2018-08-22_REVISION - M1977493 (7)

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Robinson Lake Operating Plan Climax Molybdenum Company Version R1 Permit M-1977-493 overflowing through its spillway into Robinson Lake. If it was conservatively assumed that Warren's Pump System is not being used, the entire 424 acres tributary to Robinson Lake would produce a 1 -day snowmelt runoff rate of 6,480 gpm. With this snowmelt runoff, 1,000 gpm of seepage from 1 Dam, and 480 gpm being pumped back from each the Robinson Dam and 4 Dam seepwater collection systems, the maximum inflow to Robinson Lake during 100 -year snowmelt conditions would be about 8,440 gpm. The capacity of the Robinson Lake Pump Station without using the installed spare pump is approximately 10,000 gpm. 4.0 PMP EVENT CONDITIONS During a PMP event, it has been assumed that the Chalk Mountain Interceptor would fail. Figure 2 shows the drainage basin configuration for this condition. In this scenario, approximately 696 acres is tributary to Robinson Lake; this area includes the area tributary to Chalk Mountain Reservoir. During a rainfall event, a portion of the rainfall amount is retained by vegetation or infiltrated into the ground. These losses are typically modeled through hydrologic analyses. For conservatism, however, Climax has accounted for the full volume of rainfall, without subtracting losses. In this case, the total volume of the site-specific PMP event in the Robinson Lake drainage basin would be 256 acre-feet. Climax recognizes that during and immediately after an extreme event such as the PMP, conditions may not allow immediate access for maintenance and repair of inoperable facilities. It is also recognized that power to the pump stations that remove water from the Robinson Lake basin may be unavailable for a short period of time. To account for these conditions, Climax is committed to maintaining available surcharge storage capacity in Robinson Lake equivalent to five days worth of inflow from the 1 Dam, Robinson Dam, and 4 Dam seepage collection systems. The combined 5 -day volume of seepwater inflow to Robinson Lake would be about 43 acre-feet. This amount is conservatively based on 480 gpm being continuously pumped from both the Robinson Dam and 4 Dam seepage collection systems, in addition to 1,000 gpm from the 1 Dam seep. When natural inflows are not sufficient to maintain Robinson Lake at the desired operating level, Climax could be diverting water into the lake through the Tenmile Tunnel from the Tenmile Creek basin. Climax recognizes that access to the hydraulic pump to close the bulkhead valves may be hindered during the PMP event. To account for this temporary condition, Climax is committed to maintaining available surcharge storage capacity in Robinson Lake equivalent to two days worth of diverted inflow from the Tenmile Tunnel. The 2 -day volume of diverted inflow to Robinson Lake would be about 88 acre-feet based on a flow rate of 10,000 gpm through the Tenmile Tunnel. If the PMP event happened to occur simultaneously with the peak of the 100 -year snowmelt event and the Chalk Mountain Interceptor had failed, the additional 5 -day snowmelt volume from the entire 696 acres tributary to Robinson Lake would be 191 acre-feet, using the 5 -day average snowmelt runoff rate of 0.2740 acre-feet per acre, discussed previously. 5.0 OPERATING CRITERIA Climax is committed to maintain the water level in Robinson Lake with sufficient surcharge storage capacity to store, without release, the full 256 acre-feet of rainfall from the site-specific PMP event, plus five days worth or 43 acre-feet of seepwater inflow from the 1 Dam, Robinson Dam, and 4 Dam seepage collection systems, plus two days worth, or 88 acre-feet, of diverted inflow from the Tenmile Tunnel throughout the year. To meet these criteria, the surcharge storage requirement EPP — Appendix D December 2011