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EPA Science Matters Newsletter: From Gray to Green - Helping Communities Adopt Green Infrastructure

EPA’s “Greening CSO Plans” report is a resource for communities looking for innovative, lower-cost ways to reduce combined sewer overflows.

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More than 700 cities across the United States use combined sewer systems, which are designed to funnel both sewage and stormwater in the same large pipes leading to facilities that treat the water before releasing it into nearby waterways. During moderate to heavy precipitation, these systems often exceed capacity, meaning the treatment part of the equation is bypassed. Instead, a mixture of stormwater, untreated sewage, and pollution in the form of commercial and industrial waste, toxic materials, and debris can flow directly into streams, rivers, and other waters.

For many of these cities, combined sewer overflows (CSOs) remain one of the greatest challenges in meeting water quality standards. EPA researchers and their partners are helping them find solutions. 

In March 2014, the Agency released Greening CSO Plans: Planning and Modeling Green Infrastructure for Combined Sewer Overflow (CSO) Control, a free publication that provides municipalities and sewer authorities with information about available tools and techniques to help them calculate green infrastructure contributions to overall water management strategies. 

This technical resource is intended to assist communities in developing and evaluating Combined Sewer Overflow (CSO) control alternatives that include green infrastructure. It is designed to provide municipal officials as well as sewer authorities with tools to help quantify green infrastructure contributions to an overall CSO control plan. 

From Greening CSO Plans: Planning and Modeling Green Infrastructure for Combined Sewer Overflow (CSO) Control.

EPA supports and encourages the use of green infrastructure for stormwater runoff and sewer overflow management because of its environmental, economic, and social benefits. Green infrastructure contributes to runoff reduction and often offers less expensive alternatives to replacing and upgrading sewer mains or making other improvements to traditional, “gray” infrastructure.Informed largely by EPA research on green infrastructure techniques, Greening CSO Plans will help communities make cost-effective decisions to maximize water quality benefits. The resource explains how to use modeling tools such as EPA’s Storm Water Management Model (SWMM) to enhance green infrastructure and reduce sewer overflow.

Green infrastructure also supports the principals of Low Impact Development (LID), an approach to land development (or re-development) that works to tap the “ecosystem services” of natural and naturalistic landscapes to manage stormwater as close to its source as possible.

Both low impact development and green infrastructure mimic natural hydrologic processes that absorb and or store stormwater in place, reducing the volume that flows into combined sewer systems. Green infrastructure can also provide the flexibility needed to build resiliency into plans that need to account for uncertainties related to climate change, which is expected to increase the frequency and severity of the kind of weather events that spark combined sewer overflows.

Several communities that have planned for green infrastructure as part of their stormwater runoff management strategies are presented as case studies in the report. One illustrates the use of EPA's Storm Water Management Model (SWMM), a downloadable computer application used throughout the world for planning, analysis and design related to stormwater runoff in urban areas. The newly upgraded model (SWMM Version 5.0) allows users to explicitly incorporate a variety of green infrastructure practices into their strategies for increasing the capacity of water infrastructure.

Several other case studies presented illustrate how communities can estimate the impact of green infrastructure on runoff management:

  • The San Francisco Public Utilities Commission modified its baseline collection system model for estimating the hydrology and runoff portion of its combined sewer system model. Using green infrastructure, they expect to reduce annual combined sewer overflows by some 200 to 400 million gallons (14 to 27 percent).
  • The Northeast Ohio Regional Sewer District performed a systematic evaluation of where to best implement green infrastructure measures within their service area. Using a ranking-based tool, they identified the most promising sewersheds and most appropriate practices within a given service area.
  • The Metropolitan Sewer District of Greater Cincinnati conducted a planning effort that included replacing some of the impervious cover of roadways, rooftops, parking lots and sidewalks with bioretention and permeable pavement, green roofs, and rain gardens to illustrate the effects of green infrastructure implementation.

Greening CSO Plans: Planning and Modeling Green Infrastructure for Combined Sewer Overflow (CSO) Control is one of many ways in which EPA researchers and partners are working together to advance green infrastructure to help reduce the amount of raw sewage and other pollution flowing into the nation’s waterways.

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