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runoff from storms having a return period of 100 years ranging from approximately <br />6 acre feet with 1.5 inches of rainfall occurring in one hour to 20 acre feet with 2.0 <br />inches of rain falling in 6 hours under conditions which prevailed prior to mining. <br />After mining, flood runoff volumes will be less than one-half of these amounts. (a) <br />On the rare occasions when surface runoff occurs in Streeter Gulch, the flow <br />will be conveyed in the relocated channel which will follow the meandering contact <br />of the embankment with the north abutment of the canyon. This channel will be an <br />essentially rock-lined ditch having a bottom width of 6 feet and a minimum depth of <br />5 feet. During high flow periods, most of the flow will be carried to the silt dentention <br />pond at the toe of the embankment but a portion may enter the embankment and per- <br />colate downward through the fill and ultimately appear as groundwater discharge in the <br />valley of Good Spring Creek in much the same manner as existed prior to mining. <br />In order to evaluate the internal drainage characteristics of the embankment, <br />permeability tests were made at various depths within the fill with the following results: <br />Hole Depth Tested Permeability(b) <br />No. (fe et) ft~yr. <br />PH-1 10.0 - 27.5 2,500 <br />PH-1 10.6 - 22.0 16,000 <br />PH-2 8.4 - 26.6 5,600 <br />PH-2 11.9 - 24.0 2,900 <br />PH-3 2.3 - 9.1 29,000 <br />PH-3 2.3 - 7.1 32,000 <br />PH-4 5.4 - 10.0 140,000* <br />PH-5 0.2 - 1.5 190 <br />PH-5 0.2 - 1.5 11 <br />PH-6 0.2 - 1.5 150 <br />PH-6 0.2 - 1.5 140 <br />PH-6 0.1 - 0.6 550 <br />- *Near outer slope of dumped fi II <br />`~' Data called for under MESA Regulations, Section 77.216-2, Subparagraph (a) pre- <br />pared by Tipton and Kalmbach, Inc., November 1977. <br />(b~ Results abstracted from letter Report to Colowyo Coal Company from Woodward-Clyde <br />Consultants dated August 2, 1977. 5 <br />