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Updating the Ann Arbor, Michigan WWTP Site to Manage Stormwater Using Low Impact Solutions

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The City of Ann Arbor is making improvements to a portion of its Wastewater Treatment Plant (WWTP). These improvements will affect 30 percent of the Site and occur primarily on the west area of the WWTP. The interior drainage area of the Site is approximately 28 acres; however, 9.34 acres of that is open process capture and generates zero runoff. Rainwater that enters the open process areas is treated as it goes through the WWTP, including UV disinfection prior to discharge to the river. There is one outlet for each of the drainage areas - east and west - that flow by gravity during normal water levels on the Huron River and by pump when the River experiences flood levels. These outfalls operate as authorized by NPDES General Permit (MI0022217) and the WWTP has a Stormwater Pollution Prevention Plan with certified stormwater operators working at the facility 24 hours/day, seven days/week.

Compliance with local stormwater management standards and the incorrect delineation of a FEMA floodplain and floodway on the existing site have caused significant challenges and delays for completing necessary WWTP improvements. To address stormwater management requirements, low-impact stormwater solutions will be part of the west area design to reduce stormwater quantity and improve quality. Rain gardens, grass swales, porous pavement, and zero-net runoff areas will reduce runoff at the source. In addition, hydrodynamic separators will be installed at each stormwater pump station. The quantity analysis upon which the pipe network was sized follows local design standards. The volume of stormwater requiring detention, as established by these guidelines, is not feasible or realistic to accomplish on this Site, and a preversus post-runoff analysis was performed to prove that the use of alternative low-impact solutions was acceptable. The FEMA 100-year floodplain will be revised for this project by application for a CLOMR with associated improvements to the existing floodwall and embankment system.
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Keywords: CLOMR; WWTP; floodplain; grass swales; low impact; porous paving; rain gardens; stormwater

Document Type: Research Article

Publication date: 2010-01-01

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