1991-04-22_REVISION - C1981035 (2)

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Hydrology Construction of intercept channels at the top and bottom of the waste bank and the relocation . of the natural eastside drainage channel are required. The top intercept channel will run the length of the waste bank at the top of the slope and be of sufficient size to carry the 100yr-24hr event. The contributing drainage area, above the bank, is approximately 8.3 acres. The Army Corps of Engineers ' HEC-1 Flood Hydrograph Package (HEC, 1981) was used to determine the design discharge. Total storm depth was obtained from the National Oceanic and Atmospheric Administration (NOAA) rainfall atlas (Miller, 1973) . Standard Soil Conservation Service methods, available within the HEC-1 package, were used to determine excess rainfall and unit hydrograph routing. The following table lists the watershed parameters used in determining the design discharge for the intercept channel. Watershed Parameters for Intercept Channel above Waste Pile Drainage Area = 8 . 3 acres Length to Divide z 500 ft Curve Number = 78 Average Slope = 48% Lag Time = 4 minutes The reach from section 1 to section 10 will be divided into two separate parts at section 6 and thus the contributing area to either channel will be half that listed in the table. The resulting peak discharge for each part is 4 cfs. A backwater analysis was performed to size the intercept channel using the U.S. Army Corps of Engineers ' HEC-2 , Water Surface Profiles computer model (HEC, 1982) . The channel will be triangular in shape with a 3 ft. top width, 1: 1 side slopes and will be placed on a minimum invert slope of 0. 010. The channel will be graded to drain from section 1 to section 4 and from section 6 to section 4 . Similarly the upper portion of intercept channel will drain from section 6 to section 7 and from section 10 to section 7 . Runoff collecting at sections 4 and 7 will be drained through an 18" diameter pipe down the waste bank and will terminate at the eastside drainage channel. A profile of the waste bank and the channels is shown in figure 3 . At the point where the flow exits the pipes the drainage channel must be armored with rip-rap (6" to 24" well graded) material to prevent local scour of the eastside drainage. Computer printouts for both the HEC-1 and HEC-2 runs discussed above are included in the appendix. A similar channel is required at the toe of the slope to 11