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Low-Cost Wide-Area Stormwater Retrofit Flow and Quality Management Devices
Citation:
Goodrich, J., A. Mikelonis, J. Hall, AND A. Lehmann. Low-Cost Wide-Area Stormwater Retrofit Flow and Quality Management Devices. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-23/108, 2023.
Impact/Purpose:
Within a stormwater infrastructure system, the focus has been primarily the management of peak storm event flows, not emergency response preparedness or resilience. It has been observed that management of contaminated water is often an after-thought when first responding to a natural or human caused disaster (e.g., a bomb, an industrial explosion, a cross-connection, a train derailment, or a tanker truck wreck). Treating the sick and injured, fighting fires, and preventing explosions are the immediate priority; however, decisions during the initial hours following an incident can have tremendous impacts on public and aquatic health long after the event. There is currently no tool available to stormwater managers that can help them in deciding what containment, treatment, or diversion resources should be placed in which locations to optimally reduce the risk to human and aquatic health. It would be best to integrate a variety of stormwater retrofit devices within an urban watershed to provide not only hazard mitigation, but also reap the benefits of reducing erosive flows and improving water quality, thus improving compliance with the Clean Water Act, or restoring impaired stream segments. Borrowing from the drinking water treatment multiple barrier concept of incrementally treating different contaminants via a treatment train, adsorbants can be a low-cost viable option for treating urban stormwater. Passive structural measures can be retrofit into existing grey stormwater infrastructure in urbanized watersheds to increase capacity and capabilities without the need for expanding the footprint or the cost and oversight of active controls. Three different stormwater infrastructure sites were retrofitted in a single urban catchment with “Do It Yourself” (DIY) type of devices designed to treat the stormwater while moderating flow rates: a storm sewer outlet forming the headwaters of an urban creek, a Detention Basin, and a Retention Basin. In all three installations, the retrofit devices were very inexpensive, easy to install and operate, and did not lead to any localized flooding. It was shown that low-cost retrofit devices can be safely installed throughout an urban catchment, but that there needs to be improvements in the design of the devices to improve water treatment efficacy. Longer term flow-weighted sampling of water quality and downstream benefits is also necessary to truly ascertain the value of retrofitting urban stormwater infrastructure.
Description:
The purpose of this research was to determine the viability of installing low-cost storm sewer retrofit devices in multiple locations within an urban watershed. A major component of this was the necessity for designing, fabricating, and operation of such retrofits which are not common in the stormwater industry, especially at a cost and ease of operation and maintenance that would enable an urban catchment to benefit from many installations. There is the potential for adding real-time control of valves, weirs, and pumps to increase the capacity and capability of urban stormwater infrastructure, but that remains to be proven as viable given the knowledge gap and unknow long-term capital and maintenance costs. There is a hybrid approach that incorporates real-time data with passive retrofits of existing grey stormwater infrastructure. This is similar to the drinking water multiple barrier approach where different types of unit processes are installed in a treatment train to treat raw water in a stepwise manner. Three different stormwater infrastructure sites were retrofitted in a single urban catchment with “Do It Yourself” (DIY) type of devices designed to treat the stormwater while moderating flow rates. A spreadsheet-based storm sewer retrofit device methodology framework using the Storm Water Management Model (SWMM) was also developed and tested to determine if a flap gate or riser retrofit would be safe and applicable retrofit options for the storm sewer outlet. The methodology indicated that the catchment area served in conjunction with the large outlet pipe (6’ diameter) would not be practical in cost or effectiveness and could lead to localized flooding. An alternative retrofit was designed and installed in the creek channel that consisted of primarily a corrugated pipe filled with proprietary stormwater media. A commercially available plastic storage tote was modified and installed in-line with the outlet pipe and concrete box outlet at the detention basin. Commonly available granular activated carbon and switchgrass were used separately as treatment media. The retention pond retrofit consisted of a corrugated pipe that was connected externally to the outlet structure and filled with granular activated carbon and switchgrass respectively. In all three installations, the retrofit devices were very inexpensive, easy to install and operate, and did not lead to any localized flooding. However, there was significant bypass of the media at higher storm events and minimal contact time which resulted in little improvement in water quality exiting the stormwater infrastructures. It was shown that low-cost retrofit devices can be safely installed throughout an urban catchment, but that there needs to be improvements in the design of the devices to improve water treatment efficacy. Longer term flow-weighted sampling of water quality and downstream benefits is also necessary to truly ascertain the value of retrofitting urban stormwater infrastructure.