWS Ela^afcon (Mpfi of
WG TPMtoWftl Volume)
Ontfice Ptata mod
ej Ddfcacfif KMorvcjr
ui 2-$ Dar^
wMnflUnflnfl)
Figure 6-11. Permeable interlocking concrete pavement cross-section.
Green Streets Handbook
' ' ¦' 1 ' * " » * ,* > ' » 4 * I
'j ' ' ' x ' • 9
I 1 ' | '»•'»» SJ
I * • I * TO
¦ • jO ¦ I
J Wjjf ^ |
•§
Pervious concrete 6" S
1/4" Stone choker course ? 4" §>
A. (TS
Pervious concrete
6"
3/4" Stone choker course
4*
~
i
1
Sand/grave! (filter course) T ^
18*
3/8" Stone infiUiattefi res*¦¦
6" Perforated subdraln
Native soils
4"
Figure 6-10. Typical pervious concrete pavement cross- section.
Permeable Pavement
Key Design Features
- Level storage bed bottoms
- The surface permeability should be greater than 20 inches
per hour.
- Pretreatment highly recommended to remove sediment-
laden runoff.
- Load-bearing capacity of subgrade determines design
depth.
- Infiltrative capacities of permeable pavement and aggregate
in subgrade layers.
6.8 Permeable Pavement
6 37
-------
Design Considerations
Materials
Permeable and conventional pavements require similar materials and
construction techniques with a few exceptions. Permeable pavement
requires greater depth of the aggregate subbase to provide additional
stormwater volume storage. A geotextile material might be required in
areas of unstable soils or when the groundwater table is high (University of
New Hampshire Stormwater Center 2012; MADEP 2008; RIDEM and CRMC
2010). Permeable pavement should not be installed during rain or over
frozen base material. To maximize infiltration, avoid compacting subgrade
soil during installation. If compaction is needed to support vehicle loads,
compaction density and subsequent soil infiltration should be assessed
in a test pit(s) on the site to determine an acceptable soil density and its
contribution to soil strength and infiltration.
Sizing
The at-grade contributing drainage area into permeable pavement should
generally not exceed twice the surface area of the permeable pavement
(runon from permeable areas is not recommended due to potential for
clogging of permeable pavement). This guideline helps reduce the rate of
surface sedimentation. The 2:1 ratio can be increased to no greater than
5:1 if at least one of these conditions exists:
- Permeable pavement is receiving runoff from roofs as it tends to
be very low in sediment.
- Runoff from adjacent impervious surfaces remains unburdened
with sediment due to effective pretreatment before entering the
permeable pavement.
Slopes
The permeable pavement subbase should be installed on level ground. For
slopes greater than 3 to 5 percent, check dams, baffles or terraces can
be built as part of the subgrade to provide a level area for storage area.
Otherwise, there will be little storage capacity. If excavations are necessary
Green Streets Handbook
to provide adequate storage, utilities might need to be relocated to
maintain adequate clearance.
Performance
Permeable pavement systems reduce stormwater peak discharge
and runoff volume by storing runoff within the subbase layers as it
slowly infiltrates. A larger reservoir layer allows more runoff volume to
be stored within the practice. As the runoff filters through the varying
layers, the water quality of the runoff is also improved.
PFCs, a use of permeable asphalt, achieve very little runoff volume
or peak flow reduction because they are not tied to any underground
storage (NCHRP 2009). However, they have been found to achieve
significant removal of sediment-bound pollutants, with effluent total
suspended solids concentrations in the range of 10 milligrams per liter
(Eck et al. 2012). In addition to pollutant removal, PFCs act as a level
spreader, dissipating stormwater velocity and limiting erosion.
Permeable pavers used in the parking lane of a roadway in Ann Arbor, Ml.
6.8 Permeable Pavement
6 38
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Maintenance Requirements
For More Information-Permeable Pavement
The primary goal of permeable pavement maintenance is to keep the
surface clean and free of debris to maintain efficiency. If drainage voids
or openings in the surface are not regularly cleaned and vacuumed, the
pavement surface and/or underlying infiltration bed can become clogged
with fine sediments. Signs should be posted indicating that sanding is not
required and that construction and hazardous materials vehicles should
not drive on permeable pavement. Key maintenance needs are outlined in
Table 6-12; these activities might need to occur more often depending on
the frequency and size of storm events.
Soak Up the Rain: Permeable Pavement USEPA (2015)
Permeable Pavement Systems Stormwater Manual (Chapter 3.5).
District of Columbia (2013)
Federal Highway Administration Tech Briefs:
Porous Asphalt Pavements with Stone Reservoirs (2015)
Permeable Interlocking Concrete Pavement. (2015)
Permeable Concrete Pavements (2016)
Table 6-12. Recommended maintenance activities for permeable pavement
Activity
Frequency
Additional advice
Inspect for proper drainage and potential deterioration.
4 to 6 months after
installation and then annually
Remove sediment or trash that has accumulated to prevent clogging from
pretreatment practices and inlets.
Two to four times per year
Perform vacuum sweeping.
Twice per year
_Q
aj
Conduct power hose washing.
Twice per year
Recommended after sweeping and vacuuming. Inspect the aggregate
and refill with clean stone or gravel if necessary.
o
Inspect adjacent areas, which should be kept well-landscaped to prevent
soil washout and to minimize the risk of sediment, mulch, grass clippings,
etc., from inadvertently clogging the permeable pavement.
Annually
Design pretreatment elements between landscaped areas and
permeable pavement sections to collect sediment and other organics.
Reseed bare spots on grass pavers.
As needed
Inspect surface for cracks or settling; replace any cracked or broken
sections.
Annually
k_
Avoid the use of salt and sand for snow treatment to maintain
permeability and prevent clogging.
Qi
-C
+-»
CO
aj
s
Carefully perform snow plowing.
Set blade slightly higher than usual or attach rollers to the bottoms of
snowplows to prevent catching the edges of pavers.
"D
O
o
Minimize the accumulation of snow piles on the permeable pavement to
prevent the settling of sediments and pollutants on the surface, which
could lead to clogging.
Green Streets Handbook
6.8 Permeable Pavement
6 39
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7
References
Arnold, C.L., Jr. and C.J. Gibbons. 1996. Impervious Surface Coverage: The
Emergence of a Key Environmental Indicator. Journal of the American
Planning Association 62(2):243-258.
ASCE (American Society of Civil Engineers). 2015. Permeable Pavements.
Ed. B. Eisenberg; K. Collins Lindow, P.E.; and D.R. Smith. American
Society of Civil Engineers, Reston, VA.
Barrett, M.E. 2004. Performance and design of vegetated BMPs in the
highway environment. In Proceedings of the 2004 World Water and
Environmental Resources Congress: Critical Transitions in Water and
Environmental Resources Management, Salt Lake City, Utah, June 27-
July 4, 2004, pp. 777-786.
Bergstrom, D., K. Rose, J. Olinger, and K. Holley. 2013. The
Sustainable Communities Initiative: The Community Engagement
Guide for Sustainable Communities. PolicyLink and Kirwan
Institute, http://www.policvlink.org/find-resources/librarv/
community-enaaaement-auide-for-sustainable-communities.
BES (Bureau of Environmental Services). 2006. Vegetated Swales. City of
Portland, Bureau of Environmental Services, Portland, OR.
https://www.portlandoreaon.aov/bes/article/127473.
BES (Bureau of Environmental Services). 2008. Erosion and Sediment
Control Manual. City of Portland, Bureau of Environmental Services,
Portland, OR. https://www.portlandoregon.gov/bes/article/474129.
Burden, D. 2006. 22 Benefits of Urban Street Trees. Glatting Jackson and
Walkable Communities, Inc.
https://www.walkable.org/download/22 benefits.pdf.
Green Streets Handbook
CEI and NHDES (Comprehensive Environmental, Inc. and New Hampshire
Department of Environmental Services). 2008. New Hampshire
Stormwater Manual. Volume 2, Chapter 4-4.
https://www.des.nh.goV/sites/g/files/ehbemt341 /files/
documents/2020-01/vol2-ch4-sct4.pdf.
Clary, J., J. Jones, M. Leisenring, P. Hobson, and E. Strecker. 2017.
International Stormwater BMP Database: 2016 Summary Statistics.
Prepared for Water Environment & Reuse Foundation by Wright Water
Engineers, Inc., and Geosyntec Consultants.
CNT (Center for Neighborhood Technology) and American Rivers. 2010.
The Value of Green Infrastructure: A Guide to Recognizing Its Economic,
Environmental and Social Benefits. Center for Neighborhood Technology,
Chicago, IL; American Rivers, Washington, DC.
http://www.cnt.org/publications/green-infrastructure.
Colwell, S.R., R.R. Horner, and D.B. Booth. 2000. Characterization of
Performance Predictors and Evaluation of Mowing Practices in
Biofiltration Swales. Report to King County Land and Water Resources
Division and others by Center for Urban Water Resources Management,
Department of Civil and Environmental Engineering, University of
Washington, Seattle, WA.
CWH (Council for Watershed Health). 2010. Water Augmentation Study:
Research, Strategy, and Implementation. Council for Watershed Health,
Los Angeles, CA.
https://www.usbr.gov/lc/socal/reports/LASGwtraugmentation/report.
£df.
7 1
-------
Connecticut Department of Environmental Protection. 2004. Connecticut
Stormwater Quality Manual. 2004. Connecticut Department of
Environmental Protection, Department of Energy and Environmental
Protection, Hartford, CT.
http://www.ct.aov/deep/cwp/view.asp?a=2721&q=325704.
CWP (Center for Watershed Protection). 2007. National Pollutant Removal
Performance Database, Version 3.0. Center for Watershed Protection,
Ellicott City, MD.
CWP and CSN (Center for Watershed Protection and Chesapeake
Stormwater Network). 2008. Technical Support for the Baywide Runoff
Reduction Method. Baltimore, MD. www.chesapeakestormwater.net.
Davis, A.P, Hunt, W.F., Traver, R.G., and Clar, M. 2009. Bioretention
technology: overview of current practice and future needs. Journal of
Environmental Engineering 135:109-117.
DC OP (District of Columbia Office of Planning). 2011. New York Avenue
Green Infrastructure Assessment. District of Columbia Office of Planning,
Washington, DC. http://plannina.dc.gov/sites/default/files/dc/sites/op/
publication/attachments/NY Ave GIA 5 23 part1.pdf.
Denman, L., PB. May, and PF. Breen. 2006. An investigation of the potential
to use street trees and their root zone soils to remove nitrogen from
urban stormwater. Australasian Journal of Water Resources 10(3).
Donovan, G.H. and D.T. Butry. 2010. Trees in the City: Valuing Street Trees in
Portland, Oregon. Landscape and Urban Planning 94:77-83.
Driscoll, E., P. Shelley, and E. Strecker. 1990. Pollutant Loadings and Impacts
from Highway Stormwater Runoff Volume I: Design Procedure. Federal
Highway Administration, Washington, DC. https://www.usas.aov/media/
files/fhwa-rd-88-006.
Green Streets Handbook
Eck, B.J., R.J. Winston, W.F. Hunt, M.E. Barrett. 2012. Water Quality of
Drainage from Permeable Friction Course. Journal of Environmental
Engineering 138(2).
FHWA (Federal Highway Administration). 1984. Sources and Mitigation
of Highway Runoff Pollutants. Federal Highway Administration,
Washington, DC.
FHWA (Federal Highway Administration). 2016. Highway Statistics 2016.
U.S. Department of Transportation, Federal Highway Administration,
Washington, DC.
https://www.fhwa.dot.aov/policvinformation/statistics.cfm.
Fitzpatrick, K., P. Carlson, M. Brewer, and M. Woolridge. 2000. Design
Factors That Affect Driver Speed on Suburban Streets. Transportation
Research Record 1751:18-25.
Hinman, C. 2005. Low Impact Development Technical Guidance Manual for
Puget Sound. Puget Sound Action Team. Washington State University,
Pierce County Extension, Tacoma, WA.
http://www.psp.wa.gov/downloads/LID/LID manual2005.pdf.
Holtan, M.T., S.L. Dieterlen, and W.C. Sullivan. 2014. Social Life Under Cover:
Tree Canopy and Social Capital in Baltimore, Maryland. Environment and
Behavior 1 (24).
Kaplan, S. 1995. The restorative benefits of nature: Toward an integrative
framework. Journal of Environmental Psychology 15(3):169-182.
LI DC (Low Impact Development Center). 2005. Tree Box Filters: Low Impact
Development for Big Box Retailers. Prepared for U.S. Environmental
Protection Agency, Office of Water, Washington, DC.
MADEP (Massachusetts Department of Environmental Protection).
2008. Massachusetts Stormwater Handbook. Volume 2, Chapter 2.
Massachusetts Department of Environmental Protection, Boston, MA.
http://www.mass.aov/eea/aaencies/massdep/water/reaulations/
massachusetts-stormwater-handbook.html.
I
7 References
-------
MHD (Massachusetts Highway Department). 2006. Massachusetts
Highway Project Development and Design Guidebook: Chapter 13,
Landscape and Aesthetics. Massachusetts Highway Department,
Boston, MA.
MPCA (Minnesota Pollution Control Agency). 2013. Minnesota Stormwater
Manual. Minnesota Pollution Control Agency, St. Paul, MN.
https://stormwater.pca.state.mn.us/index.php/Main Page.
MWCOG (Metropolitan Washington Council of Governments). 2007. Plan
To Improve Air Quality In The Washington, DC-MD-VA Region. State
Implementation Plan (SIP) for 8-Hour Ozone Standard. Prepared for
District of Columbia Department of Environment, Maryland Department
of the Environment and Virginia Department of Environmental Quality.
www.mwcoa.org/environment/air/SIP/default.asp.
NCHRP (National Cooperative Highway Research Program). 2009. NCHRP
Report 640 Construction and Maintenance Practices for Permeable
Friction Courses. National Academies of Sciences, Engineering, and
Medicine: Transportation Research Board, National Cooperative
Highway Research Program, Washington, DC. http://onlinepubs.trb.org/
onlinepubs/nchrp/nchrp rpt 640.pdf.
NCSC (National Complete Streets Coalition), n.d. Implementing Complete
Streets 4: Sustainable Complete Streets. National Complete Streets
Coalition, Washington, DC. http://old.smartgrowthamerica.org/
documents/cs/factsheets/cs-greenstreets.pdf.
NHDES (New Hampshire Department of Environmental Services). 2008.
Pretreatment Practices. Chapter 4-4 in New Hampshire Stormwater
Manual, Volume 2: Post-Construction Best Management Practices
Selection and Design. New Hampshire Department of Environmental
Services, Concord, NH.
https://www.des.nh.goV/sites/g/files/ehbemt341 /files/
documents/2020-01/vol2-ch4-sct4.pdf.
Green Streets Handbook
NRC (National Research Council). 2008. Urban Stormwater Management in
the United States. The National Academies Press, Washington, DC.
http://www.epa.gov/npdes/pubs/nrc stormwaterreport.pdf.
NRDC (Natural Resource Defense Council). 2013. The Green Edge: How
Commercial Property Investment in Green Infrastructure Creates Value.
Natural Resource Defense Council, New York, NY.
http://www.nrdc.org/water/commercial-value-green-infrastructure.asp
Pomerantz, M.H. Akbari, S-C. Chang, R. Levinson, and B. Pon. 2003.
Examples of Cooler Reflective Streets for Urban Heat-Island Mitigation:
Portland Cement Concrete and Chip Seals. No. 49283. Lawrence
Berkeley National Laboratory, Berkley, CA.
http://repositories.cdlib.org/lbnl/LBNL-49283.
Prince George's County, Maryland. 2010. Environmental Technical Manual.
The Maryland-National Capital Park and Planning Commission and
Prince George's County Planning Department.
http://www.pgplanning.org/DocumentCenter/View/1072.
Qin, H., Z. Li, and G. Fu. 2013. The effects of low impact development on
urban flooding under different rainfall characteristics. Environmental
Management 129:577-585.
http://www.sciencedirect.com/science/article/pii/S03Q14797130Q5495.
RIDEM and CRMC (Rhode Island Department of Environmental
Management and Coastal Resources Management Council). 2010.
Rhode Island Stormwater Design and Installation Standards Manual.
Rhode Island Department of Environmental Management, Providence,
Rl; Coastal Resources Management Council; Wakefield, Rl.
http://www.dem.ri.gov/pubs/regs/regs/water/swmanual.pdf.
RTI and Geosyntech. 2014. Low Impact Development Stormwater Control
Cost Estimation Analysis. Prepared for U.S. Environmental Protection
Agency, Office of Research and Development, by RTI International,
Research Triangle Park, NC, and Geosyntec Consultants, Lafayette, CO.
7 References
I
-3
-------
San Mateo County. 2020. San Mateo Green Infrastructure Design Guide.
San Mateo County, CA.
https://www.flowstobav.org/data-resources/resources/
green-infrastructure-design-guide/.
Santamouris, M. ed. 2001. Energy and Climate in the Urban Built
Environment. 1st ed. Routledge Publishing, Abingdon, United Kingdom.
Tilley, J.S., and E.T. Slonecker. 2006. Quantifying the Components of
Impervious Surfaces. Open-File Report 2006-1008. U.S. Geological
Survey, Reston, VA.
TFA (Transportation for America). 2019. Dangerous by Design: Solving the
Epidemic of Preventable Pedestrian Deaths. Transportation for America,
Washington, DC.
https://smartarowthamerica.ora/resources/danaerous-bv-desian-2019/.
University of New Hampshire Stormwater Center. 2012. 2012 Biennial
Report. University of New Hampshire, Durham, NH.
https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/docs/
UNHSC.2012Report.10.10.12.pdf
USDOT (U.S. Department of Transportation). 2013. Highway Functional
Classification Concepts, Criteria and Procedures. FHWA-PL-13-026.
U.S. Department of Transportation, Federal Highway Administration,
Washington, DC.
USFS (U.S. Forest Service). 2014. i-Tree: Tools for Assessing and Managing
Community Forests. U.S. Department of Agriculture Forest Service,
Washington, DC. Accessed March 2014.
http://www.itreetools.org/index.php.
USEPA (U.S. Environmental Protection Agency). 2001. Storm Water
Technology Fact Sheet: On-Site Underground Retention/Detention. EPA
832-F-01-005. U.S. Environmental Protection Agency, Washington, DC.
Green Streets Handbook
USEPA (U.S. Environmental Protection Agency). 2008. Reducing Urban
Heat Islands: Compendium of Strategies. U.S. Environmental Protection
Agency, Washington, DC.
https://www.epa.gov/heat-islands/heat-island-compendium.
USEPA (U.S. Environmental Protection Agency). 2013. Case Studies
Analyzing the Economic Benefits of Low Impact Development and
Green Infrastructure Programs. U.S. Environmental Protection Agency,
Washington, DC. https://www.epa.aov/sites/production/files/2015-10/
documents/lid-ai-proarams report 8-6-13 combined.pdf.
USEPA (U.S. Environmental Protection Agency). 2014. The Economic
Benefits of Green Infrastructure: A Case Study of Lancaster, PA. U.S.
Environmental Protection Agency, Office of Research and Development,
Washington, DC. https://www.epa.gov/areen-infrastructure/
economic-benefits-areen-infrastructure-case-studv-lancaster-pa.
USEPA (U.S. Environmental Protection Agency). 2016. Estimating Monetized
Benefits of Groundwater Recharge from Stormwater Retention Practices.
U.S. Environmental Protection Agency, Office of Water, Nonpoint Source
Control Branch, Washington, DC.
https://www.epa.gov/sites/production/files/2016-08/documents/
gw recharge benefits final april 2016-508.pdf.
Vingarzan, R. and B. Taylor. 2003. Trend Analysis of Ground Level Ozone
in the Greater Vancouver/Fraser Valley Area of British Columbia.
Atmospheric Environment 37(16):2159—2171.
WSDOT (Washington State Department of Transportation). 2016.
M31-16.04. WSDOT Design Office, Highway Runoff Unit, Olympia, WA.
https://www.wsdot.wa.gov/publications/manuals/fulltext/M31-16/
highwavrunoff.pdf
Young, G. Kenneth, Stuart Stein, Pamela Cole, Traci Kammer, Frank
Graziano, Fred Bank. 1996. Evaluation and Management of Highway
Runoff Water Quality. U.S. Department of Transportation, Office
of Environment and Planning, Federal Highway Administration,
Washington, DC.
7 References
------- |