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• ELK CREEK DRAINAGE Area D is the Elk Creek Drainage. This drainage must flow through a 9-foot diameter culvert which extends completely under the Elk Creek Facilities area. This culvert must be extended up the drainage approximately 280-feet. According the Handbook of Steel Drainage & Highway Construction Products a 108" diameter CMP will handle 500 cfs with no head. The Vegetation Map in the permit, drawing No. E8-3391 shows that the vegetation in the Elk Creek Drainage is predominantly Mountain Shrubland with intermingled pockets of Conifer forest and Aspen. The baseline vegetation survey is located in Appendix H in the permit. This survey shows that the total cover for the Mountain Shrubland community is 82.1$. Table S-3 of the Procedures for Determining Peak Flows in Colorado shows that an Oak-Aspen area with 808 ground cover and soil type D would have a Curve Number of 57. The run-off calculations for the Sanborn Creek drainage were also compiled using a CN of 57. In the case of the Elk Creek drainage the curve number, was varied as follows to analyze the effect of various curve number selections had on the peak flow calculation. It can be seen that even if a curve number of 75 were selected the existing 108" culvert would handle the calculated flow. ELK CREEK DRAINAGE ,f, Curve No. 100-YR 24-HR EVENT 57 67 CFS 60 107 CFS 65 201 CFS 70 331 CFS 75 500 CFS Appendix B-1 also contains run-off calculations performed for the reclamation phase of the project for Elk Creek. The calculated peak flow from the 100 year event show in appendix B-1 is 110 CFS which provides independent confirmation that the Elk Creek culvert is more than adequate to handle the design flow. The inlet of- the existing 108" culvert is protected from erosion by a rock headwall. A similar the rock headwall will be constructed to protect the inlet of the extended 108" culvert from erosion and piping. B-127 4/92