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`r 5174 Highway 133 Somerset, CO 81434 (970)929-SOIS Fax (970)929-5595 2. For the raw water intake gallery SAE, please justify the use of a curve nuber of 62 for SWS#2. The submittal cover letter states that the subwatershed #2 area has more vegetation and rock cover than the subwatershed #1 area. The greater vegetation cover could decrease the curve number but the greater rock cover could increase the curve number. COMPANYL.L.C. West Elk Mine A Subsidiary ofArch Western Resources. LLC P O BOx 591 ~~MOUNraiN(oa~ .~'` ~(OMPANY„ As we don't have the original data from which the original curve numbers were derived, MCC used a curve number of CN=89. A curve number of 89, was selected for SWS #2 to represent a graveled ground surface with some sparse grasses on a soil with a slow infiltration rate. 3. It appeazs that the SEDCAD muskingum routing for the raw water intake gallery SAE uses the parameters for SWS #1, when the pazameters for SWS #2 should be used. Please revise accordingly, if you agree MCC agrees with the Division's observation. The particle size distribution for SWS #1 was changed to the sandy clay loam from the North Fork Alluvium. 4. The raw water intake gallery SAE utilizes a grass filter as a structure for sediment control in the SEDCAD design. As has been discussed with West Elk previously in item number 41 of the Permit Revision No. 7 adequacy review letter, dated 15 January 1997 (incorrectly dated 15 January 1996), a grass filter can only be used for sediment control if the grass filter is on land that is already disturbed. The SEDCAD design for this SAE utilizes a grass filter that lies between the disturbed area and the river. An SAE design cannot include a grass filter that is outside the disturbed azea of the SAE. Also, a grass filter is not recommended for this site because, once the grass filter silts over and becomes ineffective, the sediment from the SAE would pass directly into the river. Please revise the SEDCAD design for the raw water intake gallery SAE so that a grass filter is not used. If silt fence or straw bales are used in the SEDCAD designs, please show their location on Map 54. The raw water intake gallery SAE includes a small, relatively flat, sparsely vegetated, pad with arip-rap apron into the river. The area subject to erosion is less than 0.04 acres. The attached SEDCAD run assumes half of this area has a CN=72 and half has a CN=89, both at a 10% slope. Run-off from a 10-year, 24-hour event is calculated to be only 0.003 acre-feet or 131 cubic feet. This volume will be contained by either silt fence or strawbales and will be located along the lowest soil contour of the SAE above the normal high water line of the river. 5. The submittal cover letter states that the soil particle size distribution selected for the Sylvester Gulch fan SAE was for the same sandy clay loam used for the raw water intake gallery SAE. However, the particle size distribution used for both subwatersheds of the raw water intake gallery SAE's for the North Fork alluvium. It would appear that the particle size distribution for SWS#] of the raw water intake gallery SAE should be for the sandy clay loam, not for the North Fork alluvium. Please revise this SEDCAD design if you agree.