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Robinson Lake Operating Plan Climax Molybdenum Company <br />Version R1 Permit M-1977-493 <br />1.0 OVERVIEW <br />In accordance with the requirements described in the Climax Mine EPP, Robinson Lake is <br />designed and operated as a zero discharge facility to preclude overflow to Eagle Park Reservoir <br />and the East Fork of the Eagle River. To fulfill this commitment, the water level in Robinson Lake <br />is maintained well below the spillway control crest to provide sufficient surcharge capacity to store, <br />without release, runoff from the probable maximum precipitation (PMP) event, plus runoff from <br />the 100 -year snowmelt event, plus seepwater from the Robinson Dam, 1 Dam, and 4 Dam <br />seepage collection systems, plus operating return flows from the Tenmile Tunnel. The following <br />technical discussion provides the basis and assumptions used to establish the Robinson Lake <br />Operating Plan. <br />2.0 BASIS AND ASSUMPTIONS <br />2.1 Probable Maximum Precipitation <br />A site-specific PMP study for the Climax Mine was completed by Applied Weather Associates in <br />2005 and revised in 2009. This study concluded that the PMP event at Climax would produce <br />4.41 inches of total rainfall. A discussion of the analyses and conclusions are presented in Permit <br />Amendment AM -06 Exhibit G. <br />2.2 100 -Year Snowmelt <br />As part of the Climax flood hydrology study completed in 2005, Wheeler developed 100 -year <br />snowmelt runoff rates for the Climax site. These rates were based on an analysis of the Black <br />Gore Creek drainage basin near Vail and adjusted for the Climax site. The snowmelt study <br />determined the 100 -year 1 -day maximum snowmelt runoff rate to be 15.28 gpm per acre (0.0675 <br />acre-feet per acre in one day). Correspondingly, the peak 100 -year 5 -day -average snowmelt <br />runoff rate was determined to be 12.40 gpm per acre (0.2740 acre-feet per acre in five days). <br />2.3 Seepwater Inflow <br />The Robinson Dam and 4 Dam seepwater systems collect seepage below the respective dams <br />and pump that water into Robinson Lake. The seepage rate from Robinson Dam has historically <br />been less than about 25 gpm. The pump system that pumps this seepwater back to Robinson <br />Lake has a nominal capacity of 480 gpm and is therefore only run on an intermittent basis. The <br />seepage rate from 4 Dam has historically been less than 25 gpm. This pump system also has a <br />pumpback capacity of 480 gpm. <br />Seepage from 1 Dam is conveyed to Warren's Pump System where it is normally pumped into <br />the Tenmile Creek basin for treatment. The rate of 1 Dam seepage has historically been less <br />than 1,000 gpm. Warren's Pump System has a nominal capacity of 4,500 gpm. <br />2.4 Operating Return Flows <br />During operation, approximately 10,000 gpm of process water will be pumped from Robinson <br />Lake to supply the demands of the mine and mill. Whenever the natural inflow to the lake is not <br />sufficient to maintain the desired operating water levels, water can be diverted into the lake from <br />the Tenmile Creek basin through the Tenmile Tunnel. Water from the Tenmile TSF water pool <br />can be released through the Tenmile Decant and water from the Mayflower TSF water pool can <br />EPP — Appendix D December 2011 <br />