Designing Holistic Bioretention
for Performance and Longevity
EPA 841-B-23-002 | November 2023
United States
Environmental Protection
Agency
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ACKNOWLEDGMENTS
DISCLAIMERS
This handbook was prepared by the U.S.
Environmental Protection Agency's (EPA's)
Office of Water/Office of Wetlands,
Oceans and Watersheds/Nonpoint Source
Management Branch in conjunction with
Tetra Tech.
*>EPA
United States
Environmental Protection
Agency
This handbook is the culmination of a
collaborative effort between several
cities, counties, and other organizations.
Credit should be given to the many local
government officials and practicing
professionals who offered their time,
provided tours of their work, and showcased
different designs to bring this document
to press. Special acknowledgment goes
to Rhea Thompson, the primary author of
the Bioretention Design Handbook. The
document was researched and written
during her tenure as an Oak Ridge Institute
for Science and Education (ORISE) participant
at EPA. Adrienne Donaghue also contributed
to completing the document during her
ORISE tenure. Other EPA contributors
include staff from the Office of Wastewater
Management, Office of Community
Revitalization, and EPA regional offices.
External contributors include ACF
Environmental, Alisha, Goldstein, Anoka
Conservation District, Camden County
Municipal Utilities Authority, Cecilia Lane,
City and County of Denver Division of Green
Infrastructure, City/County Association of
Governments of San Mateo County, City
of Atlanta, City of Austin, City of Boston,
City of Chicago, City of Chattanooga,
City of Columbia (MO), City of Fort
Lauderdale, City of Miami Beach, City of
Omaha, City of New Orleans, City of San
Diego, City of Santa Rosa, City of Seattle,
Darren Distefano, District of Columbia
Department of Energy and Environment,
District of Columbia District Department of
Transportation, Fairfax County Government,
Florida Department of Environmental
Protection, Jim Lenhart, Kansas City Water,
Low Impact Development Center, MIG|SvR,
Montgomery County, MD Department of
Environmental Protection, Middle St. Croix
Watershed Management Organization,
National Association of City Transportation
Officials, North Carolina State University,
New York City Department of Environmental
Protection, Philadelphia Water Department,
Pima County Regional Flood Control,
Portland Bureau of Environmental Services,
San Francisco Water, Thomas Liptan, Tucson
Water Department, University of Maryland,
College Park, University of Missouri, Urban
Drainage & Flood Control District, Villanova
University, Water Now, and OLIN Labs.
We would also like to thank our external
reviewers, William Hunt from NC State
University and Jason Wright of Tetra Tech.
Special thanks to Kary Phillips and Regina
Scheibner of Tetra Tech, who assisted with
document development, content review,
editing, formatting, and layout design.
This document serves as a guide to
designing, implementing, and maintaining
bioretention facilities; selection of and
specifications for individual facility 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.
Lastly, this handbook does not impose
any legally binding requirements on EPA,
states, or the regulated community and
does not confer legal rights or impose
legal obligations upon any member of the
public. EPA made every attempt to ensure
the accuracy of the examples included in
this document. In the event of a conflict or
inconsistency between this handbook and
any statute, regulation, or permit, it is the
statute, regulation, or permit that governs,
not this handbook.
It
TETRA TECH
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CONTENTS
Acronyms iii
Preface iv
Chapter 1 Introduction 1-1
1.1 Handbook Scope, Purpose, and Audience 1-2
1.2 Handbook Organization 1-5
1.3 Using the Handbook 1-6
Chapter 2 Bioretention Design Elements 2-1
2.1 Bioretention Design Elements 2-2
Chapter 3 Holistic Design Concepts 3-1
3.1 Consider the Site Context 3-2
3.2 Strive to Provide Multiple Benefits 3-4
3.3 Design for Longevity 3-7
Chapter 4 Managing Drainage Area 4-1
4.1 Drainage Area Delineation 4-2
4.2 Determining Grade 4-4
4.3 Selecting the Optimal Site Location 4-5
Chapter 5 Bioretention Geometry and Sizing 5-1
5.1 Cell Sizing and Geometry 5-2
5.2 Side Slopes and Geometry 5-4
5.3 Sizing Methods and Considerations 5-5
Chapter 6 Runoff Capture 6-1
6.1 Stormwater Conveyance 6-2
6.2 Sheet Flow Conveyance 6-4
6.3 Inlet Types for Concentrated Flow 6-6
6.4 Inlet Number, Placement, and Frequency 6-20
6.5 Inlet Size 6-22
6.6 Inlet Flow Path Modifications and Retrofits 6-25
6.7 Inlet Protection 6-30
Chapter 7 Pretreatment 7-1
7.1 Importance of Pretreatment 7-2
7.2 Pretreatment for Sheet Flow 7-3
7.3 Pretreatment for Inlets and Concentrated Flow 7-6
7.4 In-Practice Erosion Control 7-22
Chapter 8 Bioretention Media 8-1
8.1 Bioretention Soil Media Function and Composition 8-2
8.2 Assessment and Testing of Existing Soils 8-3
8.3 Media Design Considerations for Hydrologic Performance 8-7
8.4 Media Design Considerations for Pollutant Removal 8-8
8.5 Liners 8-10
8.6 Aggregate Media 8-11
Chapter 9 Vegetation 9-1
9.1 Why Is Vegetation Important? 9-2
9.2 Considerations for Vegetation Selection 9-2
9.3 Planting Plan 9-10
9.4 Planting Mechanisms 9-11
9.5 Vegetation Establishment 9-13
Chapter 10 Underdrains and Outflows 10-1
10.1 Hydrologic Performance Goals and Outlet Types 10-2
10.2 Underdrains 10-4
10.3 Internal Water Storage 10-6
10.4 Outlet Boxes 10-9
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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Chapter 11 Multimodal Transportation and Public Safety 11-1
11.1 Why the ROW? 11-2
11.2 Pedestrian Mobility and Access 11-2
11.3 Traffic Mobility and Parking Access 11-4
11.4 Public Safety 11-7
Chapter 12 Promoting Community Acceptance 12-1
12.1 Importance of Community Involvement 12-2
12.2 Building Community Engagement and Ownership 12-2
12.3 Adding Design Elements to Build Community Acceptance 12-5
Chapter 13 Managing the Construction Process 13-1
13.1 Importance of Construction and Inspection 13-2
13.2 Considerations for Construction Preparation and
Implementation 13-2
13.3 Construction-Related Inspection 13-5
Chapter 14 Long-Term O&M and Asset Management 14-1
14.1 Planning for O&M 14-2
14.2 Maintaining Specific Design Elements 14-4
14.3 Asset Management 14-9
14.4 Longevity and Continued Performance 14-10
Glossary Glossary-1
References Ref-1
TABLES
Table 3-1. Opportunities to increase environmental and social
benefits using bioretention 3-5
Table 5-1. Description of sizing elements for bioretention facilities
(planters and boxes) 5-3
Table 6-1. Curbless and curbed design challenges
and considerations 6-3
Table 7-1. Pretreatment options for concentrated flow 7-7
Table 8-1. Organic media amendment characteristics
and applications 8-5
Table 8-2. Inorganic media amendment characteristics
and applications 8-6
Table 9-1. Design considerations for various planting mechanisms 9-12
Table 10-1. Considerations for IWS design elements and goals 10-8
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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ACRONYMS
ADA
Americans with Disabilities Act
MPCA
Minnesota Pollution Control Agency
BSM
bioretention soil media
MC DEP
Montgomery County Dept. of Environmental
CCD
City and County of Denver
Protection
CDA
contributing drainage area
NACTO
National Association of City Transportation Officials
CWA
Clean Water Act
NRCS
Natural Resources Conservation Service
DOT
Department of Transportation
NYC DEP
New York City Dept. of Environmental Protection
DDOT
District of Columbia Dept. of Transportation
OLIN
Olin Partnership Limited Labs
EJSCREEN
Environmental Justice Screening and Mapping Tool
O&M
operation and maintenance
EPA
Environmental Protection Agency
PVC
polyvinyl chloride
FHWA
Federal Highway Administration
PWD
Philadelphia Water Department
GAO
Government Accountability Office
ROW
right-of-way
GSI
green stormwater infrastructure
SOP
standard operation procedure
HDPE
high-density polyethylene
SWMM
Storm Water Management Model
IDF
intensity-duration frequency
UDFCD
Urban Drainage and Flood Control District
IPM
integrated pest management
USDA
U.S. Department of Agriculture
IWS
internal water storage
VFS
vegetated filter strip
LR
loading ratio
WTR
wastewater treatment residual
MOU
memorandum of understanding
WINSLAMM
Source Loading and Management Model for
Windows
MSC WMO
Middle St. Croix Watershed Management
Organization
WQV
water quality volume
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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PREFACE
Bioretention is one of the most widely implemented green
stormwater infrastructure (GSI) practices. Nationally, the
design for bioretention varies based on factors such as local
requirements, climate, site conditions, and land use. At the
state and local levels, much innovation in bioretention design
and management has occurred in the recent past. However,
this information is not compiled and readily available to inform
practitioners about the latest trends, designs, and approaches to
optimizing bioretention design and management.
The Bioretention Design Handbook (the handbook) compiles the
current state of knowledge from a combination of literature,
interviews, and site visits with leading municipalities and
practitioners across the United States to document approaches
for bioretention design, construction, inspection, and operation
and maintenance (O&M). The handbook is organized into three
main sections: the introduction, the design phase, and the post-
construction phase. The introduction defines each bioretention
design element discussed in detail in the handbook and highlights
the important holistic design concepts such as urban heat
mitigation and material reuse. The design phase section walks
readers through each design element, working in the order from
the inlet to the outlet. The design phase section also discusses
strategies to help build community acceptance and incorporate
design aspects that accommodate multimodal transportation
and public safety concerns. The handbook also includes a post-
construction phase section that covers considerations associated
with the O&M of bioretention facilities and recommendations for
longevity. The handbook describes various resources and tools to
assist with bioretention design.
A unique feature of the handbook is the numerous photographs
of bioretention facilities from across the contiguous United States
that showcase the diversity of design techniques and approaches.
The photographs show sites from more than 20 municipalities
visited as part of an ORISE fellowship research project or provided
by other credited sources. Hopefully, these images will inspire
new ideas and further advance the GSI field. Please note that the
handbook is not intended to be used for design specifications
but rather as a starting point to help designers and planners
consider various design elements and approaches to improve the
functionality and management of bioretention systems.
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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Chapter 1
INTRODUCTION
In this chapter
1.1 Handbook Scope, Purpose, and Audience
1.2 Handbook Organization
1.3 Using the Handbook
Chapter 1 provides an overview of this handbook's scope,
purpose, and intended audience. According to the U.S.
Government Accountability Office (GAO), a recent survey
revealed that municipalities found the process of developing
green stormwater infrastructure (GSI) to be more difficult
than developing gray infrastructure. The municipalities
identified design and engineering as the most challenging
aspects (GAO 2017). An overarching goal of this handbook
is to highlight lessons learned and design concepts to help
municipalities overcome these challenges.
Photo: NYC Department of Environmental Protection
1-1
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CHAPTER i: INTRODUCTION
1.1 HANDBOOK SCOPE, PURPOSE, AND AUDIENCE
1.1 Handbook Scope,
Purpose, and Audience
Municipal stormwater programs have been installing GSI for several
decades. In contrast to gray infrastructure, which relies on piped
networks and engineered components to convey stormwater,
GSI instead depends on natural physical, chemical, and biological
processes to manage stormwater quality and quantity.
GSI, the term used in this handbook, is synonymous with the term
green infrastructure defined in the Clean Water Act1 (CWA) as
the range of measures that use plant or soil systems; permeable
pavement or other permeable surfaces or substrates; stormwater
harvest and reuse; or landscaping elements to store, infiltrate,
or evapotranspirate stormwater and reduce flows to sewer
systems or to surface waters. Some use other terms to reference
the same practices as green infrastructure for stormwater
management. Other terms include low impact development,
natural infrastructure, and nature-based solutions. The definitions
of these terms might vary slightly among organizations and
industry professionals; however, these concepts are generally
captured in the CWA definition of green infrastructure. GSI and
green infrastructure are both terms used in planning and research
to achieve various ecosystem services.
Bioretention is one of the most popular GSI practices
implemented in urban areas. For example, as explained in the
GAO's 2017 report, Stormwater Management: EPA Pilot Project
to Increase Use of Green Infrastructure Could Benefit from
Documenting Collaborative Agreements, a survey found that
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1 Water Infrastructure Improvement Act, 2019. https://www.conaress.aov/115/
plaws/publ436/PLA'W-115publ436.pdf
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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CHAPTER i: INTRODUCTION
municipalities most frequently installed, encouraged, or required
three main types of GSI: downspout disconnection, bioretention
(also referred to as rain gardens), and permeable pavements
(GAO 2017). A correctly designed, constructed, and maintained
GSI practice can be a more effective, economical, and sustainable
choice over its service life when compared to gray infrastructure
(TNC and TX A&M 2021).
Despite the popularity of GSI, the 2017 GAO report noted that 15
of the 27 municipalities surveyed reported that less than 5% of
the area subject to their municipal stormwater permit or consent
decree drained into GSI. The remaining area drained into either
gray infrastructure (i.e., combined or storm sewers) or directly into
receiving waters. Municipalities indicated that implementing some
GSI tasks were more challenging compared to gray infrastructure,
including developing capital expenditure, identifying operation and
maintenance (O&M) costs, and designing practices (GAO 2017).
The overarching goal of this bioretention design handbook-
referred to as "the handbook" throughout this document—is to
help you successfully implement bioretention projects. In addition
to sharing lessons learned from across the country, the handbook
offers recommendations for design, construction, inspection,
and O&M practices that will help you achieve performance goals,
reduce project costs, and effectively integrate bioretention into
your built environments. We will highlight common obstacles
that can cause a project to incur added expenses. For example,
undersized inlets can lead to runoff bypassing the system and
require reconstruction. The top photo shows an inlet that was
constructed incorrectly without a drop in elevation; as a result,
runoff ponds at the inlet. The bottom photo shows an inlet
constructed with a change in grade between the roadside and the
practice; it functions properly during a storm.
1.1 HANDBOOK SCOPE, PURPOSE, AND AUDIENCE
Incorrectly installed inlet.
A well-designed inlet.
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CHAPTER!: INTRODUCTION 1.2 HANDBOOK ORGANIZATION
The second overarching goal of the handbook is to promote
adaptive management in GSI projects. Adaptive management
is the process of observing how a system performs over time
and using that knowledge to adapt O&M strategies, retrofits, or
future designs. Applying adaptive management can also extend
beyond the physical boundary of a GSI site. Incorporating lessons
learned can improve future GSI implementation in public settings,
enhancing social benefits such as green space.
This handbook is intended for a multidisciplinary audience of
GSI professionals, including design professionals, municipal
officials, developers, planners, contractors, and inspectors
across states, territories, and Tribes.2 This document compiles
current knowledge from published resources detailing how to
approach bioretention design and post-construction activities.
As recognized in the acknowledgments, this handbook also
conveys the experiences and expertise of many municipalities
and GSI practitioners who generously shared information during
interviews and site visits.
Bioretention facilities used throughout this document encompass
bioretention, rain gardens, bioswales, bioretention planters/boxes,
and tree pits. Design elements specific to each of these practices
are called out when applicable. As highlighted later in the
handbook, GSI offers benefits beyond stormwater control—it can
also be implemented for programmatic reasons, such as traffic
calming, urban greening, carbon sequestration, and heat island
mitigation.
2 For the purposes of this handbook, Tribe is used as a collective term
encompassing Tribes, Nations, Pueblos, and other similar entities.
A street-side bioretention planter in Washington, DC.
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CHAPTER i: INTRODUCTION
1.2 HANDBOOK ORGANIZATION
1.2 Handbook Organization
The handbook includes three parts, described in detail below.
Chapters 1-3 offer background information. Chapters 4-12
describe the GSI design phase, and Chapters 13-14 discuss the
GSI post-construction phase.
Parti: Introduction - Provides background details:
• Chapter 1: Handbook Introduction
• Chapter 2: Bioretention Design Elements
• Chapter 3: Holistic Design Concepts
Part 2: Design Phase - Describes various design considerations
and lessons learned:
• Chapter 4: Managing Drainage Areas
• Chapter 5: Bioretention Geometry and Sizing
• Chapter 6: Runoff Capture
• Chapter 7: Pretreatment
• Chapter 8: Bioretention Media
• Chapter 9: Vegetation
• Chapter 10: Underdrains and Outflows
• Chapter 11: Multimodal Transportation and Public Safety
• Chapter 12: Promoting Community Acceptance
Part 3: Post-Construction Phase - Describes post-design
considerations:
• Chapter 13: Managing the Construction Process
• Chapter 14: Long-Term O&M and Asset Management
Bioretention in San Diego, CA.
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
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CHAPTER i: INTRODUCTION
1.3 USING THE HANDBOOK
1.3 Using the Handbook
This design handbook complements EPA's Green Streets
Handbook, which guides state and local transportation agencies,
municipal officials, tribal staff, designers, stakeholders, and others
as they select, design, and implement site design strategies and
GSI facilities for streets, alleys, and parking lots.
This design handbook focuses on efforts to implement
bioretention in right-of-way (ROW) areas for stormwater
management. Using ROWs for GSI alleviates concerns about
access and O&M because these areas are already under the
municipality's control. However, the State of the Public Sector
Green Stormwater Infrastructure 202.2 surveyed 52 public sector
entities and found that the GSI they implemented in the public
ROW accounted for less than a third of acres they managed
compared to projects implemented on parcels and redevelopment
projects (Greenprint Partners 2022a). The report also notes that
GSI in the public ROW is an expected growth area between 2022
and 2027. This handbook aims to expand the use of ROWs for GSI.
This document is not a design manual; instead, it provides
recommendations and resources for bioretention design
approaches—especially where technical expertise might
be lacking. Furthermore, this document does not replace
construction, inspection, or O&M manuals, which are necessary
to ensure project success. Planning and design professionals are
responsible for developing projects using sound judgment and
following applicable laws and regulations. Always consult local or
municipal specifications for design, construction, and O&M.
EPA 841-8-18-001 I March 2021
EPA's Green Streets Handbook focuses on GSI in
transportation networks.
isi
Street-side bioretention in Portland, OR.
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am
Chapter 2
BIORETENTION
DESIGN ELEMENTS
In this chapter
2.1 Bioretention Design Elements
Chapter 2 defines and illustrates the bioretention design
elements referenced and discussed in detail throughout
the handbook. The practices defined in this chapter include
inflows, pretreatment components, energy dissipaters,
mulch layers, vegetation, existing soil, soil media for
bioretention, choker layers, liners, drainage/storage layers,
underdrains, outflows, and overflows.
Photo: Maryland Department of the Environment
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CHAPTER 2: BIORETENTION DESIGN ELEMENTS
2.1 BIORETENTION DESIGN ELEMENTS
2.1 Bioretention Design Elements
Bioretention facilities generally include many of the following
design elements. Some elements are optional (i.e., the amount
of water stored), and the bioretention design will vary based
on regional and site-specific conditions. Many design elements
presented for bioretention are translatable to other GSI practices,
such as vegetated swales and permeable pavements.
Inflow. For a bioretention facility to capture and treat stormwater
runoff, the runoff must enter the practice. Runoff entering
bioretention facilities, also called inflow, can travel in the form of
sheet flow or concentrated flow that moves through engineered
inlet structures (curb cuts, gutters, etc.). Other methods of runoff
capture, such as runnels and depressional channels, are discussed.
(See Chapter 6 for more details.)
Pretreatment components. Pretreatment (e.g., forebays, vegetated
filters) is typically used for inflow that enters a bioretention
facility as concentrated flow via inlets such as curb cuts, trench
drains, and pipes. Pretreatment components prevent erosion by
slowing inflow velocities and dissipating the energy. Reducing
the inflow speed promotes the settling out of suspended solids,
debris, and trash in a localized area. Pretreatment also prevents
downstream clogging. (See Chapter 7 for more details.)
Energy dissipator. Including pretreatment techniques, such
as weirs, check dams, and rock rundowns, can reduce runoff
velocities and dissipate energy. Energy dissipation reduces the
likelihood of erosion and vegetation disturbance. Additionally,
slower flow velocities within the facility enhance infiltration.
(See Chapter 7 for more details.)
Mulch layer. Mulch retains water, traps pollutants, prevents
erosion, suppresses weed growth, and provides a favorable
environment for beneficial soil organisms to thrive at the mulch/
soil interface. However, mulch can be susceptible to suspension
and washout during intense storms and can leach dissolved
organic carbon, which can stimulate unwanted biological
activity in lakes and streams. Therefore, other alternatives, such
as expanded shale, gravel, or crushed rock, may be used. (See
Chapter 7.4 for more details.)
Vegetation. Bioretention vegetation includes native noninvasive
grasses, perennials, shrubs, and trees. Vegetation can reduce
runoff velocities, improve infiltration rates, and enhance water
quality through phytoremediation and nutrient uptake (via plant
roots and biological processes). Vegetation selection is influenced
by site conditions, such as soil type, available water, light, climate,
road salt application, and other environmental stressors. Other
factors to consider are the plant locations and intended plant
functions (e.g., aesthetics, nutrient removal, infiltration, wildlife
value). (See Chapters 3 and 9 for more details.)
Existing soil. A site's existing soil refers to soil conditions before
site disturbance or bioretention implementation. Existing soil
may consist of the area's native soil (a mixture of sand, silt, clay,
and organic matter) or historical fill (a mixture of dirt, debris, and
construction material). Existing soils are evaluated to determine
whether they are suitable for the bioretention practice design and
have the desired porosity, nutrient content, compaction resilience,
and other properties to support plant growth and the overall
performance goals. In many cases, well-draining, existing soils can
be used in the bioretention facility, or the soils can be amended to
meet design goals. (See Chapter 8.2 for more details.)
Bioretention soil media (BSM). BSM refers to the media mix used
in the bioretention facility. Often BSM mixes may be required
to meet local government specifications. If existing soils do not
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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CHAPTER 2: BIORETENTION DESIGN ELEMENTS
meet the design criteria, designers can use engineered BSM
to improve infiltration rates and achieve other goals, such as
pollutant removal or carbon sequestration. BSM can include
media amendments (e.g., biochar, wastewater treatment
residuals) to enhance the removal of specific dissolved pollutants.
(See Chapter 8 for more details.)
Choker layer, A choker layer separates different media layers. It
is usually placed between the BSM and the drainage layer. The
choker layer generally consists of 3 inches (minimum) of sand and
gravel that prevents BSM from migrating into the underlying
gravel drainage layer. (See Chapter 8.6 for more details.)
Liner, A liner can be a permeable or impermeable geotextile fabric
used to prevent weed growth, separate media layers to prevent
migration of material (as an alternative to a choker layer), or
restrict exfiltration. Liners can clog, particularly in areas with
clay soils, and thus are not recommended in some circumstances.
Impermeable geotextiles are suitable along slopes for blocking
exfiltration and preventing interactions with structure foundations
or preventing the mobilization of pollutants (e.g., underlying
soil that contains a hot spot from a contaminant spill). Lastly,
bioretention facilities implemented with an impermeable liner at
the bottom can increase the systems' hydraulic residence time and
prevent water from interacting with groundwater or contaminated
areas in the soil below. (See Chapter 8.5 for more details.)
Stone storage/drainage layer. A washed stone layer added beneath
the BSM promotes drainage and exfiltration. This design element
may be practical when it is impossible to store the total water
quality volume (WQV) on the surface of the practice, such as in
constrained bioretention systems (e.g., planter boxes) or when
designers wish to minimize surface storage. (See Chapter 8.6 for
more details.)
2.1 BIORETENTION DESIGN ELEMENTS
STORMWATER PLANTER
Underdrain. Underdrains are generally required when underlying
soils have an insufficient infiltration capacity. In such cases,
designers may incorporate underdrains to reduce the failure
potential. To maximize retention volumes, designers can include
caps or valves on the underdrain that stay in place unless the
practice experiences a drainage problem. Underdrains consist
of a perforated pipe set in a gravel bed installed along the
bottom of the filter bed (this design separates the BSM from the
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CHAPTER 2: BIORETENTION DESIGN ELEMENTS
2.1 BIORETENTION DESIGN ELEMENTS
underdrain). Underdrairis collect and convey effluent stormwater
back into the sewer, surface conveyance system, or the surface of
another GSI practice area. (See Chapter 10 for more details.)
thp Qtrpetcrane Q
Internal water storage (IWS). IWS is a zone in the practice designed
to hold water. IWS can be added to increase overall storage
capacity or enhance nitrate removal. A typical IWS design includes M v
a gravel layer with a perforated underdrain. The elevation of
the underdrain outlet is raised to create a saturated layer during
storm events. When denitrification is desired, the IWS layer must
include a carbon source such as woodchips. (See Chapter 10 for
more details.)
Outlet. The outlet is where the treated runoff exits the
bioretention facility. They are placed downgradient of the inflow
structure. Outlets may be on the surface (e.g., curb cut, riser) or
subsurface (underdrain). They can convey water to another GSI
practice, treatment train, or storm sewer (or approved discharge
point, such as a waterbody). (See Chapter 10 for more details.)
Overflow. Overflow refers to the water volumes associated with
storms that exceed the design volume. Overflow structures are
sized to safely manage larger storms and allow flows to bypass
the practice during extreme storm events. Designers can include
a positive overflow that enables stormwater to flow back out of
the system when the water level reaches the maximum design
subsurface elevation or surface ponding depth. Flow that moves
out through the positive overflow can connect to another GSI
practice or an approved discharge point. (See Chapter 10 for more
details.)
*Note: Because the bioretention design terms are defined in this
chapter, they are not repeated in the glossary.
Trees as part of bioretention in Kansas City, MO.
Design schematic for a stormwater tree trench.
Trees take up and
transpire water from
trench providing
shade and enhancing
the streetscape
Stormwater from
roadway flows into
the stormwater
tree trench
Internal Water
Storage
pipe distributes water
into stone or other storage media
within the stormwater tree trench
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Chapter 3
HOLISTIC
DESIGN CONCEPTS
In this chapter
B.I Consider the Site Context
3.2 Strive to Provide Multiple Benefits
3.3 Design for Longevity
While most of this handbook describes each bioretention
design element in detail, Chapter 3 introduces holistic design
concepts. Holistic design integrates existing site conditions,
multiple performance goals, and other life cycle factors
into the design process to maximize a bioretention facility's
performance and longevity. Consider these concepts as you
start a project and carry them through the construction and
maintenance phases.
Photo: Rhea Thompson
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CHAPTER 3: HOLISTIC DESIGN CONCEPTS
3.1 CONSIDER THE SITE CONTEXT
B.I Consider the Site Context
Although GSI can be implemented at a network scale, adapting
practices to the site scale takes advantage of existing conditions
and prevents over-design. The planning and design phases should
be multidisciplinary and engage municipal officials, landscape
architects, local community leaders, and professionals from
transportation, stormwater management, and public utilities. The
following holistic design concepts and engagement opportunities
should be considered during planning and design.
Site-specific design optimizes performance. A single standard
bioretention design does not apply in all cases. Conducting a site
assessment is critical for ensuring the constructed facility meets
stormwater management program requirements and accounts for
This GSI design treats runoff while guiding pedestrians safely to
the crosswalk,
site-specific conditions and local needs. When creating a system,
designers should consider the characteristics of the site and its
surrounding areas, such as topography, available space, slope,
microclimate (aspect, shading, wind exposure, and thermal gain
from adjacent buildings), proximity to utilities, conditions of local
soils (contamination and infiltration capacity), and the presence
of nearby materials that could clog the system. Designers should
also consider other programmatic goals, such as incorporating
multimodal transportation opportunities, meeting accessibility
needs, and improving community aesthetics.
Understand the existing soil conditions at the site. Identifying the
infiltration capacity of existing soils allows designers to correctly
size and model facilities. For example, San Francisco Water Power
Sewer conducts surveys and collects existing site data (e.g., local
infiltration capacity) to see if the existing infiltration rates will
satisfy stormwater quantity goals (NACTO 2017). If so, planners
can adjust the design and save money. If existing soils do not
meet acceptable infiltration rates, the design can use BSM and/or
an underdrain.
Use the site to restore connections to the natural water cycle.
One of the biggest contrasts between GSI and gray stormwater
infrastructure is how water is viewed and managed. GSI relies
on natural processes, such as infiltration, groundwater recharge,
and evapotranspiration. Alternatively, gray infrastructure simply
conveys the runoff to treatment plants or receiving waters.
To create conditions that mimic the natural hydrological cycle,
designers should account for the nature of storms (frequency
and intensity) and how water flows naturally (predevelopment).
When all the runoff cannot be managed using natural processes
alone, the design can emphasize detention—the capture and slow
release of water.
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3.1 CONSIDER THE SITE CONTEXT
Aerial image showing impermeable liners at the bottom of two
bioretention cells under construction in Denver, CO.
Groundwater interactions with bioretention media should be
avoided. The bottom elevation of a bioretention facility should
be at least 2 feet above the seasonally high groundwater table
(USEPA 2004). Groundwater interactions with BSM can mobilize
stormwater contaminants. Additionally, groundwater interactions
reduce the available void space for runoff capture. In areas with a
high year-round or seasonal water table, noninfiltration practices
are preferred. If a bioretention facility is sited near industrial areas
with existing soil contamination (i.e., potential hotspots), the
design should incorporate pretreatment and/or an impermeable
liner. EPA's The influence of Stormwater Management Practices
and Wastewater Infiltration on Groundwater Quality: Case Studies
discusses the results of three field studies investigating the
A bioswale includes gentle grades and low-growing vegetation,
which fits the parking lot site in Minneapolis, MN.
potential impacts on groundwater quality due to using GSI (Beak
et al. 2020). Bioretention facilities that emphasize exfiltration
should also be carefully sited in areas with karst geology to
avoid contamination of aquifers and potential de-stabilization
of geology The Minnesota Stormwater Manual outlines design
considerations for karst areas (MPCA 2023a).
Consider whether coupling bioretention facilities with other
GSI practices maximizes impact. Space constraints in the ROW
may limit the available surface area for bioretention. Other GSI
practices, such as permeable pavement for sidewalks or bike
lanes, can be incorporated to meet hydrologic performance goals.
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Disconnect impervious surfaces and apply GSi planning and design
approaches. Impervious areas within the area draining to a
bioretention facility should be disconnected from each other (e.g.,
ensure that roof runoff does not flow across the sidewalk and
directly into a storm sewer). Instead, route flow over the natural
landscape to slow runoff to reduce the quantity of water entering
the combined or separate sewer system. When possible, route
flow directly into a bioretention facility.
Consider other opportunities to manage runoff beyond the project
area. Look beyond a single project site to determine if GSI could
be designed to benefit a larger drainage area. For example,
in one scenario described by the National Association of City
Transportation Officials (NACTO) (2017), designers in one city
realized that large bioretention facilities installed in a project
could manage runoff from the adjacent street as well as another
area three to five times larger. The added treatment capacity
allowed for a system that would accept runoff from cross streets
and other nearby streets; therefore, these streets did not need
costly reconstruction for stormwater control.
Avoid impacts to utilities. Utility lines are a common and necessary
part of suburban and urban infrastructure, running overhead and
under streets, sidewalks, and tree spaces. Emergency responders
should always have access to utilities. Thus, bioretention designs
should avoid utility lines, even if it means moving the practice or
designing it to coexist (i.e., providing protective elements within
facilities). Maintaining the facility will help prevent impacts
to utilities through the facility's life span (e.g., trimming trees
prevents interference with overhead wires).
3.2 STRIVE TO PROVIDE MULTIPLE BENEFITS
Preserve existing trees where possible. Many urban settings
balance bioretention areas with the need for improved tree
canopy. Street trees are often protected as part of a city's
efforts to improve greening/urban canopy coverage. Trees
and bioretention both supply the benefits of stormwater
management, air quality, and urban heat mitigation. If possible,
avoid disrupting mature trees when siting bioretention. If this
is not possible, assess and evaluate the benefit and trade-offs
of removing trees before doing so. Consult with local experts to
understand the tree species, growth patterns, and on-site root
systems that may exist for the site's mature trees, for these will
affect the recommended proximity of bioretention practices.
Proper planning can allow existing trees and new bioretention
facilities to coexist and contribute to a successful green street.
3.2 Strive to
Provide Multiple Benefits
Holistic bioretention design meets performance goals (e.g.,
state or local regulatory requirements) and aims to maximize
environmental and social benefits. Table 3-1 summarizes
performance goals and benefits that are reiterated in later
chapters.
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3.2 STRIVE TO PROVIDE MULTIPLE BENEFITS
Table 3-1. Opportunities to increase environmental and social benefits using bioretention.
Goal
Description
Design considerations and resources
Reduce
Combined
Sewer
Overflows
Minimizing the occurrence of combined sewer overflows involves
capturing, retaining, and infiltrating as much volume as possible.
When infiltration is not possible, stormwater volume is managed by
detention and controlled release to the existing sewer.
• Design bioretention with permeable soils that promote infiltration.
• Add IWS to increase storage volume and exfiltration.
• Use stone wells if layers of more-permeable soil lie beneath an
impermeable layer.
Pollutant
Removal
In areas where storm sewers convey runoff to an open water body,
the primary goal of bioretention is water quality. As a result, tailor
bioretention treatment to watershed-specific pollutants. The first
flush of a rain event is often characterized by a higher concentration
of pollutants than later flushes. Bioretention media can capture and
remove common pollutants (e.g., nutrients, bacteria, metals) as well
as more emerging contaminants like microplastics and 6PPD-quinone
(see box, next page).
• Add IWS to remove nitrates when soils have low infiltration rates.
• Consider inflow and outflow configuration, soil permeability, and check
dams to allow ponding, maximize storage volume, and increase runoff
residence time.
• Select compost blends or mulch to prevent pollutant leaching.
• Incorporate vegetation and soil amendments to enhance pollutant
removal.
• Including forebays can localize sediment capture near inlets.
Improve Flood
Resilience
Localized flooding, not riverine or coastal flooding, is a concern
in areas with poor drainage, overwhelmed pipe networks, and
impervious cover. Bioretention can retain, infiltrate, or move
stormwater to reduce risks.
• Design systems with permeable soils that promote infiltration.
• Design systems to safely manage volumes from larger storm events.
Urban
Heat Island
Mitigation
GSI that incorporates canopy cover and other vegetation can reduce
urban heat island effects via processes like evapotranspiration and
shading. Maximizing these benefits will be a function of the local
site and climate.
• Find opportunities to include native trees and other vegetation to provide
a canopy for evaporative cooling and shade.
Air Quality
Where appropriate, design bioretention to help improve air quality
through the use of vegetation for air filtration.
• Find opportunities to include native vegetation and supporting soil
media that filter air pollutants and sequester carbon emissions. Refer to
Recommendations for Constructina Roadside Veaetation Barriers
to Imorove Near-Road Air Ouaiitv for best oractices when addina trees
along roadsides for air quality benefits (Baldauf 2016).
• Design systems with local, sustainable materials to offset emissions.
Green Space
& Wildlife
Habitat
When a community has tree canopy, habitat creation, or greening
compliance requirements, add green space into designs. Planting
vegetation helps to improve native wildlife habitat.
• Consider native vegetation and supporting soil media that enhance
biodiversity and attract wildlife.
• Refer to EPA's Stormwater Trees: Technical Memorandum for information on
planting and maintaining trees in urban areas.
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3.2 STRIVE TO PROVIDE MULTIPLE BENEFITS
Goal
Description
Design considerations and resources
Community-
Focused &
Equitable
Access
Green streets are part of healthy, equitable urban designs and are
vital to creating viable public spaces. When implementing GSI, such
as bioretention, ensure the benefits are supplied equitably, especially
in communities that lack green space or have historically had
disproportionately high air and water pollution levels.
• Identify environmental justice communities with the greatest need
for GSI projects or communities that currently lack GSI projects. EPA's
EJSCREEN: Environmental Justice Screenina and MaDDina Tool combines
environmental and demographic indicators.
• Engage residents and property owners early in the process to ensure
projects are collaborative.
• SuDDort urban tree Dlantina decisions usina American Forests' Tree Eauitv
Score (environmental, climate, and demoaraDhic data).
• Review the Eauitv Guide for Green Stormwater Infrastructure Practitioners
to build equity into stormwater management.
• Create a sense of community and ownership with artistic and functional
elements like seating and bike racks.
Public Health
& Safety
Bioretention elements in the ROW (e.g., bumpouts) can calm traffic
and help pedestrians by creating a visual and physical buffer
and reducing crossing distances. GSI improves public health by
connecting people to natural spaces and adding safe, accessible, and
active modes of transportation.
• Integrate crosswalks/pathways for pedestrians and bikes.
• Design and maintain vegetation to protect sight lines.
• Design systems to minimize tripping hazards.
• Incorporate proper barriers around facilities to prevent pedestrians from
stepping into ponded water.
Using Bioretention to Address Emerging
Contaminants
Several emerging contaminants are gaining research interest, such as
microplastics and 6PPD-quinone (N-(1,3-dimethylbutyl)-N'-phenyl-p-
phenylenediamine-quinone). The contaminant 6PPD-quinone, a transformation
product of the tire additive 6PPD, has demonstrated acute toxicity to several
salmonid species (ITRC 2023). Data have shown bioretention to be a viable
option for removal of 6PPD-quinone (Rodgers 2023).
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CHAPTER 3: HOLISTIC DESIGN CONCEPTS
3.3 DESIGN FOR LONGEVITY
3.3 Design for Longevity
If well-designed, constructed, and maintained, a bioretention
facility can operate for decades. The City of Portland is a notable
example of a municipality with functional systems up to 25 years
old. Holistic design considers future development, snow impacts,
maturing vegetation, and other factors that could affect the
practices' long-term performance.
Incorporate material reuse with life cycle and sustainability in mind.
GSI is an asset for cities—consider sustainability, life-cycle costs,
and benefits when planning, designing, and implementing GSI.
Ensure systems are properly designed, operated, and maintained,
and consider ways to use less material, reuse local materials,
recycle construction waste, and close waste loops where possible.
For example, Olin Partnership Limited (OLIN) Labs is leading the
Soilless Soil Initiative, a phased research project exploring ways
to close the loop of Philadelphia's glass and food waste streams.
In fall 2022, OLIN Labs and its partners retrofitted an existing
bioretention facility to test the efficacy of glass-based soil.
Bigger doesn't mean better. Some people think bioretention must
be extensively engineered to meet performance goals. However,
smaller footprints and simple designs can be effective. For
Soilless Soil Initiative project site during construction.
Soilless Soil Initiative project site after vegetation establishment
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CHAPTER 3: HOLISTIC DESIGN CONCEPTS
example, researchers compared the catchment-to-bioinfiltration
surface area ratios for the bioinfiltration units at EPA's Edison
Environmental Center, which included small (22:1), medium
(11:1), and large (5.5:1) test units. The vegetation included native
grasses, perennials, shrubs, and trees. This research showed that
the larger oversized units did not engage the entire surface area
during runoff capture and were less efficient In using all of the
bioretention media for volume control. As a result, plant growth
was slower and less dense compared to the small units (O'Connor
2022).
Design with redundancy. Including multiple ways for runoff
to enter a practice is beneficial in case of water flow changes,
clogging, or high water volumes. Drainage can flow through
multiple entrances, which helps ensure runoff does not bypass
the system. Designing bioretention practices in a series might be
more beneficial than relying on a single system to manage runoff
from a drainage area—if the runoff misses or overflows the first
system, it can be captured and treated by the next one.
Allow flexibility for adaptive management. Consider designing
systems to be adaptable in case of performance issues and to
mitigate potential impacts of natural hazards and climate change.
For example, designers can plan the systems so they can adjust
the size, curb-cut widths, vegetation, weirs, and other elements
to accommodate larger storm volumes in the future. Continuous
monitoring and experimentation will ultimately provide the best
data on which design elements work best. Designers may also
consider setting aside space that can be used for bioretention if
runoff volumes increase due to changing climatic or site conditions.
Be mindful of where water will flow if the system overflows.
Designers must anticipate where water will flow if the system
3.3 DESIGN FOR LONGEVITY
This facility in Portland, OR, illustrates how the placement of
seating and other features can help create a sense of place.
backs up or clogs, which is especially important if the water
will affect nearby critical infrastructure or create public safety
hazards. Designers should plan for the possibility that a system
does not operate as intended and add elements to reduce the
potential for unintended consequences (e.g., by incorporating
bypass elements).
Consider current and future uses of the space and its relationship
to the surrounding area. Once built, city streets are generally not
reconstructed for many decades. Therefore, when a reconstruction
opportunity arises, carefully consider future needs and capitalize
on the opportunity. To extend the street's useful life, account for
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CHAPTER 3: HOLISTIC DESIGN CONCEPTS
3.3 DESIGN FOR LONGEVITY
expected changes in mobility patterns, local climate and
precipitation, and land use. Proactive planning can prevent
many performance and maintenance problems and
minimize costs for reconstructing or retrofitting systems
as sites evolve. For example, if an area is proposed for
future development, consider how it could affect the
catchment area of the bioretention facility and if the site
is still feasible.
Incorporate design elements that allow for efficient
maintenance. Maintain GSI to ensure its long-term
functioning as a treatment facility and part of the
community. Conducting routine maintenance—removing
trash, debris, dead vegetation, leaves, and sediment-
can prevent costly performance problems and increase
public acceptance. When designing facilities, consider
the O&M needs upfront (type, intensity, and frequency)
and the budget. Consider local factors, such as snow
removal and storage, deicing salt use, street sweeping,
and the presence of clogging materials that could damage
bioretention health and longevity Also, review the types
of maintenance equipment (e.g., shovels) needed and
include access routes in the design. Ensure any grates,
outlets, runnels, or other runoff conveyances allow
maintenance access. Remember that simpler systems are
generally easier for company staff or private owners to
maintain, given possible turnover and a lack of technical
expertise.
A bioretention facility after a snowfall in Portland, OR.
CO
~o
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CHAPTER 3: HOLISTIC DESIGN CONCEPTS
3.3 DESIGN FOR LONGEVITY
This grassy street-side bioswale is easy to clean and maintain and fits in with the community aesthetic in Fort Lauderdale, FL,
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Chapter 4
MANAGING
DRAINAGE AREA
In this chapter
4.1 Drainage Area Delineation
4.2 Determining Grade
4.3 Selecting the Optimal Site Location
This chapter highlights resources to assist drainage area
delineation as weli as considerations for grading and site
location.
Photo: Kary Phillips
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CHAPTER 4: MANAGING DRAINAGE AREA
4.1 Drainage Area Delineation
The drainage area is the quantified surface area draining to
a single point or location. In this handbook, the contributing
drainage area (CDA) refers to the total area (both pervious and
impervious) draining to a bioretention facility. Maximizing
the capture efficiency of runoff from impervious surfaces
increases bioretention's benefits.
Drainage area delineation allows designers to understand
stormwater drainage patterns in a study area and quantify the
runoff volumes. A delineation will also inform the design of
bioretention elements such as pretreatment, inlet size, erosion
control measures, and outflow structures. A drainage area
delineation is conducted during the site assessment. Delineating
a drainage area requires creating a representation of the drainage
area boundary, drainage pattern based on flow direction, and the
different contributing land use types. A combination of publicly
available data sources, software programs, or site surveys can be
used. These resources are described in more detail below.
Publicly available or local data sources. Topographic maps,
elevation data, aerial imagery, and existing roadway plans are
often available. On a national scale, the USGS National Mao is the
primary online viewing source of USGS's geospatial data, which
includes topographic maps (digital and print) (USGS 2023). USGS
has updated the National Map with high-resolution elevation
data developed from nationwide liDAR (i.e., Light Detection and
Ranging, a remote sensing method using light to measure ranges
to the Earth); these data can be used for catchment delineation.
One-meter resolution data sets are available for portions of the
United States. Aerial imagery provides a resource to identify
4.1 DRAINAGE AREA DELINEATION
bioretention practice manages drainage from a street in New
>rk City, NY.
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CHAPTER 4: MANAGING DRAINAGE AREA
The Philadelphia Water Department performs in-house drainage
area delineation using a geographic information system. The
delineation is completed for the entire study area being analyzed,
including ROWs and parcel locations (PWD 2018a). This graphical
example of a Philadelphia city block CDA delineation shows a split
drainage area (indicated by the colored layers in the software
output). The high point is denoted by the yellow triangle,
and drainage inlets are denoted by the green squares. Other
examples are available in Philadelphia Water Department's
Green Infrastructure Planning and Design Manual (PWD 2021).
specific land use types and site features such as roofs or parking
lots. Local, as-built plans for roadways also offer information on
slopes, dimensions, and existing inlet locations.
Software programs. Autodesk and ArcGIS (closed sources) offer
tools to calculate the CDA. In Autodesk CivilSD, the catchment
area command determines the runoff flow paths and quantifies
the CDA. In ArcGIS, Digital Elevation Models (DEMs) can be
used to determine the CDA. For example, Villanova University
researchers coupled one-meter-resolution DEMs with a model
developed by ArcGIS Pro software to create urban micro-
4.1 DRAINAGE AREA DELINEATION
subbasins. This resulted in an automated workflow for urban
watershed delineation that incorporated GSI, building roof
drainage, and inlets by altering the DEM to make these features
part of the hydrologic landscape (Hosseiny et al. 2020; Jahangiri
et al. 2020). Lastly, some municipalities may have ROW CDAs
already delineated in-house for GSI design use, such as the
Philadelphia Water Department (see image at left).
Site surveys, Use topographic surveys to determine elevation;
contours; and site features such as trees, buildings, or streams
(when this information is unknown). Alternatively, a site survey
may simply include visiting a site to verify the information
collected from other sources. Field verification activities could
consist of observing flow patterns during a storm and obtaining
photo and video documentation. In the absence of rain,
practitioners can place a ball on the ground to identify grade
changes. Conducting field verification of site drainage patterns
and features is important because publicly available data sources
might not reflect current site conditions.
Once the CDA is known, planners and designers determine the
surface area of a bioretention facility using the loading ratio (LR).
LR is a design parameter equal to the impervious CDA divided
by the bioretention infiltration surface area. LR is an important
design consideration because LRs that are too high (i.e., greater
than 25:1) can lead to maintenance problems (e.g., excessive
sediment accumulation) or LRs that are too low can hinder plant
growth and result in nonuniform infiltration (O'Connor 2022).
Follow local or state guidelines for LRs when available; they are
often found in local stormwater management design manuals.
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CHAPTER 4: MANAGING DRAINAGE AREA
4.2 DETERMINING GRADE
4.2 Determining Grade
The grade of a CDA determines the direction of water flow
and velocity (discussed in subsequent chapters) and influences
what percentage of the targeted drainage area is captured. For
example, larger CDAs and steeper grades contribute to higher
runoff volumes and velocities. When assessing the grade and flow
paths of a CDA, consider the factors described below.
Slope. Bioretention generally requires a relatively flat site and is
generally best applied when the grade of the contributing slopes
is greater than 1% and less than 5%. For slopes greater than 5%,
the interior of the bioretention facility may need to be terraced
or include check dams or other energy dissipators. Alternatively,
another approach could shift the design from a single bioswale
to a series of terraced bioswales. Avoid sites with steep slopes
(more than 20%) that cause erosive inflows and reduce capture
efficiency.
Site grading If the targeted CDA is not adequately graded,
consider opportunities to regrade the roadbed or incorporate
design elements such as berms and runnels to improve flow
routing into the facility. Grading combined with diverters, inlet
design, and bioretention sizing can ensure that flow enters
the facility, which can accept or bypass flow without damage.
Regrading can be difficult in retrofit cases. Additionally, some
people involved in the construction process might not understand
GSI principles. Including details such as flow path arrows on
design drawings, schematics, and construction plans can help
ensure proper construction and avoid drainage problems.
A road is graded toward a bioretention practice in
Washington, DC.
Improper grading causes water to build up at the edge
and not flow into this facility.
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Poor grading allows water to build upon the road adjacent to a
bioretention practice.
Use of medians Medians (when present) can support linear
practices such as bioswales and infiltration trenches. Medians
tend to be at the high point of a road's cross-slope; thus, the
flow path is typically not toward the median. As a result, using
bioretention facilities in medians might require reversing a street's
cross-slope. If the street's cross-slope cannot be modified, seek
opportunities to intercept stormwater in an upstream conveyance
system (from another street) and daylight the collected flow into
a series of bioretention facilities in the median. To maximize the
amount of ROW available for bioretention and provide more area
to treat offsite water from upstream, either reverse the street
crown to direct flow to the median or change the roadbed to
slope to one side of the street.
4.3 SELECTING THE OPTIMAL SITE LOCATION
4.3 Selecting the Optimal Site Location
Siting a bioretention facility is not limited to the commonly
thought of site characteristics such as groundwater depth,
infiltration rate, and topography but also requires balancing other
factors such as utilities, space constraints, and roadway safety
During the planning phase, performing a desktop analysis of
the drainage area using Google Earth or ArcGIS provides a first
cut at determining the feasibility of a particular location. Using
additional data layers, such as city utility maps, bike lanes, the
100-year flood plain, and sewer networks, can also influence
siting decisions. Consulting the community as part of this
analysis can help uncover functional aspects of the project, such
as incorporating benches because a school bus stop is nearby.
(Chapter 12 discusses community engagement in more detail.)
When conducting this type of site feasibility analysis, consider the
bioretention facility siting factors below.
Existing utilities and public use corridors. Utility conflicts affect
decisions regarding how runoff is conveyed to the site and the
facility's footprint. For example, a conveyance pipe directing
runoff across a street could be a way to minimize impacts to a
bike lane or to route runoff to a parcel that accommodates more
of the drainage area.
The number of facilities implemented. Choosing whether to
implement fewer facilities that manage the entire drainage area
or multiple smaller units in series may be influenced by available
land type. For example, a park may offer sufficient space to
implement a single facility, while a congested city block may need
multiple bioretention planters implemented in the ROW.
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CHAPTER 4: MANAGING DRAINAGE AREA 4.3 SELECTING THE OPTIMAL SITE LOCATION
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 4-6
Bioretention can be configured as a series of individual facilities
(top) or a single, street-length facility with multiple inlets (bottom).
The Philadelphia Water Department installed GSI and other
stormwater management facilities at the city-owned Venice Island
Performing Arts and Recreation Center in Philadelphia, PA.
Available partnerships. Working with partners may provide
opportunities to incorporate GSI into other projects, such as
new recreational fields or transportation improvements. Seeking
partners with a stormwater interest may yield more options for
siting. The RainScapes Project in Montgomery County, Maryland,
offers technical and financial help to property owners who install
bioretention and other GSI practices.
Variable siting costs Investigating the costs of different siting
options influences placement. For example, the Philadelphia
Water Department found that managing runoff in parcels such as
city-owned parks, facilities, and schools was more cost-effective
than building in the ROW (PWD 2014a).
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Chapter 5
BIORETENTION
GEOMETRY
AND SIZING
In this chapter
5.1 Cell Sizing and Geometry
5.2 Side Slopes and Geometry
5.B Sizing Methods and Considerations
The configuration of bioretention facilities can range from
relatively large and open vegetated basins to small-scale
facilities contained within flow-through planter boxes.
They can also be designed as a series of multiple cells along
roadways or parking lots or combined with other GSI to
meet flood control requirements. This chapter defines the
different sizing elements of bioretention facilities and
presents various methods that can be used for bioretention
sizing.
Photo: Robert Goo, Washington, DC
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.1 CELL SIZING AND GEOMETRY
5.1 Cell Sizing and Geometry
Bioretentiori surface area, ponding depth, and infiltration rate
of BSM and underlying soils influence the effectiveness of
bioretention facility to temporarily store and infiltrate runoff.
When siting bioretention in the ROW, external factors such as
sidewalk widths, the presence of utilities, and surrounding land
use may constrain or limit bioretention dimensions. Additionally,
the local drainage systems will influence whether design goals
emphasize water quantity or water quality. For example, for
combined sewer systems, bioretention design emphasizes runoff
volume reductions and minimizing combined sewer overflows.
Alternatively, in settings where storm sewer systems exist,
bioretention design emphasizes water quality improvements
related to pretreatment, treatment, and maintenance to protect
the health of receiving waters.
Bioretention facility sizing is an important design element—one
that is discussed thoroughly in the National Association of City
Transportation Officials' (NACTO's) 2017 Urban Street Storm water
Guide. The following section leans heavily on NACTO's resource,
which was developed to help cities design and construct
sustainable streets (NACTO 2017).
The bioretention planter design schematic (right) illustrates
five key concepts applicable to many bioretention facilities in
the ROW, including the sizing elements of length, width, BSM
depth, ponding depth, and freeboard depth. Table 5-1 (next
page) describes these sizing elements and provides suggested
dimensions and other considerations.
Planter Boxes: Small Size, Big Impact
Bioretention planter boxes are an application of bioretention
used to handle volume and peak flow requirements for
smaller CDAs such as roofs or sidewalks. Planter boxes are
contained in a concrete box with an impermeable liner to
prevent impacts to a building foundation or utilities.
Q Cell Length Q Ponding Depth
@ Cell Width |j| Freeboard Depth
||| BSM Depth
Bioretention planter design schematic.
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.1 CELL SIZING AND GEOMETRY
Table 5-1. Description of sizing elements for bioretention facilities (planters and boxes).
Sizing
element1
Sizing element information
1,2. Cell
length,
cell width
(surface
area)
Description: The surface area is equal to the length multiplied by the width, and it represents the surface area available for infiltration and
temporary surface storage. Greater surface area increases the surface available for infiltration.
Typical range: Length: Ranges from 10 feet to the length of a city block (260-400 feet); Width: A minimum of 4 feet
Design considerations:
• If sidewalk access is a priority, several short cells can be used in place of one long cell. Short cells could increase hydraulic efficiency and require
the use of BSM.
• Check dams provide flow control on sloped surfaces.
3. BSM
depth
Description: BSM depth refers to the thickness of the soil media that extends from the surface of the facility to the bottom of the cell or other
media layer, such as IWS or a gravel drainage layer. Deeper media increases the storage capacity and pollutant-removal benefits.
Typical range: 2-4 feet
Design considerations:
• Pollutants such as total suspended solids, metals, hydrocarbons, and particulate phosphorus are generally captured within the top 2-8 inches
of BSM.
• Targeting dissolved phosphorus, nitrogen, and temperature pollution requires a minimum BSM depth of 2-3 feet; 4 feet is sometimes preferred
for targeting elevated temperatures.
• Including IWS in the design would increase the total cell depth. IWS-specific design considerations are presented in Chapter 10.
4. Ponding
depth
(surface
storage)
Description: Ponding depth refers to the depth of runoff that is temporarily stored—to meet local draw-down requirements—on the surface of
a bioretention facility before infiltration.
Typical range: Maximum ponding depth is based on soil infiltrate rates, local guidelines, and public safety requirements: 6-12 inches; 18 inches
maximum
Design considerations:
• 6 inches is recommended for areas with high foot traffic, next to sidewalks, or when a fence is not included in the design.
• For depths of 12-18 inches, consider including safety features (e.g., fencing).
• Incorporate check dams and weirs to control the desired ponding depth.
5. Freeboard
depth
Description: Freeboard depth equals the distance between the top of the facility's overflow elevation and the maximum ponding depth.
Freeboard provides a margin of safety for larger storm events.
Typical range: 2-6 inches
Design considerations:
• A freeboard more than 6 inches high might be needed for sites with frequent overflows.
Sources: NATCO 2017; Hunt et. al. 2012; DOEE 2020
Note:1 Refer to the design schematic on page 5-2 for a diagram of the sizing element numbers.
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.2 SIDE SLOPES AND GEOMETRY
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 5-4
• Typical recommendations specify a 2% or less longitudinal
slope and side slopes of 4:1 horizontahvertical, with a
maximum slope of 3:1 (some resources recommend that a
maximum side slope of 2:1 not be exceeded). Where feasible,
use gradual side slopes of 5:1 for graded surface facilities.
• The Philadelphia Water Department recommends a
maximum side slope of 4:1 for mowed facilities to prevent
damage from mower blades and a side slope of 3:1 for
facilities that are not mowed (PWD 2018b).
• To prevent erosion, design the facility's side slopes based
on expected stormwater flow rates. Applying jute or coir
erosion control mats can help stabilize soils until vegetation
is established.
A bioretention practice with side slopes in Montgomery
County, MD.
5.2 Side Slopes and Geometry
The side slopes of a bioretention facility provide a transition
from the adjacent roadway or sidewalk to the bioretention
surface. Designers can increase the ponded storage volume of
the bioretention facility by using deeper side slopes. Design
considerations include:
• Cross-sections of the bioretention surface depression can
be parabolic, trapezoidal, or flat (with a minimum 2-inch
freeboard).
• Side slopes allow for sidewall infiltration. For facilities with
graded side slopes, the cell-wetted area includes the surface
of the cell and the area on the sides when inundated (i.e.,
maximum ponding depth plus freeboard).
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.3 SIZING METHODS AND CONSIDERATIONS
• For street conditions, use a 12-inch flat shelf transitioning
between the curb or pavement and the slope when used
next to a parking lane, bicycle facility, or sidewalk.
• Where space is available, using bioretention swales with
graded side slopes provides gentler transitions from the
pedestrian path to the bioretention facility, offers safer
conditions compared to vertical walls, and allows for more
plant choices.
5.3 Sizing Methods and Considerations
Various methods and models are available to guide the design of
bioretention facilities to ensure they are adequately sized.
When sizing and designing the bioretention facility's size,
understand the facility's purpose, review the stormwater
management goals, and ensure you're conforming with
the state and local codes. For example, state and local
codes might dictate that bioretention facilities be designed
to mimic an area's original hydrological (pre-development)
conditions or to infiltrate a specified volume of water
(e.g., the first inch of precipitation or runoff).
Runoff and Water Quality Volume
The necessary cell size can also depend on the pollutant load and
desired pollutant removal targets, which are more difficult to
evaluate. WQV generally refers to the stormwater runoff volume
created from a given precipitation event that be captured and
treated to remove most of the pollutants on an average annual
basis (Ohio EPA 2018).
Alternatively, the sizing of bioretention can be guided by tools such
as performance curves. For example, the New England Retrofit
Manual (SNEP 2022) provides performance curves for various
stormwater control measures and pollutants (total phosphorus,
total nitrogen, total suspended solids, metals, and bacteria). The
performance curves can be used to identify the percent removal
of a pollutant based on a design runoff depth or determine
the runoff depth that must be captured to achieve a specific
pollutant reduction target.
Planners and designers can estimate the runoff volume (V, in acre-
inches) by multiplying the impervious cover (IC, in acres) of the
CDA by the design rainfall depth (P, in inches).
V=IC x P
The bioretention surface area can then be estimated as a function
of the runoff volume managed divided by the bioretention
ponding depth. The bioretention surface area is typically sized to
equal 5% to 10% of the CDA (Tetra Tech 2011).
The Rational Method and Curve Number Method can also be
used to calculate the size of a facility needed to manage a
specific runoff volume. When applying runoff coefficients or
curve numbers, note that grass strips or highly compacted areas
can be considered impervious unless they are modified with soil
improvements (such as sand) as part of the proposed work. Most
municipalities generally design facilities to manage 1 inch of
precipitation. EPA's Compendium of MS4 Permitting Approaches
provides other examples of post-construction performance
standards to meet on-site retention requirements from cities and
towns (USEPA 2022a).
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.3 SIZING METHODS AND CONSIDERATIONS
Effective sizing is a common challenge for communities, with many
bioretention facilities being undersized or oversized for the site.
In this example, large oversized bioretention can be perceived as
giant pits to the surrounding community.
Modeling
EPA's Storm Water Management Model (SWMM) is used
worldwide for planning, analyzing, and designing systems for
stormwater runoff, combined and sanitary sewers, and other
types of drainage (USEPA 2022c). SWMM can be used to size
detention facilities to control flooding and protect water quality.
EPA's National Stormwater Calculator is a simple screening tool
that uses EPA SWMM to calculate hydrology for sites up to 12
acres. The tool allows users to test the impact of implementing
different GSI practices for runoff capture. The tool can also
provide planning-level estimates of capital and O&M costs and
account for future climate change scenarios.
The Source Loading and Management Model for Windows
(WINSLAMM) considers both runoff quantity and water quality.
The program evaluates GSI performance for specific sources in
urban land use areas during a range of rainfall events.
The Water Environment Federation's Stormwater. Watershed.
and Receiving Water Quality Modeling document provides
straightforward guidance for modeling tools and their
capabilities. The document offers past, present, and future
perspectives on stormwater quality modeling and outlines criteria
for model selection based on project needs (WEF 2020).
Other Sizing Considerations
Weather. Local climate and weather conditions (e.g., precipitation,
temperature) affect how quickly water soaks into the ground.
Many factors influence the sizing and modeling coefficients used
for runoff and infiltration, including the annual precipitation,
storm frequency, and evaporation and infiltration rates.
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.3 SIZING METHODS AND CONSIDERATIONS
Available space In many urban settings, retrofitted bioretention
often cannot be fully sized to manage the expected runoff
volumes or WQVs because of space constraints. The length-
width-depth dimensions of practices might be restricted
by adjacent land uses; surrounding topography; pedestrian
volumes; and proximity to trees, utilities, and buildings and
other structures. Under space-constraint scenarios, size the
bioretention facilities to capture and treat as much water
as possible. Consider reducing the size of facilities to supply
sufficient setbacks from any conflicts. In constrained spaces (e.g.,
ROWs), bioretention facilities could be narrow and deep; in open
areas, they could be wide and shallow. If undersized, consider
designing the practice as an offline system; as part of a connected
series to prevent overloading; or with design elements such as
flow splitters/diverters, underdrains, and overflow structures.
Margin of safety. In jurisdictions with limited or unreliable
maintenance, consider designing oversized bioretention facilities
to supply a margin of safety if performance deteriorates. For
example, some states require that 100% of WQV be captured
within the surface storage rather than allowing some of it to be
stored in the media; this ensures adequate storage volume in case
of over-mulching (poor maintenance) or faulty construction.
Effective sizing Consider the total cost and feasibility of
maintaining bioretention at a given size. For example, the
Philadelphia Water Department (2018a) found that systems are
cost-effective when they manage an area of at least 8,000 square
feet overall, and the individual drainage areas entering each inlet
are at least 5,000 square feet.
Public use. Consider public safety and multimodal transportation
access when needed. Refer to Chapter 12 for design considerations
related to public safety and transportation.
In this example, a series of bioretention practices in Kansas City,
MO, were sized appropriately and constructed on once-vacant land
to manage a very large CDA.
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CHAPTER 5: BIORETENTION GEOMETRY AND SIZING
5.3 SIZING METHODS AND CONSIDERATIONS
A bioretention facility (left) within Oklahoma City's Scissortail Park is situated alongside public benches and a playground (right), which
encourage community gathering.
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Chapter 6
RUNOFF CAPTURE
In this chapter
6.1 Stormwater Conveyance
6.2 Sheet Flow Conveyance
6.3 Inlet Types for Concentrated Flow
6.4 Inlet Number, Placement, and Frequency
6.5 Inlet Size
6.6 Inlet Flow Path Modifications and Retrofits
6.7 Inlet Protection
Runoff must flow into a bioretention facility to be effective.
This chapter provides a comprehensive overview of
the various inlet designs used to manage sheet flow or
concentrated flow via inlets. Many different inlet types are
presented in detail, along with photographs illustrating
various design modifications from across the country.
Photo: NYC Department of Environmental Protection
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CHAPTER 6: RUNOFF CAPTURE
6.1 STORMWATER CONVEYANCE
6.1 Stormwater Conveyance
The best design choice for capturing and conveying stormwater
runoff into bioretention facilities depends on various factors,
such as location, land use, grade, and flow velocity. Curbless
bioretention facilities are designed without a curb and gutter,
while curbed bioretention facilities incorporate a curb and gutter
in the design.
A curbless design allows sheet flow—runoff that flows uniformly
over the ground surface—to drain freely into the practice.
Curbless bioretention facilities in low-density areas are associated
with larger footprints and little or no vertical separation from
the sidewalk and street. Curbless bioretention facilities are also
often used along major arterials and highways where sidewalks
are absent or separated from the roadway. Bioretention designed
with gentle side slopes are amendable to curbless practices. More
structured facilities, such as straight-sided bioretention planter
boxes, are less common in this context.
Curbed bioretention designs are prevalent in high-density urban
environments with space limitations or other site constraints.
Stormwater curb extensions and bioretention planter boxes are
generally more appropriate for this context. Curbed bioretention
facilities require inlets to convey runoff to practices from the
surrounding catchment area. Proper inlet design is essential for
ensuring runoff is effectively captured.
Design Challenges and Considerations
Table 6-1 summarizes the challenges often encountered in curbless
and curbed bioretention facilities and the design options that
help overcome those challenges.
Curbless bioretention in Kansas City, MO.
Curbed bioretention with inlets for runoff in College Park, MD.
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CHAPTER 6: RUNOFF CAPTURE
6.1 STORMWATER CONVEYANCE
Table 6-1. Curbless and curbed design challenges and considerations.
Design challenge
Low Capture Efficiency for inlets occurs when runoff
flows across the pavement and does not enter the
bioretention facility; low capture efficiency is a common
problem for both curbless and curbed systems.
For curbless systems, a slight elevation difference
between the edge of the pavement and the edge of
the bioretention facility can contribute to low capture
efficiency; debris buildup and plant growth along the
edges can also be factors that reduce runoff capture.
For curbed systems, problems such as too-small inlets,
improperly placed and/or numbered inlets, 90-degree
turns, and debris buildup can prevent runoff capture.
Design consideration
Regrade the roadbed to improve the capture efficiency of runoff (i.e., gentle grading affects the
capture of small flows). Regrading will depend on existing development and whether grades can be
adjusted at the property line.
Design the inlets to meet stormwater capture requirements.
Understand the hydrology to inform inlet design and enhance overall system performance (i.e.,
preventing erosion).
Design inlets to function in concert with pretreatment to capture most incoming debris and
sediment, allowing for easier maintenance.
Consider how ice and snow may block inlets and design to minimize these impacts to the extent
practical.
Specify routine visual inspections and maintenance every few months or after large storms
to minimize impacts from vegetation and debris.
Erosion occurs due to fast-flowing water. In curbless
systems, the slope or land cover can direct sheet flow to
form channels within the practice. Although the inlets of
curbed practices are designed to capture high-velocity
runoff flows, incorrect or inadequate designs can lead to
erosion around the inlet or within the practice.
Protect the system with pretreatment, energy dissipators, and erosion control devices (e.g.,
forebays).
Design inlet width, number, and placement to mitigate erosive flows that could scour the surface
of bioretention facilities.
Damage, such as compaction from pedestrians and
vehicles, can occur in bioretention facilities.
jbCL ICM Consider measures such as visual barriers, low fencing, and bollards to protect the system
from damage.
Include elements to minimize vehicle entry. Wheel guards (e.g., steel plates, concrete lids)
reduce the risk of incursion.
Ease of Maintenance ensures inlets allow access for
cleaning and clearing of debris.
Check inlets 3-4 times per year for accumulated grit, leaves, and other debris that could block
inflow. Clear blockages as necessary.
Use inlets that are wide, free of enclosures, and allow for access by maintenance tools to ensure
efficient long-term maintenance.
= sheet flow
= inlets
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CHAPTER 6: RUNOFF CAPTURE
6.2 SHEET FLOW CONVEYANCE
6.2 Sheet Flow Conveyance
A bioretention facility receiving sheet flow (without inlets) takes
advantage of how water drains naturally across the land, uses a
simpler design, and is less prone to failure. Sheet flow enters the
facility uniformly, evenly distributing sediment, so bioretention
erosion-control measures can often be omitted.
Suitability
Sheet flow conveyance is often associated with smaller drainage
areas where runoff can be safely conveyed via overland flow
over short distances (e.g., 400 feet or less), such as parking lots,
residential areas, or curbless roadway sections (PDHonline 2020).
Design and Maintenance Considerations
A grass filter strip or gravel filter strip are common pretreatment
for sheet flow. A grass filter strip is a uniformly graded vegetated
buffer that traps sediment and reduces runoff velocities entering
the bioretention facility. A gravel filter strip consists of a trench
(2-4 feet wide by 1 foot deep) placed between the edge of the
pavement and the edge of the bioretention side slope.
A catchment area's grade, length, and roughness influence the
time of concentration, or how long it takes for a raindrop to travel
from the furthest point of the catchment area to the collection
point (the bioretention facility). Gently sloped grades (1%-5%
for paved surfaces) help maintain a thin, even sheet flow across
level entrance areas (PDHonline 2020). Sheet flow conveyance is
unsuitable for travel distances more than about 400 feet because
the water concentrates into erosive flows. For transitioning from
the catchment surface to the bioretention facility, a 3:1 side slope
allows runoff to enter and encourages even sediment distribution.
Sheet flow is directed off a residential street by using a gentle
slope along the street crown and a curbless gutter (valley gutter) in
Portland, OR.
A valley gutter concentrates sheet flow from upstream and can
cause localized deposition of debris as it flows into a bioswale in
Portland, OR.
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CHAPTER 6: RUNOFF CAPTURE 6.2 SHEET FLOW CONVEYANCE
Grade and Routing of Inflow
When stormwater flow is routed incorrectly and results in low
capture efficiency, the problems are usually related to variations in
microtopography (landscape irregularities) or errors in construction
or design. Minor elevation differences between the edge of the
pavement and the curb's edge can greatly influence inflow capture.
For example, an elevation increase of as little as one-fifth of an
inch (5 mm) between the asphalt and the invert of a concrete inlet
can lead to suboptimal capture efficiencies. A smail expansion gap
between the asphalt and the curb's edge will also interfere with
runoff capture. Moreover, routing water into bioretention facilities
can be challenging when stormwater flows from different slopes
converge and cause variable velocities.
The pavement's slope next to the curb or the gutter might need to
be adjusted to orient flow paths toward the bioretention facility.
Typically, curb cuts sloped (2%) toward the bioretention facility or
designed with a minimum 2-3 inch drop in grade between the curb
entry point and the bioretention facility's finishing grade provide
optimal conditions for conveyance and minimal sediment buildup.
Also, consider whether multiple inlets or curb cuts will improve
runoff routing into the facility (see photos at right).
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
A design that allows water to turn 90 degrees and flow
through the inlet, resulting in high capture efficiency.
An inlet design where water cannot turn 90 degrees,
resulting in low capture efficiency.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
63 inlet Types for Concentrated Flow
Curb Extension Inlet (Rhea Thompson) Depressed Drain (Portland Bureau of inlet Sump (ACF Environmental)
Environmental Services)
Inlets can be open, closed, precast, cut, retrofitted, or cast
in place. Inlet construction materials vary and include stone,
concrete, steel, and aluminum steel. Choose the best design for a
site based on your desired inflow, cost and feasibility, community
aesthetics, pedestrian safety, functionality, durability, and ease of
maintenance.
Inlets can generally be classified into the following types:
1. Curb cuts 5. Curb extension inlets
2. Covered inlets 6. Depressed drains
3. Trench drains 7. Inlet sumps
4. Pipes and downspouts
Curb Cut (Rhea Thompson)
Covered Inlet (Rhea Thompson) Trench Drain (City of Seattle and MIG/SvR)
Pipe (City of Seattle and MIG/SvR)
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CHAPTER 6: RUNOFF CAPTURE
6.3 INLET TYPES FOR CONCENTRATED FLOW
Curb Cuts
A curb cut is any break along a uniform curb that allows runoff
from the street or sidewalk to enter a stormwater management
practice next to the back edge of the curb break. For the purposes
of this handbook, curb cuts are defined as curb openings that
are poured, cast, or cut into an existing curb.
Suitability
• Curb cuts convey both street and sidewalk runoff to the
surface of a bioretention facility.
• Curb cuts work well for relatively shallow facilities that do
not have steep side-slope conditions.
• Curb cuts are generally an easy retrofit for existing
neighborhoods and provide a way to implement bioretention
facilities without major reconstruction.
Design arid Maintenance Considerations
• Consider designing wide openings and angling the curb cuts
to help facilitate stormwater entry For example, curb cut
openings placed parallel to the gutter's stormwater flow
typically result in a lower capture efficiency because the
water must make a 90-degree turn into the facility.
• Space the curb cut openings to distribute inflow evenly
within the bioretention facility.
• The bottom of the concrete curb cut generally is sloped
toward the GSI practice.
• Depressed or angled curb cut geometries help facilitate the
flow of stormwater into bioretention facilities.
A curb cut on the sidewalk side of a bioretention practice in San
Francisco, CA.
Depressed Curb Cut
Depressed curb cuts are often used to convey runoff into
bioretention facilities next to curbed roadways. A depressed
curb cut is one that is poured with one or more sides tapered
down, which allows stormwater to enter a facility directly behind
the back edge of the depressed curb. Cuts are usually made at
45 degree angles (forming a trapezoidal channel shape), but the
dimensions vary by location.
70500087
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
Flow enters a well-designed curb cut in New York City, NY.
Water flows through a curb cut in Seattle, WA.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 6-9
Depressed curb cuts have been documented for their
effectiveness by NYC DEP (2011). After evaluating the first
generation of inlets, the city replaced some of them with
depressed curb cuts to increase stormwater conveyance. Some
inlets were retrofitted to include either a depressed curb or a
modified back-plate on the cast iron curb pieces to increase
clearance (minimum of 3 inches). These modifications increased
the overall conveyance of stormwater to the underground storage
areas without affecting vegetation.
Angled Curb Cuts
Angled curb cuts are constructed by angling or bending the curb
cut in an orientation towards the facility to help route flow and
increase capture efficiency. Many variations of angled curb cuts
are illustrated in the following photographs.
Covered Inlet
Covered inlets are openings in sidewalks that direct runoff from
streets or parking lots into a bioretention facility located directly
behind the back edge of the opening.
Suitability
• Covered inlets are useful for sidewalks with high pedestrian
volume, such as parking zones, because they allow runoff to
flow underneath the sidewalk into the bioretention facility.
Design and Maintenance Considerations
• Openings are often placed parallel to the stormwater flow
along the curb, which can reduce capture efficiency.
• Covered inlets are prone to clogging from trash and debris
and are difficult to clean.
These examples illustrate how covered inlets can become easily
clogged with trash and debris and are difficult to clean.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
A trench drain crosses a bioretention practice in San Francisco, CA.
Trench Drain
A trench drain, also called a grated curb cut, is a long, shallow
channel with a grate or solid cover over the top. This drain collects
stormwater and directs it into a stormwater management practice.
Trench drains convey stormwater over a distance while the grate
or solid cover maintains the grade of the ground surface.
Suitability
• Trench drains can be installed over an existing runoff channel
or gutter.
• On sidewalks they support pedestrian traffic along the
surface.
• Aesthetically they are less attractive in residential
neighborhoods.
Design and Maintenance Considerations
• Trench drains are manufactured and sized for many different
site conditions and can be selected accordingly. The choice
of trench drain depth depends on the volume of runoff
expected (i.e., more runoff requires deeper trench drains).
Note that deeper trench drains can accumulate sediment and
must be maintained frequently.
• Trench drain channels are usually concrete and covered with
a heavy-duty bolted metal grate or a solid cover.
• The concrete strength required for trench drains will vary
by municipality For example, the Philadelphia Water
Department and the City of Columbus specify 3,500 and
4,000 pounds per square inch, respectively (PWD 2018c; City
of Columbus 2022).
• Grate and covers can include decorative patterns or colors.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 6-11
All bolts are typically stainless or galvanized steel (or as the
manufacturer recommends).
Although the length varies, it is recommended that
trench drains be less than 20 feet long. Trench drain grate
covers are typically cast in 2-foot sections (with varying
widths); therefore, design the trench drain length in 2-foot
increments to avoid the need to cut grate castings to fit.
Trench drains may be placed across or parallel to the flow
direction to collect and direct the runoff to a single inlet
point in the bioretention facility.
Review current Americans with Disabilities Act (ADA) codes
for grate requirements if trench drains are within sidewalk
zones and/or ADA travel paths. All grates or covers used must
be heel-safe and have sufficient slip resistance. Consider
using slip-resistant materials, textured surfaces, or other
measures to minimize slippery surfaces and enhance safety
To support accessibility and address safety concerns, a trench
drain can serve as part or all of a detectable edge treatment
(i.e., to indicate pedestrian street crossing is not intended).
They can also provide a visual separation on a roadway,
especially on curbless or shared streets (i.e., a road with a
designated bicycle lane).
If projects use federal funds (e.g., state revolving funds),
American Iron and Steel requirements apply.
Avoid grade changes in the location of trench drain grates.
Trench drains generally work well; however, if not
maintained, debris can get trapped in the covered zone and
block runoff flow. When the trench drain is installed with
a removable cover, debris can typically be extracted with a
shovel. Trench drains with permanent covers usually require
washing/flushing to clear.
A trench drain allows for runoff conveyance and pedestrian
circulation in Seattle, WA.
A trench drain across a sidewalk in Atlanta, GA.
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CHAPTER 6: RUNOFF CAPTURE
6.3 INLET TYPES FOR CONCENTRATED FLOW
A pipe conveys and discharges roof runoff in San Diego, CA
Pipes and Downspouts
Pipes are commonly used to convey surface runoff from roadways,
rooftop surfaces, and other impervious areas before discharging
it directly to a bioretention facility A downspout is a pipe that
conveys runoff from a rooftop to a ground drainage system.
Suitability
• Pipes carry runoff to a bioretention facility when the
practice can't be fed by sheet flow or when flow comes from
different subcatchments.
A pipe discharges runoff over an energy-reducing concrete
splash pad in Seattle, WA.
• Pipes and downspouts are useful in areas where water needs
to be intercepted from rooftops. Many buildings, especially
in commercial districts, have existing buried downspout
connections that discharge to the street curb line or a curb
and gutter underdrain. The downspouts from rooftops can
be reconfigured to route stormwater into a GSI facility, such
as a bioretention planter.
• Pipes convey runoff over a distance from an upstream source.
• Pipes work well in high-traffic areas because they do not
interfere with multimodal transportation. However, existing
underground utility infrastructure might prohibit the
installation of additional storm drain pipes.
Design and Maintenance Considerations
• Existing downspouts can be modified to divert runoff to a
GSI practice. Coordination with property owners is needed
when downspouts are on privately owned buildings (City of
Columbus DPU 2015).
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CHAPTER 6: RUNOFF CAPTURE
6.3 INLET TYPES FOR CONCENTRATED FLOW
Modify downspouts according to their distance and
elevation in relation to the GSI practice.
- For downspouts discharging directly to a bioretention
facility, cut the existing downspout pipe parallel to the
adjacent grade and smooth the edges.
- For downspouts located away from the practice but at a
higher elevation, add piping that carries the water and
discharges it at or above the surface of the practice.
- For downspouts with an elevation lower than the proposed
GSI surface, fit the downspout with a perforated pipe
that discharges into a gravel layer in the subsurface of the
practice (City of Columbus DPU 2015).
Use check valves when connecting downspouts to
bioretention to prevent stormwater backflow due to
ponding in the GSI during storm events (City of Columbus
DPU 2015).
Bubble-up risers, also called pop-up emitters, are internal
downspouts that may be used as an alternative to below-
grade distribution piping. They function by allowing water
to back up from underneath the ground. Although they can
direct roof runoff to bioretention, they can be problematic
because they need significant pressure for water to flow.
Moreover, they are not suitable near building foundations.
The use of energy dissipation is generally needed to mitigate
erosion.
Monitor and maintain pipes to avoid blockages. Also, consider
installing grates over pipe openings to exclude pests.
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A pipe under a street conveys runoff between bioretention
practices in Seattle, WA.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
Curb Extension Inlet
A curb extension inlet is a type of opening found on a curb
extension, which is a bioretention facility in the ROW that
extends into the street (often referred to as a bump-out) to
capture runoff and calm traffic.
Suitability
• They are suitable for any stormwater curb extension practice.
Design and Maintenance Considerations
• Some inlet designs have been modified to be stormwater
curb extensions. The most common design allows water to
flow straight along the gutter, such as in the image to the
left.
• Curb extensions placed in the street can be susceptible to
damage from drivers, especially in locations near parking
spaces. Consider adding elements such as bollards or metal
bars secured across the top of the inlet for protection when
designing practices.
Stormwater curb extension in New York City, NY.
Stormwater curb extension in Phiiadelphia, PA.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 6-15
An early curb extension inlet design allowed flow
bypass.
An inlet design that extends into the gutter Metal bars added across inlet opening A metal bar is included in all new curb
to intercept flow. prevents vehicle access. extension designs.
Lessons Learned
The City of Portland's preferred stormwater curb extension inlet design
has evolved over time to meet local needs. In Portland's early curb
extension design (right), the water had to turn to flow into the curb cut
and most runoff flowed past the opening and bypassed the practice
completely. To prevent inlet bypass, the City of Portland modified the
curb extension inlets so the opening was offset and intercepted runoff
within the flow path along the gutter (bottom left) However, the City of
Portland then realized that vehicles could drive into the facilities through
the opening. To solve this problem in the existing inlets of this type,
Portland retrofitted the inlets with a metal bar extension that prevented
vehicles from entering the facilities (bottom middle). The current standard
for new stormwater curb extension inlets is to include a metal "staple"
bar across the top and install a splash pad for sediment collection just
downgradient (bottom right).
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CHAPTER 6: RUNOFF CAPTURE
Depressed Drain
In depressed drains, the runoff flowing along the gutter drains
down through a grate-covered inlet and discharges into a
bioretention facility.
Suitability
• Depressed drains are helpful when water flows parallel to a
cell because the water drops in without needing to make a
90-degree turn.
• They are useful for bioretention facilities on sloped streets,
where directing runoff into cells can be challenging.
6.3 INLET TYPES FOR CONCENTRATED FLOW
Design Considerations
• Drain cover design must be safe for bicyclists and
pedestrians. Grid-type covers are generally preferred.
• Depressed drains can be shallow or deep. Deep depressed
drains generally use pipes to convey runoff to the facility,
making them challenging to maintain.
• If retrofitting a site to implement a depressed drain, consider
the cost and labor needed to cut into the road.
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Shallow depressed drains include inlet openings to maintain runoff capacity into the cell if debris accumulates on the grate, Portland, OR.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
Water flows into a depressed drain and through a pipe to a
bioretention facility in Minneapolis, MM.
A pipe conveys runoff from a depressed drain to a bioretention
facility in San Francisco, CA.
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CHAPTER 6: RUNOFF CAPTURE 6.3 INLET TYPES FOR CONCENTRATED FLOW
Inlet Sump
Inlet sumps, also known as manufactured inlets, include
pretreatment for sediment and erosion control. They are
designed to route runoff into a catch basin to collect debris. After
pretreatment, water is directed into the bioretention facility.
Suitability
• When a lot of debris is expected in stormwater flows, inlet
sumps are especially useful because they settle and separate
the sediment from the runoff before it enters a bioretention
facility.
• Avoid siting inlet sumps where pedestrians will interact with
them, such as at intersections or next to a curb ramp.
Design Considerations
• Many inlet sumps are proprietary devices; some
municipalities have replicated and modified these designs.
• The pretreated runoff typically drains out of the catch basin
through a pipe or opening and into the bioretention facility.
Some inlet sumps are designed to release pretreated runoff
through a perforated underdrain directly into the subsurface
of bioretention facilities (NACTO 2017).
• The grate, filter, and chamber of inlet sumps capture
sediment and debris. Maintenance efforts can remove
accumulated sediment and debris from the chamber with a
shovel and clean the drop-in filter with a broom or hose.
Proprietary inlet sump in Minneapolis, MN.
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CHAPTER 6: RUNOFF CAPTURE
6.3 INLET TYPES FOR CONCENTRATED FLOW
A bioretention practice in Minneapolis, MN, with a proprietary inlet
sump often used in residential areas.
The Middle St. Croix Watershed Management Organization
developed this low-cost shallow sump to capture runoff and debris.
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CHAPTER 6: RUNOFF CAPTURE
6.4 INLET NUMBER, PLACEMENT, AND FREQUENCY
6.4 inlet Number, Placement,
and Frequency
During the design phase, pay careful attention to the proposed
number, placement, and frequency of inlets.
Use multiple inlets to maximize capture efficiency and minimize
erosion, Placing inlets at intervals along a bioretention facility
helps to capture more stormwater and evenly distribute the flow
within a GSI practice. Installing multiple inlets also ensures water
can enter the celi if an upstream inlet becomes blocked.
Position inlets to capture the majority of runoff. Ensure they are
placed in the pathway of stormwater flow and alongside the
gutter line at the upstream end of the facility. Placing an inlet
at a low point or depression in a road or parking lot might be
necessary. As previously noted, stormwater flows can be altered
by small factors in the landscape (e.g., pavement cracks or divots),
which could direct runoff away from the inlets.
Review the slope of the street when siting inlets. On a street that
is higher in the middle and drains to the sides (i.e., a crested/
crowned street), note that one side of the street could be at a
slightly higher elevation and might affect runoff flow.
Use hydrological modeling tools to help place inlets, Various
municipalities and researchers are studying how to design inlets
to accept desired volumes and velocities based on the hydrology
(i.e., the volume, velocity, time of concentration, and direction of
water flow) of the modeled drainage area. Although effective,
modeling can be costly and impractical for many municipalities.
This bioretention facility in Seattle, WA, has multiple inlets that
allow runoff to enter if an upstream inlet gets clogged or is at
capacity. Although some bypass is expected, designing inlets with
redundancy can help maximize conveyance.
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CHAPTER 6: RUNOFF CAPTURE
6.4 INLET NUMBER, PLACEMENT, AND FREQUENCY
Lessons Learned e
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Examine examples of the number, placement, and frequency of inlets in existing bioretention facilities to determine if the ^
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designs could be improved. Incorporate these lessons into future designs. In Example 1, a bioretention facility has a single M
undersized and enclosed inlet downstream of an existing catch basin draining to the city's sewer system. Stormwater easily 8
bypasses the inlet, especially when clogging occurs. The design could be improved by placing the bioretention inlet upstream of £
the catch basin, designing a wider and more open structure, or converting the catch basin into an inlet by adding a fiow splitter.
In Example 2, a street-side bioswale is isolated from the street runoff. The design could be improved by adding multiple curb cut
inlets to increase runoff capture efficiency.
Catch Basin
Inadequate
Inlet Size
Example t. Arrow shows an undersized, poorly functioning inlet
Example 2. Arrows show possible locations for new curb inlets,
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CHAPTER 6: RUNOFF CAPTURE
6.5 INLET SIZE
6.5 inlet Size
The following factors should be considered when designing the
size of the inlet opening, which is the key to ensuring water flows
into the practice.
Inlet size varies based on the type of bioretention facility. Inlet
size can allow some (i.e., an offline system) or all (i.e., an online
system) the stormwater to enter the facility. In offline systems,
fewer, smaller inlets would allow storm flows that exceed the
design storm to bypass the system. This excess runoff would
continue downgradient to another bioretention facility or flow
into a storm drain, in online systems, using wide, frequently
spaced inlets would allow all stormwater to enter the practice.
Include overflow structures to manage excess flows and add
pretreatment to slow flow and minimize erosion.
Inlet size varies based on drainage area. The flow rate, longitudinal
slope, and the number and frequency of inlets placed along
a bioretention facility affects the size needed. The following
resources are available to help with inlet sizing:
• The City and County of Denver (CCD) and the Urban
Drainage and Flood Control District (UDFCD) developed
the Ultra-Urban Green Infrastructure Guide—a resource for
determining inlet width when the upstream drainage area is
assumed to be 100% impervious (CCD and UDFCD 2016).
• The Philadelphia Water Department requires the use of the
Rational Method to determine inlet capacity (PWD 2018a).
Once the flow is known, the inlet width (specifically, for
type R inlets) can be identified using equation 3 in Hydraulic
Efficiency of Street Inlets Common to UDFCD Region (UDFCD
2011).
Small inlet openings convey runoff into a bioretention practice
within a traffic median in Atlanta, GA.
• The District of Columbia Department of Transportation
(DDOT) recommends using the method in the Federal
Highway Administration (FHWA) Hydraulic Engineering
Circular No. 22 to determine the size of the inlet opening
that achieves 100% interception for a 15-year storm (DDOT
2017; FHWA 2009).
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CHAPTER 6: RUNOFF CAPTURE
6.5 INLET SIZE
Avoid small inlets where water must turn 90 degrees to flow
into the bioretention facility. To prevent clogging with trash or
street debris, ensure inlets are wide enough to accommodate
the expected stormwater volume and debris. Curb opening
widths of 18-24 inches (and no smaller than 12 inches) have been
recommended to reduce the chance of sediment and debris
clogging an entry point. Moreover, designing curb openings
with angles and other modifications can facilitate inflow. Several
examples are discussed in the remaining pages of this chapter.
Oversized inlets can accommodate high volumes and may
exacerbate erosion. Oversized Inlets can also be hazardous to
pedestrians and drivers.
Pretreatment type and maintenance plans (e.g., the use of shovels
or vac trucks) can influence inlet size. Inlet size can be influenced
by pretreatment type and maintenance plans and capabilities
(e.g., the use of shovels or vac trucks). Chapter 7 provides details
on pretreatment and maintenance considerations.
A bioretention practice in Philadelphia, PA, with a wide curb opening and angled edges to facilitate runoff capture.
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CHAPTER 6: RUNOFF CAPTURE
6.5 INLET SIZE
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BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 6-24
A depressed curb cut in Seattle, WA.
An angled inward curb cut at Villanova University, PA.
An angled inward curb cut in Washington, DC.
An angled curb cut in Seattle, WA.
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CHAPTER 6: RUNOFF CAPTURE
6.6 INLET FLOW PATH MODIFICATIONS AND RETROFITS
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 6-25
6.6 inlet Flow Path
Modifications and Retrofits
Enhancing an inlet's design (e.g., incorporating gutter aprons
and saw cuts) or changing the road surface (e.g., adding berms
and speed bumps) can help guide stormwater runoff into inlets.
These modifications are useful where flows are potentially erosive
or inlets require runoff to make 90-degree turns. Additionally,
modifying an existing inlet (i.e., retrofitting it) may be needed
to Improve stormwater runoff capture. When considering the
following options, note the added cost they may incur. Additionally,
modifications and retrofits can be viewed as an adaptive
management approach that responds to changes or lessons learned
and helps maintain the functionality of a facility long-term.
Channels and Runnels
Channels and runnels are surface depressions designed to convey
concentrated flow to a bioretention facility or a drain. Channels
are used to collect and carry moderate-to-large flows. Runnels are
typically shallow systems designed for small spaces and small-to-
moderate flows.
Suitability
• Channels and runnels are beneficial because they quickly
and unobtrusively direct water where desired and are easily
combined with GSI. They are useful in commercial or mixed-
use areas with high traffic where runoff along paths of travel
(e.g., sidewalks) must be conveyed to a GSI facility.
• They are placed along the surface, so they are suitable in
areas where underground utility infrastructure prohibits
installing buried drainpipes.
Retrofitted inlet in Montgomery County, MD.
Retrofitted inlet in Montgomery County, MD.
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CHAPTER 6: RUNOFF CAPTURE
• Channels and runnels are not appropriate across designated
ADA pathways or emergency egresses. A covered trench
drain is necessary when traversing a pedestrian travel path.
• On low-volume streets such as alleys, runnels can be
combined with bioretention in the center of the roadway.
The road must be graded and crowned to direct runoff to
the roadway's center.
A runnel directs runoff to a bioretention practice in San Diego, CA.
6.6 INLET FLOW PATH MODIFICATIONS AND RETROFITS
Design Considerations
• Artistic or educational stormwater designs can incorporate
channels and runnels.
• Where pedestrian crossing or accessibility is needed, cover
channels or runnels with durable ADA-compliant linear
covers such as steel grates, trench drain grates, boardwalks,
or other walkable surfaces at least 4 feet wide, or fill them
with stone to reduce tripping hazards. Consider American
Iron and Steel requirements if projects use federal funding,
including state revolving funds.
• Channels and runnels are commonly constructed using
concrete or stone. Appropriate materials include pavers,
bricks, recycled cobblestone, river rock, or any other durable,
impermeable material. In highly urban areas, concrete-
mortared work well for durability.
• Channels and runnels installed using contrasting material to
the main path of travel enhance visibility for pedestrians.
• Maintain the bottom of the covered channel at or below the
grade of a pre-existing gutter pan to maintain drainage to
the storm drain inlet.
• Channels and runnels are typically 10-36 inches wide with
gentle slopes (0.5%-3%) to move water effectively toward
the discharge point.
• Runnels design typically incorporates a smooth, sloping cross-
section with maximum depths of 2-2.5 inches for safety
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CHAPTER 6: RUNOFF CAPTURE
6.6 INLET FLOW PATH MODIFICATIONS AND RETROFITS
Gutter Apron
A gutter apron, also known as an inlet apron, is a depressed gutter
section of concrete placed along the gutter line in front of curb
openings to increase inlet capacity. It helps to guide concentrated
flow towards the curb opening.
Suitability
• Gutter aprons are usually placed in front of inlets that are
closed along the top of the opening (e.g., trench drains,
wheel guard in place).
Design Considerations
• A depressed concrete apron can be cast in place or retrofitted
by grinding down the existing concrete pavement. Cast-in-
place gutter aprons provide an increased cross-slope and
tapered sides that slope toward the curb opening.
• Aprons typically drop 2 inches into the bioretention facility,
with another 2-inch drop behind the curb to maintain inflow
as debris collects.
• Stormwater capture requirements and design entrance
velocities typically govern a gutter apron's cross-slope. The
slope of aprons parallel to the curb is recommended to be a
maximum of 8% (City of Columbus DPU 2015).
• Limit the slope and extent of gutter aprons to prevent
hazards to pedestrians or bikers.
• To give bicyclists adequate clearance from the curb and any
pavement seams, protected bike lanes along concrete aprons
are typically implemented with a minimum widths of 6 feet.
Top: A schematic of a gutter apron. Bottom: A depressed curb
cut in Seattle, WA, showing how concrete aprons can be created
(retrofitted) by grinding down the existing concrete pavement.
• Aprons that create a drop of more than 8 inches along a
parking lane can cause a vehicle wheel to drop below the
curb, preventing the door from opening.
• For aprons leading into bioretention swales, the curb may
angle into the facility to improve the conveyance of flow.
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CHAPTER 6: RUNOFF CAPTURE
Flow Splitters
Flow splitters are devices used to direct the design WQV into a
GSI practice while splitting flows from larger events and routing
them around the practice into a bypass pipe or channel. The
bypass typically connects to another GSI practice, storm sewer,
or receiving water and will vary depending on the design and
management requirements.
Suitability
• Flow splitters can divide runoff volume and divert it to
different destinations to alleviate downstream flooding.
• Flow splitters can also be used to separate the first flush
volume, which contains the majority of the runoff pollutants.
Flow splitters allow the first flush to be sent to a facility
offering more intensive treatment or allowing treatment
over a longer duration without being diluted by additional
runoff (which can be diverted downstream or to another
GSI practice).
Design Considerations
• They can be constructed by installing bypass weirs in
stormwater control structures, such as manholes.
• Flow splitter design components include the elevation of
the bypass weir, the capacity of the pipe routing to the GSI
practice, and the capacity of the pipe bypassing flow that
discharges over the weir.
• The elevation of the bypass weir dictates the maximum
elevation of the water in the GSI practice. The bypass
elevation typically equals the design storage elevation in
the practice. The flow will begin to bypass the facility once
6.6 INLET FLOW PATH MODIFICATIONS AND RETROFITS
it exceeds the design storage elevation of the practice. The
design storage elevation is the water surface elevation at
which the facility storage area contains the runoff volume
from a design storm event (for example, the WQV or the first
1.5 inches of precipitation).
• Flow splitters are sized to provide enough capacity to
transmit larger flows over the bypass weir without
surcharging (i.e., overflowing) the top of the flow-splitter
control structure.
• When designing flow splitters, construction materials that
are corrosion resistant are best (e.g., reinforced concrete,
galvanized steel, brick and epoxy mortar).
A schematic of a flow splitter for a stormwater management
practice (SMP).
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CHAPTER 6: RUNOFF CAPTURE
6.6 INLET FLOW PATH MODIFICATIONS AND RETROFITS
Berms
An optional 'I- to 2-inch high asphalt or concrete berm placed on
the downstream side of a curb opening can help direct runoff into
a bioretention facility. Berms are particularly useful in areas with
steep slopes.
A berm on the downstream side of a curb opening directs runoff.
• Flow splitters have the potential to cause flow reversal
under certain circumstances (e.g., due to lack of a backflow
preventer or one-way valve) in which water will flow from a
facility back through the flow splitter
Saw Cuts
Saw cuts are ridges cut into the surface next to the inlet and
are often used on offline stormwater retrofits to direct water
into bioretention facilities. While effective, saw cuts may not be
appropriate in colder climates because the uneven surface can
catch plows during snow removal.
Saw cuts help direct water flow into an inlet.
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CHAPTER 6: RUNOFF CAPTURE
Speed Bumps
Speed bumps are a simple and inexpensive retrofit strategy
to convey water to bioretention, They can be installed as a
"backstop" near curb cut entries to direct water into practices.
They can also be installed near the beginning of a facility to
increase treatment time. In the example pictured below, without
the speed bump in place, runoff would enter the bioswale much
lower within the system, bypassing some of the area available
for treatment. A 2-inch speed bump is typically adequate for
directing stormwater flow, and it can be set on a diagonal to
further facilitate stormwater capture.
A speed bump intercepts and redirects runoff.
6.7 INLET PROTECTION
6.7 Inlet Protection
Inlet protection includes measures along concrete curb cuts and
gutters that protect at-grade inlets from damage. This section
reviews wheel guards, grates, and winged curb cuts.
Wheel Guard
Wheel guards, or wheel stops, consist of a steel plate or bar
extending along the top of a curb opening that allows water to
pass through while protecting the entrance—they help to prevent
car wheels from unintentionally entering curb openings.
Suitability
• Wheel guard plates protect at-grade inlets from damage
caused by vehicles; thus, they are useful on high-traffic
streets or in areas where parking is common along the curb.
• Wheel guard installation occurs on the curb cuts on the
street side of a bioretention facility.
Design Considerations
• The Philadelphia Water Department recommends using
steel-plate wheel guards but notes that other materials
(e.g., strong composite plastics), patterns, and colors can add
aesthetic interest (PWD 2014).
• Wheels guards are prone to damage as shown; thus, entrance
protection that can withstand the expected loading from
vehicles or pedestrians will be more durable (i.e., a tensile
strength greater than 35,000 pounds per square inch).
• Consider American Iron and Steel requirements if projects
use federal funding (e.g., state revolving funds).
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CHAPTER 6: RUNOFF CAPTURE
6.7 INLET PROTECTION
A damaged wheel guard over a curb cut in Washington, DC.
Wheel guards protect openings from vehicles on this stormwater
curb extension in Montgomery County, MD.
Wheel guards are installed over inlets that are parallel to traffic,
such as on this median bioswale in Prince George's County, MD.
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CHAPTER 6: RUNOFF CAPTURE
Grates
Grates are often applied over inlets to remove floatables.
Although they can be effective, they must be maintained, or
they will clog. The Philadelphia Water Department generally
recommends that grated inlets use a clogging reduction factor
of 0.5 (assuming only half of the opening is available for the
conveyance of stormwater to the practice) (PWD 2018). This factor
must be applied to the unclogged inlet capacity of the inlet and
the resulting clogged interception capacity compared to the
design intensity flow rate.
Winged Curb Cut
A curb cut with wings helps retain the side-slope grade on each
side of the opening while directing concentrated runoff into the
bioretention facility. Winged curb cuts are particularly good for
routing runoff into bioretention without eroding the sides. Curb-
cut wing walls also allow soil to be pulled up to the top of the
curb. They work well with relatively shallow bioretention facilities
that do not have steep side-slope conditions. Designs vary, but
openings are usually at least 18 inches wide.
6.7 INLET PROTECTION
Debris clogs a grate leading into a bioretention facility.
A winged curb cut in San Mateo County, CA.
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Chapter 7
PRETREATMENT
In this chapter
7.1 Importance of Pretreatment
7.2 Pretreatment for Sheet Flow
7.3 Pretreatment for Inlet
and Concentrated Flow
7.4 In-Practice Erosion Control
Inlets opening into a bioretention facility are often
coupled with pretreatment to capture solids and dissipate
the energy of the incoming flow. This chapter presents
pretreatment options for both sheet flow and concentrated
flow. Various forebay pretreatment design options are
discussed in more detail.
Photo: Rhea Thompson
7-1
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CHAPTER 7: PRETREATMENT
7.1 IMPORTANCE OF PRETREATMENT
7.1 importance of Pretreatment
Pretreatment consists of an aboveground area or a belowground
structure designed to capture solids from runoff and dissipate
flow velocities before the water contacts the BSM and
vegetation. As a result, pretreatment is not a standalone
practice but an upstream design component; the pretreatment
type will differ depending on whether the facility is receiving
sheet flow or concentrated flow. Pretreatment provides the
following benefits when included in bioretention facilities.
Reducing sediment. Pretreatment can work together with in-
practice erosion control measures, such as check dams and
weirs, to reduce flow velocities, collect water, and promote
sedimentation. Settling sediment-bound pollutants, such
as metals and phosphorus, in pretreatment can increase the
treatment lifespan of BSM and reduce the pollution entering
municipal stormwater and natural drainage systems.
Prolonging the service life of BSM. Pretreatment plays a critical
role in removing substances from runoff (leaves, coarse sediment,
trash, etc.) that can cause bioretention clogging, which shortens
the lifespan of BSM and can result in costly repairs.
Protecting vegetation. Pretreatment reduces inflow velocities and
protects vegetation from erosive flows.
Localizing maintenance efforts Pretreatment concentrates
sediment and debris at the upstream end of the facility, where it
can be easily accessed and removed.
Inadequate pretreatment allowed sediment and debris to
accumulate on the surface of a facility in Baltimore, MD.
Accumulated sediment and trash needs to be removed from this
forebay to ensure proper function.
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7-2
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CHAPTER 7: PRETREATMENT
7.2 PRETREATMENT FOR SHEET FLOW
When is Pretreatment Necessary?
The pretreatment type used is influenced by factors such as
the CDA, land use, slopes, and soils. Some bioretention may not
require pretreatment, such as facilities that drain small stabilized
tributary drainage areas. Alternatively, an urban site with a large
LR would benefit from adding pretreatment practices to minimize
clogging and focus maintenance efforts. Other examples may
include areas with leaf drop or high traffic or truck volumes
(NACTO 2017). Areas with CDAs of more than 0.5 acre and steep
slopes also need pretreatment to mitigate erosion by inlet and
sheet flows.
Check local requirements and guidelines. Some cities or
municipalities may provide specific guidance for pretreatment.
For example, the Minnesota Stormwater Manual recommends that
flow entering a pretreatment vegetated filter strip (VFS) should
not exceed 1 foot per second (MPCA 2013).
The pretreatment design types described in the following sections
are categorized into pretreatment for sheet flow and pretreatment
for inlets or concentrated flow. Using multiple pretreatment
elements together can yield multiple benefits, such as energy
dissipation (i.e., splash pad) and sedimentation (i.e., forebay).
7.2 Pretreatment for Sheet Flow
Sheet flow pretreatment is designed to manage incoming flows
that enter the practice as a diffuse layer. Thus, it is important
to avoid concentrated flow, which can cause channelization or
erosion in the pretreatment device. Designing for sheet flow
pretreatment emphasizes sedimentation, energy dissipation, and
the redistribution of concentrated flow, if needed.
Vegetated Filter Strip
VFS, also called grass filter strips or buffer strips, are often
used to pretreat sheet flow entering a bioretention facility.
They are suitable alongside roadways or parking lots where
the contributing area is uniform and the flow path is less than
150 feet. A VFS is a common pretreatment application for sheet
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A VFS in a curbless bioretention facility treats incoming sheet flow
in Kansas City, MO.
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CHAPTER 7: PRETREATMENT 7.2 PRETREATMENT FOR SHEET FLOW
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 7-4
flow conditions. Adding a gravel diaphragm or level spreader-
described below—at the upstream end of a VFS can improve
pretreatment effectiveness. Conducting street sweeping also
helps reduce sediment loads entering the VFS in sheet flow.
Design and Maintenance Considerations
The length of the VFS needed from the edge of the drainage
area to the bioretention practice is a function of the VFS slope,
the vegetation type, and the drainage area's soil type. The CDA's
vegetation and soil type also determines the needed VFS slope;
typical values are 4%-8% slope. The New Jersey Stormwater
Best Management Practices Manual (see Chapter 9.10; NJDEP
2021) provides lookup curves for VFS slopes, VFS lengths, and
the maximum slopes for different vegetation types (turf grass,
meadow cover, or forest).
Design a VFS to fall at least 2 inches below the contributing
impervious surface. If, over time, the grade of the VFS rises above
the adjacent impervious catchment, regrade the VFS to restore
proper drainage.
Debris buildup and plants growing along the edge can cause
performance issues. For example, small incongruities or pieces of
debris can alter flow paths and result in flow diversions. Perform
visual inspections every few months to inform maintenance
needs. Maintenance crews should clear the accumulated sediment
and trash from the VFS's edges at the same time they remove
debris from the bioretention practice—approximately twice
per year. Maintenance staff should also check for erosion in the
VFS. If erosion is visible, it should be repaired with topsoil and
revegetated.
Curbless bioretention with a VFS that treats sheet flow in Fort
Lauderdale, FL.
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CHAPTER 7: PRETREATMENT
7.2 PRETREATMENT FOR SHEET FLOW
VFS may not be practical for retrofit projects and urban sites
constrained by small footprints.
Gravel Diaphragms
Gravel diaphragms consist of a gravel trench (1 foot deep by 2
feet wide) positioned between the CDA and the VFS. The gravel
diaphragm's primary purpose is to remove sediment, maintain
sheet flow, and reduce erosion potential. It's important to note
that this device is not a conveyance practice but rather a measure
to distribute flow evenly before it enters the VFS.
Level Spreader
Level spreaders are an engineered practice that discharge sheet
flow evenly into a bioretention facility. It can be used to convert
concentrated flow into sheet flow; for example, when added
just downgradient of a curb cut. Typically, the design includes
a rigid material, such as concrete, wood, or metal, where runoff
collects in a trench on the upstream side and spills over the lip of
the level spreader as diffuse flow. The level spreader is situated
between the CDA or conveyance pipe and the GSI practice. Gravel
can be placed immediately downgradient of the level spreader to
provide a transition to the VFS or bioretention practice.
Curbless bioretention with a level spreader and VFS in Omaha, NE.
Curbless bioretention with a level spreader in Portland, OR.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
7.3 Pretreatment for
Inlets and Concentrated Flow
Pretreatment is generally a necessary element for practices
with concentrated flow and inlets. A pretreatment practice on
the ground surface is sometimes referred to as a presettling
zone, which consists of a designated area for collecting debris,
dissipating energy, and preventing erosion of BSM directly
downgradient of the inlet. The following design and maintenance
considerations apply to all pretreatment options selected for
concentrated flow through inlets.
ideally, install pretreatment at the primary inlet of a single
bioretention facility or at the inlet of the first facility in a series.
This design focuses the maintenance efforts in one location and
dissipates energy in the first cell before the flow moves to other
cells in a series.
Consider the land use when selecting the pretreatment type for
inlets. Runoff from busy urban streets can contribute high loads
of sediment and debris, which necessitates installing larger
presettling zones at inflow points. Also, design the pretreatment
areas to withstand the flow velocities expected from the design
storm.
Ensure regular maintenance of the inlet pretreatment area.
Minimizing the buildup of sediment or debris is important for
maintaining system performance. The pretreatment type selected
will influence the maintenance tasks and equipment needed, so
consider a town or city's operations and available maintenance
equipment during the design phase. For example, city agencies
using vacuum trucks might prefer forebays with concrete pads
rather than rocks or cobbles.
Stone presettling zones collect debris and dissipate the energy of
runoff entering a bioretention facility in Olney, MD.
Table 7-1 provides photos of and describes the various methods
available to pretreat concentrated flow, along with method-
specific design and maintenance considerations. Forebays and
splash pads—two of the most common pretreatment methods-
are presented in more detail after the table.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Table 7-1. Pretreatment options for concentrated flow.
Pretreatment type
Description
An aboveground pretreatment area
separated from the BSM by a berm or
gabion. It is the first compartment of a
bioretention practice; often used when
a stormwater pipe or swale discharges
directly into bioretention.
Note: More detailed information about
forebays is provided on pages 7-10 to 7-20.
Design and maintenance considerations
Surface material options include vegetation, stone,
pavers, or concrete pads next to the inlet.
Helps to maximize sedimentation and localize
maintenance efforts.
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Splash pad or splash block
Dissipates energy to prevent erosion and
channelization.
Often used directly below conveyance pipes
connecting into a bioretention facility or at
the bottom of downspouts.
Note: More detailed information about splash
pads and blocks is provided on page 7-21.
Typically made of concrete; can be designed with
embedded stones, cobbles, rocks, or bricks.
The roughness of surface materials slows the
stormwater, reducing erosive potential.
* \ /:
Catch basin
(sump or chamber of inlet sump)
An underground chamber or sump
connected to conveyance piping. Debris and
sediment collect in a sump; oil and grease
float on the surface.
After pretreatment, water drains into the
facility via a piped discharge or an opening
in the catch basin's walls.
Note: See Chapter 6 for more details about
inlet sumps.
Suitable for ultra-urban settings where
aboveground space is limited.
Can be paired with a splash pad for energy
dissipation (as pictured).
Routine maintenance prevents foul odors.
Size will vary depending on the land use (e.g.,
residential versus commercial).
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Table 7-1. Pretreatment options for concentrated flow.
Pretreatment type
Leaf screen or inlet filter guard
Description
• Consists of a grate over the inlet opening.
• Useful in residential settings with high leaf
drop.
Design and maintenance considerations
Can prevent larger debris (plastic bottles, leaves,
trash) from entering the practice.
Can be used in conjunction with forebays and/or
splash pads.
Placed across curb cuts or between the
presettling zone and the BSM.
Suitable in areas where trash and large
debris is expected.
Can protect outlet structures.
Screen's small footprint allows trash to collect in
one location for easy removal.
Trash rack/screen
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Table 7-1. Pretreatment options for concentrated flow.
Pretreatment type
Description
Design and maintenance considerations
• Underground flow through pretreatment
chamber that can be installed online or
offline.
• Capable of reducing sediment and
floatables (e.g., oil and grease).
• Primarily proprietary structures.
• Several types are available using varied separation
methods (swirl/vault systems).
• Requires routine maintenance. Sediment is
removed using a vacuum truck.
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Vortex chambers
or hydrodynamic separators
• Typically above ground with a check dam or
weir at the downstream end.
• Place next to the inlet pipe or curb cut to dissipate
concentrated flow.
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• Larger than a forebay; designed to detain
15% of the design volume (VA DEQ 2011).
• Not often used in small-scale or residential
applications.
• No need for underlying engineered BSM, unlike
the main bioretention practice. If the bioretention
storage volume includes the pretreatment cell's
volume, the cell must dewater between storm
events to avoid permanent ponded volume.
Pretreatment cell
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Forebays for Pretreatment
Forebay pretreatment is the most diverse and common type
of pretreatment device. The following section describes the
suitability and design considerations common across all forebay
design variations and highlights examples.
Suitability
• Forebays can be used in most curb cut settings or when
a stormwater pipe or swale discharges directly into the
bioretention facility.
• Forebays are acceptable in ultra-urban environments when
space allows and there are no strict aesthetic requirements.
• Forebays can enhance suspended solids removal within the
bioretention facility when total suspended solids removal is
a priority
• In colder climates, forebays can offer dual functionality and
be used to store plowed snow.
Design and Maintenance Considerations
Typically, a forebay is sized to capture and temporarily detain a
portion of the water quality volume to satisfy sedimentation.
For example, the Georgia Department of Transportation sizes its
forebays to hold 0.1 inch of runoff per impervious area managed
(GDOT, n.d.). Local guidance documents should be consulted for
specific sizing requirements.
Stone forebay along Elmer Avenue in Los Angeles, CA.
An effective design technique includes an energy dissipation zone
followed by a sedimentation zone. The energy dissipation zone
is a deeper pool (2-18 inches) that transitions to a shallow zone
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
separated from the BSM by an earthen berm, gabion, or concrete
structure. The berm structure at the edge of the sedimentation
zone allows water to spill over into the main bioretention area.
Be mindful of the forebay's depth near pedestrian or public
access areas.
Select the material for the energy dissipation zone so it
withstands incoming flow velocities and resists erosion or
scouring. Where cobblestones are the desired material, using a
mortar treatment secures the cobblestones in place and reduces
the need to replace cobbles during maintenance. The choice
of forebay material may also be influenced by the setting. For
example, stone or gravel forebays might not be the best choice
for schools and playgrounds, as children can climb or pick up
stones. Concrete offers a good alternative for these types of site
conditions.
A routine maintenance schedule helps maintain the functionality
of the pretreatment practice, prevents the accumulation of too
much sediment, and minimizes the resuspension of sediment.
Forebay stones near playground were moved by children.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Forebay Types
Forebay pretreatment design options are diverse. This
handbook discusses 10 main forebay types, including stone,
gravel, limestone, concrete, curb well, shallow sump, flagstone,
utility box, vegetated bag, and inlet sump forebays. A photo
comparison of forebay types is presented on this page; the
following pages provide a detailed description of each type.
Stone Forebav (Steve Epting, EPA): Gravel Forebav (Rhea Thompson)
Limestone Forebav (City of Omaha) Concrete Forebav (Kansas City Water)
Flagstone Forebav (Citv of Omaha) Utility Box Forebav fCitv of Omaha)
Curb Well Forebav (City of Omaha)
Vegetated Baa Forebav
(City of Omaha)
Shallow Sump Forebav (MSC WMO)
Inlet Sumo Forebav (Rhea Thompson)
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Stone Forebay
Stone-fiiled forebays, also referred to as stone spreaders,
rock rundowns, or rock aprons, are often found at the end of
conveyance pipes or other concentrated inflow points. Orienting
them perpendicular to the flow path promotes settling, with
an allowable ponding depth of 2-4 inches from the pavement
or other hard-edged surface to the top of the stone. Select the
stone size according to the expected rate of discharge. A typical
design for the bottom of the presettling area includes large rocks
(streambed or round cobbles, 2-4 inches in diameter) with a
porous berm or weir that ponds the water to a maximum depth
of 12 inches. When stone forebays are the same width as the
curb cut or greater, place the stone 1-2 inches below the curb
opening to prevent soil erosion. Stone forebays are not an optimal
pretreatment for facilities expected to receive high sediment
loads. The cracks and crevices in stone forebays clog easily and
are more cumbersome to maintain.
A stone forebay dissipates the energy of incoming flow.
A sediment-clogged stone forebay.
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CHAPTER 7: PRETREATMENT
Gravel Forebay
Gravel forebays are similar to a gravel diaphragm or gravel
flow spreader. Gravel forebays are usually designed as a small
shallow-graded, non-planted area with stone that can be placed
at curb cuts, downspouts, or other concentrated inflow points.
For concentrated flow applications, attention should be given
to stone size to avoid washout at high inflow rates. The gravel
should extend the entire width of the opening and create a level
surface to distribute flow.
A gravel forebay receives runoff flow and distributes it evenly
across this bioretention facility in New York City, NY.
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Limestone Forebay
The City of Omaha uses limestone slabs positioned at the base
and slabs oriented vertically, with the sides higher than the inlet,
to create a forebay sump and weir. Open-graded stone is installed
below the base for drainage. A critical component of this design
is the free-draining rock under the forebay that requires regular
maintenance. Limestone is readily available in some areas and is
durable, which can be an advantage. If implementing this design
in locations with pedestrians, be mindful of elevation changes.
A limestone forebay captured significant amounts of sediment
(see inset for typical depth of a newly installed forebay) and needs
maintenance in Omaha, NE.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Concrete Forebay
Concrete forebays are common because they are easy to vacuum.
The example pictured below shows an older forebay in Kansas
City, MO, which was retrofitted because it was not draining well.
For 56 sites where this was implemented in Kansas City, 27 of the
concrete forebays had documented drainage problems. The initial
design included weep holes that clogged, causing ponded water
that contributed to a mosquito problem. The retrofit design raised
the pretreatment base elevation to facilitate drainage and mitigate
the ponded water. Paver joints and gravel were also added to the
downstream end to allow infiltration of ponded water.
A poorly draining concrete forebay (left) was retrofitted in Kansas City, MO (right),
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 7-16
Concrete forebay in New York City, NY.
Concrete forebay in New York City, NY.
Concrete forebay in New York City, NY.
Concrete forebay in New York City, NY.
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CHAPTER 7: PRETREATMENT
Curb We!! Forebay
A curb well forebay resembles a window well design with a
custom stainless steel settling area that collects runoff. Curb wells
are generally designed so that the bottom of the presettling area
is a volume of large rock (2-4 inch streambed or round cobbles)
or a concrete pad with a porous berm or weir that ponds the
water to a maximum depth of 12 inches. The original design
used by the City of Omaha used a porous concrete bottom with
clean open graded stone, and perforated pipe connected to an
underdrain. This type of forebay is typically 2 feet long by 4
feet wide and 8-10 inches deep. Curb well forebay materials are
readily accessible, and installation is quick and straightforward.
Regular maintenance will typically suffice to ensure long-term
performance. To improve runoff dissipation, small baffles can be
incorporated on the sides of the forebay.
Shallow Sump Forebay
Shallow sumps are a low-cost forebay alternative used in
Minneapolis, MN. The design consists of pavers on the ground
surface and bricks to create a shallow pan as shown in the picture.
The pan creates a zone for sediment collection, and they are
relatively easy to maintain (crews can manage a large number of
system clean-outs in a day) with a flat shovel and a broom. The
limited capacity requires frequent clean outs for systems with
larger CDAs and to maintain aesthetics. Shallow sump forebays
are less durable and may require replacement in the spring, but
the costs are low and the materials are readily available.
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Facility with a retrofitted shallow sump in Minneapolis, MN.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
A flagstone forebay in Omaha, NE.
A vegetated bag forebay in Omaha, NE.
A utility box forebay in Omaha, NE.
Flagstone Forebay
Flagstone forebays are a design used by
the City of Omaha. They are designed
with high sides and an engineered v-notch
weir to pond and drain water slowly
Alternatively, a retaining wall block can
be used to create a shallow 2-inch sump.
Materials for the design can be expensive
and are susceptible to deterioration due to
salt and grime and can be easily broken.
In areas with pedestrians, be mindful of
ponded water depth to avoid safety issues.
Vegetated Bag Forebay
Vegetated bags with planting media can
be used as a simple, low-cost, and green
pretreatment method. In the example
pictured, vegetated bags were placed at
the back (three bags) and downgradient
side (two bags) of the forebay. The
forebay was then planted with lily turf to
promote water filtration. Vegetated bags
can break down quickly and as a result
are not as durable as other pretreatment
methods and will require more frequent
maintenance/replacement.
Utility Box Forebay
Off-the-shelf utility boxes are a low-
cost, accessible, and durable means of
pretreatment in Omaha, NE. They are
usually designed with no bottom, over
a clean stone, and can be connected to
an underdrain, A screen can be placed
on top if desired. They pair well with
large curb cuts (as pictured). Ensure that
plastic applications can withstand the
sun's ultraviolet radiation; inspect them
periodically to assess degradation and the
need for replacement.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Inlet Sump Forebay
Inlet sumps were described in Chapter 6 as an inlet type. Note
that these structures serve a dual functionality as an inlet
flow (i.e., the grate or screen that serves as an inlet) and a
pretreatment device (i.e., the underground chamber or sump).
Proprietary pretreatment systems such as inlet sumps or catch
basins are becoming increasingly common. Although these
systems have high upfront costs, they tend to be cost effective
long-term due to more efficient maintenance. Maintenance
simply requires removing the top grate, scooping out sediment
and debris, and scrubbing any fines from the filter.
Example of proprietary inlet sump forebays with demonstrated capability of reducing sediment and hydrocarbons to provide pretreatment.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Inlet sump forebay paired with a rock forebay in Minneapolis, MN.
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CHAPTER 7: PRETREATMENT
7.3 PRETREATMENT FOR INLETS AND CONCENTRATED FLOW
Splash Pads for Pretreatment
A splash pad or splash block is an energy dissipation method to
reduce inflow velocities. The splash pad is a rigid material able to
withstand inflows at the inlet point (i.e., concrete, larger stones,
or pavers). Energy dissipation is helpful for instances when
concentrated stormwater flows entering a GSI facility might
cause erosion of planting media within the facility. Concrete can
be considered for situations where the entrance velocity exceeds
6 feet per second.
Design and Maintenance Considerations
• Splash pads are typically concrete, but they can also be
designed with stones, cobbles, rocks, or bricks embedded in
concrete at the entrance of the GSI practice. The roughness
of the surface material reduces the stormwater velocity.
Material for a splash pad can be installed over an aggregate
bedding with a media liner or may be embedded in concrete
• Widths and lengths of energy dissipation will vary based
on the type and size of inlet used and the velocity of
stormwater entering the bioretention practice. At a
minimum, it is recommended the energy dissipation method
extend the full width of the concentrated flow path.
• Refer to local construction and material specification
guidelines for concrete mix, placement, and testing.
• Typical design elements include the following:
- A minimum separation of 3 inches of freeboard between
the top of the energy dissipation material and the
inlet grade elevation allows for sediment accumulation
between maintenance events.
Concrete splash pad in San Mateo, CA
A concrete pad in a stormwater curb extension system in
Portland, OR.
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CHAPTER 7: PRETREATMENT
A concrete splash pad is paired with a stone forebay in
Kansas City, MO.
• Splash pads that are not embedded in concrete can be
loose surface stone, such as local washed gravel or river
rock, which is well-graded with stone sizes of 1-4 inches.
Additionally, stones that are not embedded in concrete can
be surrounded with a permanent edging (e.g., concrete,
anchored angle irons) to prevent the materials from
migrating into the planting area of the facility.
7.4 IN-PRACTICE EROSION CONTROL
7.4 In-Practice Erosion Control
In some situations, forebays and other pretreatment devices
may not adequately prevent the erosion and scouring of surface
layers containing soil, mulch, vegetation, or other materials.
In such cases, adding in-practice erosion control devices, such
as check dams (discussed below), can help to slow stormwater
velocity and promote the settling of coarser materials. In-practice
erosion control devices can encourage water to pond to promote
infiltration or detention. Other available in-practice erosion
control practices that will not be featured in this document
include berms, elevated terraces, rock rundowns, and rip rap.
Check Dams
A check dam is a small structure constructed across a GSI practice
that helps to reduce velocities, pond water, and maximize
sedimentation and infiltration. By ponding water in a segment
of the facility, water maintains contact with more surface area
and experiences longer residence times through the system,
maximizing treatment efficiency.
Suitability
• Bioretention may be built in steeper/sloped areas with check
dams that create ponded areas and step-down points to
adjust to street slopes. These designs effectively serve as
terraced infiltration systems.
• On streets with longitudinal slope (more than 5%), consider
installing check dams at intervals to help slow the water
flow to avoid wash-out from occurring and allow runoff
ponding and infiltration throughout the entire cell area
rather than flowing directly to the downstream end.
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CHAPTER 7: PRETREATMENT
7.4 IN-PRACTICE EROSION CONTROL
• Check dams can be applied to sites with minimal longitudinal
slopes to promote infiltration where soils are suitable.
Underdrains can be used in areas with poorly draining soils.
• Terraced or check dam designs can be used in systems where
more filtering and settling of nutrients, sediment, and other
pollutants is desired.
• Check dams can be a suitable to incorporate when slopes
range between 3% and 10% (MPCA 2023b).
Check dams in a bioretention facility in Paso Robles, CA. These
types of in-practice erosion control measures create terraces on the
steep grades of sloped streets, prevent erosive flows from entering
and damaging bioretention practices, and help control the desired
ponding depth. This example also illustrates how check dams can
be designed in an artistic manner.
Check dam used for energy dissipation in a bioretention planter box
in Kansas City, MO.
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CHAPTER 7: PRETREATMENT
7.4 IN-PRACTICE EROSION CONTROL
Design and Maintenance Considerations
• Check dams are often concrete walls with v-notch weirs
spanning the width of the cell perpendicular to the flow.
Weirs can be designed with adjustable heights to provide
flexibility on sites with variable soil conditions.
• Check dam material options include wood, metal, waterproof
membrane, polyvinyl chloride (PVC) sheeting, acrylic
sheeting, and permeable materials (e.g., rocks, stone, soil
berms). Membranes and sheeting are the most cost-effective
and generally preferred options. Using more durable
materials is advised on steeper slopes.
• Place check dams in sloped facilities at intervals to maintain
ponding and facility depth within allowable limits. Space
the check dams based on channel slope and ponding
requirements.
• Check dams can create a series of small, temporary pools
along the length of the facility, and they should drain
down effectively within 24-48 hours (depending on local
requirements).
• Bioretention cells on steep slopes with check dams that
are not stabilized in the BSM or sidewalls are likely to be
less effective than a bioretention surface without check
dams, unless: (1) the check dam extends deep into the soil
profile, forcing the "upstream" media to saturate or (2) the
bioretention practice is "over excavated" on the upstream
end of the cell.
d IjO
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Rock check dam in a bioretention facility in
Portland, OR.
Check dam in a bioretention facility in
Atlanta, GA.
Rock check dams on a steep facility in
Kansas City, MO.
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Chapter 8
BIORETENTION
MEDIA
In this chapter
8.1 Bioretention Soil Media Function and
Composition
8.2 Assessment and Testing of Existing Soils
8.3 Media Design Considerations for
Hydrologic Performance
8.4 Media Design Considerations
for Pollutant Removal
8.5 Liners
8.6 Aggregate Media
Bioretention media selection and design influences
infiltration rates, water quality, and plant health. Chapter
8 presents methods for assessing and testing existing soil
media and notes considerations for achieving hydrologic
and water quality performance goals. Media amendments,
such as biochar and wastewater treatment residuals, are also
introduced.
Photo: Adrienne Donaghue
8-1
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CHAPTER 8: BIORETENTION MEDIA
8.1 BIORETENTION SOIL MEDIA FUNCTION AND COMPOSITION
8.1 Bioretention Soil Media
Function and Composition
BSM typically includes a soil media layer for filtration and plant
growth and sometimes an aggregate layer for more water
storage or underdrain placement. Additionally, design elements
may include liners to restrict water flow (impermeable liner)
or create a barrier that separates BSM layers while allowing
water to flow between them (permeable liner). When runoff
flows into a bioretention facility, it ponds on the surface and
then infiltrates through the BSM. Captured runoff leaves the
bioretention facility through exfiltration (i.e., slow drainage) into
the underlying subsurface or by slow release via the system's
underdrain or outlet. Additionally, evapotranspiration (the sum of
evaporation and transpiration) moves captured runoff from the
soil and surface to the air. The intended hydrologic function of the
bioretention facility, such as storage or peak discharge mitigation,
will influence design choices regarding BSM, liners, and the
presence or absence of an underdrain. Hydrologic performance
goals are discussed in Chapter 10.
BSM is a blend of sand, silt, and clay mixed with organic material
to promote infiltration and influence the physical, biological,
and chemical processes affecting pollutant fate and removal (see
the bioretention schematic). For example, solids are generally
removed through the physical processes of sedimentation and
filtration, whereas nitrate is removed via plant uptake or the
microbial pathway of denitrification. In addition to providing
volume retention and pollutant removal, the chosen BSM should
have the chemical and physical properties necessary for soil
and plant health. Although BSM can be derived from the site's
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Bioretention facility schematic showing media layers, hydrologic
processes, and pollutant-removal mechanisms.
existing soils, designers might need to include amendments or
use engineered soils to ensure adequate infiltration rates and
optimize treatment—particularly for dissolved pollutants. This
chapter highlights considerations for BSM selection and design
Geotextile
fabric
Exfiltration
Biological
Plant uptake and
microbial processes
impact nitrogen
and phosphorus
removal
Physical
Sedimentation
and infiltration
Chemical
Functional groups, surface charge,
and cation/ion exchange capacity
impact contaminant sorption,
precipitation, and ion exchange
Evapotranspiration
pore space
BSM particle
Infiltration
Choker
layer
Impermeable
liner
vA-w.
Aggregate
layer
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CHAPTER 8: BIORETENTION MEDIA
8.2 Assessment and
Testing of Existing Soils
Because soil characteristics vary between sites, evaluating the
existing soil in the proposed location is important for determining
the feasibility of bioretention. The following steps are typically
used when testing and assessing soils; however, consult your local
guidelines for the specific required testing protocols in your area.
1. Use publicly available soil survey data. Consult resources
such as the Natural Resources Conservation Service (NRCS)
Soil Survey Geographic Database (also known as the SSURGO
Database) to assess potential conditions during the planning
phase (NRCS, n.d). The NRCS Web Soil Survey, an online
mapping application, provides soil surveys with digital spatial
data (see the Soil Availability Map for data in your area) (NRCS
2019). You may also download the Web Soil Survey data from
the NRCS Geosoatial Data Gateway (NRCS 2023). Your local
NRCS or soil and water conservation district office or local
library might also have soil survey information available.
2. Analyze the existing soil composition. Many municipalities
specify required ranges for soil media composition. For
example, in its Urban Storm Drainage Criteria Manual: Volume
3. Best Management Practices, the Urban Drainage and Flood
Control District in Denver, Colorado, specifies a blend of
80%-90% sand, 396-1796 silt, and 396-1796 clay (UDFCD 2010).
Analyzing existing soil composition can reveal if changes are
needed to satisfy media blend requirements. When possible,
use existing soils to help reduce earth-moving costs.
3. Confirm the site's existing infiltration potential. Typically,
soil textures that facilitate infiltration include sand, loamy
sand, sandy loam, and silty sands. Minnesota's stormwater
8.2 ASSESSMENT AND TESTING OF EXISTING SOILS
»«. • - , " _>
Bioretention facility under construction in Atlanta, GA.
manual recommends using infiltration-based practices for
hydrologic soil groups A (0.8-1.6 inches/hour) and B (0.3-
0.45 inches/hour) (MPCA 2023a). PWD (2014) recommends
infiltration-based practices when infiltration rates are equal to
or greater than 0.25 inches/hour. When soils have infiltration
rates of less than 0.25 inches/hour, PWD (2014) recommends
using temporary storage and slow water release, typically via
an underdrain. Regardless of the facility's intended function,
the drawdown times of ponded water should follow local
requirements (typically 24-48 hours).
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CHAPTER 8: BIORETENTION MEDIA
8.2 ASSESSMENT AND TESTING OF EXISTING SOILS
"In North Carolina, construction can reduce infiltration
rates by as much as a factor of 10. Thus, a pre-construction
infiltration rate of 2 inches per hour is required to forego
the use of underdrains. Infiltration rates of 0.25-0.5
inches per hour may be good as long as that is the rate
post-construction."
— William Hunt, North Carolina State University
4. Perform a test boring to determine the depth to
groundwater or the presence of restrictive layers.
Test borings provide valuable information on subsurface
conditions such as restrictive layers (e.g., bedrock) and
changes in infiltration conditions (e.g., presence of clay).
Additionally, test borings also indicate the water table depth.
As previously noted, the bottom of the bioretention facility
should maintain a separation of at least 2 feet from the
seasonally high groundwater table (USEPA 2021b).
5. Verify if soil contamination is a concern. The areas
around the potential location might show visual evidence
of possible soil contamination, such as stressed vegetation.
Checking historical records might uncover previous
activities on and around the site that warrant soil testing.
Alternatively, inspecting soil cores from the test boring
could reveal an odor or sheen that suggests contamination.
If contaminated soil is present, the designer can change the
practice goal from infiltration to detention and release (by
adding an impermeable liner) or relocate the practice to
prevent contaminants from moving into surrounding soils or
groundwater.
6. Amend soils to target specific pollutants, increase water
retention capacity, or improve soil fertility. Organic
matter soil amendments, such as biochar, can be added
to BSM to increase porosity, improve water retention, and
enhance pollutant removal. Additionally, amending soils with
organic material promotes plant growth and microbiological
processes. Tables 8-1 and 8-2 provide more information on
biochar and other media amendments.
Installing BSM amended with biochar, zeolite, and
coconut coir in Denver, CO.
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CHAPTER 8: BIORETENTION MEDIA
8.2 ASSESSMENT AND TESTING OF EXISTING SOILS
Table 8-1. Organic media amendment characteristics and applications.
Media
amendments
Description
Target pollutant(s)/
benefit(s)
Operational considerations
Source3
Organic
Compost
Substrate produced from biological decomposition
of organic material. Provides nutrients (N, P, and
C) to support soil health and plant growth.
• Metals
• Soil fertility
• Plant growth
Not all compost is created equal and can
contribute to nutrient leaching under saturated
conditions. The BSM specification for compost
will vary by jurisdiction.
1, 5
Biochar
Biochar is a carbon-rich and porous absorbent
produced from the pyrolysis of biomass (crop
residues and wood). In addition to pollutant
removal, biochar provides benefits of water
holding capacity, organic carbon content, and
carbon sequestration.
• Plant growth
• Soil fertility
• Organic contaminants
• Metal/metalloids
• N03", NH4+, P043-
Like compost, not all biochar is equal. Properties
vary based on feedstock and pyrolysis
temperature, which affect contaminant
specificity (e.g., metals and nutrients noted at
left). Feedstock also influences whether biochar
is a sink or source for N03" or P043-.
3, 4
Coconut Coir
Is a byproduct of processing/recycling of coconut
fibers. Provides a rich carbon source, and the
surface functional groups make it effective for
metal binding.
• Heavy metals
• Soil fertility
• Plant growth
• Organics
Susceptible to leaching of dissolved organic C;
has demonstrated limited effectiveness for lead
or copper. Application in the top 5 centimeters of
media can replace mulch.
2, 5
Notes: C = carbon; Ca = calcium; Fe = iron; N = nitrogen; P = phosphorus; = P043- = phosphate; NH/ = ammonium; N03" = nitrate
a Information sources:
1 Hurley, Stephanie, Paliza Shrestha, and Amanda Cording. "Nutrient Leaching from Compost: Implications for Bioretention and Other Green Stormwater Infrastructure." Journal of
Sustainable Water in the Built Environment 3, no. 3 (August 2017): 04017006. httDs://doi.ora/10.1061/JSWBAY.0000821.
2 Lim, H.S., W. Lim, J.Y. Hu, A. Ziegler, and S.L. Ong. "Comparison of Filter Media Materials for Heavy Metal Removal from Urban Stormwater Runoff Using Biofiltration Systems."
Journal of Environmental Management 147 (January 2015): 24-33. https://doi.ora/10.1016/i.ienvman.2014.04.042.
3 Marvin, Jeffrey T., Elodie Passeport, and Jennifer Drake. "State-of-the-Art Review of Phosphorus Sorption Amendments in Bioretention Media: A Systematic Literature Review."
Journal of Sustainable Water in the Built Environment 6, no. 1 (February 2020): 03119001. https://doi.ora/10.1061/JSWBAY.0000893.
4 Mohanty, Sanjay K., Renan Valenca, Alexander W. Berger, Iris K.M. Yu, Xinni Xiong, Trenton M. Saunders, and Daniel C.W. Tsang. "Plenty of Room for Carbon on the Ground:
Potential Applications of Biochar for Stormwater Treatment." Science of The Total Environment 625 (June 2018): 1644-58. https://doi.ora/10.1016/i.scitotenv.2018.01.037.
5 Tirpak, R. Andrew, ARM Nabiul Afrooz, Ryan J. Winston, Renan Valenca, Ken Schiff, and Sanjay K. Mohanty. "Conventional and Amended Bioretention Soil Media for Targeted
Pollutant Treatment: A Critical Review to Guide the State of the Practice." Water Research 189 (February 2021): 116648. https://doi.ora/10.1016/i.watres.2020.116648.
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CHAPTER 8: BIORETENTION MEDIA
8.2 ASSESSMENT AND TESTING OF EXISTING SOILS
Table 8-2. Inorganic media amendment characteristics and applications.
Media
amendments
Description
Target pollutant(s)/
benefit(s)
Operational considerations
Source3
Inorganic
Wastewater
Treatment
Residuals
Byproducts from coagulation/flocculation
treatment process in drinking water treatment
plants. WTR can be Al, Fe, or Ca-based.
• Dissolved P (P043- and
dissolved organic
phosphorus)
Al and Fe WTR are suited for neutral to acidic
pH. Fe WTR should be separated from anoxic
zone to avoid reduction of Fe(lll) to Fe(ll) and
subsequent release of dissolved P. Ca-based WTR
can increase pH to alkaline conditions.
1
Zeolite
A naturally derived or synthetic aluminosilicate
sorbent. Zeolite is effective for adsorption due to
its high surface area, cation exchange capacity,
and porous structure. Also, surface modifications
can enhance bacteria removal.
• Heavy metals
• nh4+
• Bacteria
The primary removal mechanism is ion exchange;
therefore, basins with high salt loadings could
potentially leach contaminants.
2
Iron-Based
Amendments
Examples include zero valent iron or iron oxide
coated sands.
• Heavy metals
• Dissolved phosphorus
• Bacteria (£. coli)
The presence of dissolved organic carbon can
alter surface charge and reduce the efficacy of
amendment.
2
Notes: Al = aluminum; Ca = calcium; Fe = iron; P = phosphorus; P043- = phosphate; NH4+ = ammonium; WTR = wastewater treatment residuals
a Information sources:
1 Marvin, Jeffrey T., Elodie Passeport, and Jennifer Drake. "State-of-the-Art Review of Phosphorus Sorption Amendments in Bioretention Media: A Systematic Literature Review."
Journal of Sustainable Water in the Built Environment 6, no. 1 (February 2020): 03119001. https://doi.ora/10.1061/JSWBAY.0000893.
2 Tirpak, R. Andrew, ARM Nabiul Afrooz, Ryan J. Winston, Renan Valenca, Ken Schiff, and Sanjay K. Mohanty. "Conventional and Amended Bioretention Soil Media for Targeted
Pollutant Treatment: A Critical Review to Guide the State of the Practice." Water Research 189 (February 2021): 116648. httDs://doi.ora/10.1016/i.watres.2020.116648.
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CHAPTER 8: BIORETENTION MEDIA
8.3 MEDIA DESIGN CONSIDERATIONS FOR HYDROLOGIC PERFORMANCE
83 Media Design Considerations
for Hydrologic Performance
Media design will influence the storage capacity and hydrologic
function of a bioretention facility Drawdown time (infiltration)
and storage capacity are described here. More details on
hydrologic performance can be found in Chapter 10, Typically, it
is recommended that ponded water draws down within 24-48
hours. The following equation can be used to calculate drawdown
time (t, hours).
i
prevents water seepage around built structures. When soils
with moderate-to-high swell potential are present—where the
presence or absence of water causes soil volume to expand or
contract significantly—avoid exfiltration to minimize damage
to adjacent structures. In these cases, an impermeable liner and
underdrain system may be warranted. Additionally, consider the
potential effects on local drinking water supplies if there is a risk
of groundwater contamination from pollutants captured in the
BSM. The presence of an impermeable media layer (e.g., clay) at
a site does not necessarily rule out exfiltration. Gravel or stone
wells can be included to create conduits to deeper permeable
layers (PWD 2014). Depending on the design and application,
these stone wells could be classified as Class V injection wells and
subject to EPA underground injection control requirements.
Where;
• V represents the storage volume on the surface
(in cubic feet),
• As represents the infiltration surface area
(in square feet), and
• i represents the BSM infiltration rate
(in inches/hour).
The BSM depth and infiltration rate are design parameters that
can be modified to achieve drawdown requirements. Increasing
BSM depth can also expand storage capacity. For bioretention
facilities with an underdrain, adding IWS can increase storage and
promote exfiltration. IWS is discussed more in Chapter 10.
When evaluating exfiltration, existing soils are a critical factor.
Regions with fast-draining soils, especially gravel or sandy
soils, may offer rapid exfiltration but require that the design
A modified
Philip-Dunne
infiltrometer, which
measures the soil's
saturated hydraulic
conductivity.
Photo: NIC State
Department of Biological
and Agricultural Engineering
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CHAPTER 8: BIORETENTION MEDIA
8.4 MEDIA DESIGN CONSIDERATIONS FOR POLLUTANT REMOVAL
8.4 Media Design Considerations
for Pollutant Removal
Water quantity and water quality aspects of storrnwater manage-
ment are often discussed separately, but the two are linked by
the system's infiltration processes and hydrologic performance.
Hydraulic residence time, or the average time a molecule of water
resides in the system, is critical for removing many pollutants.
Residence time is partly controlled by infiltration rates, but it
can also be influenced by modifying the travel pathways by
changing the BSM thickness or altering the bioretention surface
by adding a check dam or vegetation. Longer residence times
increase the effectiveness of the removal mechanisms illustrated
in the bioretention facility schematic in Chapter 8.1 (i.e., filtration,
sorption, and chemical and biological uptake).
Designing BSM for pollutant removal will vary depending on the
targeted pollutant. Common pollutants of Interest in storrnwater
include solids, nitrogen, phosphorus, bacteria, metals, and organic
compounds, such as polyaromatic hydrocarbon (PAHs). Often,
the pollutant of interest will be dictated by existing data or
requirements related to total maximum daily loads, nutrient-
sensitive watershed classifications, or pollutants associated
with the watershed's dominant land use. For instance, BSM for
industrial areas may include multiple layers specifically designed
for pollutants not common to residential and ROW locations.
Understanding the target pollutant and the primary removal
mechanism will guide BSM design or optimization. The Water
Research Foundation's International Storrnwater BMP Database:
2020 Summary Statistics report provides consolidated information
about types and removal mechanisms (WRF 2020).
Soil media is visible during facility construction in Denver, CO (left) and Santa Rosa, CA (right).
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CHAPTER 8: BIORETENTION MEDIA
8.4 MEDIA DESIGN CONSIDERATIONS FOR POLLUTANT REMOVAL
Phosphorus is used as an example to illustrate how the
pollutant type and mechanisms of removal impact BSM design.
Total phosphorus includes dissolved and particulate forms.
The dissolved forms of phosphorus include soluble reactive
phosphorus (inorganic phosphate) and soluble unreactive
phosphorus (polyphosphate and organic compounds). Soluble
reactive phosphorus is a readily bioavailable form. Particulate
phosphorus represents a combination of bacteria, inorganic
particulate, algae, etc. The different forms of phosphorus are
highlighted because dissolved and particulate components
have different removal mechanisms. Particulate phosphorus is
primarily removed via physical processes such as sedimentation
and filtration within shallow depths of BSM. Soluble reactive
phosphorus is removed via plant uptake (biological processes)
and adsorption/precipitation (chemical processes). BSM
cation exchange capacity is an important factor influencing
phosphate sorption. Additionally, as noted in Table 8-2, other
media amendments can be added, such as iron-coated sand, to
increase phosphate specificity. Hunt et al. (2012) recommend BSM
infiltration rates and media depths of 1-4 inches/hour and 2-3
feet, respectively.
Generally, less permeable soils increase residence time and
enhance potential water quality benefits for nitrogen. For
example, when an anoxic layer and carbon source is present,
infiltration rates of 1-2 inches/hour are generally effective for
denitrification (the microbial conversion of nitrate to nitrogen
gas). Studies in natural and nonengineered soils have shown
a negative correlation between microbial activity and sand
percentage and a positive correlation between microbial
respiration and soil organic carbon (Deeb et al. 2018). Retrofits
can be implemented to increase residence times and are discussed
more in Chapter 10.
Bioretention practices can export nutrients to groundwater or via
an underdrain if present. If the bioretention practice is to be sited
in a nutrient-sensitive watershed, evaluate both the existing soils
and the BSM to determine the potential for nutrients leaching
into groundwater or surface water. For example, if the existing
soils are already rich in phosphorus, and phosphorus is a target
pollutant, avoid using existing soils in the BSM. Promoting the
growth of mycelium, a fungus with a rootlike structure, in BSM
has been shown to help mitigate phosphorus export (Poor et al.
2018). Also, if the practice is situated in areas that apply road salts
during winter months, ammonium and phosphorus leaching can
occur (Donaghue et al. 2023; Erickson et al. 2022).
BSM can incorporate soil media amendments to enhance
removal of dissolved pollutants such as heavy metals, nitrate
(N03 ), phosphate (P043 ), and organics (e.g., PAHs). Tables 8-1
and 8-2 summarize the most common media amendments and
their operational considerations. Media amendments can be
categorized into organic and inorganic amendments. Organic
amendments biodegrade over time but tend to be low-cost.
Alternatively, inorganic amendments are not biodegradable
and can be implemented as a polishing step to target specific
pollutants. Note that compost and biochar media are not all
equal, and the properties of these media varies significantly
based on the source material and preparation. Therefore,
it is important to analyze and test amendments before
implementation. Examples of amendments and their key
considerations are noted below.
Compost is an example of an organic material that has historically
been mixed with sand in bioretention facilities to support plant
health. However, compost contains dissolved organic matter,
nitrogen, and phosphorus, all of which can leach from BSM and
compromise bioretention water quality benefits—especially when
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CHAPTER 8: BIORETENTION MEDIA
8.5 LINERS
reapplied incorrectly during maintenance. As a result, compost
should be applied with caution in nutrient-sensitive watersheds.
Additionally, compost should be tested to ensure it has low metal
concentrations and no pathogens. Erickson et al. (2022) tested
various compost materials in outdoor mesocosm experiments
that included leaf compost, sphagnum peat, reed sedge peat,
and food compost. The media design with a layer of 10% leaf
compost/90% sand (by volume), followed by a bottom layer of
5% iron fillings/95% sand, was effective in phosphorous capture in
the presence and absence of road salts.
Biochar can serve as a soil amendment to enhance soil
aggregation, water holding capacity, and organic carbon content.
Researchers have been studying the effectiveness of biochar in
BSM. Biochar can also be used for carbon sequestration, serve as
a substrate binding site for microorganism, and be used in the
remediation of contaminants. Contaminants adsorbed by biochar
include heavy metals, pesticides, and organics. However, studies
have not shown biochar to be effective in reducing the leaching
of nutrients and dissolved organic matter (Iqbal et al. 2015). The
Minnesota Stormwater Manual provides a comprehensive summary
of biochar application in stormwater management (MPCA 2013).
It is important to note that biochar is generated from a variety of
source materials and temperatures. As a result, all biochar is not
equal and specificity to certain pollutants will vary.
Inorganic additives can be used to target metal cations. The
most common amendments include calcium and magnesium (Ca/
Mg), which remove phosphorus via precipitation, and aluminum
and iron (Al/Fe), which remove phosphorus via adsorption. The
amendments may be naturally occurring (e.g., limestone, gypsum)
or be derived from industrial and process waste materials such as
water treatment residuals, fly ash, steel slag, acid mine drainage
residuals, and zeolite.
8.5 Liners
Media liners are permeable or impermeable synthetic fabrics
used to: (1) provide stabilization, (2) separate soil and aggregate
media within a facility, or (3) prevent stormwater migration to
groundwater or adjacent infrastructure.
Permeable liners (also called filter fabrics) are nonwoven
geotextile fabrics that allow stormwater infiltration within a
facility and provide separation between varying media and
drainage layers. Used burlap coffee bags have worked in some
regions as permeable liners (although these types of natural
materials will degrade over time and become less effective). They
prevent the sediment and clays in the top media layers from
migrating into underlying coarser media, where they could cause
clogging. Permeable liners placed below gravel, mow strips, or
other landscaping materials help limit weed growth within the
GSI surface area. Permeable liners can be installed along the side
slopes of bioretention or horizontally between media layers.
Impermeable liners are impermeable membranes or
geomembranes that prevent water migration to a particular area.
They are used for scenarios such as hotspot areas of contaminated
soils, high groundwater tables, or when a facility is next to
structures (roadways/pavements and buildings). The four primary
types are compacted till liners, clay liners, geomembrane liners,
and concrete liners. When bioretention is next to pavements
within the public ROW or a building, it is recommended that an
impermeable liner be placed along the side of the facility.
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CHAPTER 8: BIORETENTION MEDIA
8.6 AGGREGATE MEDIA
BSM installation for a bioretention planter implemented by the
Riverside County Flood Control and Water Conservation District,
Riverside, CA.
8.6 Aggregate Media
Using aggregate media in BSM design can serve several purposes,
including separating distinct media layers, providing more
stormwater storage, and maintaining drainage around underdrains.
Note that all aggregate material should be clean, double-washed,
and free of fines to prevent clogging of the media.
Choker layers are horizontal transition layers of aggregate media
that prevent the migration of particulates from finer media
layers into the coarser storage aggregate media layers without
restricting flow. A choker layer is typically used between overlying
soil or sand layers and the coarser storage aggregate media
layers. The choker layer typically includes sand and aggregate
numbers 7, 8, 9, or 89 (approximately 1/4-inch to 1-inch diameter
stone). The depth of the layer varies as a function of material:
sand depth typically ranges from 4-6 inches, and numbers 7, 8,
9, and 89 aggregate depths typically range from 2-6 Inches. A
choker layer of sand is a thin layer (2-4 inches) that acts as a
transition layer between finer and coarser media, used primarily
to prevent finer media from migrating to subsurface layers. Sand,
which typically makes up 50%-80% of the total BSM design mix,
encourages infiltration and storage.
Storage aggregate media is a coarser-graded stone placed in
bioretention to provide more storage capacity within the cross-
section of a cell. Some communities offer regulatory credit for
retention storage. Storage aggregate materials include numbers
2, 3, 56, 57, and 67 aggregate (approximately 3/4-inch to 2.5-
inch diameter stone). Storage aggregate media has a variety of
applications in GSI. The Number 57 aggregate can be used as the
primary storage aggregate layer of the facility and as the bedding
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CHAPTER 8: BIORETENTION MEDIA
8.6 AGGREGATE MEDIA
Installing aggregate media in Atlanta, GA
for the underdrain or distribution piping to mitigate clogging of
the perforated pipe system. The numbers 2 and 3 aggregates are
typically used below the primary storage aggregate layer. The
depth of aggregate storage media depends on the BSM depth,
CDA, the plan area of the facility, and the practice's designed
storage volume. Aggregate bedding depths for underdrain
systems will vary based on the diameters and configuration of the
underdrain(s) within the bioretention facility. A minimum 4-inch
offset from the outside diameter of the pipe is recommended.
Aggregate media in Santa Rosa, CA.
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iHi
KM
Chapter 9
VEGETATION
In this chapter
9.1 Why Is Vegetation Important?
9.2 Considerations for Vegetation Selection
9.3 Planting Plan
9.4 Planting Mechanisms
9.5 Vegetation Establishment
Plant selection, development of a planting plan, and
establishment are important steps to a healthy and
vibrant vegetated bioretention facility. Chapter 9 discusses
important considerations during each of these steps.
Photo: Adrienne Donaghue
9-1
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CHAPTER 9: VEGETATION
9.1 WHY IS VEGETATION IMPORTANT?
9.1 Why Is Vegetation Important?
Vegetation in bioretention design promotes evapotranspiration,
reduces flow velocities, stabilizes soil, and improves water quality
through nutrient uptake via plant roots and other biological
processes. Root systems also encourage infiltration, and the
vegetation helps capture trash and debris before it can enter the
storm drain system. Plants offer more than just water quality
benefits by creating urban wildlife habitat, mitigating the urban
heat island effect, offering aesthetic appeal, and calming traffic.
This chapter discusses how to select vegetation (typical plants,
shrubs, and trees), choose planting plans and mechanisms, and
establish vegetation.
Stormwater curb extension bioretention in Philadelphia, PA.
9.2 Considerations for
Vegetation Selection
Plant selection influences the performance and public acceptance
of bioretention practices, especially within the public ROW. The
following considerations can guide vegetation selection and
optimize long-term success.
Use vegetation that is resilient across various site and microclimate
conditions Plant selection should be based on water and light
availability, site conditions (land use, habitats, and aesthetics),
and the species' tolerance for the site's soil characteristics.
For example, drought-tolerant plants are suitable for drier
climates. Choose vegetation that can adapt to local climate
and microclimate conditions, such as an extended dry season
or severe cold. In urban environments, growing conditions are
often harsh, and the long-term viability of the practice could
depend on incorporating hardy vegetation that can tolerate the
accumulation of sediment and debris. Similarly, in snowy climates,
select plants that can tolerate salt and magnesium chloride
deicers. Choosing coastal vegetation that grows on roughly the
same latitude may be appropriate. For example, when choosing
salt-tolerant plants for Northern Ohio facilities, designers may
favor plants that grow along Long Island Sound in Connecticut
due to their salt tolerance (unless native plants are required or
preferred). In residential settings, deer-resistant vegetation may
be necessary (MC DEP 2019).
Use native, noninvasive vegetation whenever possible. Native
plants are typically noninvasive, and they are acclimated to the
local climate and need less maintenance. Also, the deep-rooted
systems associated with native plants can filter pollutants and
require less fertilizer and pesticides to remain healthy. For ideas,
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9.2 CONSIDERATIONS FOR VEGETATION SELECTION
refer to local GSI design resources or tools for lists of suitable
plants to fit your project needs (e.g., Central California Coast LID
Plant Guidance for Bioretention. Fresh Coast Guardians Plant
Selection Tool). Although native vegetation is often encouraged,
site conditions could favor a diverse plant mix that includes
nonnative, noninvasive plants that are easy to manage. For
example, in the northwestern United States, designers initially
used many native wetland plants in bioretention practices. The
wetland vegetation became too dense and overgrown in the
practices placed in narrow ROWs. These conditions hindered
pedestrian travel and increased maintenance demands.
Diversify the plant mix to include specialist and generalist species.
Specialist species require habitats with a specific and often
narrow range of temperatures, soil types, and precipitation to
survive, in contrast, generalist species thrive in a broader range
of habitats and environmental conditions. Some specialist
species, such as those with high evapotranspiration rates or deep
roots, can be incorporated into bioretention designs to increase
infiltration. Other specialist species can target certain pollutants
via phytoremediation (an active area of research). In general,
bigger plants (both in size and density) with extensive root
systems provide more evapotranspiration potential. Including
generalist species, such as those that can tolerate occasional
flooding and dry periods, will ensure your bioretention facility
supports a healthy plant community long-term.
Mix plant types to enhance performance and co-benefits. Using
a mixture of groundcovers, trees, sedges, shrubs, ornamental
grasses, and/or other herbaceous plants is generally suggested to
create a microclimate that can combat environmental stressors
(e.g., drought, extreme temperatures, high winds, sun exposure),
minimize susceptibility to insect and disease infestation, reduce
A street-side planter with drought-tolerant plants in arid Pima
County, AZ.
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9.2 CONSIDERATIONS FOR VEGETATION SELECTION
Bioretention facility in Washington, DC
ilil
SP
Bioretention with low-height plants in a parking lot in
Montgomery County, MD,
Aesthetically pleasing naturalized vegetation in a pedestrian area
in Portland, OR.
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9.2 CONSIDERATIONS FOR VEGETATION SELECTION
weed growth, and reduce maintenance needs. For example,
monocultures typically do not survive well and may supply fewer
water quality benefits. Using turf grass, although generally not
recommended, might be acceptable if the designer can show it
meets all applicable requirements. Diversifying plant species'
size, color, and texture also increases a site's aesthetic appeal.
Trees and deep-rooted plant systems play a critical role in carbon
sequestration and improve soil health, biodiversity, infiltration,
and water retention.
Design for aesthetic appeal and performance year-round. Public
acceptance is crucial to GSI success. When implementing
bioretention in various climates, consider incorporating some
plants that are green year-round to ensure the facilities do not
appear dormant or unmaintained. For example, designs may
include vegetation that has varying colors and textures through-
out the seasons. Some species may perform strongest during the
spring/summer, while others maintain functionality throughout
the year. Ideally, plants should be native, become established
quickly, offer long flowering periods (if applicable), be aesthetically
pleasing in all seasons, and have lifespans of 5-10 years.
Predict plant growth and maturation. Vegetation will look
different once established. When developing the planting
palette, anticipate the future growth of the planted area to avoid
problems such as trees interfering with overhead electric lines
or plants needing intensive maintenance (a plant that grows
and disperses seeds). Also, ensure the potential plant height
does not exceed 42-48 inches above the sidewalk elevation
near intersections to maintain required visibility (some resources
recommend limiting heights to 2-3 feet or 24 inches near curbs).
S
Vegetation can provide year-round seasonal interest and variation.
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9.2 CONSIDERATIONS FOR VEGETATION SELECTION
Select diverse plant types to create a healthy plant community.
Include lower-growing plants to maintain sight lines for driver and
pedestrian safety.
Consider community acceptance and preferences. Some
communities may be open to naturalized types of planting like
grasses and sedges. Other communities may prefer traditional
turfgrass or ornamental plantings. Accommodating community
preferences where possible and maintaining the practices will
help to avoid complaints and increase acceptance. In general,
planted bioretention areas are recommended for higher-profile
settings where sufficient resources (financial and personnel)
can be allocated to build community buy-in and ensure regular
inspection and maintenance. Incorporating trees, shrubs, and
ornamental grasses can also help reduce noise and pedestrian
travel across bioretention facilities.
Incorporate vegetation that attracts pollinators. Planting
bioretention areas with flowering vegetation that attracts
butterflies and other pollinators can enhance the ecosystem
services of the facility by increasing habitat diversity and
community enjoyment.
Select vegetation that minimizes maintenance needs. Anticipate
necessary vegetative maintenance to avoid the use of pesticides,
intensive pruning, leaf litter removal, and high labor costs. Using
aesthetically pleasing plants and maintaining manicured edges
shows an intentional landscape design. Thus, select plants based
on the intended level of facility care: (1) a low level of care
(annual maintenance; no irrigation), (2) a medium level of care
(quarterly maintenance; some water available), or (3) a high level
of care (monthly maintenance; site may require irrigation). To
ensure that areas do not become overgrown, select plants that
grow slowly and require less mowing, pruning, and irrigation.
Furthermore, choose plants that O&M staff can easily distinguish
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CHAPTER 9: VEGETATION
9.2 CONSIDERATIONS FOR VEGETATION SELECTION
from weeds. Understand the resources needed to implement
an effective maintenance plan, including equipment, personnel,
training, educational and reference materials, tasks, and a
schedule. Also, developing and implementing an integrated pest
management (IPM) plan can help ecologically suppress pests and
reduce or eliminate pesticide use. IPM plans integrate biological,
operational, physical, and chemical controls with an integrative
approach to control pests and reduce risks to the environment.
For an excellent resource providing guidance, training materials,
and other resources related to IPMs, refer to the Seattle Public
Utilities' Integrated Pest Management web page.
Consider options for capturing runoff to irrigate plants during dry
periods Landscapes can be designed to encourage the collection,
filtering, and storage of runoff for future use. In Washington, DCs
Canal Park, linear bioretention facilities and tree pits implemented
along the site's perimeter successfully capture, treat, and direct
runoff to underground cisterns. The captured water is used to
meet the site's irrigation and other water demands and saves
more than 8,000,000 gallons of potable water annually (LAF, n.d.)
Integrate safety for pedestrian and roadside travel. Visibility is
important around roadsides and other sensitive areas where
vegetation may negatively impact lines of sight. Low-lying plants
(turfgrasses, low-to-ground shrubs) are generally recommended
for maintaining sight lines and maximizing visibility. Additionally,
vegetation can be a visual barrier and deter pedestrians from
traffic areas. Visibility is also an Important factor near parks,
schools, or other settings with children.
This low-maintenance swale ensures sight lines and fits the
character of this neighborhood in Seattle, WA,
A highly visible bioretention area that has clearly marked plant
groupings for easy maintenance in Washington, DC.
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CHAPTER 9: VEGETATION
9.2 CONSIDERATIONS FOR VEGETATION SELECTION
'Frees
Trees provide stormwater volume and pollution
control through rainfall interception and redistribution,
enhanced infiltration, evapotranspiration, and nutrient
uptake. Additionally, tree canopies offer shade and
evaporative cooling, and they provide carbon
sequestration benefits. However, unlike plants and
grasses, trees require more space for growth above
and below ground. The following resources highlighted
below provide in-depth technical detail of tree
benefits, tree crediting, design consideration for tree
health, and more.
• U.S. Department of Agriculture's Urban Forest-
Systems and Green Infrastructure describes urban
trees' stormwater benefits, tree crediting tools,
and case studies (USDA 2020).
• EPA's Stormwater Trees: Technical Memorandum
focuses on planting and maintaining trees
in urban areas and includes soil amendment
recommendations and an inspection checklist
(USEPA 2016b).
• The Bioretention Design for Tree Health.
developed for the Bay Area Stormwater
Management Agencies Association, identifies and
describes six critical requirements to improve tree
health (BASMAA 2016).
• The Deep Root Bloa bv DeepRoot Green
Infrastructure, LLC, covers recent research, Bioretention facility with trees in
projects, or design concepts (DeepRoot, n.d.). Philadelphia, PA.
Bioretention facility with trees in
Kansas City, MO.
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CHAPTER 9: VEGETATION
9.2 CONSIDERATIONS FOR VEGETATION SELECTION
Shrubs
Including deep-rooted, woody shrubs in bioretention helps infiltrate and retain stormwater
while creating wildlife habitat and adding visual interest (i.e., varying shrub heights,
color, and growth patterns). The Cornell University Publication, WoodvShrubs for
Stormwater Retention Practices Northeast and Mid-Atlantic Regions Second Edition, offers a
comprehensive look at the use of woody vegetations/shrubs in GSI practices. Although the
woody vegetation is specific to the Northeast and Mid-Atlantic, the highlighted approaches
can be applied in other geographic areas (Cornell 2017). Consult with local horticulturists and
plant specialists to select the woody vegetation most appropriate for your location.
Woody Shrubs for Stormwater
Retention Practices
Northeast and Mid-Atomic Regions
Trees and shrubs are integrated into Grasses and woody shrubs in this bioretention facility offer visual
a series of bioretention facilities in interest on the James Madison University campus in Harrisonburg, VA.
Washington, DC.
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CHAPTER 9: VEGETATION
9.3 PLANTING PLAN
93 Planting Plan
Planting plans inform initial plant quantities, plant species,
and planting frequency and impact the long-term viability and
maintenance needs for a successful vegetated practice. The
following components can guide the development of a planting
plan.
Plant diversity. Planting plans typically include vegetation of
varying sizes, colors, and textures and consider compatibility
among species. A diverse, dense plant cover reduces pollutants
in stormwater, withstands urban stresses (e.g., insect and disease
infestations, drought, temperature, wind, sun exposure) and adds
aesthetic appeal to a site. Avoid choosing plants that will require
excessive thinning, trimming, or removal due to site constraints.
Planting plans may need to be adjusted if plants die off during
the initial establishment period. In such cases, an alternative plant
species might be better suited for the site conditions.
Placement and layout Placing vegetation based on the species'
tolerance to inundation increases survivability. Consider placing
streambank-edge species or species tolerant of water flows at
the facility's entrance, facultative wetland species at the facility's
bottom, and decorative ornamental plants or uplands species
and sod at the upgradient edges. The orientation of planting
layout and spacing should correspond to site dimensions and
conditions (i.e., the plants selected should be appropriate for the
bioretention in terms of their mature size, growth characteristics,
and maintenance requirements). Also consider grouping similar
plants rather than intermixing species so O&M staff can more
easily recognize and remove weeds.
Similar species are grouped in this bioretention practice in Atlanta, GA.
Similar species are placed in dense, colorful groupings in Seattle, WA.
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CHAPTER 9: VEGETATION
9.4 PLANTING MECHANISMS
Vegetative cover Balance plant density to minimize weed growth,
promote plant health, and prevent erosion. Placing a dense
vegetative cover on the bottom and side slopes of a bioretention
facility filters pollutants and reduces flow velocities, preventing
erosion. Fine-leaved, close-growing grasses are often ideal
because they increase the surface area of vegetation exposed
to runoff and improve the system's effectiveness. Tightly
spaced plantings promote efficient maintenance, ensure a neat
appearance, and reduce areas available for weed growth. The
initial planting density can be decreased for more cost-effective
planting, especially when using plugs and/or plants that naturally
self-seed or spread through rhizomes.
9.4 Planting Mechanisms
Various methods are used for establishing vegetation in
bioretention facilities. These methods, also referred to as planting
mechanisms, include placing seeds directly in the uncovered
soil, installing sod over the soil, and planting already-established
potted plants and plugs. Table 9-1 highlights the factors that can
help determine the best planting mechanism for a bioretention
site, given the site conditions, the availability of maintenance
staff, and the project budget.
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This Harrisonburg, VA, facility features small trees, a dense layer of
grasses and bushes, and colorful native flowers.
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity 9-11
lEntrance Zone ' -- "
•flints located where water enters and in the stormWat^
flow tath should be able to stabilize the sod and preveTft
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CHAPTER 9: VEGETATION
9.4 PLANTING MECHANISMS
Table 9-1. Design considerations for various planting mechanisms.
Planting
mechanism Design considerations
Seeding
• A cost-effective method for large bioretention facilities.
• Requires more maintenance early in the facility's lifespan to prevent weed establishment.
• Requires careful seedbed preparation and pre-planting weed control to avoid excessive weed growth and
confusion in differentiating bioretention plants from weed seedlings.
• Vegetation establishment can occur slowly with seedlings, especially at the bottom of the basin. It typically
takes native plants up to two years to fully establish their root structure before they expand foliage and bear
flowers or fruit.
• Seeds have a low survival rate if the facility receives heavy flows that scour soil and create bare areas. Consider
using both the seeding and plugging mechanisms to increase success.
Sod
• Provides instant coverage of a bioretention facility and is easy to install.
• Provides immediate visual appeal and soil protection, which limits the potential for erosion.
• Can be difficult to establish if the facility receives flows immediately. If the practice receives flows upon
installation, fully open any valves to limit stress on the sod until it is established and tolerant of longer
inundation periods.
• Becomes established more slowly than potted plants because sod must grow roots into the underlying BSM.
• Plant choice is limited. Some nurseries are experimenting with bioretention plants to create a native plant sod,
which might be a cost-effective, intermediate step between using the less-costly seeding option and the more-
costly potted plant option (if native and adapted plants are desired over turfgrass).
Potted plants
and plugs
• Available in various sizes, from deep cell plugs to more-established potted plants (gallon size).
• Deep cell plugs (deep, narrow pots that drive root growth downward) are a cost-effective option for live plants
that enhance early plant health and establishment.
• Larger potted plants, although more expensive, offer instantaneous aesthetic appeal, are tall enough to limit
stresses from initial inundation, and can be more tolerant of irregular irrigation during establishment.
• When selecting a potted plant size, consider soil conditions, the growth rate and vigor of the plant, the time of
year, irrigation requirements, and plant availability.
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CHAPTER 9: VEGETATION
9.5 VEGETATION ESTABLISHMENT
9.5 Vegetation Establishment
Successful vegetation establishment will be influenced by the
BSM and frequency of watering or irrigation.
Bioretention Soil Media
The BSM (including existing soils) composition and application
will influence plant selection and contribute to the long-term
plant mix health and performance of the bioretention facility.
The following considerations can help guide the selection and
management of BSM to optimize success.
Ensure the soil composition and chemistry align with species-
specific habitat preferences Choosing the correct BSM will help
establish and grow healthy plants. For example, in many cases,
predominantly sandy soils will not support plant growth. If
possible, use existing soil—when it represents natural conditions
(i.e., is not historical fill)—or a similar BSM substitute, when native
vegetation is planted.
Ensure sufficient soil volume is in place to support proper growth,
especially for trees. The Minnesota Storm water Manual provides a
literature summary of soil volume requirements for tree trenches
and tree boxes. A synthesis of studies indicates that 1-3 cubic feet
of soil is needed per square foot of tree canopy (MPCA 2013). If
the site does not provide ample root volume, use structural cells
or suspended pavements to provide space for root growth and
minimize soil compaction.
A good planting or seedbed improves the success of bioretention
plants. Topsoil is often removed in development sites, resulting in
compacted subsoil with a high clay content. Most plants do not
thrive in these environments, which can promote weed growth.
A mesh mat was installed to suppress weed growth in a newly
planted bioretention practice in Santa Rosa, CA.
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CHAPTER 9: VEGETATION
9.5 VEGETATION ESTABLISHMENT
A newly planted bioretention practice in Fairfax, VA.
In areas where the receiving waters have nutrient impairments,
consider the possible effect of fertilizer or compost before use.
Native plants should thrive without fertilizer because they will
obtain most of what they need from decomposing organic
matter—adding fertilizer will only promote weed growth. Also,
the BSM likely already contains compost or other amendments
as determined during the design and installation phases. As a
preliminary assessment, light brown or yellowish-brown soil
suggests a low organic content. Compost is the most common
amendment used to enhance soil carbon and nutrient content
(see Chapter 8). A soil assessment can be used to determine if
adding compost is necessary
Mulch can be added over the BSM to reduce weed growth in
planting beds. Mulch may be omitted if the plant density is
sufficient to cover 75% or more of the bioretention media;
otherwise, place 1-2 inches of wood mulch over the BSM to
control weeds. Mulch application is less effective in areas of
concentrated flow or slope surface because these site conditions
contribute to washout into the stormwater system. EPA's 2021
fact sheet, Stormwater Best Management Practice: Mulching.
offers guidance for using mulch as an erosion control practice.
Many practitioners use wood chips, gravel, and other alternative
ground cover materials that are less likely to wash away.
Avoid compacting the soil during planting and maintenance
activities. To minimize compaction, plant in the middle of the
garden, work toward the edges, and keep all equipment/foot
traffic on planks, plywood, or other supports. When performing
activities such as mulching, begin applying at the bioretention
edges and then move inward by walking on the applied mulch.
Irrigation
To ensure survivability of new plants, irrigation is sometimes
needed. The following considerations can help guide irrigation
planning and implementation.
When planting, keep roots moist and provide adequate room. Dig
the planting holes deep and wide enough to provide adequate
backfill and allow full extension of root systems as they grow.
Thoroughly water the plant after firming the backfill around the
base. Watering helps create good soil-root contact.
Watering new plants is essential. Irrigation will be necessary
to establish new plantings and sustain plant health, especially
during periods of dry weather. Water regularly during the first
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CHAPTER 9: VEGETATION
9.5 VEGETATION ESTABLISHMENT
and second growing season until 95% vegetative cover is in place.
Water can be supplied by an automatic irrigation system or by a
maintenance staff with an available water source and equipment
such as a hose or sprinklers. Conducting weekly or biweekly
inspections during the first growing season may be necessary.
Water the established plants on an as-needed basis. Monitoring the
plants for water needs at least once every 10 days in the second
season and once every 2 weeks thereafter. To support irrigation
needs, consider the location of the facility's inlet(s) when placing
plants (i.e., place more drought-tolerant plants further away from
incoming water flows).
Other Considerations
The long-term health and survival of vegetation is also affected
by other factors that can influence growth, including weather,
sunlight, and damage. The following considerations can help
inform your plans for vegetation establishment.
Weather conditions. When planting bioretention facilities, consider
the time of year the vegetation is planted and determine any
operational measures needed to ensure adequate establishment.
For example, consider allowing the plants to become established
before bringing the facility online.
Plant sizes and condition. Although small plants are initially more
cost-effective, problems such as excessive weed growth can occur
if proper maintenance is not provided during the establishment
period. Plants should be durable, with well-developed roots that
are not root-bound. Root-bound plants may be acceptable if not
overgrown in the pot. If root-bound, soil balls should be scored
or broken along the edges of the root mass to encourage new
rooting during planting.
Lighter wood mulches can float and wash away into outlet drains,
as is shown in this bioretention facility in Harrisonburg, VA. Using a
heavier material such as gravel can reduce mulch loss.
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9.5 VEGETATION ESTABLISHMENT
Sunlight availability, Because bioretention relies on dense
vegetation for pollutant removal and flow attenuation, proper
sun exposure for selected plantings must be carefully considered.
If grasses are used, the bioretention should receive a minimum of
6 hours of sunlight daily during the summer months throughout
the length of the swale. Consider using alternative vegetation if
sun exposure is limited due to shading by surrounding buildings
or structures.
Staff follow a plan as they prepare potted bushes and grasses for
planting. Potted plants offer instantaneous aesthetic appeal and
can better tolerate irregular watering during establishment.
Perimeter protection. Install low fencing, edging, boulders,
or other barriers to delineate and protect vegetation from
unsolicited mowing, trampling, or animal incursion during
establishment.
Plant location Place plants at grade (preferably) or slightly above
grade. For wetter bioretention practices, planting with a fraction
of the rootball above grade ensures better vegetation survival.
Grass plugs were planted in a newly constructed bioretention
facility in Denver, CO.
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Chapter 10
UNDERDRAINS
AND OUTFLOWS
In this chapter
10.1 Hydrologic Performance Goals
and Outlet Types
10.2 Underdrains
10.3 internal Water Storage
10.4 Outlet Boxes
Chapter 10 discusses considerations for outlet design
when a bioretention facility does not rely on exfiltration
alone. Information on underdrains, IWS, and outlet boxes is
included.
Photo: Montgomery County, MD, DEP
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.1 HYDROLOGIC PERFORMANCE GOALS AND OUTLET TYPES
10.1 Hydrologic Performance
Goals and Outlet Types
Once runoff has been routed to a bioretention facility, water can
exit the system via multiple pathways, including exfiltration,
evapotranspiration, and outflow. Outflow from the bioretention
facility is controlled by an outlet structure that can be located
on the surface or subsurface. A facility's performance goals often
necessitate balancing multiple hydrologic aspects, including:
• Drawdown time, which is the amount of time needed to
infiltrate water ponded on the surface (or surface storage).
Many jurisdictions require ponded water to drain from the
surface within 24-48 hours.
• Peak discharge, which is the detention and controlled
release of water from the outlet. In addition to water
quantity, controlled release influences the residence time of
runoff in the facility and can enhance water quality.
• Storage, which is associated with the retention of
a specified water volume to promote exfiltration,
evapotranspiration, or water quality improvement.
• Water budget, which is the balance of water flowing in and
out of the facility, including changes in storage. To maintain
plant health and vitality, the design should account for water
budgets within the system, so the vegetation is not stressed
with too much or too little water.
• Overflow or high flow conditions, which are runoff volumes
beyond the design volume. Runoff can be managed via
bypasses within the facility, such as curb openings connected
to the gutter or emergency spillways.
Defer to Local Requirements and Site Conditions
Local requirements, adjacent land use, and design goals
will determine whether a subset or all the hydrologic
components listed here are relevant to outflow design.
An outlet's type, structure, and design are influenced by factors
such as site location (e.g., in the public ROW versus a park) and
performance goals such as design volume or peak discharge
reductions. Surface outlets include curb cuts, orifices, weirs,
and risers (summarized below). Subsurface outlets include
underdrains, IWS, and outlet structures (described on pages 10-4
to 10-10).
Curb cuts are surface outlets that are commonly included in
practices implemented in the ROW. Curb cuts used as outlets are
positioned downstream of the inlet and direct water to the curb
or storm sewer (CCD 2021).
Orifices on the surface are openings on an outlet box structure
that manage the design ponding depth and bypass larger storms
for online bioretention facilities. The elevation and size of the
opening will vary based on the function (e.g., design ponded
depth versus bypass for the 100-year, 24-hour storm event).
Weirs include a raised wall or check dam on the surface to pond
water, reduce surface velocities, encourage infiltration, and
increase residence time. Weirs are most common for bioretention
cells used in series.
Risers include an outlet or orifice raised a certain height above
the ground surface. Risers can help to control ponding and can be
used alongside other outlet structures.
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.1 HYDROLOGIC PERFORMANCE GOALS AND OUTLET TYPES
O
cL
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Curb cut
Riser
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS 10.2 UNDERDRAINS
Underdrains being installed in Atlanta, GA.
10.2 Underdrains
Underdrains collect and release water that has infiltrated
through the BSM, and they are used particularly when a site is
characterized by poor exfiltration. Underdrains typically connect
to an outlet control structure or convey water to another GSI
practice. The section below outlines general applications and
design considerations; consult local and municipal guidelines for
specific requirements in your area.
Suitability
Underdrains are appropriate for site conditions when:
• The existing soil's infiltration rates below the facility are very
low (for example, less than 0.2 inches/hour).
• An impermeable liner is needed under the practice to avoid
risks of contaminant mobilization due to the presence of
"hot spots" in the underlying soil.
• The design goals include detaining and slowly releasing
water.
• A seasonal high water table exists.
• IWS is included in the design (discussed more in Chapter 11.2).
Design and Maintenance Considerations
When adding underdrains to your bioretention facility, consider
the following:
• An underdrain is typically a 4- to 6-inch diameter PVC or
a high-density polyethylene (IHDPE) perforated pipe with
equally spaced holes. Diameters greater than 4 inches are
recommended to avoid clogging. The orifice equation for
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.2 UNDERDRAINS
a single orifice relates the peak discharge rate (Q) and the
underdrain or orifice opening (A) (in square feet).
Q = CdAy[2gh
Where Cd is the discharge coefficient (typically 0.6-0.65), g is
gravity (32.2 feet per second squared), and h is the hydraulic
head (in feet). For an underdrain, the hydraulic head
represents the depth of water from the bottom elevation
of the underdrain to the water surface elevation or the
overflow orifice (if present).
Underdrains are installed in a gravel layer or envelope below
the BSM.
Including a valve at the discharge point of the underdrain
can provide flow control. The valve can be adjusted to
increase or decrease the flow, change the hydraulic residence
time, and enhance exfiltration.
The upstream end of an underdrain is typically installed with
a capped cleanout to allow inspections and maintenance.
The cleanout location should avoid dense vegetation for
easy access and provide adequate clearance from other site
features, such as curbs or gabion baskets. The cleanout pipes
on the surface can connect to the underdrain via a 45-degree
elbow in the direction of flow. The cleanout opening is located
above the design ponding depths (for example, 6-18 inches
above the ground surface).
Deep root systems, particularly trees, can encroach on
underdrains and should be located in areas offset from the
underdrain.
A bioretention practice with an underdrain cleanout in Fairfax, VA,
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.3 INTERNAL WATER STORAGE
Design Idea: Infiltration Cells
Municipalities often experiment with innovative designs to meet
performance goals based on local conditions. The City of Omaha
uses an infiltration cell (also termed "the bathtub drain") that
provides capacity for 100% of the design volume to drain from
the system into the underdrain within 24 hours. The infiltration
cell is localized around the underdrain and occupies less than 5%
of the bioretention media volume. The infiltration cell contains
BSM that is often a mix of sand (80% by volume) and compost
(10%-20% by volume). For more details, refer to Bioretention
Gardens: A Manual for Contractors in the Omaha Region to Design
and Install Bioretention Gardens (Hartsig and Rodie 2016).
10.3 Internal Water Storage
IWS is an optional subsurface design element included to increase
storage capacity or enhance water quality. IWS is created by
raising the underdrain outlet elevation (see conceptual IWS
schematic, next page). The runoff captured in IWS is released via
exfiltration to the underlying soils and the underdrain once the
IWS water level reaches the outlet elevation. IWS can also be
created by including a weir or stop logs in the outlet structure.
Suitability
Consider the following factors when deciding if including IWS is
appropriate for the site:
• IWS provides added storage capacity when water quantity
is a primary design objective. The presence of permeable
underlying soils enhances exfiltration and allows the IWS to
empty before subsequent storms. Estimates of drawdown
times for IWS can be determined based on the infiltration
rate for the existing soil below the BSM using the drawdown
time equation in Chapter 8.
• IWS also offers water quality improvements, such as nitrate
removal and thermal pollution abatement. Nitrate removal
in IWS occurs via denitrification, the microbial reduction
of nitrate to nitrogen gas. IWS design for nitrate removal
requires a saturated layer to promote anoxic conditions (low
oxygen), the presence of a carbon source such as wood chips,
and longer hydraulic residence times (more than 7 hours) to
increase denitrification efficiency. Additionally, IWS buffers
temperature by mixing warm runoff with the cooler water
stored in the IWS and reducing discharge volumes.
• IWS sizing will depend on state or local post-development
retention requirements and water quality performance goals.
Generally, the saturated thickness ranges from 2 to 2.5 feet.
• Extension of IWS into unsaturated media, i.e., the BSM,
can mobilize phosphorus or negatively affect plant growth
and root health. Recommendations in published literature
prescribe that the top 1.5-2 feet of media remain unsaturated
(Hunt et al. 2012; Kim et al. 2003; Passeport et al. 2009).
• A separation between the bottom of the IWS and the
seasonally high groundwater table (1-2 feet) allows for
exfiltration and reduces the potential for mobilizing
contaminants in groundwater (USEPA 2021b). Including IWS
is unsuitable when the seasonal high water table interacts
with bioretention media. Additionally, IWS should not be
implemented near structures' foundations. Impermeable
liners are recommended when the required separation
distance cannot be achieved.
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.3 INTERNAL WATER STORAGE
bioretention
media
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native soil
exfilt ration
perforated
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Conceptual IWS schematic
• The height of the underdrain with respect to the IWS depth
can influence hydraulic efficiency (i.e., the effective use of
the IWS volume). Hydraulic efficiency is most sensitive for
narrow bioretention facilities, such as a bioretention swale
(where the IWS width-to-depth ratio is less than 1). Under
these scenarios, underdrains located near the top of the
IWS create immobile zones, or stagnant water areas, which
reduce IWS treatment volume (Donaghue et al. 2022).
• For bioretention facilities with a wider footprint (where
the IWS width-to-depth ratio is greater than 1), hydraulic
efficiency is less sensitive to underdrain height. Raised
underdrains or underdrains located towards of the top of the
IWS can provide O&M benefits. For example, a raised IWS
underdrain can reduce sediment clogging and gas buildup
from biological processes.
• IWS can be incorporated into existing bioretention facilities
as a low-cost retrofit by raising the elevation of the
underdrain outlet with PVC piping in the outlet structure or
adding a weir to the outlet structure (Hirschman Water &
Environment 2018). Other options could include raising the
inlet to the storm sewer or adding IWS when it comes time
for media replacement.
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10.3 INTERNAL WATER STORAGE
IWS design goal Design elements
Water quantity and/
or sewer overflow
reduction goals
(e.g., reducing peak
flow, managing runoff
volumes to mimic the
site's predevelopment
hydrology)
Underlying permeable soils are needed to promote exfiltration.
Thicker IWS zones (i.e., higher outlet elevations and larger volumes of
retention media/void space) increase the storage capacity, enhance
exfiltration, and reduce the volume and frequency of discharges from the
underdrain.
Raking the bottom of the cell during construction minimizes soil compaction
and increases exfiltration (Hunt et al. 2012).
Water quality goals
(note: IWS-related water
quality improvement
is limited to mitigating
nitrate or thermal
pollution)
• Under this scenario, exfiltration is less of a design focus. The IWS is designed
to maintain saturated conditions, create an anoxic zone, and increase
residence time. If soil exfiltration rates are too fast, then including a liner may
be necessary.
• In cases where the design goal is nitrate removal, a carbon source (e.g.,
woodchips, sawdust) is necessary to promote denitrification (the microbial
reduction of nitrate to nitrogen gas).
• Having restrictive or less-permeable underlying soils reduces exfiltration and
increases residence time, which enhances denitrification.
• For narrow systems, such as a bioretention swale, the underdrain's location
within the IWS layer can reduce hydraulic performance by underutilizing
the IWS treatment volume. Underdrains at the bottom of the IWS optimize
hydraulic performance (Donaghue et al. 2022).
• Runoff can mobilize sodium from roadways in areas where deicers are applied
during winter months. Runoff that contains elevated sodium concentrations
can replace positively charged pollutants, such as ammonium, on bioretention
media and cause them to be flushed from the bioretention cell (Donaghue
et al, 2023). Capping the underdrain during colder months allows water to
infiltrate and prevents pollutants from discharging via the underdrain.
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Design and Maintenance Considerations
IWS Design elements will vary depending on water quantity or water quality goals
(Table 10-1).
Table 10-1. Considerations for IWS design elements and goals.
Underdrain outlet in an outlet box.
An IWS system with an outlet box.
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.4 Outlet Boxes
Outlet boxes tend to be the most engineered outlet structure,
with both surface and subsurface components. The surface
components may include orifices with different elevations to
accommodate and safely pass various storms (e.g., 25-, 50-, and
100-year storm events). The subsurface component can connect
to the underdrain and provide a tie-in to the storm sewer. Outlet
boxes can be constructed on-site or prefabricated; they house
operational features to control flow, such as orifice plates, valves,
or stop logs.
Suitability
Consider the following when deciding if an outlet box is
appropriate for the site:
• Outlet boxes can manage multiple hydrologic goals,
including drawdown times, peak discharge, detention,
storage, and overflow. Proper sizing and elevation of outlet
types, such as orifices, is critical to ensuring the facility
operates as intended.
• Outlet box overflow features are applicable for on-line
systems (systems that receive runoff from storms greater
than the design storm). Because these systems manage water
quantity from larger storms, they help reduce flooding risks.
10.4 OUTLET BOXES
An outlet box in a bioretention facility in Winston-Salem, NC.
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CHAPTER 10: UNDERDRAINS AND OUTFLOWS
10.4 OUTLET BOXES
Design and Maintenance Considerations
When adding outlet boxes to your bioretention facility, consider
the following:
• Reference the local guidelines for overflow structure design
requirements. For example, some municipalities require
the handling of overflow up to and including the 100-year,
24-hour storm event. When applying the orifice equation
(page 10-5) for an overflow opening on the surface of an
outlet box, the hydraulic head represents the depth of water
bottom of the orifice to the water surface elevation.
A multistage outlet control may include several orifices for
controlled flow and a positive overflow to quickly pass flow
during extreme events.
Ensure the outlet box provides enough space to allow for
maintenance needs.
Schematic illustrating water flow entering through a curb-cut inlet, ponding in the facility, and overflowing into a
raised outlet drain (also known as an overflow riser).
Source: NACTO (2017)
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Chapter 11
MULTIMODAL
TRANSPORTATION
AND PUBLIC SAFETY
In this chapter
11.1 Why the ROW?
11.2 Pedestrian Mobility and Access
11.3 Traffic Mobility and Parking Access
11.4 Public Safety
GSI installed in ROW settings requires balancing safety,
mobility, connectivity, and traffic flow across various modes
of transportation, including walking, cycling, and driving
vehicles. This chapter highlights key considerations for
successfully integrating bioretention that minimizes impacts
on the public.
Photo: NYC Department of Environmental Protection
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY
11.1 WHY THE ROW?
11.1 Why the ROW?
Green streets is a term used to describe the implementation of
GSI to manage stormwater from transportation infrastructure,
such as roads and parking lots. ROW settings include public lands
that are adjacent to transportation infrastructure with a drainage
network already in place. In addition to the environmental
benefits of managing stormwater, integrating bioretention
facilities into the ROW provides many social benefits, such as
traffic calming, reduced crossing distances for pedestrians, and
beautification. The Green Streets Handbook provides more detailed
information on the benefits of green streets and considerations
for developing a green streets program (USEPA 2021a).
11.2 Pedestrian Mobility and Access
The following considerations can help designers integrate
bioretention that supports pedestrian use.
Check permitting requirements. Implementing bioretention in
ROW areas may require a sidewalk landscaping permit or a minor
encroachment permit, depending on the scale and complexity of
the project.
Maintain accessibility. Maintain access points across bioretention
facilities for sidewalk crossings and direct connections between
destinations, such as parking lots and building entrances.
Provide access points using breaks between the planters,
dedicated crossings, pedestrian bridges or walkways, or other
techniques. Pedestrian bridges can be constructed of decking,
grates, or other acceptable materials. When siting GSI adjacent
to sidewalks, maintain a comfortable sidewalk width (typically
8-12 feet in dense contexts) based on the pedestrian level of use.
Stormwater curb extension facility that accommodates pedestrian
crossing in Hoboken, NJ.
A bioretention practice designed with a pedestrian bridge in Kansas
City, MO.
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY
11.2 PEDESTRIAN MOBILITY AND ACCESS
A pedestrian walkway was integrated into the design of this hioretention Raised curbs and seating help delineate the edges of the
practice in Kansas City, MO. GSI while providing a place for respite in Portland, OR.
Alternatively, when the GSI footprint competes for space with
the pedestrian sidewalks, consider adding deeper bioretention
facilities. Lastly, ensure pedestrian pathways are ADA-compliant
and accessible to people who are blind by integrating extensive
tactile warnings into the design.
Minimize compaction. Compaction is common in areas where
pedestrians step off the sidewalk or curb into a facility. Avoid
pathways through the facility's filter bed to avoid compaction
and safety risks due to ponded water.
Prevent overflows Maintaining bioretention facilities ensures
both performance and functionality with adjacent pedestrian
pathways. Stormwater ponding and overflows into pedestrian
crossings and ramps limit mobility and can create barriers to
transit stations and bus stops. Be aware that overflows into
pedestrian pathways may result from debris-blocked curb cuts
and basins, wear and tear of roadway pavement, or faulty
stormwater drainage systems.
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY
11.3 TRAFFIC MOBILITY AND PARKING ACCESS
11.3 Traffic Mobility and Parking Access
The following considerations can help designers integrate
bioretention that supports motorized vehicles and facilitates
parking access.
Ensure bioretention in the ROW accommodates vehicle use and
parking demands. To avoid affecting parking or curbside access,
use stormwater curb extensions in areas where on-street parking
is already prohibited, such as near fire hydrants. For on-street
parking applications, place and design bioretention to maintain
sidewalk access. Sidewalks must be wide enough to accommodate
minimum widths of a step-out area, a bioretention area, and a
sidewalk zone. A 12-inch to 48-inch step-out zone is common for
on-street parking to allow access from vehicles to the sidewalk.
In addition, account for regular spacing for crossings in the
sidewalk about every 40 feet, or approximately the length of
two parking stalls (ideally, access paths should align with private
entrances and stairways). Provide sidewalk access along the curb.
Bioretention cell crossings can be at the sidewalk level or may
slope down to street level but should be elevated above the
ponding depth of the cell.
Consider alternatives to bioretention when needed. Sometimes
practices such as suspended or permeable pavement are more
appropriate for ultra-urban streets that experience intensive
demands for curb space. Curbside access is an especially high
priority for urban streets, which typically support frequent stops
by for-hire vehicles (e.g., taxis), commercial freight loading and
deliveries, transit (including paratransit) and bicycle access, bike
share stations, and on-street parking.
A bioretention practice includes a pedestrian bridge for foot traffic
access to a parking lot in Montgomery County, MD.
A vehicle backed into a bioretention practice lacking adequate
protection.
AtttSSA-XiOt
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11.3 TRAFFIC MOBILITY AND PARKING ACCESS
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Examples of two step-out zone designs (left, right) in Portland, OR. Concrete is generally preferable over gravel due to weed growth.
These three bioretention facilities lack proper step-out zones, creating a safety hazard for individuals entering and exiting vehicles.
Providing easy access to step-out zones and pathways allow for pedestrian circulation and minimize trampling-related system damage.
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY 11.3 TRAFFIC MOBILITY AND PARKING ACCESS
Design bioretention elements for compatibility with frequent
curbside traffic Maintain clear paths and avoid placing vertical
elements at accessible parking spaces and designated loading
zones for freight or passenger pick-up. For local businesses that
generate heavy freight and delivery activity, consider reducing
parking duration, shifting deliveries or freight loading to off-peak
hours, or siting designated freight loading zones on side streets
to minimize conflicts. Provide separate pedestrian and bike-traffic
zones to enhance mobility and safety. Features such as planter
boxes or tree pits can function as stormwater retention and
physical barriers.
Design to accommodate pedestrian movement entering/exiting
vehicles. To limit bioswale damage along streets and parking
lots, designers must ensure that the bioretention practice design
leaves enough space between the parking stall edge and the
bioretention facility for people to enter and exit their vehicles
without stepping into the system. Smaller parking lots are
often the most difficult to retrofit because of the high demand
for available space. Larger parking lots require considerable
investment to manage stormwater.
Avoid placing bioretention in historical snow storage areas.
In colder climates, consider where salt has been stored or,
historically, where snowplows have piled snow/salt mixtures for
melting.
Reflective bollards alert drivers about a stormwater curb extension
bumpout in the road in Philadelphia, PA.
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11.4 PUBLIC SAFETY
11.4 Public Safety
Assess the existing street design for opportunities to improve
safety. Where possible, leverage bioretention projects with street
design projects to realize complementary goals, including safe
mobility. Addressing multiple objectives as part of a single retrofit
project, such as combining a GSI retrofit with a mobility project,
is efficient from a design and construction perspective and can
unlock funding and resources.
As described below, key considerations for ensuring public safety
around bioretention facilities include visually defining the facility
and protecting pedestrians, bicyclists, and drivers from accidental
incursions or other dangers.
Delineate the edge of facilities. Incorporating physical or visual
protective barriers to delineate the edge of facilities prevents
people from inadvertently entering into a system and potentially
injuring themselves or trampling the filter bed. Establishing
noticeable edges is especially important during construction and
for facilities on a slope, in school yards or recreation spaces, or
with curbless or in-ground planter designs. Protective barriers
include raised curbs, walls, low fencing, railings, bollards, boulders,
or dense edge plantings around the perimeter or the accessible
side of the bioretention facility. Additionally, adding benches,
streetlights, paving materials, trees, bicycle parking, and artistic
elements can help further define the edges while enhancing
aesthetics. Alternatively, placing high-visibility or retro-reflective
safety measures at the leading edge of facilities can reduce the
risk of motor vehicle incursion.
A raised curb edge delineates a facility in Philadelphia, PA.
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY
Bioretention facilities can provide traffic-calming benefits and
improve safety conditions. Adding bioretention along streets or
in parking lanes can reduce street widths and provide a visual
and physical buffer between pedestrians and moving traffic.
Generally, as vehicle speed decreases, the risk of vehicles entering
GSI facilities decreases. Traffic-calming elements should reduce
target speeds to safe and appropriate urban speeds (typically
20-25 miles per hour; rarely more than BO miles per hour) by
reassigning and narrowing motor vehicle traffic lanes.
Design inlets and outlets to resist incursions by vehicles and
bicycles. For example, motor vehicle wheels may be prone to
enter, especially during parking maneuvers in paved areas with
no curb or wheel stop). Metal lids are an effective design strategy
to block vehicle entry. Additionally, because bicycle wheels can
get stuck or slip when wet, avoid installing grates near bike lanes.
Chapter 6 discusses inlet design in more detail.
Maintain visibility requirements by highlighting bump-outs and
preserving sight lines. For example, select low-growing plants
and shrubs that do not block the view of oncoming vehicular
or pedestrian traffic near intersections, medians, or pedestrian
crossings. Additionally, maintain plant heights no more than 24
inches above the ground surface where pedestrians will gather
or cross the intersection. Place larger trees along the back of the
facility. Ensure trees do not obstruct street lighting or lines of
sight for drivers, cyclists, or pedestrians. Moreover, select trees
with branch heights or widths that will not cross into pathways
or bikeways.
11.4 PUBLIC SAFETY
Dense low plantings discourage people from walking into this
facility in San Diego, CA.
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY 11.4 PUBLIC SAFETY
Seating is used to delineate a facility in Washington, DC.
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
Account for tree growth during the plant selection phase. Ensure
that trees planted next to sidewalks have sufficient subgrade root
space to reduce the likelihood of broken or launched (elevated)
sidewalks. Many urban soils can be compacted, especially in areas
with sidewalks and high traffic, which can affect root growth.
Suspended pavement systems or structural soils can be integrated
to support tree growth for some species. Finally, designers should
ensure that future tree canopy growth will not interfere with
existing overhead utility lines. Chapter 9 offers more detail about
incorporating trees into bioretention practices.
Minimize tripping hazards and prioritize vulnerable groups for
safety. When space is available, facilities with graded side slopes
are generally preferred because they allow for gentler transitions
from the pedestrian path to the bioretention practice, which
minimizes tripping hazards. Alternatively, bioretention facilities
with steep side walls in tighter geometries can have shallower
freeboard to reduce tripping and injury risk. Design the edges to
be navigable by people who are blind or have poor vision, and
integrate cues to distinguish zones traversed by vehicles.
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CHAPTER 11: MULTIMODAL TRANSPORTATION AND PUBLIC SAFETY
11.4 PUBLIC SAFETY
Aerial view of the installation of five bioretention facilities in the ROWs in conjunction with the reconstruction of
a city-owned recreation center in Denver, CO. Barriers are being added around the perimeter of the construction
staging area for public safety. Note the space reserved for pedestrian pathways to the right of the four
bioretention units in the top left.
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Chapter 12
PROMOTING
COMMUNITY
ACCEPTANCE
In this chapter
12.1 Importance of Community Involvement
12.2 Building Community Engagement
and Ownership
12.3 Adding Design Elements to Build
Community Acceptance
Bioretention may attract attention in visible public spaces
by incorporating colorful plants, attractive seating, art
installations, and open spaces. Bioretention can also receive
negative attention when trash and debris accumulate,
vandalism occurs, and parking spaces are removed to make
way for the bioretention practices. This chapter highlights
strategies to engage the local community, meet community
needs, offer aesthetic and recreational benefits, and gain
public acceptance.
Photo: Rhea Thompson 12-1
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE
12.1 IMPORTANCE OF COMMUNITY INVOLVEMENT
12.1 Importance of
Community Involvement
Establishing community support for bioretention development
can help attract local partners (faith-based institutions, schools,
nonprofit organizations, etc.) willing to provide potential sites,
funds, and volunteer labor. In contrast, community pushback can
prevent a project from being implemented. Involving local groups
early in the process allows the public to learn about the potential
benefits of bioretention and contribute to the decision-making
process—giving them a sense of ownership. Additionally, local
residents can offer site-specific knowledge that builds awareness
and improves the design to better fit into specific neighborhoods.
12.2 Building Community
Engagement and Ownership
EPA offers several documents to help you engage with your
communities, including Storm Smart Schools: A Guide to Integrate
Green Stormwater Infrastructure to Meet Regulatory Compliance and
Promote Environmental Literacy. Enhancing Sustainable Communities
with Green Infrastructure, and Saving the Rain: Green Stormwater
Solutions for Congregations (USEPA 2014, 2017b, 2021). The following
key recommendations will help build community engagement and
ownership in bioretention and GSI projects.
Design GSI to blend in seamlessly with the surrounding community
and meet local needs. Community-centered GSI design
incorporates social benefits to serve the surrounding community
without compromising stormwater function. When planning
GSI for a neighborhood, designers should ensure the practices
and landscape materials reflect the image and character the
Community members working in a bioretention facility in
Camden, NJ.
community desires while also creating lively, safe, pedestrian-
oriented spaces. Community outreach and site assessments are
critical to this process. Site assessments should include evaluating
factors like recreational space, safety, visibility, art opportunities,
parking, and access (sidewalks, step-out zones, etc.).
Provide opportunities for community ownership and co-design.
Creating a public engagement strategy can maximize community
involvement and ensure local stakeholders are consulted during
all stages of the planning and design process (Greenprint Partners
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE
12.2 BUILDING COMMUNITY ENGAGEMENT AND OWNERSHIP
2022b). Community meetings foster relationships across groups,
inform designers about local needs, and allow community
members to develop ownership. Stakeholder engagement may
include local watershed groups, faith-based institutions, business
districts, and other community groups. Degrees of involvement
will vary across communities. Multiple strategies are available
for engaging local groups, including electronic surveys, email,
social media, printed media, community meetings, phone
conversations, stoop surveys, and block parties. Community
outreach should allow public input (when possible) on topics
such as plant selection and design. For example, when promoting
its green streets program, the Arlington County Department of
Environmental Services developed four plant palettes (Sunny
Meadow, Shrub and Wildflower Garden, Bright and Bold, and
Shade Garden) and asked residents to vote for their favorite (City
of Rockville 2017). This approach provided neighborhood residents
the opportunity to choose the vegetation for the bioretention
facility and fostered a deeper sense of ownership.
Advance the equitable implementation of bioretention to improve
flood and climate resilience. Disadvantaged communities are
often more vulnerable to flooding hazards, water pollution, and
poor air quality. Identify environmental justice communities
with the greatest need for bioretention projects by using state-
developed mapping applications or EPA's Environmental Justice
Screening and Mapping Tool (EJSCREEN). EJScreen provides
a nationally consistent dataset and approach for combining
environmental and demographic indicators (USEPA 2022b).
Recognize that communities might be concerned about lower-
income residents being displaced through the gentrification
of their neighborhoods. The Equity Guide for Green Stormwafer
Infrastructure Practitioners, developed by the Green Infrastructure
Bioretention was retrofitted with artistic signage in a mall parking
lot near Minneapolis, MN.
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE
Leadership Exchange and Greenprint Partners, provides best
practices and approaches for advancing equity through GSI
(Greenprint Partners 2022b).
Build constructive partnerships and work through a trusted
community liaison Community liaisons often know the area's
historical context, especially with disenfranchised populations,
and can educate team members on the proper techniques for
engaging community members. As a result, these representatives
can help champion a project through completion when there
is community hesitation. In some cases, building constructive
partnerships can lead to the liaisons taking ownership of a project
post-construction. Recognizing private landowners' creative and
innovative GSI projects is another way to showcase the value of
GSI. For example, in 2014, the Philadelphia Water Department
created the Stormwater Pioneers program to award and showcase
the city's best stormwater management projects.
Educate and communicate co-benefits. When engaging
community members, understand that many individuals
may be unfamiliar with the purpose of GSI and the added
environmental, social, and economic benefits—also known as
co-benefits—it provides. Educating stakeholders on co-benefits,
such as shade, beautification, and contact with nature in urban
environments, can build community buy-in. When reaching out
to the community, your messages should target the site-specific
benefits delivered by the project. For example, in flood-prone
communities, your educational message can emphasize flood
mitigation benefits. From a social perspective, design plans may
integrate high-quality public gathering spaces with natural
12.2 BUILDING COMMUNITY ENGAGEMENT AND OWNERSHIP
cu
A bioretention site integrated into a school playground in
Philadelphia, PA.
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE
12.3 ADDING DESIGN ELEMENTS TO BUILD COMMUNITY ACCEPTANCE
features and create opportunities for community development
and social cohesion. Demonstration projects are usually important
for promoting understanding; therefore, consider placing GSI in
building entryways, parking areas, or other visible locations to
educate the public about their function and promote acceptance.
Many places have integrated GSI facilities in schoolyards to
teach the next generation about the importance of stormwater
management. When communicating co-benefits, avoid overly
technical terms. For example, some municipalities have found that
using the term "rain garden" instead of "bioretention" increased
comprehension and acceptance of a project. Ensuring public
benefits (seating areas, pathways, art installations, flowers, etc.)
are visible during the first phase of an ambitious project can help
build community and political support and provide momentum
for similar future projects.
Manage expectations and concerns. Educational materials, such as
webpages and visual renderings, can help community members
better conceptualize the project in their neighborhood. Show
community members images of bioretention practices at all
stages of construction and at different times of year to help
manage community expectations and gain project support.
Furthermore, address any parking and maintenance concerns
early and ensure trash, debris, and overgrown vegetation are
removed routinely to prevent bioretention from being perceived
as an unsightly nuisance. Safety is also a concern. As previously
emphasized, ensure that bioretention facilities drain within locally
required timeframes (24-48 hours), maintain visibility, and have
adequate barriers. Finally, consider the extensive community
outreach and communication needed during construction,
such as notifying local residents about street closures. Sharing
information about funding sources and costs is also important.
12.3 Adding Design Elements
to Build Community Acceptance
Previous chapters provided details on vegetation, multimodal
design, safety, and other elements that are key to integrating
a bioretention facility into a community. The most important
community-focused design elements needed to help ensure
community acceptance are summarized below.
Incorporate signage. Each bioretention facility should be stenciled
or otherwise permanently marked to designate it as a stormwater
management facility. The stencil or plaque could explain its
water quality purpose, warn people that the facility may pond
briefly after a storm, or explain that pedestrians should not walk
on the basin floor. Educational signs are often necessary when
implementing bioretention in a highly visible area.
Choose vegetation carefully. Plants should enhance the site and
reflect the surrounding community. Vegetation can include
a diverse palette of native plants and locally adapted plants
varying in sizes, textures, and colors to offer aesthetic appeal and
attract beneficial wildlife such as butterflies or other pollinators.
In addition to being a visual amenity, trees and other tall greenery
make the walking environment more inviting and pleasant by
offering shade, reducing temperature, attenuating noise, and
improving air quality. Community preference will vary depending
on demographics and cultural factors.
Add multimodal elements. Including ample parking, bike lanes,
pedestrian crossings, step-out zones, and other features to
maintain pedestrian, bicycle, and vehicle movement is crucial to
public acceptance. Where possible, align bioretention projects and
funding with placemaking initiatives to incorporate improved
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE
12.3 ADDING DESIGN ELEMENTS TO BUILD COMMUNITY ACCEPTANCE
walking and bicycling conditions including wider sidewalks,
pedestrian-only paths, refuge islands (protected spaces in the
center of the road for crossing pedestrians and cyclists), new bike
lanes, bike parking, bike share stations, enhanced transit stops
with high-quality shelters, and reduced motor vehicle speed and
volume (using stormwater curb extensions).
Include safety elements. Low fencing, railings, benches, dense
edge plantings, and bollards help delineate practices for drivers,
cyclists, and pedestrians. Furthermore, ensure facilities drain
effectively within 24-48 hours (depending on local requirements)
so that they do not become a safety hazard. Finally, ensure
visibility is maintained for all site users.
Cleanliness is crucial Accumulated trash and unmanicured
vegetation can lead to complaints from the surrounding
A bioretention practice with beautiful plantings and decorative
seating in San Francisco, CA.
community. For high-traffic sidewalks, expect more frequent trash
removal needs. Consider enacting O&M agreements for debris
removal and light weeding with specific businesses, business
improvement districts, or merchant organizations to reduce costs
and ensure longevity. Additionally, for settings with pedestrians
and pets, incorporate signage and trashcans to promote proper
disposal of pet waste.
Incorporate public features and amenities. Functional elements like
pathways and seating can promote public use and acceptance.
For example, a perimeter wall could be designed to function as
a seat wall, or benches can be placed adjacent to the practice.
Bioretention can also be integrated in a way to enhance the
topography of a park, such as providing picnic and play areas,
adding visual or physical barriers to designate spaces for
meditation or wildlife viewing, and creating community gathering
Bioretention with low fencing and walls for safety and benches for
pedestrians in Washington, DC.
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE 12.3 ADDING DESIGN ELEMENTS TO BUILD COMMUNITY ACCEPTANCE
A sculptural installation provides a safe space to sit in Scissortail
Park in Oklahoma City, OK.
The City of Chicago installed a decorative trench drain with
a walkway to allow users to see water flowing through this
facility. In an area traditionally lacking investment, this allows
community members to connect with nature and beautifies
the surrounding areas as well.
areas. These features offer a garden appearance and provide
spaces for public enjoyment and recreation.
Add play/interactive areas. Engage and educate the public
about stormwater management by including interactive design
elements, such as features that allow users to see the water
flowing through a garden.
Include art. Use design elements such as sculptures, murals,
concrete imprints, memorials, or the layout of the stormwater
feature itself. These elements create a sense of place, build
community, and enhance the aesthetic appeal of the site.
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CHAPTER 12: PROMOTING COMMUNITY ACCEPTANCE
12.3 ADDING DESIGN ELEMENTS TO BUILD COMMUNITY ACCEPTANCE
Permeable pavers were installed around this bioretention facility to create a stage and event space for the
community in Chicago, IL.
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Chapter 13
MANAGING THE
CONSTRUCTION
PROCESS
In this chapter
13.1 Importance of Construction and Inspection
13.2 Considerations for Construction Preparation and
Implementation
13.3 Construction-Related Inspection
Construction inspection is critical to the long-term operation
of bioretention, A system can be well designed, but
construction brings the plan to life. Poor communication,
a lack of oversight, and improper construction phasing can
result in poor performance and unplanned maintenance,
consequently increasing costs. Construction oversight and
inspection should be performed by experienced staff.
Photo: Adrienne Donaghue
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CHAPTER 13: MANAGING THE CONSTRUCTION PROCESS
13.1 Importance of
Construction and Inspection
Conduct routine inspections at important milestones during
construction and post-construction to ensure the facility was built
properly. System failures can often be attributed to errors during
construction, such as the following, that can result in costly
maintenance-related repairs.
Over-compaction of the subsoils and BSM. For example, placing
heavy construction equipment on the practice compresses the soil.
Bioretention under construction in Camden, NJ.
13.1 IMPORTANCE OF CONSTRUCTION AND INSPECTION
Using incorrect materials. For instance, failures can occur if BSM
has a high percentage of fines, the drainage stone is the Incorrect
size or is not washed free of fines, or the vegetation is dead or
dying when planted.
Incorrect construction sequence. For example, excavating the
bioretention cell before stabilizing the surrounding road can allow
fines to clog the subgrade.
Improper grading or inlet construction. Improper grades can
cause runoff to bypass inlets and observed LRs to be lower than
designed. It is important that grades and direction of flow are
clearly communicated on design plans and drawings. Contractors
with less GSI experience might overlook these details.
13.2 Considerations for Construction
Preparation and implementation
Before construction can begin, assemble construction documents,
conduct environmental assessments, and finalize needed
agreements with partner agencies, adjacent property owners,
or private partners so the project can be put out to bid and
contractors selected. Moreover, consider any final approvals
needed from agency administrators and elected officials for the
project to enter construction or go out to bid. Other important
factors to consider are described below.
Development approvals may be required. For example, a
memorandum of understanding (MOU) may be needed for
shared GSI. MOUs usually include agreements on funding of
administrative costs, construction costs, and O&M costs by
property owners for improvements along their property frontage.
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CHAPTER 13: MANAGING THE CONSTRUCTION PROCESS
13.2 CONSIDERATIONS FOR CONSTRUCTION PREPARATION AND IMPLEMENTATION
Ensure that contractors are reputable (e.g., have certifications,
provide a portfolio). Proper selection is important; once
construction is complete, the contractor will likely be responsible
for vegetation establishment. Where possible, ensure that
contractors have experience designing and constructing GSI
with trained and certified staff. It is not uncommon that the
general contractor and the landscaping subcontractor are not as
experienced with GSI construction. Consider providing training;
for example, this handbook could serve as an educational tool.
Provide construction oversight with experienced personnel.
During construction, communicate with contractors regularly to
ensure they understand the goals, the materials are delivered,
and they stay on schedule for proper construction phasing.
Perform milestone check-ins and inspect systems during and after
construction. In many municipalities, the jurisdiction's engineering
staff reviews and approves the contractor's work. If available, a
design and engineering consultant team will typically provide
construction administration assistance to the jurisdiction's staff.
Inspection is covered more in-depth in Chapter 15.3.
Coordinate projects to avoid impacts from other local or municipal
development projects. If other projects such as road re-grading or
road widening are planned, bioretention should be constructed
last when possible so that grading does not change after
implementation. The fines from the asphalt can clog the subgrade
or BSM if not properly stabilized before the bioretention is
constructed.
Build more time into the project schedule for first installations. To
clarify expectations, consider conducting an initial review of the
cell grading and mock-ups of key elements before proceeding
with constructing all systems. Although adding this review step
Bioretention under construction in Atlanta, GA
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CHAPTER 13: MANAGING THE CONSTRUCTION PROCESS 13.2 CONSIDERATIONS FOR CONSTRUCTION PREPARATION AND IMPLEMENTATION
Bioretention under construction in Denver, CO.
to provide sufficient setback from the conflicting element(s).
Proactive accommodation of a subsurface space can generally
save reconstruction or retrofit costs as streets evolve over time.
Consider installing design elements such as empty sleeves that
provide conduits and protection for future utility connections
and power needs (such as transit shelters and curbside kiosks)
that may run through GSI installations in the ROW. Additionally,
incorporate access points such as utility access holes for
maintenance needs. ROW systems are typically placed adjacent
to sidewalks and roadways and may extend under these features
when more storage is needed. Storage included under paved
sidewalks or roads should typically not take up more than half
of the cartway to allow room for other utilities in the ROW, and
storage designs should factor in the thickness of the overlying
material (e.g., an asphalt roadbed is thicker than a concrete
sidewalk).
Eliminate impacts to nearby structures and foundations. If
bioretention facilities are in close proximity to buildings with
basements or subsurface structures, line the practices along
curbs or next to utility trenches with a thin, impermeable plastic
liner to prevent migration of infiltrated stormwater to sensitive
areas. Under these conditions, water can be directed downward
to avoid lateral flow or to prevent vertical flow. Bioretention may
require deeper walls to prevent lateral water seepage into nearby
basements.
Avoid compacting BSM and the subgrade so the design infiltration
capacity is not compromised. Over-compaction often results from
heavy equipment being placed on infiltration beds during fill and
grading activities. It is generally recommended to first rototill any
imported soil media with the existing soil in 6-inch layers, then
use foot compaction in 6-inch layers or a landscape roller to finish
requires extra time at the beginning, construction is more likely
to run smoothly and quickly once the approach is reviewed and
approved.
Strive to implement simple shapes and minimize excavation needs.
Systems should generally be arranged in simple shapes. Complex
configurations are difficult to construct. When planning the
grades for surface systems, the designer should minimize the
excavation where possible and work with existing contours to
reduce the overall system depth.
Be flexible with design if conflicts arise. Consider reorienting
or reducing the size of facilities (rather than relocating them)
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CHAPTER 13: MANAGING THE CONSTRUCTION PROCESS
13.3 CONSTRUCTION-RELATED INSPECTION
Bioretention under construction in Atlanta, GA.
the grade of gardens. Be aware of other construction or utility
work in the ROW, which can damage the bioretention facility
during construction (e.g., vehicles driving over BSM).
Communicate with other relevant municipal departments and the
public Scaffolding, construction fences, and other equipment
associated with private property development adjacent to ROW
GSI may limit accessibility to the site. To combat this issue, the
New York City (NYC) Department of Environmental Protection
(DEP) provides all proposed ROW GSI locations to the NYC
Department of Buildings to ensure GSI construction is coordinated
with private property development. Additionally, ensure the
public knows about upcoming construction-related street closures
and other Impacts to their daily activities. Prior to closure,
contractors should proactively contact affected homeowners and
identify any special needs. The public should also be made aware
of maintenance schedules and who is responsible for the work.
Chapter 12 provides strategies for communication.
Consult local construction resources when possible. Many
municipalities have guidance manuals listing implementation
guidelines, material specifications, and applicable requirements.
13.3 Construction-Related inspection
Inspect bioretention facilities during construction milestones. For
example, to ensure proper installation and function according
to plans, inspect the site: (1) after excavation, (2) after subgrade
preparation is complete, (3) after BSM installation, (4) before and
after planting, and (5) after construction is complete. Ongoing
inspection throughout the life span of systems is necessary to
ensure maintenance tasks are being conducted and identify
adaptive changes to the maintenance plan.
During Construction
Inspect bioretention facilities before and after the BSM and
vegetation are installed, as follows:
S Perform a walk-through before installing the BSM and
preparing the subgrade. Inspect the soil mix and subgrade
materials to ensure the proper type is used, such as washed
number 57 stone (correct) instead of ABC stone or crusher-run
gravel (incorrect). All stones used in the bioretention should
be washed and free of fines.
V Ensure BSM and the subgrade are not compacted after
installation. Test the infiltration rate of the subgrade before
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CHAPTER IB: MANAGING THE CONSTRUCTION PROCESS
13.3 CONSTRUCTION-RELATED INSPECTION
installing the drainage layer or BSM; then, test the infiltration
rate of the surface of the soil media after installation is
complete.
¦/ Conduct walk-throughs before and after planting. Before
planting begins, check that the plants are the types specified
in your planting plan and are healthy.
After Construction
Once the bioretention facility is complete, regular inspections are
essential, as described below:
¦/ Confirm that water is not bypassing the bioretention facility
due to improper grading and/or inlet construction (e.g.,
proper angle and slope are essential). If possible, examine
how water flows during a storm through the system and
ensure that runoff has a residence time before entering
overflow structures. Alternatively, a synthetic runoff test
can be performed using a hydrant or water truck to fill the
bioretention facility to observe water flow.
-/ Ensure adequate infiltration is occurring. No ponded water
should be visible 24-48 hours after rainfall (depending on
local requirements). Be aware that poor infiltration can
be related to improper BSM mix, compaction, sediment
buildup due to inadequate pretreatment, and clogging of
the underdrain. Look for sediment buildup, which can cause
clogging over time. Synthetic runoff tests or double ring
infiltrometer tests can be used to determine soil infiltration.
Double ring infiltrometer
The double-ring infiltrometer is an instrument used to
measure the rate of infiltration. Two rings are secured
into the area to be tested; water is poured into each ring,
and the drawdown time is measured. The double ring is
preferred because it minimizes the errors associated with
lateral flow.
¦/ During each inspection, identify and immediately repair
eroded areas inside and downstream of the facility. Identify
and immediately repair any damage to the structural
elements of bioretention (pipes, concrete drainage structures,
retaining walls, etc.). To prevent more structural damage,
patch or fill cracks, voids, and undermined areas.
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Chapter 14
LONG-TERM
O&M AND ASSET
MANAGEMENT
In this chapter
14.1 Planning for O&M
14.2 Maintaining Specific Design Elements
14.B Asset Management
14.4 Longevity and Continued Performance
This chapter outlines O&M considerations across design
elements and notes considerations for continued
performance across the lifecycle of GSI. Conducting routine
O&M is as important as quality design and construction for
maintaining bioretention aesthetics and functionality over
the facility's life cycle. A lack of maintenance can lead to a
higher risk of failure and increase operating costs. Chapter
14 also describes how asset management—applying a
business approach to managing GSI—can be implemented to
strategically target maintenance efforts as the GSI inventory
increases. Finally, this handbook concludes with holistic
design concepts such as accommodating a changing climate
and monitoring the practice over time.
Photo: MSCWMO
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CHAPTER 14: LONG-TERM O&M AND ASSET MANAGEMENT
14.1 PLANNING FOR O&M
14,1 Planning for O&M
Previous chapters noted examples of designing with maintenance
in mind for specific bioretention elements such as inlets and
vegetation. This chapter highlights O&M considerations for
designating responsibilities, conducting training, and managing
staff turnover. The EPA webpage Operation and Maintenance
Considerations for Green Infrastructure provides helpful
information on O&M planning (USEPA 2023).
Establish an O&M plan and designate roles and responsibilities
to reliable entities Written O&M plans and standard operation
procedures (SOPs) provide detailed direction for maintenance
needs and activities and play a key role during periods of staff
turnover by ensuring consistency in efforts. They can also serve as
a record of what entities are responsible for specific O&M tasks.
In addition to O&M plans and SOPs, records of design, as-builts,
completed maintenance, and staff activities should be properly
filed. While municipalities are usually responsible for maintaining
publicly installed GSI, O&M can be performed by landscaping
contractors, residents, businesses, local parks staff, school staff,
workforce development crews, summer youth, or community
stewards. Long-term maintenance will likely involve sustained
public education, deed restrictions, and maintenance covenants or
ongoing maintenance contracts with the owner for bioretention
facilities on private property. Ideally, maintenance should be taken
on by entities that can commit to responsibilities long-term using
interagency agreements or MOUs. An interagency agreement may
involve routine maintenance by landscape crews with experience
or O&M training for GSI facilities. They can also remove trash and
water the site during dry periods. An MOU can be enacted with
specific businesses, business improvement districts, or merchant
organizations for debris removal and light weeding to reduce
A facility experiencing erosion, unhealthy vegetation, and
unwanted bike parking.
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CHAPTER 14: LONG-TERM O&M AND ASSET MANAGEMENT
14.1 PLANNING FOR O&M
costs and improve performance. The municipality can perform
nonroutine maintenance activities requiring technical expertise
and specialized equipment.
Ensure sufficient funding sources are available for ongoing O&M
costs. Local tax and utility fees can provide a funding stream
for GSI maintenance. For example, Tucson implemented a green
stormwater fee (approximately $1 on each water bill) to finance
maintenance. Other funding strategies could include splitting
costs via cost-sharing or interagency agreements.
Provide O&M training where necessary. Although hiring
knowledgeable contractors and personnel who understand
GSI maintenance requirements is ideal, maintenance personnel
may sometimes be working with GSI for the first time. Training
is crucial for new staff or when assets are transferred from
contractors. Provide site training, pictorial training materials,
and interpretive signs to explain the hydrologic and horticultural
systems so they are maintained properly. Consider having
maintenance programs be developed by jurisdictions (e.g.,
municipalities, states) where qualified instructors provide "hands-
on" mentoring; landscaping contractors can also be trained or
managed by the municipality. Partnerships with local universities,
community colleges, or cooperative extension offices can also be
an avenue to develop and deliver training materials. Local and
national GSI training programs and published maintenance guides
and inspection checklists are available. Novice municipalities can
also learn from more GSI-savvy municipalities nearby.
Balance O&M demands across staff and other resources.
Maintenance personnel often provide O&M for hundreds to
sometimes thousands of constructed GSI facilities, often in the
A site with high maintenance demands due to weed growth
and no routine maintenance in Maryland.
ROW. Hiring and training should be paced with new
construction—the maintenance team should grow as the number
of new practices increases if needed to keep up with demand. To
keep pace with new construction, some municipalities use drones
for O&M. Drones have the potential to offset staffing limitations
and assist in GSI inspection and asset management. Alternatively,
drive-by inspections could be used to identify O&M tasks and
then send out O&M crews as needed. In dire circumstances, highly
visible systems can be maintained more frequently than less-visible
systems to make maintenance more efficient and cost-effective.
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14.2 MAINTAINING SPECIFIC DESIGN ELEMENTS
Incorporate community priorities into O&M plans. The aesthetics
and cleanliness of GSI facilities are often more important to
adjacent businesses and street users than functionality. O&M
plans that incorporate engagement strategies can provide a
feedback loop to ensure community needs are being met.
14.2 Maintaining
Specific Design Elements
O&M plans wili vary based on GSI design, location, season,
and land use. Long-term inspection and maintenance activities
specific to each design element of the bioretention facility are
noted here for consideration. When applicable, recommended
frequencies of activities are noted but local design guides should
also be consulted. In general, facilities should be inspected once
every six months. Additionally, facilities should be inspected,
at least annually, during or immediately following a significant
rainfall event to evaluate facility operation (e.g., 0.5 inches or
greater rainfall event). Some municipalities require a minimum of
one annual inspection completed by a third-party inspector.
EPA's technical memorandum, Operation and Maintenance of
Green Infrastructure Receiving Runoff from Roads and Parking
Lots, answers common O&M questions and offers guidance for
evaluating the O&M needs for GSI facilities that service roadways
and parking lots (USEPA 2016a).
Even with pretreatment, this example shows a facility clogged
with sediment and experiencing severe erosion due to lack of
maintenance.
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14.2 MAINTAINING SPECIFIC DESIGN ELEMENTS
Inlets
Routine cleaning of inlets and pretreatment is required to prevent
occlusion, bypass, clogging, and channelization of flow into a
facility. Curbless systems should also be cleared regularly to
maintain clean edges.
Inspection and Maintenance Activities
• Visually inspect all inlet components. Ensure the water is
not bypassing the system due to changes in grading and or
inlet damage. If possible, examine the system during/after a
storm or perform a low-flow hydrology test on inlets.
• Look for erosion and sediment buildup at the pretreatment
area and throughout the bioretention facility on an annual
basis. Repair and reinforce as needed.
• Check inlets and pretreatment areas for accumulated grit,
leaves, and debris that could block inflow, and regularly clear
any substances that could lead to occlusion and prevent the
free flow of stormwater into basins (3-4 times/year).
• Remove sediment from the inlet structure and sedimentation
chamber when buildup reaches a depth of 6 inches, is more
than 50% full, or when proper functioning of inlet and outlet
structures is impaired.
• Street sweeping can be an effective tool in preventing trash,
grit, leaves, debris, or any other substances from entering GSI
facilities. Municipalities' street sweeping programs generally
guarantee routine maintenance for the municipality.
• In colder climates, snowplows may not work well with
bumpout designs and cause damage. Additionally, ice and
snow may block inlets. Mitigate this issue if necessary.
Example of erosion and sediment build up in pretreatment area.
Erosion near inlet has exposed geotextile fabric.
Bioretention Soil Media
Unless damaged by unusual sediment loads, high flows, or
vandalism, BSM can be left undisturbed and allowed to age
naturally.
Inspection and Maintenance Activities
• During quarterly inspections, check for signs of erosion
of the filter bed, settlement (depression in media), or
compaction.
• BSM can become clogged when runoff carries high quantities
of sediment and collect debris and trash on the surface.
One approach to check for clogging may include digging a
hole to identify signs of a clogging layer. Maintenance of
the BSM, including media replacement, might be necessary
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if drawdown time exceeds local requirements (provided all
other components of the system are functioning correctly).
• If clogging occurs, excavate the BSM to a depth that would
remove the clogging layer (the top 3- to 6-inch layer of
media is common).
• Remove the previous mulch layer and apply a new layer
by hand in the spring (once every two to three years). Be
mindful of not overapplying mulch, as it will float into the
stormwater system and/or cause the storage volume of the
bowl to decrease. Shredded wood chips, native wildflowers,
and ground covers may be used as an alternative.
• If dewatering of the system is necessary due to prolong
drawdown times, ensure dewatering is properly conducted.
Pretreatment maintenance often requires removing the grate and
scooping out sediment and debris for disposal off-site.
14.2 MAINTAINING SPECIFIC DESIGN ELEMENTS
Vegetation
Treat vegetation as assets. They protect filtration media from
surface crusting and sediment clogging. Plant roots also provide a
pathway for water to permeate into the media, further enhancing
a system's hydraulic performance. Because bioretention is
often placed in highly visible areas, maintenance demands
may be high—similar to any well-manicured landscaped area—
to maintain aesthetics year-round (O&M depends on design
factors like the selected plant palette). Many state and local
municipalities have published vegetation maintenance schedules
for reference. Providing training for less-experienced maintenance
staff may be necessary. For example, personnel can quickly
identify flowering plants, while intermixed plant designs can
be more confusing and make weeding difficult. Offering plant-
identification training and reference pictures might be necessary.
Inspection and Maintenance Activities
• Once established, regular maintenance may include mowing,
watering, pruning, and weed and pest control. Ideally,
maintain systems with minimal fertilizers, pesticides, and
organic herbicides where possible to prevent leaching of
contaminants.
• Check vegetation for invasive and indicator species. For
example, the presence of cattail species typically indicate
that the system is too wet and not draining effectively.
• Water plants during the crucial establishment period.
Remove weeds by hand. Manage the vegetation to
preserve a dense, healthy plant cover while performing
regular maintenance to prevent plant overgrowth or weed
establishment, protect the garden-like curb appeal, and
preserve sight lines to keep pedestrians and vehicles safe.
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14.2 MAINTAINING SPECIFIC DESIGN ELEMENTS
• As previously mentioned, an IPM approach is encouraged to
control pests and diseases in the landscape before turning
to pesticide use. When pesticides are required, apply the
least-toxic and least-persistent pesticide that can provide
adequate pest control.
• Conserve water use where possible. As previously discussed,
re-use collected water (e.g., rain barrels, cisterns) and
consider native and low-water-use vegetation when
designing the system and developing the planting plan.
• Mow the grassed facilities at least once yearly or when grass
heights exceed 6 inches. Avoid cutting the grass shorter than
3-4 inches; otherwise, the effectiveness of the vegetation
in reducing flow velocity and removing pollutants may be
reduced.
• Vegetation may need to be thinned and/or replanted over
time. Ensure overgrown vegetation does not block outlets or
overflow structures.
• Check vegetation health. If necessary, remove and replace
dead or diseased vegetation that is considered beyond
treatment (semi-annually). Treat all diseased trees and
shrubs mechanically or by hand, depending on the type of
insect or disease infestation.
• More frequent maintenance in the fall is recommended due
to the presence of trees, especially collecting leaves before
anticipated storms if they could clog the facility's primary
inflow. Leaves can be vacuumed and ground to be used as
compost.
• In the late winter, trim bunch grasses, mow turf grasses,
and harvest other types of vegetation according to
Vegetation overgrowth is blocking an overflow structure.
recommendations in the planting specifications. Remember
that vegetation may appear dormant in the dry season-
train personnel so that they do not mistakenly remove the
dormant vegetation.
• It is recommended to install salt-tolerant vegetation in
bioretention sites where chlorides may be an issue. Deicing
chemicals, salt, and sand can impact plants that are not salt-
tolerant, especially if salts are over-applied. If salt appears
to be detrimental to plant growth, consider replacing
vegetation with salt-tolerant species. For example, consider
using coastal vegetation found on the same latitude.
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14.2 MAINTAINING SPECIFIC DESIGN ELEMENTS
Underdrains and Outflow
Outflow structures should be maintained to meet hydrologic
performance goals and drawdown requirements.
Inspection and Maintenance Activities
• If drainage is poor, check the BSM and outlet structures
(weirs, riser, underdrains, etc.) for clogs.
• At least once annually, complete a drawdown report in
conjunction with a rainfall event equal to or greater than
the design capture depth of the facility, or perform a test
of the facility after filling with a secondary water source (at
minimum a hose and double-ring infiltrometer). Note the
date and time the facility was observed as full and the date
and time it was observed as empty, verifying that drawdown
occurred in the required time frames.
• Inspect the cleanouts to ensure they are capped and properly
connected to the underdrain. Damaged clean outs can cause
the runoff to bypass the soil media. Adding concrete donuts
can protect cleanouts from mower damage.
• Clean the underdrain piping network to remove any
sediment buildup as needed to maintain the designed
drawdown time.
Ponded water remained for days after a storm, indicating poor drainage due to
soils or outflow design.
A broken cleanout
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CHAPTER 14: LONG-TERM O&M AND ASSET MANAGEMENT
14.3 ASSET MANAGEMENT
14.3 Asset Management
As bioretention assets build overtime, incorporating green
assets into an existing asset management plan or implementing
an asset management plan is an effective tool for agencies
or municipalities to cost-effectively operate, maintain, and
protect GSI over its life cycle. When resources are limited, asset
management provides a decision tool to determine the effective
allocation of staff, money, and equipment for maintenance, repairs,
or replacement. The Green Infrastructure Leadership Exchange's
2021 Greenstormwater Infrastructure Asset Management
Resources Toolkit provides lessons learned and examples for each
of the different components of asset management for GSI assets.
The components and additional information on asset management
are discussed in the EPA webcast Stormwater Asset Management:
Letting Your Green Infrastructure Work for You.
The components include:
• Level of service
• The current state of the asset
• Criticality
• Life-cycle costing
• Long-term funding
Asset management plans should promote flexibility and an
adaptive management approach to enable lessons learned to
be incorporated into future assets or maintenance routines. EPA
offers an online training resource, Asset Management 101 - Basics
for Small Water and Wastewater Systems, to provide introductory
information for identifying and managing assets. Additionally, the
Asset Management Switchboard, developed by the Southwest
Environmental Finance Center in partnership with EPA, offers a
In colder regions, bioretention areas are often used for snow
storage.
repository of documentation and tools covering a range of asset
management topics.
Another critical asset management element is coordinating
with utility and construction entities to take precautions
and safeguard practices, preventing damage that requires
expensive maintenance and repairs. Meet with public works
and utility entities early in the process to avoid conflicts during
and after construction. More specifically, a higher level of GSI
implementation coordination will be needed for dense utility
infrastructure, duct banks, subterranean basements, and
underground transit infrastructure. NYC DEP is a good example
of how to coordinate efforts. In the past, construction by other
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CHAPTER 14: LONG-TERM O&M AND ASSET MANAGEMENT
14.4 LONGEVITY AND CONTINUED PERFORMANCE
utilities in the ROW caused damage to constructed (or under
construction) GSI facilities. NYC DEP now provides a list of all
ROW GSI facilities to the Department of Transportation (DOT)
Street Permitting group. The NYC DOT Street Opening Permit
protects streets or blocks where reconstruction or resurfacing
occurred within the last five years.
Consider digital tools to keep track of nearby work. Kansas City,
for example, uses geospatial data to track all ROW projects near
GSI. They use excavation notifications provided by Missouri One
Call (the state's 811 "call before you dig" provider), where polygons
for GSI projects are added to the Missouri One Call map. When
companies call to report planned digging, a polygon is created
for their proposed dig site. If the polygons overlap, they receive a
notification with contact information, and they reach out if there
are conflicts that could impact GSI. This enables them to quickly
engage with personnel to avoid conflicts before beginning
construction or utility work, which proactively protects assets and
prevents damage.
An artist's rendition of a street median that could be transformed i
high-profile community asset.
14.4 Longevity and
Continued Performance
Using a holistic design approach increases the likelihood of
bioretention longevity and resilience. Anecdotal evidence from
the City of Portland, Oregon, where the first documented
bioretention facilities were implemented, indicates that good
design and careful management can lead to practice life spans
of over two decades (see the photo of the Reed College facility
installed in 1996, next page). This handbook concludes with the
following considerations to ensure longevity and continued
performance.
Use trained installers. Engaging trained and certified personnel
to perform construction oversight and inspections helps ensure
facilities are correctly implemented as designed. Conducting
routine site inspections after the facility is first brought on-
line helps to identify and correct performance errors quickly.
Proper construction and inspection are especially important for
a low-maintenance bioswale to capture runoff and serve as a
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CHAPTER 14: LONG-TERM O&M AND ASSET MANAGEMENT
14.4 LONGEVITY AND CONTINUED PERFORMANCE
demonstration or high-profile GSI projects. If done correctly, these
initial projects will likely spur future GSI investment and influence
approaches to subsequent projects.
Minimize watershed disturbance. Land-disturbing activities in the
watershed (clearing, re-grading, changing vegetative cover, etc.)
can alter water flow volumes and pollutant transport; minimizing
these types of watershed changes will help protect bioretention
longevity
Maintain the practice. Inconsistent and improper maintenance
impacts a bioretention facility's ability to perform as intended,
increases the likelihood of failure, poses risks to adjacent
properties and the public, and negatively influences the public's
perception of bioretention. Designate routine maintenance
responsibilities early on, and identify a schedule and needed
labor, costs, and equipment. Maintenance tasks may include
monitoring plant health, ensuring curb Inlets are clear, removing
weeds and debris, and monitoring drainage. Establishing
partnerships with other local entities or developing a funding
mechanism to support maintenance ensures success. For example,
in 2020, the City of Tucson added a GSI fee based on water usage
to finance O&M needs.
Monitor the facility. If resources permit, monitoring is a tool for
understanding a system's water balance, the impact of a new
design element, and improving maintenance efforts over time.
Monitoring can also help to assess system response to changes in
climate patterns. Long-term continuous monitoring (20 years) of
a bioinfiltration traffic island at Villanova University showed the
practice managed 86% of all rainfall and discharged 14% of the
rainfall (Wadzuk et al. 2023). The cumulative analysis demonstrated
that three years of continuous monitoring would produce the
same water balance with a 5% uncertainty (Wadzuk et al. 2023).
A facility completed in 1996 is still functioning properly at Reed
College in Portland, OR.
Monitoring methods vary from deploying continuous sensors to
collecting discrete samples, and the lessons learned can be applied
to future sites. Establishing partnerships with other local groups,
state agencies, community volunteers, or universities can provide
additional funding and technical resources to support monitoring.
Retrofit as needed A retrofit is defined here as a new installation
or an upgrade to an existing GSI practice in a developed area
lacking stormwater management. Bioretention facilities are
dynamic systems; thus, adapting to potential shifts in performance
helps maintain functionality long-term. For example, a retrofit
may include amending BSM to improve pollutant removal or
raising the outlet's elevation to promote denitrification.
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
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CHAPTER 14: LONG-TERM O&M AND ASSET MANAGEMENT
14.4 LONGEVITY AND CONTINUED PERFORMANCE
Account for climate resiliency. Climate change is expected to
exacerbate the increased runoff quantities that result from
altered landscapes and impervious surfaces. For example,
precipitation in the northeastern United States is expected to
intensify, with annual amounts predicted to increase by 10%-
15% by 2100 (Frumhoff et al. 2007; Guilbert et al. 2015). Sizing
bioretention facilities to account for future design storms can
improve practice resiliency and avoid failures. For example, the
Chesapeake Bay Program and partners developed projected
intensity-duration frequency (IDF) curves to reflect two future
climate change scenarios (2020-2069 and 2050-2099) (MARISA
2021). This effort was motivated by the need to provide
consistent design standards within the watershed that account
for future climate conditions.
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Holistic design incorporates stormwater management and
community use in Oklahoma City, OK.
Consider future land use. When identifying prospective sites
for bioretention, designers should ensure that the systems can
accommodate future development and societal demands without
overwhelming the facility's storage and treatment capacity.
For instance, Canal Park in Washington, DC, was revitalized as
part of the Anacostia Waterfront Initiative. Redevelopment of
the site involved installing bioretention and other GSI practices
to capture, treat, and reuse stormwater to meet up to 95% of
the park's water demands, including irrigation and fountains.
Designers planned for the development of adjacent parcels that
treats runoff while adding aesthetic value to an outdoor space
can be tied into the existing Canal Park stormwater system to
meet up to 99% of the water needs (LAF, n.d.).
An infiltration planter treats runoff while adding aesthetic value to
an outdoor space at the Navy Yard in Washington, DC.
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
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GLOSSARY
Adaptive management - The process of observing and learning
how a system performs over time and using that knowledge to
adapt O&M strategies, retrofits, or future designs to improve
overall functionality and performance.
Bioretention facility - Within the context of this handbook,
bioretention facilities encompasses the following GSI practices:
bioretention, rain gardens, bioswales, bioretention planters/boxes,
and tree pits.
Bioretention soil media - See Chapter 2.
Choker layer - See Chapter 2.
Contributing drainage area (CDA) - Refers to the total area
(both pervious and impervious) draining to a bioretention facility.
Energy dissipator - See Chapter 2.
Existing soil - See Chapter 2.
Freeboard - Is defined as the distance between the top of the
facility's overflow elevation and the maximum ponding depth.
Freeboard provides a margin of safety for larger storm events.
Gray infrastructure - Includes piped networks and man-made
engineered components in the built environment, such as drains,
storage tanks, and gutters, that collect and convey stormwater.
Green stormwater infrastructure (GSI) - GSI used in this
handbook is synonymous with the term green infrastructure
defined in the CWA1 as 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. Some use other
terms to reference the same practices as green infrastructure
for stormwater management. Similar terms may include low
impact development, natural infrastructure, and nature-based
solutions. 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. GSI and green infrastructure are both terms used in
planning and research to achieve various ecosystem services.
Green streets - A term used to describe the implementation of
GSI to manage stormwater from transportation infrastructure
such as roads and parking lots.
Loading ratio (LR) - LR is a design parameter equal to the
impervious CDA divided by the bioretention infiltration surface
area. LR is an important design consideration because LRs that are
too high (i.e., greater than 25:1) can lead to maintenance problems
(e.g., excessive sediment accumulation); LRs that are too small
can hinder plant growth and result in nonuniform infiltration
(O'Connor 2023).
1 See Water Infrastructure Improvement Act, 2019.
https://www.cona ress.gov/115/Dlaws/Dubl436/PLAW-115DU bl436.pdf
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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GLOSSARY
Heavy metals - Include metals with a high density such as
arsenic, cadmium, chromium, copper, lead, and zinc. Some heavy
metals are essential to living organisms (copper and zinc) while
others are toxic at trace levels (cadmium and lead).
Hydraulic efficiency - Describes the effective use of the BSM
or IWS volume under saturated conditions. Hydraulic efficiency is
the ratio of the observed hydraulic residence time (for example,
measured during a tracer study) to the theoretical residence time.
A hydraulic efficiency of 1 indicates that stormwater infiltrating
into BSM and/or IWS flows through 100% of the provided media
volume.
Hydraulic residence time - Describes the average length of time
stormwater resides within the bioretention facility.
Inflow - See Chapter 2.
Internal water storage (IWS) - See Chapter 2.
Operation and Maintenance (O&M) - Encompasses the routine
or restorative activities taken over the lifespan of a bioretention
facility to maintain long-term performance. O&M activities vary
based on factors such as the design components implemented,
surrounding land use, and climate. Inadequate or lack of O&M can
lead to system failure.
Outlet - See Chapter 2.
Overflow - See Chapter 2.
Pretreatment components - See Chapter 2.
Right-of-way (ROW) - ROW settings here are defined as public
land adjacent to transportation networks and infrastructure
such as sidewalks, bike lanes, and pedestrian pathways. Typically,
ROWs already have a drainage network in place.
Stone storage/drainage layer - See Chapter 2.
Liner - See Chapter 2. Underdrain - See Chapter 2.
Memorandum of understanding (MOU) - Describes Vegetation - See Chapter 2.
an agreement between two or more parties (for example
a municipality and landowner) that states each party's
commitments and responsibilities. When used in the context
of GSI, MOUs may define agreements on funding of education
and outreach signage, construction, or O&M and define
responsibilities for long-term O&M and facility access.
Mulch layer - See Chapter 2.
BIORETENTION DESIGN HANDBOOK ¦ Designing Holistic Bioretention for Performance and Longevity
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GLOSSARY
mwM-
Educational signs explain the benefits of rain gardens and native plants in Omaha, NE.
BIORETENTION DESIGN HANDBOOK • Designing Holistic Bioretention for Performance and Longevity
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Photo: Kary Phillips
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