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JD -7 Mine — Drainage Design Plan 6 <br />2. DESIGN CRITERIA & 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 <br />(USDC, 1973), for the purpose of runoff calculations and engineering design. Per DRMS <br />recommendations, control structures for the JD -7 Mine presented in this design report are engineered to <br />contain the 10 -year 24 -hour storm event and safely pass a 100 -year 24 -hour storm event. <br />Table 3. Design Storm Parameters <br />Recurrence <br />Interval <br />Duration <br />(Hours) <br />Storm Magnitude <br />NO AA Atlas <br />i Anches <br />100- ear <br />24 <br />3.0 <br />50-year <br />24 <br />2.7 <br />25-year <br />24 <br />2.4 <br />10- ear <br />24 <br />2.0 <br />5-year <br />24 <br />1.6 <br />2-year <br />24 <br />1.2 <br />Rainfall is assumed to follow the 24 -hr Type II rainfall -time distribution curve which is applicable to this <br />area of the United States (USDC, 1973). <br />2.2 Runoff Estimates <br />2.2.1 Volume <br />Catchment runoff was estimated using the methods described in Technical Release 55 (TR -55), which was <br />developed by the U. S. Department of Agriculture Natural Resources Conservation Service (MRCS, 1986). <br />TR -55 presents simplified procedures to calculate storm runoff volume, peak rate of discharge, <br />hydrographs, and storage volumes required for floodwater reservoirs in small watersheds. The methods <br />described in TR -55 were implemented using a combination of the latest versions of the WinTR -55 and the <br />HydroCAD® software packages, which are described in Section 2.2.3. <br />Runoff depth (qd) is calculated in TR -55 by the Curve Number (CN) method, using the following equation: <br />(P - 0.25)2 <br />qd — (P + 0.8S) <br />where: qd = Runoff depth, inches (qd = 0, if P < 0.25), <br />P = Rainfall depth, inches <br />S = Potential maximum rainfall retention after runoff begins, inches <br />This method assumes initial abstraction (losses before runoff begins due to retention in surface depressions, <br />interception by vegetation, evaporation, infiltration etc.) is equal to 0.25. <br />The parameter S is related to curve number (CN) by: <br />S= 1000_10 <br />CN <br />4149A.140207 Whetstone Associates <br />