2011-03-14_REVISION - M1977493 (48)

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Permit Amendment (AM-06) - Climax Mine The Tenmile Creek drainage basin area is generally typical of high mountain basins in the Rocky Mountains. The slopes of the basin are generally steep and elevations range from a maximum of nearly 14,000 feet along the eastern boundary, to a minimum of about 10,600 feet at the crest of 5 Dam. The drainage basin was divided into 18 sub-basins to model flood impacts on the three primary TSFs and Clinton Reservoir. Figure AM-06-G-01 shows the configuration of the various sub-basins summarizes the drainage areas. The mean basin elevation for the areas tributary to the three primary TSFs are 11,200 feet for the Robinson TSF, 11,440 feet for Tenmile TSF, and 11,520 feet for Mayflower TSF. Note that in the 2005 flood study, 18 sub-basins were used. For this analysis, basins LEI and MTP were combined to promote continuity between current, mineral production, and closure conditions. site-specific PMP design storm for the Climax site (Applied Weather Associates, 2005). The results of this • study were subjected to a third party review, conducted by Mr. George Taylor, a certified consulting meteorologist with Applied Climate Services. The review, documented in a written report (Applied Climate Services, 2009), concluded that the study conformed to the requirements and guidelines specified in Hydro- Meteorological Report (HMR)-50 and HMR-75, with no inconsistencies found. As a result of this study, minor editorial changes and corrections were made and an amended report was issued in August 2009. Additional information regarding the PMP event can be found in Exhibit K: Climate Information. The hydrology study indicated that, under the configuration at that time, the water storage facilities and TSF water pools at Climax were capable of passing the site-specific PMP in accordance with DRMS and Colorado Division of Water Resources requirements (COSEO, 2007). Although no significant impacts to the drainage basins and hydrologic regime are anticipated due to mine production, the water storage capacity and dam crest elevations of the TSFs will continuously change due to tailing deposition. In order to evaluate the implications of mine operation and closure on the system's flood handling capabilities, the hydrology model has recently been revised to model the expected site conditions for operating and closure scenarios. The flood hydrology model uses the U.S. Army Corps of Engineers' HEC-1 (HEC, 1990) computer program to evaluate the site's rainfall-runoff relationship. G-7.2 Drainage Basins • G-7.3 Soils and Infiltration The uniform infiltration method was used in the HEC-1 model to approximate infiltration losses. Infiltration rates were based on the soil type in each sub-basin. The soil information was developed from the "Survey of Holy Cross Area, Colorado" prepared by the U.S. Forest Service (USFS, 1995). The infiltration rates are based on the minimum infiltration values recommended in the USBR Flood Hydrology Manual (USBR,1989). For Group A soils, which generally have high permeability and low runoff, an infiltration rate of 0.30 inches per hour was used. For Group B soils, which have moderate permeability and moderate runoff, a rate of 0.15 inches per hour was used. For Group C soils, which have moderate permeability and rapid runoff, a rate of 0.05 inches per hour was used. For Group D soils, which are nearly impermeable with very high runoff, no infiltration was assumed. To be conservative in the analysis and to account for generally shallow soils, these rates represent the minimum recommended values. In order to simulate saturated soil conditions, no initial rainfall losses were considered and rock outcroppings, structures, roads, water surfaces, and other facilities were considered to be impervious. G-7.4 Flood Development The flood unit hydrograph for each sub-basin was determined using either the kinematic wave or USBR unitgraph method. The kinematic wave method was used when the basin configuration was such that the flow was routed to the basin outlet through several parallel channels and a well defined main channel was not present. This method involves the development of overland flow planes, as well as channel flow in collector channels and a main channel, if one is present. The factors used to develop the overland and channel flow characteristics are flow distance, slope, and an estimation of a representative roughness coefficient. An example of this type of basin is one that lies along the side of the large tailing impoundments, where runoff would flow onto the pond through several smaller channels. CMC000088 The unitgraph method was used when a well-defined main channel was present. The USBR Flood Hydrology Exhibit G G-15 May 2010