1991-05-16_REVISION - C1981035 (2)

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1. General Plan and Scope To satisfy the Mine ; Land Recla*ation (MLRB) regulations pertaining to hydrology of mine sites„ the east channel, and east and west sediment ponds are analyzed and redesigned. Temporary diversion channels, such as the eastside channel, must pass the 10yr-24hr event as stated in regulation 4 . 05.4 (2b) . In addition they must be designed with stable geometry and protected from erosion at critical locations. According to regulations in section 4. 05. 6 a sediment pond must be able to store the runoff volume of a 10yr-24hr rainfall event. In addition, room for sediment inflow from the watershed must be included in the design. A principal and emergency spillway which can pass the 25yr-24hr event is required. The pond embankment slopes are redesigned to meet the MLRB regulations. Both the east and west ponds are currently undersized according to these regulations. This report addresses the hydrologic design of the ponds and the eastside channel as requested by MLRB. 2 . East Drainage Channel Hydrology The Eastside Channel currently terminates in the field northeast of the East Pond. Since storm flows in this channel carry sediments derived from the mine site they must be treated. To accomplish this, the channel will be routed through the East Pond. The pond will be enlarged to handle the increased volume of water. According to the MLRB regulations, the design of a temporary diversion channel should be capable of passing the 10yr-24hr rainfall event. However, in order to be compatible with the section of the Eastside Channel adjacent to the mine waste pile, upstream of the East Pond, a 100yr-24hr event was used (see technical revision 1) . The Army Corps of Engineers' HEC-1 Flood Hydrograph Package (HEC, 1981) was used to determine the design discharge. Total storm depths were obtained from the National Oceanic and Atmospheric Administration (NOAA) rainfall atlas (Miller, 1973) . The 100yr-24hr depth is 3.25 inches. Standard Soil Conservation Service methods, available within the HEC-1 package, were used to determine excess rainfall and unit hydrograph routing through the watershed. The basin characteristics are given below. Peak design discharge was found to be about 9 cfs. Results of the computer run can be seen in the appendix 1. A backwater analysis was performed to size the channel using the U.S. Army Corps of Engineers' HEC-2, Water Surface Profiles computer model (HEC, 1982) . A trapezoidal channel with a 6 ft. topwidth, 1: 1 side slopes and a depth of 2 ft carries the design 1