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JD-8 Mine - Drainage Design Plan 5 <br />• 2.2 Design Criteria and Design Storms <br />The magnitude of design storms for the site are shown in Table 3. The rainfall intensity for 24-hour storms <br />with 2-year to 100-year recurrence intervals were interpolated from the NOAA Atlas 2, Vol. III, for the <br />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... stormwater control [be] engineered to contain the 10yr/24hour <br />event and must safely pass a 100yr/24hour event" (DRMS, 2008). <br />The design criteria for channels shall include limiting flow velocities to less than 12 ft/sec for rip-rap lined <br />channels. Riprap sizing will follow the recommended riprap gradation limits in the Surface Mining Water <br />• Diversion Manual (OSM, 1982) for steep slopes or mild slopes (as applicable on site). <br />2.3 Peak Discharge Estimates for Mine Runoff Areas <br />Runoff from each of the mine areas was estimated using the "SCS curve number method" developed by the <br />U. S. Department of Agriculture Soil Conservation Service2 for estimating surface runoff volumes (SCS, <br />1972). <br />Q= (P - 0.2S)2 <br />(P + 0.8S) <br />where: Q = Runoff depth, inches (Q=O, 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 />• <br />where: CN = Curve number <br />S = Potential maximum rainfall retention after runoff begins, in inches <br />2 The Soil Conservation Service (SCS) was later renamed Natural Resource Conservation Service (MRCS) <br />4122A.110601 Whetstone Associates