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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 />where: Q = <br />P = <br />S= <br />where: CN = Curve number <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 10yr /24hour <br />event and must safely pass a 100yr /24hour event" (DRMS, 2008). <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 Service for estimating surface runoff <br />volumes (SCS, 1972). <br />(P - o.2s) <br />Q= (P +o.8s) <br />Runoff depth, inches (Q =0, if P <0.2S), <br />Rainfall depth, inches <br />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 />1 The Soil Conservation Service (SCS) was later renamed Natural Resource Conservation Service (NRCS) <br />4122A.120702 Whetstone Associates <br />