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