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Detain H20 Detention Basin Retrofit Device
Citation:
Goodrich, J. AND J. Hall. Detain H20 Detention Basin Retrofit Device. U.S. EPA Office of Research and Development, Washington, DC, EPA/600/F-20/134, 2020.
Impact/Purpose:
Detention ponds are stormwater management structures that temporarily collect runoff and then release a reduced flow to decrease the risk of flooding. Detention ponds are frequently used as a stormwater runoff best management practice to provide general flood protection, lessen extreme floods, and improve water quality. This brief describes studies to design and test detention pond outfall retrofit devices for their effectiveness in eliminating stream erosion, improving receiving stream water quality, and providing the capability to respond to and to mitigate wide-area contamination incidents.The Detain H2O Retrofit Device improves the performance of existing detention basins by reducing erosive flows in receiving channels and improving water quality. The low-cost technology prolongs storage times in the basins by restricting the discharge below the critical threshold for erosion in the receiving stream (Qcritical). The device can abate downstream bank erosion and total suspended solids loads, enhance channel stability and aquatic habitat, and restore biota. Low to medium storm events (~<2-yr occurrence) will pass through the throttled retrofit device and optional filter media and be released to reduce channel erosion. A portion of larger flows (such as the 100-yr recurrence) can be routed through the bypass to provide similar performance to the original flood control detention basin design. Various types of media were shown to be able to remove multiple contaminants such as nutrients, bacteria, and radionuclides that can be found stormwater runoff.
Description:
Bench-scale, pilot-scale, and field-scale tests were performed to evaluate the function of two innovative outfall retrofit devices that can be quickly deployed to control stormwater contamination events within existing detention basin structures. The devices were designed to improve long-term stream water quality by reducing scouring of stream beds, providing treatment of contamination that lead to stream impairment, and reducing the spatial extent of large volumes of contaminated water from wide-area contamination incidents and mitigation efforts. A wide variety of media can be installed within the devices to remove the targeted contaminants expected to be in the stormwater. An experimental system was installed at the U.S. Environmental Protection Agency’s Test & Evaluation Facility located in Cincinnati, Ohio, to simulate a stormwater basin and associated detention basin retrofit device. The pilot-scale system was designed to evaluate flow rates and media performance prior to field-scale deployment. Different types of media were evaluated in this experimental system. Flow rates were also developed to determine if the media would impede flow exiting the detention basin too much and cause flooding. Full-scale retrofit devices were also installed in actual detention basins. Full-scale installations of two variations of the detention basin retrofit prototype device demonstrated that outlet flow rates were maintained below Qcritical while doubling the detention time within the basin without causing flooding of the adjacent area. All the media exhibited > 72% removal of nitrogen and >56% of phosphorous; The natural zeolite, switchgrass, ferric oxide powder, and coated gravel exhibited the best removal (>90%) of cesium (radioactivity surrogate). Iron composite metal reduced E. coli (used as a bacterial contamination surrogate) levels by 8 logs followed by ferric oxide powder and natural zeolite (6 logs). Switchgrass exhibited an unexpectedly high removal capacity (4 logs).Subsequent investigations of the stream downstream from the retrofitted detention basin showed that by restricting discharges below the threshold flow for erosion in the receiving channel, a concurrent benefit was the conversion of the stream from one that used to go dry approximately 10% of the time to a perennial resource with pools supportive of native minnows observed during seasonal low flow periods. It can also provide enough time for vegetation to successfully colonize recently deposited sediment at the toes of otherwise unstable streambanks.Traditional flow control strategies requiring significant excavation can cost 5 to 10 times as much as the Nessie. The cost of the constructing and installing the retrofit device with media for a 24-inch outfall was $2000 and the fully installed retrofit costs were another $8000. It is difficult to envision a comparable level of ecological recovery and improvement from a $10,000 conventional in-stream habitat restoration project.