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7. PEAK STORMWATER RUNOFF CONTROL. <br />(a) Peak stormwater runoff rates for the proposed project at the project site boundary, in <br />aggregate, must not exceed existing peak runoff rates for the 2 -year, 10 -year and 100 -year, <br />24-hour rainfall events, or a different critical event duration at the discretion of the District <br />Engineer. Notwithstanding, peak runoff may be controlled to this standard in a regional <br />facility consistent with paragraph 7(b). Aggregate compliance for all site boundary <br />discharge will be determined with respect to runoff not managed in a regional facility. <br />(b) Any increase in a critical duration flood event rate at a specific point of discharge from the <br />project site must be limited and cause no adverse downstream impact. Table C3 shows <br />the maximum curve numbers that may be utilized for existing condition modeling of those <br />project site areas not covered by impervious surface. <br />(c) Within the Flood Management Zone only (see Figure C2), the applicant shall provide peak <br />rate control for the 2, 10 and 100 year 24-hour rainfall events beyond the existing condition <br />peak rate of runoff by reducing the peak rate to <_80% of the existing condition. This <br />requirement does not apply if the project is a Public Linear Project. <br />TABLE C3. CURVE NUMBERS FOR EXISTING CONDITION PERVIOUS AREAS. <br />Hydrologic Soil Group <br />Runoff Curve Number* <br />A <br />39 <br />B <br />61 <br />C <br />74 <br />D <br />80 <br />* Curve numbers from NRCS Technical Release #55 (TR -55). <br />TABLE C4. HYDROPERIOD STANDARDS. <br />Wetland <br />Susceptibility Class <br />Permitted Storm <br />Bounce for 2- <br />Year and 10 -Year <br />Event * <br />Inundation Period <br />for 2 -Year Event * <br />Inundation Period <br />for 10 -Year Event <br />Highly susceptible <br />Existing <br />Existing <br />Existing <br />Moderately susceptible <br />Existing plus 0.5 ft <br />Existing plus 1 day <br />Existing plus 7 days <br />Slightly susceptible <br />Existing plus 1.0 ft <br />Existing plus 2 days <br />Existing plus 14 days <br />Least susceptible <br />No limit <br />Existing plus 7 days <br />Existing plus 21 days <br />Source: Adapted from: Stormwater and Wetlands Planninq and Evaluation Guidelines for <br />Addressing Potential Impacts of Urban Stormwater and Snowmelt Runoff on Wetlands. <br />* Duration of 24 -hours for the return periods utilizing NOAA Atlas 14. <br />19 <br />