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., III IIIIIIIIIIIIIIII 999 STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Department of Natural Resources 131 3 Sherman 51., Room 215 Denver, Colorado 80203 Phone: 13UJ1 8663567 FAk 1]031832-8106 DATE: November 9, 1998 ~I~ DEPARTMENT OF NATURAL RESOURCES Roy Romer Governor lames 5. LochheaA E:ecuuve D~recior TO: Jim Burnell Michael B. Long Divismn Director FROM: Joe Dudash V V RE: Water Tauk SURE, West Elk Mine, Permit No. C-80-003, Mountain Coal Co. At your request, I reviewed the SAE designs for the water tank azea, in order to access whether silt fence should still be required for sediment control. My conclusion is that silt ,'ence should still be required in that area. There are two SEDCAD designs in the permit application for the water tank area, one, which includes the cut slope and pad, and the second, which includes the slope downgradient of the pad. In reviewing the approved SAE designs, I believe I discovered an erroneous entry in each of the designs that resulted in lower predicted sediment levels than would be expected. The control practice factor is a measure of the sediment yield from an area as compared to that from a fallow field. The higher the number, the more sediment yield is to be expected from a precipitation event. An entry of zero had been used for one of the control practice factors in each of the two designs. With a more reasonable number, the sediment level in the runoff discharge would increase. I used a control practice factor of .04 instead of zero for the one entry in each SEDCAD design. This value was derived from a table, using a ground cover of about 60 percent and no appreciable cover. For the SEDCAD run for the area down gradient of the water tank pad, I used a curve number that reflected the current state of vegetative ground cover in that area. The curve number is a measure of the runoff potential from a given area. The lower the curve number, the less runoff would be expected from a precipitation event. I changed the curve numbers from 98 and 91 to 7~.