2010-05-07_REVISION - M1977493 (10)

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Permit Amendment (AM-06) - Climax Mine • than about 100 years should be considered an approximation. As additional rainfall data are collected, the results of a statistical analysis may change. Also, NOAA Atlas 2 provides broad regional rainfall depths, which are not site-specific and only for return periods up to 100 years. The NOAA Atlas 2 rainfall depths shown for the 200-year, 500-year, and 1,000-year events were extrapolated and should be considered approximate. The largest 24-hour rainfall depth recorded by the Climax Weather Station is 2.00 inches, which occurred in May 1973. This fact as well as a review of other peak rainfall events at the Climax Weather Station indicates that the NOAA Atlas 2 results may be conservatively high. The detailed design depth-duration data developed from the Log Pearson Type III and NOAA Atlas 2 approaches are provided in Tables K-7 and K-8. K-5 Probable Maximum Precipitation As part of the flood hydrology study conducted for the site in 2005, Applied Weather Associates performed a site-specific probable maximum precipitation (PMP) evaluation. This analysis transposes actual historical storms to the Climax site and maximizes the amount of precipitation that could potentially fall. This approach has been frequently used and accepted widely throughout Colorado for determining site-specific PMP events. The analysis resulted in a 6-hour local storm PMP depth of 4.41 inches and a 48-hour general storm PMP depth of 4.35 inches. K-6 Prevailing Winds Climax began recording average wind speed and direction at the site in about October 2008. Those data are shown on Table K-9. The most common prevailing wind direction is from the north-northwest with average wind speeds between five and ten miles per hour. Data on wind gusts are not available. Figures AM-06-K-06 through AM-06-K-10 provide an annual and four seasonal wind rose data figures that were developed from the wind data collected from October 2008 through September 2009. Prior to the recent study, however, wind data had not been recorded at the Climax site. The nearest known wind station with a relatively long period of recorded wind data is at the Leadville Airport, which lies about 13 • miles south and west of the Climax site. At Leadville, the prevailing winds typically blow from the north or northwest and range in average speed of six to nine miles per hour. However, because wind patterns in the Colorado mountains are almost always controlled by topography, the Leadville wind data may not be representative of conditions at Climax. K-7 Evaporation Evaporation data are not currently measured at the Climax site. In 1958, a standard evaporation pan was installed at the Climax Weather Station and measurements of summertime evaporation were recorded through 1971. Those data were later used as part of a water rights study to develop monthly evaporation coefficients for use with the Blaney-Criddle methodology to estimate monthly pan evaporation from temperature data. Pan evaporation rates, however, are typically greater than evaporation rates from open water bodies. To correlate the monthly pan evaporation to actual evaporation from the open water bodies, a conversion coefficient of 0.76 can be applied. This value is taken from NOAA Technical Report NWS 33. Based on the water rights study, which used a study period of 1963 through 1982, the average summertime (May through October) evaporation from open water bodies at Climax has been estimated to be 20.98 inches. Note that due to the cold temperatures and typical ice cover over the open water bodies, evaporation is negligible during the winter months. In addition to evaporation from open water bodies, a significant amount of evaporation at Climax occurs from the areas of the TSFs that are wetted by the deposited tailing slurry. These areas, commonly called beaches, are characterized by broad open expanses with shallow water depths (less than a few inches). In addition, the tailing slurry is typically warmer than the exposed water bodies. These conditions provide an ideal opportunity for evaporation. As part of the water rights study, a conversion coefficient of 1.00 was concluded to be appropriate to convert the calculated pan evaporation values to actual evaporation for the wetted beach areas. Through this approach, the average summertime evaporation from wetted beaches at Climax has been estimated to be 27.60 inches. • Exhibit K K-2 May 2010