2007-11-14 CC Packet - Laserfiche WebLink

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~. " "t' , " "'. .,:.: ,: ::"-, ' '''';. ..:.,.." '::' " : '. " .', ' .' "'... . "-'';:;'aridMinnesota Pollution Control Agency National Pollution Discharge Elimination System (MPCA NPDES) General Construction Permit requirements. The most stringent 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. The stormwater management features for the site will be designed to meet RCWD's proposed new rules, dated June 13, 2007. Some highlights from these rules are: . Water quality and infiltration best management practices (BMPs) designed to infiltrate and/or retain the runoff volume from the two-year (2.8 inch) storm event. . For existing impervious surfaces that will remain after the redevelopment, the treatment requirement is to provide infiltration volume adequate for the 0.8-inch event. · Proposed discharge rates must be less than or equal to pre-development discharge rates. Therefore pre-developed conditions were assumed for the downtown area with a curve number associated with wooded/grass areas. 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, pretreatment and infiltration basins. The BMPs will be designed to drawdown within 48 hours as required by the RCWD rules and the NPDES requirements. As discussed above, the project will require an NPDES permit because it will disturb more than one acre of land. The NPDES permit requires the preparation of a Stormwater Pollution Prevention Plan (SWPPP) which includes both temporary and permanent erosion and sediment control plans and stormwater management measures. The proposed project will comply with NPDES requirements. The entity responsible for ensuring compliance will be specified in the permit. Both Centerville Lake and Clearwater Creek are listed as impaired waterbodies by the MPCA - a waterbody that does not meet water quality standards and designated uses because of "pollutant( s), pollution, or unknown causes of impairment". Clearwater Creek is impaired for fish and invertebrate index of biotic integrity and Centerville Lake is impaired for nutrients. RCWD is in the process of completing a TMDL study for Centerville Lake. With the use of the proposed stormwater management strategies and implementation of Best Management Practices erosion and sediment control measures, the project is not expected to worsen the lake or creek's impairment condition. Centerville Lake is listed as a priority lake by the Metropolitan Council. Lakes designated as priority lakes generally require a nutrient budget analysis to determine if the project will have an adverse ~ffecton thela,ke's water quality. A nutrient budget analysis was not completed for several reasons: 1. The existing downtown project area WaS constructed prior to requirements for water quality treatment prior to discharge to Centerville Lake. The proposed 13