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Sunrise Over Gervais December 28, 2023 <br />Little Canada, Minnesota Page 3 of 3 <br />Plowe Engineering, Inc. <br />6776 Lake Drive, Suite 110, Lino Lakes, Minnesota 55014 <br />Office (651) 361-8210 • Fax (651) 361-8701 • Website: www.plowe.com <br />B. RAIN GARDEN TREATMENT (P2) <br /> <br />The following calculations for the required water quality volume for an infiltration BMP: <br /> <br />New/Reconstructed Impervious Area in Drainage Area = 1,838 sq. ft. <br /> <br />1,838 sq. ft. Impervious x (1.1/12) = 168 ft3 required <br /> <br /> 182 ft3 provided (890.00 – 891.55) <br /> <br />DRAWDOWN TIME (P1): <br />The following is a summary of drawdown time for the rain garden, P2: <br /> <br />HWL = 891.7 <br />Surface Area of Infiltration = 284 ft2 <br />Equivalent Drawdown height = 182 ft3 / 284 ft2 = 2.376 ft <br />0.45 in/hr infiltration (media) = (0.45/12) = 0.0375 ft/hr infiltration <br />Total infiltration time = (0.64/0.0375) = 17.06 hr < 48 hr [OK] <br /> <br />RATE CONTROL: <br />Discharge or Rate Control Standard is required for the 2-, 10- and 100-year 24-hour rainfall events. <br /> EXISTING 2-YEAR <br />(CFS) <br />10-YEAR <br />(CFS) <br />100-YEAR <br />(CFS) <br />WEST (1R) 0.25 1.20 4.30 <br />NORTH (2R) 1.54 3.54 6.97 <br /> <br /> PROPOSED 2-YEAR <br />(CFS) <br />10-YEAR <br />(CFS) <br />100-YEAR <br />(CFS) <br />WEST (1R) 0.27 1.12 3.87 <br />NORTH (2R) 1.47 2.96 6.16 <br /> <br />Rate control is met within model tolerance. <br />Note: The 2-year event in proposed conditions exceeds the existing rate by 0.02 cfs. The area was <br />designed to maximize the amount of impervious to be treated in the BMPs to the maximum extent <br />possible given the range of grades present on site. Due to the large wetland 25+ feet below the lowest <br />FFE, no adverse downstream effects are anticipated.