2007-11-28 CC Packet - Laserfiche WebLink

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The majority of soil types within the project area do not represent steep slopes or highly erodible soils. There is one area in the north part of the project site where an eroded soil type is identified: HdC2- Hayden fine sandy loam, 6-12% slopes, eroded. Soils on steep slopes are designated with a "D" or an "E" in their mapping symbol and generally have slopes greater than 12%. Soils on slopes greater than 12% are not identified at the project site. Because construction of the project will disturb more than one acre of land, an application for a National Pollutant Discharge Elimination System (NPDES) General Permit for Construction Activities will be submitted to the MPCA. This permit requires the applicant to develop a Stormwater Pollution Prevention Plan (SWPPP) which includes both temporary and permanent erosion and sediment control plans; the NPDES permit also requires the applicant to perform inspections and maintain inspection records. Erosion control measures to be used during construction are expected to include rock construction entrances, silt fence, rock ditch checks, temporary sediment traps, biorolls, riprap, inlet protection, temporary seeding and mulch. Initially, these erosion control devices will be placed at the downstream end of the construction limits; the devices will be installed prior to the start of any land disturbing activity. Additional devices as noted on the construction drawings may be added as construction progresses. The devices will be maintained as directed under permit requirements to maintain their effectiveness. Maintenance will be the responsibility of the Contractor. Within the timeframes stipulated in approved plans, graded areas will be paved, seeded and mulched, or covered with fiber blankets, stabilization mats, or sod. Erosion control devices will remain in place until site stabilization has been achieved and vegetation has been reestablished. Permanent strategies to control erosion and manage stormwater will include the use of constructed ponds and stormwater infiltration areas (such as raingardens) where the water can infiltrate. 11. Water Quality - Surface Water Runoff. As this is a redevelopment project, the existing site is primarily developed with a mix of residential and commercial land uses. Construction of the medium and high density residential and mixed use commercial sites will increase the impervious surfaces and therefore stormwater runoff will increase. 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 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 D.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. Centerville Downtown Redevelopment Page 22