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Memorandum :� Bonestroo 2335 Highway 36 W To: Mark Statz Project: Centerville Downtown Date: 9/20/2007 St. Paul, MN 55113 From: Earth Evans Client: City of Centerville Tel 651-636-4600 Fax 651-636-1311 Re: Stormwater System File No: 616-07-161 www.bonestroo.com Remarkr. This memo summarizes our preliminary stormwater system design and analysis for the Centerville Downtown Redevelopment project. The existing site is primarily developed with a ' mix of residential and commercial land uses. Proposed development includes medium and high density residential and mixed use commercial sites. Proposed stormwater management measures will be designed to meet the City of Centerville, Rice Creek Watershed District (RCWD) and Minnesota Pollution Control Agency National Pollution Discharge Elimination System (MPCA NPDES) General Construction Permit requirements. The most sVingent of these requirements in terms of stormwater management is RCWD. Compliance with these requirements will meet or exceed the requirements of the City and NPDES. City and Bonestroo staff met with RCWD to discuss their proposed new rules on 9/13/07. These rules are slated to be accepted by the RCWD Board in December of 2007 and will therefore apply to the proposed site. Based on these discussions, infiltration and bioretention practices for the site were sized on the following criteria. • For all new im�ervious - best management practices (BMPs) designed to infiltrate and/or retain the runoff volumefrom the two-year (2.8 inch) storm event. Proposed new impervious (assuming 50-75% impeNious) = 7 acres • For all existing impervious - the treatment requirement is to provide infiltration volume adequate for the 0.8-inch event. Existing impervious (including offsite drainage area) = 9 acres. Rate control is also required, but this is generally met by providing BMPs sized to meet the above requirements. A total of approximately 2.2 ac-ft of bioretention volume is required. Note that this sizing criterion is different than the verbiage in the rules dated June 13, 2007. At the writing of this memo, RCWD had not made a final decision on the redevelopment standard, but based on discussions with Carl Almer at EOR this method was the original irrtent of the rule and has therefore been pursued. Based on the Soil Survey, soils are roughly 60% hydrologic soil group (HSG) B/D and 40% HSG B. The BMPs provided to meet the above RCWD requirements will include a combination of several features — rain gardens, depressed parking lot islands, pervious pavement with underground storage, and stormwater ponds as shown on the attached figure. An innovative method of utilizing the stormwater generated from downtown to irrigate nearby LaMotte Park is also proposed.