Flashback to 2006: Research Report on Decentralized Stormwater Source Controls Defined the State-of-the-Practice for CSO Reduction
Capture Rain Where It Falls
In the United States and parts of Canada, combined sewer systems are the legacy of past engineering practices which have reflected a collect and convey philosophy and associated approach to rainwater management. Rainwater runoff that is diverted from roads, parking lots, and the roofs of buildings during storm events results in ‘combined sewer overflows’ (CSOs).
To address this issue, the Water Environment Research Federation (WERF) in 2005 commissioned a research initiative that had two main objectives:
- provide a document that defines the state-of-the-practice regarding decentralized source controls for capturing rainwater where it falls; and
- present a plan for implementation of decentralized controls in an urban environment specifically for the goal of Combined Sewer Overflow (CSO) mitigation.
The research was funded in part by the United States Environment Agency; and was undertaken by a team led by Neil Weinstein, Executive Director of the Low Impact Development Center in Maryland. The research findings were published in 2006.
To order a copy of the report from WERF, click on Decentralized Stormwater Controls for Urban Retrofit and Combined Sewer Overflow Reduction.
Develop Greener Communities
“Decentralized controls have the potential to reduce the occurrence and volume of CSO events by using natural hydrologic cycle elements to dampen the stormwater surges that overwhelm the conveyance capacities of combined systems. The decentralized 
approach is flexible, dynamic, and allows communities to be responsive to evolving economic and environmental conditions,” explained Neil Weinstein.
“The primary focus of the applied research was how decentralized controls can reduce the volume of rainwater runoff generated and, consequently, entering the combined sewer system in urban areas.”
“Capturing rainwater where if falls offers appealing technical alternatives to stormwater runoff capture than conventional end-of-pipe measures. Decentralized controls have the potential to reduce the frequency and volume of CSO events. In addition, a decentralized approach to stormwater management allows communities the flexibility to respond to everchanging economic and environmental conditions.”
“Capturing runoff where it falls in urban areas introduces ancillary benefits into the community that extend beyond runoff volume reduction. The use of decentralized source controls in conjunction with redeveloping land in urban regions creates opportunities, over time, to develop greener communities that will achieve higher levels of ecological and receiving water protection,” added Kim Stephens, the Canadian member of the research team. His responsibilities included synthesizing the research findings in all-encompassing Executive Summary.
“Benefits stem from the fact that implementation of green infrastructure reintroduces natural processes and functions into highly urbanized environments. The goal in ‘designing with nature’ is to achieve a community vision for sustainability and livability that will yield tangible environmental, community, educational and economic benefits.”
Overcoming Fear and Doubt
“The challenges in retrofitting collection areas with decentralized controls are considerable because most combined systems are found in highly urbanized areas that are typically characterized as highly connected impervious surfaces, aged infrastructure, and limited pervious or open areas,” continued Kim Stephens.
“Because the retrofit focus is primarily on individual properties, physical conditions and perceived concerns take on added significance.”
“A key challenge is overcoming fear and doubt regarding hydrologic performance, mainly because decentralized controls for rainfall capture and runoff volume reduction rely to a large extent on landscaping-type solutions. This means their feasibility often depends on soil characteristics.”
“Fear and doubt, even if without foundation, can have a material bearing on community acceptability of an implementation strategy for runoff volume reduction.”
Five-Step Framework
“Urbanized areas have a broad mix of land uses, distinctive community characteristics, complex environmental and design regulations, and a wide range of community and economic goals. Hence, development of a customized implementation strategy for retrofitting decentralized controls in an urban area is an iterative process because quantity and quality benefits in capturing rain where it falls must be balanced with planning goals and site constraints.” stated Neil Weinstein.
“The research team created a five-step framework for evaluating and selecting source control elements of a rainwater management implementation strategy.”
“Step 1 defines the problem by clarifying the drivers—reasons why the retrofit is occurring. The CSO mitigation strategy must consider primary and secondary watershed goals, which may be long term and independent of CSO mitigation goals.”
“Step 2 characterizes the site by evaluating the type of project – that is, whether redevelopment or retrofit; analyzing land cover and soil; and identifying hot spots such as flood prone areas or industrial districts.”
“Once the drivers, the watershed planning goals, and the site characteristics are well understood, specific decentralized controls can be evaluated for their suitability, considering feasibility and design variables in Step 3.”
“The project is then, in Step 4, analyzed for cost effectiveness in the context of the goals and site characteristics.”
“In Step 5, the project planners will select one or more appropriate decentralized controls, or none if they select a no-build option. The selected controls should reflect the work done in Steps 1 through 4, and be based on the overall watershed and natural resource protection goals.”
To Learn More:
To download a 2-page overview of the research prepared by WERF, click on Executive Summary: Decentralized Stormwater Controls for Urban Retrofit and Combined Sewer Overflow Reduction (2006).
To download a copy of the complete report, click on Decentralized Stormwater Controls for Urban Retrofit and Combined Sewer Overflow Reduction.
Acknowledgement: Low Impact Development Center, Maryland, USA