Technical BRIEF INNOVATIVE RESEARCH FOR A SUSTAINABLE FUTURE www.epa.gov/research Detain H20 Detention Basin Retrofit Device The 'Nessie" BACKGROUND 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 better response and mitigation efforts for wide-area contamination incidents. Contaminants within detention ponds could be removed prior to discharge of the collected pond water to surface water bodies or to municipal wastewater treatment systems. Contaminants could also enter the ponds from the discharge of water used in cleanup or mitigation operations during homeland security events, such as biological, chemical, or radiological incidents. Contaminated stormwater can be generated as a result of intentional incidents (e.g., terrorist attacks) as well as unintentional incidents (e.g., natural disasters, industrial spills, transportation accidents) from: • Washdown activities involving chemical, biological, or radiological agents from indoor- outdoor areas • Water from decontamination activities, such as extinguishing industrial fires • Stormwater runoff during an incident or water infrastructure decontamination activities The Detain H20 Retrofit Device, nicknamed Nessie (Figure 1), improves the performance of existing detention basins by reducing erosive flows in receiving MJ. rT^SBB^Sji Figure 1. Nessie deployment prior to and during a large storm event. 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 U.S. Environmental Protection Agency EPA/600/B-18/320 | May 2020 ------- performance to the original flood control detention basin design. RESEARCH APPROACH 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 led 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 (Figure 2). RESULTS MEDIA EVALUATION The media evaluated included: • gravel coated with an adsorptive media • switchgrass • granular activated carbon • natural zeolite • iron composite metals • ferric oxide coated media Figure 2. Modified Detain H20 device with perforated pipes containing media. As indicated in the table (Figure 3), below, all the media exhibited > 72% removal of nitrogen and >56% of phosphorous; these nutrients are typically related to harmful algal blooms in drinking waters. 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). U.S. Environmental Protection Agency ------- Media Description Nutrients Radioactive Bacteria Total N (% Removal) NH3-N (% Removal) Total P (% Removal) P04-P (% Removal) Cesium (% Removal) E. coli (Log Removal) Coated Gravel 90.0 78.0 100.0 86.0 92.0 0.0 Ferric Oxide Powder 76.0 78.0 100.0 98.0 94.0 6.0 Switchgrass 92.0 76.0 64.0 90.0 94.0 4.0 Activated Carbon 94.0 76.0 90.0 84.0 80.0 4.0 Natural Zeolite 94.0 80.0 88.0 86.0 96.0 6.0 Granular Ferric Oxide 66.0 74.0 100.0 100.0 NT 2.0 Sintered Metal with Cu 72.0 78.0 56.0 54.0 NT 2.0 Iron Composite Metal 80.0 80.0 100.0 100.0 NT 8.0 Figure 3. Effectiveness for removal of chemical, radioactive, The media exhibited a wide range of permeability, which reflects how quickly the treated water can exit the detention basin via the media. Most localities require detention basins to be emptied within 48 hours. The coated gravel, switchgrass, granular ferric oxide, activated carbon, and natural zeolite adequately allow flow to exit the detention basin within that time frame. The iron composite metal and sintered metal with copper may require an additional 24 hours, whereas the ferric oxide powder is not likely to be able to meet these flow requirements. Another practical consideration for the widespread use of media to treat contaminated stormwater is the cost. The ferric oxide powder was by far the most expensive media at $16.33/lb with switchgrass being the least expensive at $.20/lb. The remaining media were primarily around $3.00/lb with none exceeding $5.00/lb. oiogica/ contaminants in media. REAL-WORLD INSTALLATIONS 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. Post-retrofit detention basins safely detained storm events that exhibited more than twice the total precipitation and rainfall intensity of pre- retrofit storm events. A detailed depiction of a post-retrofit storm event (Figure 4, below) highlights the three hours of ponding that was induced by the retrofit device, resulting in a prolonged release of a peak discharge that was over five times less than the peak inflow (3.88 ft3/s compared to 20.5 ft3/s). Total precipitation of this storm was 1.3 inches with a peak intensity of 0.55 in/hr. 25 0.5 20 0.4 I -£ £ . 2 §lS 0.3 a ro C & f 10 0.2 1 i e 2 ¦ "¦ E 5 i f k ji 0.1 g 0 0 6/4/2014 0:00 6/4/2014 12:00 6/S/2014 0:00 Data/Tim* Outflow 1 nf low j Pipes 1 & 2) Rainfall at Basin In summary, post-retrofit events had greater rainfall depths, peak intensities, and shorter durations than the pre-retrofit events, but were discharged at less than or equal to the peak discharge of the pre-retrofit events. Figure A. Incremental rainfall depth at basin with flow rates for inflow and outflow. U.S. Environmental Protection Agency ------- contaminants from roads and vehicles; pesticides and fertilizers from agricultural and residential application; and chemicals from industrial, transportation, or nuclear incidents. The retrofit design approach recognizes the role of the geomorphic setting in connecting watershed hydrology with stormwater infrastructure. A Nessie retrofit strategy that is able to meet ecologically and geomorphically relevant hydrologic design goals within the limits of the existing facility has the potential to be much more cost-effective. Traditional flow control strategies requiring significant excavation can cost 5 to 10 times as much as the Nessie. The cost of constructing the Nessie 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. Figure 5. Stream stabilization 3 years after retrofit (adapted from Haw/eyetal 2020, Geomorphology 32\ 106998.) AQUATIC AND COST BENEFITS FROM RETROFIT DEVICES 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 would 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 (Figure 5). Future federal or state stormwater regulations are likely to require some level of water quality improvement. Although the retrofit devices will increase the residence time and reduce sediment in the water column to some degree, there still exists the need to reduce dissolved water quality contaminants such as synthetic and volatile organic U.S. Environmental Protection Agency ------- CONTACTS TECHNICAL CONTACTS • Jim Goodrich, goodrich.iames@epa.gov • John Hall, hall.iohn(a)epa.gov COMMUNICATIONS CONTACT • Lahne Mattas-Curry, mattas-curry@epa.gov ADDITIONAL INFORMATION Sinha, Rajib, James A. Goodrich, and John S. Hall, 2018. Evaluation of Stormwater Detention Basins to Improve Water Quality and Enable Emergency Response During Wide-Area Contamination Incidents, EPA/600/B-18/320 | October 2018 Homeland Security Research Hawley, Robert J., James A. Goodrich, Nora L. Korth, Christopher J. Rust, Elizabeth V. Fet, Craig Frye, Katherine R. MacMannis, Matthew S. Wooten, Mark Jacobs, and Rajib Sinha, 2017. Detention Outlet Retrofit Improves the Functionality of Existing Detention Basins by Reducing Erosive Flows in Receiving Channels. Journal of the American Water Resources Association (JAWRA) 1-16. https://doi.org/10.llll/1752-1688.12548 Disclaimer: The U.S. Environmental Protection Agency (EPA) through its Office of Research and Development funded and managed the research described herein under Contract EP-C-12-014 with Aptim. It has been subjected to the Agency's review and has been approved for publication. Note that approval does not signify that the contents necessarily reflect the views of the Agency. Any mention of trade names, products, or services does not imply an endorsement by the U.S. Government or EPA. B U.S. Environmental Protection Agency ------- |