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JD -6 Mine — Drainage Design Plan 3 <br />Runoff volume (Q) is obtained from: Q = q / / 12 • A • 43,560 <br />where: Q = Runoff volume, cubic feet <br />qd = Runoff depth, inches <br />A = Catchment area, acres <br />2.2.2 Peak Discharge <br />Peak discharge is calculated using Time of Concentration (T j which is the time it takes for runoff to travel <br />from the most hydraulically distant point in the watershed (or sub - basin) to a point of interest. T. is the <br />sum of travel time for sheet flow (Tsh) plus the travel time for shallow concentrated overland flow (Ts,) plus <br />the travel time for channel flow (T,h). <br />-4-AW/ A0 <br />,fha / /ems <br />ceneenliales <br />Cknrrnti <br />e/ Iles <br />.,.U.,., <br />wern/ra/e s <br />f/,90 <br />Sheet flow occurs over plane surfaces in the "headwaters" or uppermost reaches of the watershed, as shown <br />above. For the JD -6 Mine drainage design, sheet flow is assumed to occur in the upper 80 feet of the sub - <br />basin. Sheet flow travel time is calculated using the simplified form of Manning's kinematic solution from <br />Overton and Meadows (1976) which is Equation 3.3 of TR55 (MRCS, 1986): <br />where: <br />0.007 (nL)O•$ <br />Tsh = POSSOA <br />Tsh = sheet flow travel time (hr) <br />n = Manning's roughness coefficient for surface flow <br />L = flow length (ft) <br />P = 24 -hour rainfall (in) <br />s = land slope (ft/ft) <br />Shallow concentrated flow has been assumed to occur from the end of sheet flow until the flow path <br />reaches a stream channel mapped as intermittent or a channel with a known or required cross section, such <br />as the diversion or drainage ditch at the JD -6 Mine. The travel time for shallow concentrated flow is <br />calculated as: <br />4148A.131028 Whetstone Associates <br />