1991-09-26_REVISION - C1981035 (3)

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1 a Treatment of Yard Runoff All runoff generated from the yard area will be routed to the sedimentation ponds. Two small natural areas, one located on the hillside west of the mine entrance and the other on the hillside east of the yard will also be collected and routed through the ponds. Nearly all of the runoff from this area will be channeled into the west pond. Only the small yard storage area below the offices will be routed to the east pond. The pond hydrology and design is discussed below. 3. Hydrology General The drainage basin is divided into three parts, the undisturbed eastside, the undisturbed westside and the yard area. To size the drains for the east and westside diversions, the peak 100yr-24hr discharge is needed. To size the sediments ponds the 10yr-24hr volume and the 25yr- 24hr peak flow area required for the yard area. The Army Corps of Engineers' HEC-1 Flood Hydrograph Package, discussed on TR 2, was used to determine the design discharges and volumes. Total storm depths were obtained from the National Oceanic and Atmospheric Administration (NOAA) rainfall atlas. The 10yr-24hr depth is 2.30 inches, the 25yr-24hr depth is 2.90 inches and the 100yr-24hr depth is 3.25 inches. The standard Soil Conservation Service curve number method, available within the HEC-1 package, was used to determine excess rainfall. Runoff routing was accomplished using the SCS unit hydrograph technique. The curve number for each of the three parts was determined using a composite average which accounts for the different land use/covers in that area. This resulted in a curve number for the natural undisturbed east and west areas of 63 and curve number for the total composite yard and adjoining areas of 74. Detailed computations for these curve numbers are given in appendix 1. Eastside The eastside diversion system consists of both buried pipe and open channels as described above. This system will provide conveyance for all runoff resulting from the 100yr-24hr event intercepted above the waste pile and terminating in the field below the east sediment pond. The drainage characteristics are given the table 1. Peak design discharge was found to be approximately 8 cfs. Results of the computer run can be seen in the appendix 2. Westside Flow from the west portion of the basin is intercepted above the mine at the existing diversion site. The hydrology for this side is similar to that of the eastside. Table 2 lists the watershed parameters. Peak discharge from the 100yr-24hr event is approximately 6 cfs. HEC-1 printouts for the westside can be found in appendix 3. 3