Urban Street Stormwater Guide

Urban Street Stormwater Guide

by National Association of City Transportation Officials


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Urban Street Stormwater Guide by National Association of City Transportation Officials

Streets make up more than 80 percent of all public space in cities, yet street space is often underutilized or disproportionately allocated to the movement of private motor vehicles. Excess impervious surface contributes to stormwater runoff, posing a threat to the environment and human health, and often overwhelming sewer systems. This excess asphalt also poses a threat to public safety, encouraging faster speeds and dangerous conditions for people walking and biking. 

The Urban Street Stormwater Guide begins from the principle that street design can support—or degrade—the urban area’s overall environmental health. By incorporating Green Stormwater Infrastructure (GSI) into the right-of-way, cities can manage stormwater and reap the public health, environmental, and aesthetic benefits of street trees, planters, and greenery in the public realm. With thoughtful design, GSI can bolster strategies to provide a safe and pleasant walking and biking experience, efficient and reliable transit service, and safer streets for all users.

Building on the successful NACTO urban street guides, the Urban Street Stormwater Guide provides the best practices for the design of GSI along transportation corridors. The authors consider context-sensitive design elements related to street design, character and use, zoning, posted speed, traffic volumes, and impacts to non-motorized and vehicular access. The Guide documents and synthesizes current practices being developed by individual agencies and recommends design guidance for implementation, as well as explores innovative new strategies being tested in cities nationwide. The guidance will focus on providing safe, functioning and maintainable infrastructure that meets the unique needs and requirements of the transportation corridors and its various uses and users.

The state-of-the-art solutions in this guide will assist urban planners and designers, transportation engineers, city officials, ecologists, public works officials, and others interested in the role of the built urban landscape in protecting the climate, water quality, and natural environment.

Product Details

ISBN-13: 9781610918121
Publisher: Island Press
Publication date: 06/29/2017
Pages: 168
Product dimensions: 8.20(w) x 10.80(h) x 0.60(d)

About the Author

The National Association of City Transportation Officials, NACTO, is a membership network that provides support and resources for city transportation officials in cities of all sizes. 

Read an Excerpt

Urban Street Stormwater Guide

By National Association of City T Officials


Copyright © 2017 National Association of City Transportation Officials
All rights reserved.
ISBN: 978-1-61091-812-1


Streets as Ecosystems

Green Street Principles

Streets are Ecosystems

Why Sustainable Stormwater Management Matters
The Role of Streets
Complete Streets are Green Streets

Green Street Principles


Capturing, filtering, and infiltrating stormwater is critical in urban environments where impervious surface covers 60% or more of all land area. Sustainable stormwater infrastructure filters pollutants from water and restores the natural hydrological cycle, protecting water resources. Green infrastructure also improves air quality, mitigates the urban heat island effect, and increases species habitat, from small oases for birds and insects to the large water bodies that eventually receive stormwater runoff.


Green streets are part of healthy, equitable urban design that views streets as vital public spaces. Incorporating green elements into streets improves mental and physical health through better air quality, valuable shade, beautification, and contact with nature in areas where access to parks is limited. Ensure that the benefits of green stormwater infrastructure are provided equitably, especially in neighborhoods that have historically borne disproportionate air and water pollution or that lack green space.


Street reconstruction projects that incorporate green infrastructure should be aligned with citywide traffic safety and mobility efforts, especially where opportunities arise to move curbs and reallocate street space for people walking and biking. Green infrastructure can be leveraged in conjunction with other street design projects to realize complementary goals, including transit access and safe mobility, providing greater value from city projects.


Green stormwater infrastructure is an asset for cities, providing quantifiable financial benefits. Stormwater management strategies should be planned and implemented with consideration for life-cycle costs and benefits, including the potential impacts of climate change and storm events. Green street elements that are properly designed, operated, and maintained extend the useful life of other infrastructure, especially graywater systems and pavement surfaces.


As the intensity and frequency of storms increases in many cities, and as drought conditions intensify in other cities, sustainable stormwater management is critical for climate change mitigation and adaptation. Incorporating natural systems into the built environment promotes ecosystem health and urban resilience.


Green stormwater infrastructure should be implemented at a network scale, but must be tailored to the specifics of its site. Use an understanding of topography and microclimates, available space, accessibility needs and the many human functions of a street, and desirable infiltration capacity to design appropriate green stormwater systems. Use the street to restore connections to the natural water cycle, and make comprehensive, citywide investments to see watershed-level benefits.

Streets are Ecosystems

The opportunity is ripe to reimagine how streets function in cities, not just as mobility corridors and public spaces, but as part of the natural ecosystem. With re-urbanization, aging infrastructure, and a changing climate, sustainable stormwater management is a core challenge for resilient cities.

Historically, streets have formed an impermeable paved layer on top of green space, disrupting hydrological cycles and requiring expensive stormwater infrastructure to manage stormwater runoff and protect ground and surface water quality. As cities face storm events of increasing frequency, duration, and intensity, as well as more persistent drought conditions, it is time to ask more of our streets.

Urban streets can reconnect rainfall to the environmental life of the city. Forward-thinking planners, engineers, and designers are treating streets as part of the ecological fabric of cities, integrating green infrastructure into the street alongside transit infrastructure and safe places for people walking and biking. By thinking of streets as ecosystems, we can build cities that are more resilient, sustainable, and enjoyable places to live.

Why Sustainable Stormwater Management Matters

Cities are defined by water. Waterways define city edges and boundaries, shape growth and development, and provide essential resources for human populations and the built environment. However, development patterns have too often removed water from urban places, channeling stormwater out of the human environment and therefore restricting natural functions and ecosystem services at great economic expense.

In the past, stormwater has been treated as waste, and stormwater management has meant dispensing of runoff as quickly as possible after a rainfall. This approach has required expensive "graywater" infrastructure: concrete and metal pipes, gutters, tanks, and treatment plants to convey, detain, and treat stormwater before discharging it into local water bodies. In many cities, intense storms overwhelm the gray infrastructure system, resulting in an outfall of polluted water into nearby streams and rivers, and potentially causing impassable streets and flooded homes and businesses. In cities across the country, gray infrastructure systems are under-maintained and reaching the end of their useful lives. Replacing this aging infrastructure can be a prohibitively expensive proposition.

Such a singular approach to stormwater management is no longer possible or desirable. In an age of climate change, urbanization, and increasingly frequent, and intense storms and prolonged, devastating droughts, cities are now treating stormwater as a resource to be valued, not waste to be managed.

Green stormwater infrastructure (GSI) reintroduces ecological functions back into the built environment. Soil-water-plant systems — including biofiltration planters, bioretention swales, trees, and permeable pavements — intercept stormwater before it reaches gray infrastructure. Some water is infiltrated into the ground, some is evaporated into the air, and some is temporarily stored before being slowly released into the sewer system. Green stormwater infrastructure helps to reduce runoff volume to gray infrastructure and filter pollutants, protecting water quality and mitigating risks of flooding. Investments in green stormwater infrastructure complement gray infrastructure and may extend the useful life of major capital street and sewer projects. In addition to its hydrological role, green stormwater infrastructure can offer valuable co-benefits, like calming traffic and beautifying the urban landscape. An integrated approach to green stormwater management in the public right-of-way is central to the design of resilient urban landscapes.

The Role of Streets

Cities are uniquely positioned to take action on sustainable stormwater management.

Concrete and asphalt dominate urban landscapes. Typically in urbanized areas, 60% of land or more is impervious surface. Water that falls on roofs, streets, and parking lots cannot soak into the ground, and instead becomes stormwater runoff, collecting pollutants like oil, grease, heavy metals, and bacteria before flowing through gutters and storm drains, and eventually discharging into local water bodies.

Streets comprise one-third or more of all land and half of the impervious surface in many cities.

Streets are the interstitial spaces that enable cities; they provide a network for all of the dynamic social, economic, and physical activities that make cities vital human habitat. By design, streets channel and convey stormwater, providing a network along which all the rain that falls on the city can be routed. While streets have traditionally functioned to collect and drain stormwater to water treatment facilities and designated outfalls, streets that capture and infiltrate stormwater back into the urban ecosystem can generate enormous ecological, economic, and public health benefits.

Streets present both a barrier to natural hydrology and an enormous opportunity for a better approach to stormwater management. Public rights-of-way are controlled by city agencies, from design to construction to operations to regular maintenance and permitting. Interdepartmental coordination enables more streamlined and holistic projects, ensuring that streets not only collect and infiltrate stormwater, but also realize the potential health, safety, and mobility benefits of urban stormwater street design. Integrated design strategies address water quality and regulatory compliance along with traffic calming, bike and pedestrian access, safety, urban greening and aesthetic improvements, air quality, urban temperature, public health, community development, and equity.

Streets can be changed; the time to act is now.

Complete Streets are Green Streets

A flooded street is not a complete street. During storm events, people walking, bicycling, and using transit are the first users to encounter barriers and lose access to the street, and are the last to regain it. Green street design tools, which integrate stormwater control and management within the right-of-way, are a critical component of complete street design, ensuring the street remains usable and safe for all people during storm events, regardless of mode.

Take into consideration both the impacts of stormwater on multi-modal travel and the potential for green street investments to transform the public realm and create economic, social, and environmental benefits for all street users.


Planning for Stormwater

Developing a Sustainable Stormwater Network

Setting Goals for Stormwater Management
Regional Climate & Ecology
Aligning with Goals for Streets
Case Study: Greenways to Rivers Arterial Stormwater System (GRASS)
Solving the Street Design Puzzle
Retrofitting Streets for Stormwater
Reconstructing Streets for Stormwater

Developing a Sustainable Stormwater Network

Sustainable stormwater management works to reconcile natural hydrology with human land use and development. Typically, a city's stormwater management strategy is driven by federal regulatory requirements, existing sewer infrastructure, and the regional climate and ecology. Plans and strategies take shape within the context of the street, where transportation and utility infrastructure compete for space and zoning codes shape development patterns.

Integrating green stormwater infrastructure into the right-of-way requires a coordinated approach and a holistic vision for sustainable urban design. Planning a stormwater network concurrently with an active transportation network unlocks new opportunities for cities and their streets.

Setting Goals for Stormwater Management

A city's stormwater management goals are generally rooted in regulatory requirements for water quality under the Clean Water Act. City objectives and strategies vary based on the type and capacity of the sewer system, the risk of local flooding, and the need to comply with National Pollutant Discharge Elimination System permits.

Combined Sewer System

Some cities discharge their stormwater into pipes that also receive and convey wastewater or sanitary sewer flows. These types of systems are called combined sewer systems and are most commonly found in older cities; new developments do not build combined sewer systems.

Combined sewer systems are typically connected to waste-water treatment plants that then discharge treated water into a receiving water body. However, during heavy rains, water flows can overwhelm the infrastructure capacity. When pipes and treatment plants are unable to manage the flows, water is discharged directly into receiving water bodies without being treated, in an event called a combined sewer overflow (CSO).

Depending upon an agency's permit requirements with EPA, the number of CSO events that can occur at each overflow point varies. Many agencies that have CSOs exceeding EPA's overflow thresholds are utilizing bioretention facilities and other green stormwater infrastructure (with and without gray storage facilities) to come into regulatory compliance by reducing the volume of stormwater runoff that reaches the combined system, and therefore reducing the frequency and volume of CSO events.

Municipal Separate Storm Sewer System (MS4)

Current gray stormwater infrastructure guidelines require that stormwater runoff be collected and conveyed separately from sanitary sewer lines. In municipal separate storm sewer systems (MS4s), stormwater runoff is often discharged into receiving water bodies with limited or no water quality treatment.

Most cities have MS4 permits, which require new development and redevelopment projects to install water quality treatment facilities and/or flow control facilities prior to discharging runoff into receiving water bodies. The permits may also require retrofits to existing sites or streets to treat and reduce runoff.

Green stormwater infrastructure is a cost-effective way to come into compliance with regulatory requirements and create other ecological and social benefits. The EPA strongly encourages the use of green infrastructure to manage stormwater and meet federal water quality requirements. Green infrastructure projects are generally designed to complement gray infrastructure systems performing a combination of volume management, water quality improvement, and flood control.


Since the volume of stormwater runoff increases as impervious surface area increases, the two major strategies related to volume management are to increase pervious area or to divert runoff into the green infrastructure system. Green stormwater infrastructure systems are designed to convert surface area from impervious to permeable, and reduce the volume of runoff that reaches the sewer system or downstream water bodies, reducing the burden on gray infrastructure systems and infiltrating stormwater runoff directly back into the soil. Green infrastructure projects and programs set specific stormwater volume management goals, such as the first inch of rainfall on a given area.


Stormwater runoff from streets carries sediment, debris, chemicals, and pollutants (such as heavy metals from brake pads, oil dripping from engines, and grit from tires). Green stormwater infrastructure projects are designed to capture pollutants in runoff and prevent them from reaching downstream water bodies. Water quality treatment requirements vary based on the type of receiving water body (ocean, salt water bay, river, stream, wetland, or lake) as well as its existing condition. Green infrastructure projects often set a specific water quality goal, such as removing 80% of total suspended solids (TSS).


Especially heavy rainstorms can cause combined sewer overflow events as well as flooded streets, parking lots, private property, and basements. Green stormwater infrastructure programs aim to reduce peak flow rates and mitigate flooding. Cities may design to accommodate the high runoff flows and flood risks of a given peak storm event, such as a ten-year storm (a storm that has a 10% chance of occurring in any given year).

Regional Climate & Ecology

Sustainable stormwater management aims to reconnect the natural water cycle. Green infrastructure intercepts stormwater runoff at its source and does one or more of the following, depending on project goals:

inform stormwater management needs and goals at the programmatic and project levels. Designers must consider the regional climate conditions, today and in the future.

At a most basic level, it is important to consider the amount of precipitation that falls, and the rate at which water evaporates from the surface or infiltrates back into the ground. Local weather and climate determine precipitation patterns and temperature, and soil composition affects how quickly water is absorbed into the ground. These factors determine regional sizing and modeling coefficients for runoff and infiltration.


Evapotranspiration rate, or the amount of water that can be evaporated or transpired from a surface over a specified duration, varies by latitude, topography, altitude, wind conditions, and time of year, as well as surface characteristics such as permeability, albedo, and types of vegetation.

In some climates, evapotranspiration varies widely through the seasons; green infrastructure should be designed with consideration for high and low expected conditions. Irrigation may be necessary during the very dry season, while fast infiltration is desirable during the very rainy seasons. In cold climates, snow storage is critical during winter.


Green infrastructure elements and systems may be designed to infiltrate a specified amount of stormwater; for example, a system goal may be to infiltrate the first inch of precipitation of each storm event in a tributary area, or to match pre-development runoff conditions.


Excerpted from Urban Street Stormwater Guide by National Association of City T Officials. Copyright © 2017 National Association of City Transportation Officials. Excerpted by permission of ISLAND PRESS.
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Table of Contents

About the Guide
How to Use the Guide
Chapter 1. Streets as Ecosystems
Green Street Principles
Thinking of Streets as Ecosystems
-Placemaking vs. Performance
-Planning the Water Network
-Integrating Green Stormwater Infrastructure
-Regulatory Layers
Chapter 2. Stormwater Streets
The Stormwater Network
-Ultra Urban Green Street
-Green Transitway
-Multi-Way Boulevard
-Neighborhood Main Street
-Neighborhood Street
-Commercial Shared Street
-Residential Shared Street
-Green Alley
-Stormwater Greenway
-Industrial Street
-Reclaimed Intersection
Chapter 3. Designing for Stormwater
General Design Considerations for a Street
Siting GSI in the Right-of-Way
-Planting Strip/Amenity Zone
-Curb Extensions
-Curbside parking/bike lane
-Travel Lanes
-Floating BMPs
Green Infrastructure Elements
-Bioretention with Graded Side Slopes
-Bioretention with Vertical Walls
-Bioretention with Hybrid Sides
-Tree Wells
-Tree Trenches
-Permeable Pavements
Green Infrastructure Design
-Length of a Bioretention Cell
-Bottom Area
-Presettling or Pretreatment
-Inlet/Inflow Design
-Overflow/Outlet Design
-Trees and Plantings
-Subsurface Infrastructure
Chapter 4. Design Considerations
-Street hydrology and stormwater runoff
-Volume Managed vs. Water Quality Design
-Conveyance systems
-Drainage basin type
-Geotechnical/Hydrogeological conditions
-Service and franchise utilities
-Weather/Climate of a region
-Existing infrastructure and adjacent buildings
-Regulatory Framework
Chapter 5. Partnerships & Performance
-Citywide Plans & Strategies for Stormwater Management
-Interdepartmental/Interagency coordination
-Public Outreach
-Operations and Maintenance
Performance Measures

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