1991-04-22_REVISION - C1981035 (2)

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intercept surface flow off of the pile itself. Runoff parameters for the waste pile are shown in the following table. Watershed Parameters for Waste Pile Intercept Channel Drainage Area = 2.7 acres Length to Divide = 450 ft Curve Number = 90 Average Slope = 33 . 3% Lag Time = 2 minutes The 100yr-24hr discharge for the entire waste pile is 3cfs. A triangular channel with a 2 .5 ft. top width, 1: 1 side slopes, and a minimum invert slope of 0.010 is adequate. See computer runs in appendix. Flow in the lower interceptor channel, will be diverted to the eastside drainage channel at stations 4 and 7 utilizing the pipes installed to drain the upper intercept channel and sunken stilling basins at these locations see figure 5. Runoff flowing to the lower end, station 1, must be routed back to the eastside drainage channel. Since the entire pile will be divided into 3 parts, the channel at any location will actually receive only about 1/3 of the computed discharge. Thus, this channel has a more than adequate capacity. In the vicinity of sections 3-6 the eastside drainage channel will be moved approximately 30 feet to the north, so that it is adjacent to the bottom of the north side of the valley. The HEC programs were used to model the 100yr-24hr event for the eastside drainage basin and channel. The basin characteristics are given below. The peak 100yr-24hr discharge is 8 cfs. A trapezoidal channel with a 6 ft. topwidth and 1: 1 side slope carries this flow with more than 1 ft. of freebroad. See appendix for computer runs. Watershed Parameters for Eastside Drainage Drainage Area = 43 .5 acres Length to Divide z 3000 ft Curve Number = 78 Average Slope = 12 .8% Lag Time = 15 minutes The south bank of the channel should be protected from erosion with rock rip-rap (6" - 24" well graded) . Rip-rap should extend 12