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Green Retrofit Technology for Detention Basin Outlet Control Structures
Citation:
Hawley, R., J. Goodrich, J. Beaulieu, AND K. MacMannis. Green Retrofit Technology for Detention Basin Outlet Control Structures. Presented at Mississippi River Gulf of Mexico Watershed Nutrient Hypoxia Task Force meeting, Louisville, KY, April 18, 2013.
Impact/Purpose:
To inform the public
Description:
Urbanization and improperly managed impervious surfaces alters the hydrology of a watershed, leading to increased runoff volumes, higher and/or longer lasting peak flows, and more frequent runoff events. These hydrologic and hydraulic modifications can impact every aspect of stream health including physical, chemical, and biological processes. Increased erosive flows can amplify stream bank erosion, stream bed down cutting, and stream instability, which negatively impact water quality (i.e. increased suspended solids), biological communities (through habitat disruption and/or loss) and endanger infrastructure located adjacent to streams necessitating costly repairs. Investments in conventional stormwater controls such as detention and retention basins have not adequately protected receiving waterways from the undue effects of the excess runoff due to the nearly exclusive focus on flood control. By optimizing designs to match pre-developed peak flows for large events such as the 2-, 10-, 25-, 50-, and 100-year storms, conventional detention basins designed for peak-flow control have little to no attenuating effect on 97-99% of precipitation volume in a typical year (Emerson et al., 2003; Hawley, 2012) and can cause substantial increases in durations of geomorphically-effective flows resulting in corresponding channel instability and enlargement (MacRae, 1997). Consequently, many stream channels across the U.S. have become unstable, with active erosion, channel enlargement, and degraded habitat. Furthermore, stream bank collapse, widening, and downcutting are causing major impacts to our nation’s transportation and utility infrastructure (estimated damages on the order of $1B/year to state highways alone (Hawley et al., 2013)). Conventional stormwater detention basins are ubiquitous in the developed portions of U.S. particularly those areas developed since the 1980s. They are abundant stormwater assets that are not being utilized to their fullest potential, as most detention basins were designed for flood control alone and do not include any measures to mitigate erosive flows causing channel instability from hydromodification. Underutilized detention basins present an opportunity to use a cost-effective retrofit device to mitigate the erosive energy and durations of small and intermediate storm events without adversely impacting the facility’s flood control capacity. The EPA National Stormwater Rule Revisions to the Clean Water Act are anticipated to include a component that requires some level of retrofitting of existing impervious areas due the widely documented and detrimental impacts that poorly mitigated impervious surfaces have on receiving waters. Detention basin retrofits that are optimized to reduce the duration and frequency of erosive flows on receiving streams could prove to be an extremely cost effective strategy to achieving the goals of the Clean Water Act, particularly when compared to alternatives such as 1) installing new storage (e.g. bioinfiltration basins, underground chambers, etc.), or 2) reducing runoff volume by converting impervious surfaces to pervious (e.g. porous pavement, green roofs, etc.).
