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• B. Design of Collection Ditches <br />The drainage ditches located around the stockpile area <br />are designed to transport the peak discharge from a 10 <br />yr, 24 hr rainfall event from the entire stockpile area <br />and associated drainage areas. This peak discharge is <br />determined in Table 2. The ditches are sized using <br />Manning's equation for a triangular channel as shown: <br />Design Discharge = 2.58 cfs <br />Manning's n = 0.03 <br />Freeboard > 0.5 ft <br />Design slope = 1.5~ <br />Channel side slope = 3:1 <br />Normal depth = 0.58 ft <br />Velocity = 2.54 ft/sec <br />Channel depth = 1.5 ft <br />Top width (with freeboard) = 9 ft <br />A berm will be placed between the North Fork Gunnison <br />River and the ditch located along the northern perimeter <br />of the site. The berm will be 2 ft high with 2:1 side <br />slopes and a top width of approximately 2 ft. <br />Spillway Design Specifications <br />A spillway is provided to safely pass the peak discharge <br />from a 25 yr, 24 hr precipitation event. The location of <br />the spillway is shown on Attachment D. The design speci- <br />fications are determined using Manning's equation for a <br />triangular channel and are provided below: <br />Design Discharge <br />Manning's n <br />Freeboard <br />Design slope <br />Channel side slope <br />Normal depth <br />Velocity <br />Spillway depth <br />Top width <br />= 3.77 cfs <br />= 0.03 <br />> 1.0 ft <br />= 3S <br />= 3:1 <br />= 0.59 ft <br />= 3.61 ft/sec <br />= 2 ft <br />= 12 ft <br />Outlet protection is not required because of the low <br />velocities. Furthermore, the spillway channel is an <br />excavated channel and will not be located on earthen fill <br />material. <br />-5- <br />