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Design Basis for Design The design used as a basis for improvements at the site is the 10 year 24 hour • storm. This was selected because there is little potential for damage greater than any existing damage and also because it is the recommended design storm in 30 CFR Sec. 717.17 (e) (f). Prediction of Design Discharge The method of determining flows was the soil conservation service publication, procedures for determining peak flows in Colorado, Ref 2. The rainfall was determined from the NOAA Rainfall Atlas, Ref 3. All design parameters and the resulting flows are "sted on the attached sheet marked "Hydrologic ]nformation." Existing Structures There are now six 24" culverts at the mine site. Two culverts are used to cronvey flows along Curtis Creek and four to carry flows under State Highway 13 from the west side to the main channel of Curtis Creek. Design Discussion Because of severe restrictions imposed by the State Highway Department regarding any work in the R.O.W. of State Highway 13, Northern Coal has decided to attempt no construction in the R.O.SY. The narrow flood plain and the restriction imposed by not working in the R.O.W. required that the sediment pond be constructed on the east side of the_highway. To • construct the pond on the west side would not have been feasible because of the steeply sloped canyon walls. A diversion ditch to prevent flows from passing through the disturbed area was also not possible because of the steepness. Since all mining facilities will be located on the west side of the highway there is no need to improve Curtis Creek Channel which runs along the east side of the highway. The location of the sedimentation pond is out of the Highway R.O.W. and has a minimal affect on the flood plain of the intermittently flowing Curtis Creek. Recommended Design We recommend limiting all mining activities to the west side of Curtis Creek and building a sedimentation pond on the east side of the Creek. We feel this is the only design that will insure capturing all sediment produced by mining and also insure a safe, efficient design. To get flows from the west side of the highway to the pond we suggest taking advantage of the existing culverts as often as possible. The culvert under State Highway 13 at Station 17+38 will divert runoff from upstream so that it doesn't pass through the disturbed area. The culvert at 2+19 will be lengthened using a 36" diameter pipe and will be the inlet into the sedimentation pond. By using the existing pipes no disruption of traffic on the highway would be required. • Location of the pond and pipes are shown on sheets 2 and 3 of the plan set.