02/23/2011 Env Bd Packet - Laserfiche WebLink

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• • • FINAL DRAFT 02 -23 -2011 (6) Water Quality And Volume Control. (a) Activity creating impervious surface shall address the use of Better Site Design (BSD) techniques as outlined in Chapter 4, "Minnesota Stormwater Manual" (MPCA, 2006 and subsequent revisions). Better Site Design involves techniques applied early in the design process to reduce impervious cover, conserve natural areas and use pervious areas to more effectively treat stormwater runoff and promote a treatment train approach to runoff management. (b) Better Site Design principles will be incorporated and water quantity BMPs (as defined in this ordinance) must be incorporated to the following standards: 1. BMP volume must retain the one -year event by providing at least the volume equal to the runoff from a 2.3 -inch, 24 -hour storm over the tributary area within the site under proposed conditions. a. Infiltration BMPs (see City BMP standard plates and design criteria) are to be incorporated in areas with A & B hydrologic soil groups. Stormwater from impervious surfaces other than rooftops must be pretreated before discharge to infiltration BMPs, to remove sediment and floatables, or other materials that would restrict the BMP's capacity or contaminate ground water. b. In the following areas, the volume requirements shall be provided by bio- filtration features or two -cell wetland treatment systems (see City standard plates and design standards): (i) Areas of C or D hydrologic soil groups that cannot be routed by a gravity system to onsite A or B hydrologic soil groups; Areas with a groundwater table within three (3) feet of surface, or otherwise at an elevation that poses a threat of groundwater contamination or renders the infiltration BMP ineffective; (ii) Areas where soil contamination is present or land use history indicates a likely threat of soil contamination. (c) In addition to the BMPs required under (6)(b), volume control measures are required reducing runoff by at least the volume from 0.5- inches of rainfall over impervious surfaces on the site (0.5 -inch multiplied by impervious area). Volume reduction required under this paragraph may be achieved only by the following measures: Reestablishment of Effectively Drained Wetlands and Floodplain Meadows Upland Restoration/Conservation, Restoration of Degraded Wetlands, Impervious Disconnection, Soil Amendments /Tilling. 15