06-12-2002 Council Agenda - Laserfiche WebLink

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4.12 Additional Special Trout Stream and State Outstanding Resource Value Water Requirements. (Note to rewriters: Only include this part if either trout waters or state Outstanding Resource Value Waters are present.) A.) For discharges directly upstream of, directly to or to tributaries directly to Minnesota Department of Natural Resources designated trout streams, and State Outstanding Resource Value Waters there shall be no increase in either the volume or rate of discharge from any design storm with a statistical recurrence interval of less than ten (10) years (i.e., for the two (2) year, five (5) year and ten (10) storm events), unless diversion is impractical and /or the soil is not suitable for storm water infiltration and /or evaporation/transpiration techniques. This pertains to discharges directly to or upstream of such waters. (Commentary: The intent is to encourage either storm water infiltration or diversion, since urban trout streams are a unique resource and therefore deserve special consideration. Residential development increases the total volume of runoff resulting from a given storm. Since there is a larger volume of water to deal with, limiting the rate of storm runoff to predevelopment rates means that high flows (and therefore scouring velocities) will persist for longer periods of time than during preclevelopment conditions. This increases channel erosion. Infiltration or diversion deals with this increased scouting problem by lessening the volume of runoff and therefore the duration of the scouring velocities. In the case of trout streams, increasing the inputs of warm storm water increases the impact of thermal shocks. Since trout are temperature sensitive, increasing thermal shocics adversely impacts trout habitat. 1.) The phrase, "tributaries directly to," refers to tributaries within at least one Minnesota Department of Natural Resources Division of Waters minor watershed of the designated water. At its discretion the city may extend this area of protection. 2.) The phrase, "soil not suitable for storm water infiltration techniques," means soils with permeability values worst that Group B soils (less than two and a half (2.5) inches per hour) as defined by the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) or a high water table is present. If permeability is worst than Group B soils and a high water table is not present, the volume restriction target becomes the two (2) year storm event. a.) Where soil permeability is "not suitable for storm water infiltration techniques," additional design considerations to enhance the infiltration rate or other measures may be required by the city engineer. Such measures would include grassy swales or similar techniques which use evaporation/transpiration or other approaches to achieve the same goal. 13.) During construction temporary sedimentation basins are required for disturbed areas over one (1) acre.