2015-08-31_PERMIT FILE - C1981010A (23)

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A cumulative flow of 113 acre -ft of runoff was calculated for the period August 1, 1985 to July 31, 1986 for the Middle East Flume drainage. Figure 2.7-15b is a hydrograph of daily mean flows from May 1985 to October 1986. A baseflow of 35 gpm from springs in the drainage is obtained from the graph. The annual cumulative runoff from precipitation is then 56 acre -ft or 1.2 inches. This value is higher than the value for the East Pyeatt drainage and is thought to be due to local variations in infiltration rates and a smaller basin size. The precipitation depth for this period was 19.72 inches. The 44 year mean precipitation for the rainfall gauge in Craig, prior to the installation of the Trapper rainfall gauge, is 13.56 inches. The 8 year mean precipitation value for the mine's gauge was 18.49 inches. This length of record is not considered to be a period analyzed, an above average precipitation depth occurred. By noting the variation in the runoff volumes for East Pyeatt and Middle East Flume and by comparison with the other references noted previously, an average yearly runoff depth of 1" is appropriate. In 1981, during Trapper's original permit submittal, a curve number of 70 was used for estimating runoff from Trapper Mine. Woodward -Clyde, in a 1976 surface runoff investigation at the mine site, recommended this curve number. An antecedent moisture condition, AMC II, was assumed to establish a rainfall -runoff relationship. During the summer of 1986, Trapper had spent considerable time in discussions • 2-414c D Frm- 9 Y +" long enough to determine a mean precipitation value for the mine site. A comparison of the precipitation at the mine with the Craig mean value shows that, for the period analyzed, an above average precipitation depth occurred. By noting the variation in the runoff volumes for East Pyeatt and Middle East Flume and by comparison with the other references noted previously, an average yearly runoff depth of 1" is appropriate. In 1981, during Trapper's original permit submittal, a curve number of 70 was used for estimating runoff from Trapper Mine. Woodward -Clyde, in a 1976 surface runoff investigation at the mine site, recommended this curve number. An antecedent moisture condition, AMC II, was assumed to establish a rainfall -runoff relationship. During the summer of 1986, Trapper had spent considerable time in discussions • 2-414c D Frm- 9 Y +"