01-08-2018 Council Meeting Packet

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(a) Stormwater runoff rates for the proposed project at the site boundary, in aggregate, must not exceed existing runoff rates for the critical two-, ten-, and 100-year frequency events. (b) Any increase in a critical event rate at a specific point of discharge from the site must be limited and cause no adverse down gradient impact. The project must meet the hydroperiod standards found in Table 4 with respect to all down-gradient wetlands. Table 4. Hydroperiod standards Wetland Susceptibility Class Permitted Storm Bounce for 2-year and 10-year Event* Inundation Period for 2-year Event* Inundation Period for 10-year Event* Highly Susceptible Existing Existing Existing Moderately Susceptible Existing + 0.5 ft Existing + 1 day Existing + 7 days Slightly Susceptible Existing + 1.0 ft Existing + 2 days Existing +14 days Least Susceptible No Limit Existing + 7 days Existing + 21 days * Duration of 24-hours for the return periods utilizing NOAA Atlas 14 precipitation data. Source: Adapted from Rice Creek Watershed District 2013 Rules. (c) Wetland Susceptibility Class is determined based on wetland type, as follows: 1. Highly susceptible wetland types include: sedge meadows, bogs, coniferous bogs, open bogs, calcareous fens, low prairies, coniferous swamps, lowland hardwood forests, and seasonally flooded water basins. 2. Moderately susceptible wetland types include: shrub-carrs, alder thickets, fresh (wet) meadows, and shallow and deep marshes. 3. Slightly susceptible wetland types include: floodplain forests and fresh wet meadows or shallow marshes dominated by cattail giant reed, reed canary grass or purple loosestrife. 4. Least susceptible wetland includes severely degraded wetlands. Examples of this condition include cultivated hydric soils, dredge/fill disposal sites and some gravel pits. (d) Exceptions. Rate control criteria of division (5) may be waived if the site discharges directly to a water body with large storage capacity (such as a public water) that has a time-to-peak elevation greater than that for an on-site pond and the volume discharged from the on-site pond is negligible, relative to the volume of runoff entering the water body. (6) Design criteria. (a) Infiltration BMPs. Infiltration BMPs must be designed to provide: 1. Adequate pretreatment measures to remove sediment before runoff enters the primary infiltration area; 2. Drawdown within 48-hours or 72-hours from the end of a storm event, for surface or sub-surface features, respectively. Soil infiltration rates shall be based on the appropriate HSG classification and associated infiltration rates (Table 5). Infiltration area will be limited to the horizontal areas subject to prolonged wetting;