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JD-8 Mine—Drainage Design Plan 5 <br /> 2. DESIGN CRITERIA AND METHODOLOGY <br /> 2.1 Design Storms <br /> The magnitudes of design storms for the site are shown in Table 3. The rainfall intensity for 24-hour <br /> storms with 2-year to 100-year recurrence intervals were interpolated from the NOAA Atlas 2,Vol. III, for <br /> the purpose of runoff calculations and engineering design. <br /> Table 3. Recurrence Interval for Design Storms <br /> Recurrence Duration Storm Magnitude <br /> Interval (Hours) (inches) <br /> 100-year 24 2.6 <br /> 50-year 24 2.3 <br /> 25-year 24 2.1 <br /> 10-year 24 1.8 <br /> 5-year 24 1.4 <br /> 2-year 24 1.2 <br /> The existing surface water control structures were designed for the 10-year 24-hour storm. DRMS now <br /> requires that "prior to any reactivation...storm water control [be] engineered to contain the l0yr/24hour <br /> event and must safely pass a 100yr/24hour event"(DRMS, 2008). <br /> 41110 <br /> 2.2 Run-on/Run-off Volumetric Estimates <br /> Runoff volume from each of the mine areas was estimated using the "SCS curve number method" <br /> developed by the U. S. Department of Agriculture Soil Conservation Services for estimating surface runoff <br /> volumes(SCS, 1972). <br /> (P—0.2S)2_ <br /> Q (P+0.8S) <br /> where: Q= Runoff depth, inches(Q=0, if P<0.2S), <br /> P= Rainfall depth, inches <br /> S= Potential maximum rainfall retention after runoff begins, inches <br /> Equation 1 assumes that initial rainfall abstraction = 0.2S. The parameter S is related to the curve number <br /> (CN)by: <br /> CN = 1000 <br /> (10+S) <br /> where: CN= Curve number <br /> 1111 'The Soil Conservation Service(SCS)was later renamed Natural Resource Conservation Service(NRCS) <br /> 4122A.120702 Whetstone Associates• <br />