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drainage structure DS2 may convey runoff to discharge point DP 1 or DP2, or possibly <br />both, depending on the practicability to effectively grade the relatively flat Upper Bench. <br />Subbasin A2 and A12 will produce the same runoff regardless of what part of the <br />drainage structure is used. Therefore, channel design for DS2 will be the same for both <br />the north and south segments of the structure and should not be interpreted as sepazate <br />discharge rates that will be produced by the design storm. <br />Peak discharge for drainage structure DS7 was also sepazated into two segments, a road <br />(r) segment and a bench (b) segment. This is because runoff in the southern part of the <br />drainage structure originates from the narrow subbasins of A7 (the steep terrain between <br />the Upper and Lower Bench and roads) and A10 (the Lower Bench road), but on the <br />Lower Bench DS7 will receive additional runoff from DPI and DP2 from the Upper <br />Bench. The additional runoff from these points will necessitate an expanded capacity of <br />DS7 on the Lower Bench. <br />The rational method peak discharge for drainage structure DSS does not take into account <br />that this Subbasin has a retention (stock) pond located near the middle of the basin. <br />Therefore, the peak dischazge for this Subbasin is a conservative estimate for short <br />duration storms. <br />3.0 CHANNEL DESIGN <br />Stormwater control structures at the mine site will consist of open channels constructed <br />of native materials. The constructed channels will vary in size and shape depending on <br />the estimated peak dischazge. Channel shape will depend on available construction space <br />which is limited laterally by existing roads and vertically by shallow or exposed bedrock. <br />Therefore, the channel designs presented here are conceptual. Modifications will be <br />made in the field to best accommodate the peak discharge rates. Because of this <br />uncertainty, individual drawings of each channel have not been prepared. Table 4.0 and <br />the accompanying channel sketch should be used as a guide to proposed channel <br />geometry. Trapezoidal-shaped channels are anticipated in open azeas where the ground is <br />composed of thick unconsolidated soil, colluvium, or waste rock. Vee-shaped (triangulaz- <br />shaped) channels or trapezoidal channels with a small base are anticipated in areas where <br />space is limited or ground is composed of bedrock or very shallow "soils". <br />Table 4.0 shows the proposed construction parameters for each drainage structure uses a <br />trapezoidal design. These parameters were derived through the use of an electronic <br />spreadsheet (Excel®) by repeated modification of the base, depth, or side slope of the <br />trapezoidal profile until an acceptable configuration was achieved that provided adequate <br />(equal to or greater than) peak discharge (Qf). Input parameters, such as slope were <br />measured directly from site topographic maps. Note that a degree of conservative design <br />is included in the channel designs because the calculated discharge (Qd) for each channel <br />is greater than the actual anticipated flow from Che 10-year event (Qf). <br />Western Water & Land Inc. 11 <br />