2007-11-28 CC Packet - Laserfiche WebLink

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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, irrigation and re-use of stormwater, 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 effect on the lake'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 project area will be designed with water quality features to meet the requirements listed above. This results in a significant improvement in the water quality of the runoff discharged from the site. Runoff volumes, total phosphorus and total suspended sediments will all be less than existing, even with the increase in impervious surface. 2. The drainage area to Centerville Lake will decrease from existing conditions. 3. The project area draining to Centerville Lake is considerably smaller than the entire drainage area to Centerville Lake. Runoff from the site, after treatment in the several infiltration basins and other stormwater best management practices, is routed via storm sewer to either Centerville Lake or Clearwater Creek. The majority of the site is tributary to Clearwater Creek which travels roughly 4,400 feet before discharging downstream to Peltier Lake. With the stormwater management strategies to be implemented for the project, it is not expected to impact the quality of receiving waters. Centerville Downtown Redevelopment Page 23