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Coal Refuse Pile Surface Area (from Genera] Facilities Map (Map 2.05-MI (5 of 5)) <br />• Flat Areas (top bench and catch benches) - 87,100 ftZ <br />Slope Areas (bench outslopes at 2H:1 V) - 155,700 ftZ <br />Slope Correction (based on 2H:1 V slope, 8 = 26,56°, correction factor is lJcos 0) <br />- 1.12 <br />Corrected Slope Area - 155,700 x 1.12 = 174,400 ftZ <br />Total Refuse Pile Surface Area - 87,]00 + 174,400 = 26],500 ftZ <br />Maximum Infiltration (Using Darcys Equation) <br />Q=Aki <br />where Q is design flow (cfs) <br />A is surface azea of refuse pile (ftZ) <br />k is permeability of the refuse (Typical k values for coal refuse <br />range from 10"3 to 10.6 cm/sec. In order to reflect maximum <br />infiltration rates and underdrain flows, a conservative value of <br />103 cm/sec or 3.3 x 10"S fUsec was used) <br />• i is flow gradient in fUft (for vertical infiltration i = 1) <br />Q = 261,500 ft Z x 3.3 x 10"S fUsec x 1 fUft = 8.6 cfs <br />Underdrain Sizing (Using Wilkens and L.eps Equation from "Engineering and Design <br />Manua] for Disposal of Excess Spoil", TM-2183 (OSM, 1983}) <br />V - Wmo.s ~ o.so <br />~- <br />Where Vv is average water velocity through rockfil] voids (fUsec) <br />W is an empirical constant for rockfill material (1/sec) <br />m is hydraulic mean radius of rock voids (inches) <br />i is hydraulic gradient (from Map 2.05-MI {5 of 5) - (6,250'- <br />6,040'/800' = 0.26 ft/ft) <br />Values for Wm°'S for various DSO rock sizes are provided in Table 4.3 and shown <br />on the figure, Wm°'S vs Rock Size, in the "Engineering and Design Manual for <br />Disposal of Excess Spoil", TM-?/83 (OSM, 1983) <br />Assuming a DSO of 18 inches, Wm°'S = 50 in/sec or 4.2 fUsec <br />V„ = 4.2 fUsec x (0.26 ftlft )°'S4 ='2.03 fUsec <br />~J <br />