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EXISTING STRUCTURES - ELK CREEK LIGHT USE ROAD <br />• <br />~~ <br />There are four culverts associated with the light use road that <br />extends up the Elk Creek drainage. The culverts are shown on Map <br />18 (Elk Creek Facilities Area - Riprap Borrow Area) and Map E8-3207 <br />(Elk Creek Facilities Area - Existing Facilities and Topography). <br />Two of the culverts, one 18" and one 46", provide for drainage to <br />cross under the road from the west to Elk Creek. The other two <br />108" culverts are installed in the Elk Creek drainage where the <br />light use road traverses the creek. <br />The Elk Creek Drainage is shown on Map 5-036 (Elk Creek Facilities <br />- Elk Creek Drainage Area Map). This drainage must flow through <br />the 108" diameter culverts mentioned above. <br />According the Handbook of Steel Drainage b Highway Construction <br />Products a 108" diameter CMP will handle 500 cfs with no head, a <br />48" diameter CMP will handle 63 cfs with no head and an 18" <br />diameter CMP will handle 5.5 cfs with no head. <br />The Vegetation Map in the permit, Map E8-3391, shows that the <br />vegetation in the Elk Creek Drainage is predominantly Mountain <br />Shrubland with intermingled pockets of Conifer forest and Aspen. <br />The baseline vegetation survey is located in Appendix H in the <br />permit. This survey shows that the total cover for the Mountain <br />Shrubland community is 82.1$. Table 2-2d. of the Procedures for <br />Determining Peak Flows in Colorado shows that an Oak-Aspen area <br />with fair cover and soil type D would have a Curve Number of 63. <br />A curve number of 63 was used to calculate the runoff for all four <br />of the existing culverts associated with the Elk Creek Light Use <br />Road using the computer program Storm ver 6.21 as follows: <br />Elk Creek Light Use Road - 108" CMP's <br />• <br />Watershed Data <br />Curve Number <br />Area <br />Hydraulic Length <br />Elevation Change <br />Concentration Time <br />Concentration Time Type <br />Unit Hydrograph Type <br />= 63 <br />= 3776 acres <br />= 27000 feet <br />= 3700 feet <br />= 1.16 hours <br />= SCS Upland Curves <br />- Forested <br />Storm Data <br />Total Precipitation <br />Storm Type <br />Peak Discharge <br />Discharge Volume <br />108" CMP Capacity <br />= 2.4 <br />= SCS Type <br />= 83.38 <br />= 66.56 <br />= 500 <br />B-159 <br />inches <br />2, 25 Yr 24 Hr Storm <br />cfs <br />acre-feet <br />cfs <br />6/94 <br />