2011-03-14_REVISION - M1977493 (48)

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Permit Amendment (AM-06) - Climax Mine CM0000089 Manual provides unit hydrograph lag relationships for both the general and local storms. For the Rocky • Mountain Region, Kn ranges from 0.130 to 0.260 for general storms, and from 0.050 to 0.073 for local storm events. Based on the suggested ranges for Kn, a value of 0.050 was selected to be used for both the general and the local storm for all sub-basins for which the unitgraph method was applied. G-7.5 Baseflow The Climax area experiences an annual snowmelt period that typically occurs in May and June. The runoff associated with this snowmelt must be considered as the baseflow condition for storms that could occur during this period. The site-specific PMP analysis indicates that the design storm could occur during mid-summer and could, therefore, potentially occur during a period of high snowmelt runoff. For this hydrology analysis, it was assumed that a 100-year snowmelt flood would be the appropriate baseflow to use for the PMP event. The snowmelt baseflow was generated using USGS streamflow data from the Black Gore Creek near the Minturn gaging station. This basin is located approximately nine miles north of Climax and has a drainage area of 12.6 square miles. The mean basin elevation of the Black Gore Creek drainage is similar to that of the Tenmile Creek basin at Climax. Streamflow data for Black Gore Creek is available for the period 1948 to 1956 and 1964 to the present. The 100-year snowmelt flood for the Black Gore Creek basin was developed using the procedures set forth in the USGS Bulletin #17B "Guidelines for Determining Flood Flow Frequency." This calculation was made on a unit discharge basis so that the results could be applied to the various drainage basins in the Climax flood routing analysis. A minor adjustment was made to the unit discharge to account for the higher average annual precipitation of the Black Gore Creek basin. This adjustment was based on the precipitation data compiled by the Colorado Climate Center and published in map form titled "Colorado Average Annual Precipitation 1951-1980." This calculation procedure resulted in a 100-year peak average daily flow of 21.74 cfs per square mile. G-7.6 Flood Routing Assumptions Fresh water runoff from the basins tributary to the Climax site is generally intercepted and conveyed to either Tenmile Creek or the Eagle River basin through the fresh water bypass and interceptor systems. The • interceptor systems are generally not designed to withstand PMP magnitude flood events. Therefore, it has been assumed that all of the interceptors would fail during the PMP and water that would normally be intercepted would then report directly to the TSFs. It was also assumed that Buffehrs Lake would fail and the contents of the reservoir would flow into the Robinson TSF water pool. The storage volume of Buffehrs Lake is about 111 acre-feet. The flood wave generated by the dam failure was not specifically modeled because its volume is significantly less than the volume of water flowing through the Robinson Pond water pool during a PMP event. The reservoir volume, rather, was simply added to the volume of water entering Robinson Pond. The initial level of each of the TSF water pools was set to match the water level that would be necessary to convey the 100-year snowmelt flood. Also, the spillway and decant structures, which have movable control sections, were assumed to be set at a conservatively high setting. The coupling of these assumptions provides conservatively high initial water levels. Several changes have occurred at the site since the 2005 hydrology study was performed. These changes were incorporated into the revised model and include: construction of the 2 Dam Spillway, abandonment of the West Gravity Line (a former decant structure that delivered Robinson Pond water to Robinson Lake), updated information regarding the storage capacities of each of the TSF water pools, and construction of a new decant structure on Tenmile TSF. G-7.7 Flood Routing Results During mining, the storage capacities of the TSF water pools will be continuously changing as additional tailing is deposited in the facilities. Rather than attempt to model each expected change, a generalized model was established to determine the volume of flood storage that will need to be maintained on the TSFs to accommodate the PMP event with an additional one foot of residual freeboard. The peak flow capacities needed by the decant and spillway structures on each water facility are also determined. is Exhibit G G-16 May 2010