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j) No Net Loss of Storage Capacity: If encroachments within storm water retention basins are <br /> approved by the City Engineer, then calculations indicating the volume of encroachment and <br /> plans for volume mitigation must be submitted. <br /> 5) Volume Control/Infiltration Practices Design Criteria. <br /> a) Infiltration systems are prohibited: <br /> 1) Where the bottom of the infiltration basin is less than three (3) feet to bedrock or the <br /> seasonally high water table; <br /> 2) Low permeability soils (i.e., Hydrologic Soil Group D soils) or where a confining layer exists <br /> below the proposed basin; <br /> 3) Within fifty (50) feet of a public or private water supply well (Minn. Rules, Chapter 4725); <br /> 4) Potential storm water hot spots or contaminated soils; <br /> 5) Within ten (10) feet of a property line or building foundation; and <br /> 6) Within thirty-five (35) feet of a septic system tank or drain field. <br /> b) Infiltration practices must be designed to draw down to the bottom elevation of the practice <br /> within forty-eight (48) hours. The pond depth shall be based on the soil infiltration rate <br /> determined from site-specific soils investigation data taken from the location of proposed <br /> infiltration practices on the site (e.g., double ring infiltrometer test). The maximum pond depth, <br /> regardless of infiltration rate shall be two (2) feet unless otherwise approved by the City <br /> Engineer. The soils investigation requirement may be waived for residential property practices <br /> where the maximum pond depth is one (1) foot or less. The following infiltration rates shall be <br /> used for the most restrictive underlying soil unless otherwise supported by an in-situ infiltration <br /> test: <br /> ASTM Unified <br /> Soil Group Rate Soil Textures Soil Class <br /> Symbols <br /> A 1.63 in/hr Gravel, sand, sandy gravel, silty GW, GP <br /> 0.80 in/hr gravel, loamy sand, sandy loam GM SW SP <br /> 0.60 in/hr SM <br /> B Loam, silt loam <br /> 0.30 in/hr ML, OL <br /> C 0.20 in/hr Sandy clay loam GC, SC <br /> D 0.00 in/hr Clay loam, silty clay loam, sandy CL, CH, OH, <br /> clay, silty clay, or clay MH <br /> Source: Minnesota Storm water Manual, November 2005. <br /> c) Infiltration practices shall have provisions for pretreatment of the runoff. Examples of <br /> pretreatment include: a mowed grass strip between a curb-cut and a small rain garden, a sump <br /> Page 5 of 8 12/11/14 <br />