02/23/2011 Env Bd Packet - Laserfiche WebLink

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FINAL DRAFT 02 -23 -2011 groundwater - dependent natural resource. In evaluating the appropriateness of peak flow and water quality management in an existing regional basin, the City will consider whether it previously approved the basin and whether the basin was designed for build -out of the site to the extent proposed. (e) Landlocked Basins. A landlocked basin may be provided an outlet only if it: 1. Retains a hydrologic regime that complies with the requirements of the Local Governmental Unit (LGU) responsible for administration of the Wetland Conservation Act (WCA); and 2. Provides sufficient dead storage volume to retain back to back 100 year, 24 hour rainfalls and runoff; and 3. Does not create adverse downstream flooding or water quality conditions as a result of increased discharge rate or volume, or other factors. (f) All plans shall be consistent with National Pollutant Discharge Elimination Permit (NPDES) requirements, the Lino Lakes Engineering Design Standards, the Lino Lakes Stormwater Design Standards, and the filing or approval requirements of Rice Creek Watershed District, Vadnais Lakes Watershed Management Organization, Anoka County, Minnesota Department of Natural Resources, Minnesota Department of Transportation, U.S. Army Corps of Engineers, State of Minnesota Stormwater Manual or other regulatory agencies. (5) Stormwater Management Plan Modeling Requirements. (a) A hydrograph method or computer program based on Natural Resources Conservation Service Technical Release #20 (TR -20) and subsequent guidance must be used to analyze stormwater runoff for the design or analysis of flows and water levels within and off the project site. Composite Curve Numbers shall not include directly connected impervious surfaces. (b) In determining Curve Numbers to model runoff in the post- development condition, the Hydrologic Soil Group (MSG) of areas within construction limits is to be shifted down one classification (or '/2 classification for HSG A) to account for the impacts of grading on soil structure unless the project specifications incorporate soil amendments in accordance with Rice Creek Watershed District Soil Amendment Guidelines. (c) The 100 -year critical event analysis of flood levels, storage volumes, and flow rates for waterbodies and stormwater management basins must include both the 24 -hour rainfall and the 10 -day snowmelt events. The 10 -day snowmelt event is simulated by a 7.2 -inch, 10 -day spring runoff event during which it is assumed the ground is frozen solid and no infiltration occurs (CN set to 100 for all areas). 14 • •