Laserfiche WebLink
practicable stormwater management measures for plans shall be designed, <br />engineered and implemented to achieve the following results: <br />1. Volume Control - Stormwater runoff volume retention shall be achieved <br />onsite in the amount equivalent to the runoff generated from a one inch <br />rainfall over the impervious surfaces of the development. The required <br />stotmwater runoff volume reduction shall be calculated as follows: <br />Required Volume (le) = impervious surfaces (ft2) x 0,9 (in) x1/12 (ft /hil <br />a, When using infiltration for volume reduction. the following requirements <br />Hurst be met: <br />Infiltration volumes and facility sizes shall be calculated <br />using the appropriate hydrological soil <rroup classification <br />and design infiltration rate from Table 1. Select the design <br />infiltration rate from Table 1 based on the least permeable <br />soil. horizon within the first: five feet below the bottom <br />elevation of the proposed infiltration BMP. <br />ii. The applicant may complete double -ring infiltrometer test <br />measurements at the proposed bottom elevation of the <br />infiltration BMP to the requirements of ASTM D3385. The <br />measured infiltration rate shall he divided by the appropriate <br />correction factor selected from the Minnesota Stornnvater <br />Manual. This test trust he completed by a licensed soil <br />scientist or engineer. <br />Tablet. Design Infiltration Rates <br />Soil Group <br />Rate <br />Soil Textures <br />ASTM Unified Soil <br />Class Symbols <br />1.60 in /hr <br />(.iravel, sandy gravel, or silty gravel <br />GW. GP, GM SW <br />0.80 in hr <br />Sand. loatikys:and or sandy loam <br />S1' <br />F3 <br />0.60 i Mu <br />Silt loam <br />SM <br />0.30 iniltr <br />Loam <br />MH <br />C <br />0.20 in/hr <br />Sandy clay loam <br />ML <br />D <br />0.00 in /hr <br />Clay loam, silty clay loam, sand.v <br />GC SC, Cl. Gt. al. <br />clay silty_clav, or clay <br />OR <br />Source: MinnesotaStorrmater Manual November 2005. <br />iii. The in- filtration area shall be capable of infiltrating the <br />required volume within 48 hours for surface BMPs and 72 <br />hours for subsurface13t19Ps. <br />iv. infiltration areas shall be limited to the horizontal areas <br />subject to,nrolc» lgedwetting, <br />Model Stormwater Ordinance Page 6 NEMO Ordinance Project <br />-10- <br />