2006-08-02_PERMIT FILE - C1980007 (8)

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using broadly used techniques. A 100-year return period, 24-hour duration peak discharge, Q~oo, • was modeled, which was requested by the U.S. Forest Service to be the design storm. 2.2 Hydrologic Analysis Input Data Storm Tvne SEDCAD 4 uses the National Resources Conservation Service (NRCS, formerly Soil Conservation Service, or SCS) storm type distributions that are considered very conservative - i.e. the peak flow prediction based on these type distributions will be higher than almost any actual measured storm. In Colorado, SCS Type II storm distribution should be used. Design Storm The precipitation data was obtained from the National Oceanic and Atmospheric Administration (NOAA) Atlas 2 for Colorado, which was published by NOAA National Weather Service Hydro- meteorological Design Studies Center (Appendix A). At the project site, the depth of a 24-hour 100-year storm event is 2.9 inches. Particle Size Distribution Soil gradation tests were conducted for the site (IGES, 2003) and the particle size distribution chart from one of these tests is included in Appendix B. SCS Runoff Curve Number • Published soil data is not available for the study area. Soil grouping was determined by analyzing the gradation test results and findings from site inspections. The soil types in the study area aze estimated mainly in hydrologic group C with some in hydrologic group D. The project drainage basins are vegetated mountain azea with fair hydrologic conditions. The SCS runoff curve number for this kind of soil cover listed the U.S. Department of Agriculture (USDA) National Engineering Handbook (SCS, 1972) is 63 (Arid and Semiarid Rangelands, Cover Type: Oak- aspen). Because there is a lack of published data, a conservative SCS rnnoff curve number 66 was used in this study. Sub-watershed Area and Time of Concentration USGS 7.5 Minute Topographic Quadrangle Maps (USGS, 1964 and 1964) were used to delineate the contributing watershed and the sub-watersheds (Plate I). The area and the hydraulically longest flow path for each sub-watershed were measured and calculated based on the delineations and summarized in Table 1. Time of concentration for asub-watershed can be calculated within SEDCAD 4 by using SCS runoff curve method and a Routing Calculator included in the program. The time of concentration consists of an overland flow component and a channel flow component. The flow lengths and slopes for each component were measured from the USGS map. SEDCAD 4 Routing Calculator includes nine different categories that can be selected to estimate the time of concentrations. In the Sylvester Gulch roadway drainage model, category 1 (forest with heavy ground litter) was chosen to model the overland flow and category 8 (large gullies, diversions and low flowing streams) was used for channel flow for the sub-watersheds. • Sunrise Engineering, Inc. Sylvester Gulch Access Road Hydrologic /Hydraulic Analyses for Roadway Drainage