Laserfiche WebLink
.. APPENDIX A <br />Mt. Gunnison Drainage Ditch Relocation <br />Drainage within the Nlt. Gunnison Mine facilities area will be <br />routed down the slope to the Main Access road as shown on the <br />attached map. The drainage area above the ditch consists of <br />about 1.3 acres of undisturbed area and 0.25 acres of road. The <br />ditch is designed to safely pass the peak discharge resulting <br />from the 10-year, 24-hour precipitation event from the drainage. <br />The design of the ditch was prepared using the procedures <br />developed in "Procedures for Determining Peak Flows in Colorado", <br />USDA Soil Conservation Service (1980), and the "Surface Water <br />Hydrology and Sedimentology Manual", OSM Region V(1981). Design <br />rainfall magnitudes were obtained from NOAA Atlas 2 "Precipita- <br />tion Frequency Atlas of the United States", Volume III, Colorado <br />(1973). The 10-year, 24-hour design rainfall magnitude to the <br />location of the Mt. Gunnison Mine facilities area was estimated <br />to be 1.8 inches. <br />Peak flows from the 10-year, 24-hour precipitation event were <br />determined as follows: <br />Curve Number Calculations <br />Cover Area Curve No. <br />*Forest-Oak & Aspen 7.3 57 <br />Road-Dirt .26 87 <br />Weighted Curve 7.56 <br />68 <br />LAG Time = 0.95 hours (from Figure 3.3 in SCS (1980)) <br />for a slope of 25A and length of 950 feet. <br />Time of Concentration (TC) = 1.667 x LAG = 0.16 hrs. <br />Runoff (10 yr., 24 hr. event) = 0.13 inches <br />Peak Discharge = 1.35 cfs (from figure <br />5-2 in SCS (1980) <br />*Oa'~ aspen Cover, with 30A cover and Hydrologic Soil Group C, <br />from C'SN, Region V(19fi1). <br />