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Section 2 <br />Topaz Mine <br />i• <br />] 18.4(A)o"15 <br />Qioo <br />where: <br />Qioo =100-year peak flow in cubic feet per second (cfs) <br />A = drainage area in square miles <br />The standard error of the above regression equation is approximately 78 percent. If <br />the data are assumed to be normally distributed, the standard error means the <br />difference between the mean and upper (or lower) 68 percent confidence limits. Due <br />to the error associated with the regression equation, CDM increased the calculated <br />discharge from the flood-frequency equation by the standard error (78 percent). CDM <br />used the increased discharge as the design peak discharge. Table 2-1 presents Tetra <br />Tech's and CDM's estimated 100-year peak discharges for each basin of the Topaz <br />Mine. <br />Table 2-1 100-near Peak Discharaaes for the Topaz Mine Basins <br /> 00-year peak disc arge <br /> Area Tetra Tech CDM Increased by SE <br />Basin ac s -mi cfs cfs cfs <br />1 4.20 0.00656 3.01 3.26 5.79 <br />2 2.72 0.00425 1.99 2.39 4.25 <br />3 4.88 0.00763 . 6.85 3.62 6.45 <br />4 2.63 0.00411 2.39 2.33 4.15 <br />CDM's estimate of the 100-year peak discharges is greater than the discharges <br />determined by Tetra Tech for all basins with the exception of Basin 3. While the <br />difference is not considerable, CDM used the USGS approach as a conservative <br />measure (e.g. larger discharges) to design the storm water drainage structures. <br />2.4 Surface Water Control Structure Design <br />Tetra Tech designed two diversion channels (Channels 1 and 2) to catch offsite run-on <br />from Basin 1 and Basin 2 and route flows around the surface mine facilities to the <br />ephemeral drainages. Storm water runoff from Basin 3 will be captured and detained <br />by a pro posed earthen berm along the crest of the waste rock area (WRA). Storm <br />water runoff from Basin 4 will be captured in a berm and routed to a catchment basin. <br />2.4.1 Channel Design <br />Channels 1 and 2 were designed to convey the 25-year peak discharge from Basin 1 <br />and Basin 2 to the two ephemeral drainages. Tetra Tech used Manning's equation to <br />calculate the flow velocity and depth in Channels 1 and 2 based on the values found <br />in Table 2-2. CDM reviewed the design by Tetra Tech to determine if Channels 1 <br />and 2 have adequate capacity to convey the 100-year discharges listed in Table 2-1. <br /> <br />2-2 <br />• <br /> <br />0184986-D..1-kT.sk Order 1 -Sunday Mines Drainage StudVYUport%F1m1 ReponlFiml Sunday Mines Dminege Repor_052208.doc