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<br />July 1996 CYVCC -Sediment Yield Demonstration Rev.1- BlockC • 4 <br />Hydrograph Response Shape. A unit hydrograph was chosen for each drainage area or sub-area model <br />to predict the runoff response. The hydrograph responses available in the SEDCAD+ model are slow, <br />medium, and fast. A slow response corresponds to a forested area, or an area with a number of <br />obstructions, whereas a fast response corresponds to an unvegetated or poorly protected area. For the <br />demonstration a medium runoff response was chosen. <br />Drainage Basin Ares. The drainage areas were determined by digitizing duectly from 1" = 500' scale <br />maps supplied by CYVCC. <br />Time of Concentration. Time of concentration (tc) was calculated by using the SCS Upland Method <br />as presented in Barfield, et al., (1981) page 100. All hydraulic lengths, drainage heights, and slopes were <br />measured directly from the 1" = 500' scale maps. <br />Curve Number. The nmofl'curve number (CN) is a factor relating the amount of rainfall to the amount <br />of runoff for a given area. it is dependent upon the infiltration characteristics ofsite-specific soil types <br />and the type and amount of vegetation. <br />Prior to disturbance both areas encompassed the following soil types (Ref: Soils Map, CYVCC Mine <br />Permit): <br />Soil Name <br />Winevada-Splitro Complex <br />Aaberg Silty Clay <br />Cochetopa Loam <br />FIVdr OIOR.rC SDII GfDDD <br /> C <br /> D <br /> C <br />Hydrologic Soil Groups based on Routt County SCS designations. <br />Ground cover values for Block C and pastureland reference area were obtained from the 1992 vegetation <br />survey conducted by CYVCC. <br />Curve numbers were obtained from Procedures Cor Determining Pcak Flows in Colorado and Technical <br />Release No. 55, (USDA-SCS, 1980). Based on soil type and vegetative cover, curve numbers were <br />determined using Procedures for Determining Peak Flows in Colorado, for the following conditions. <br />Block C, <br />Soil Type: Predominately C <br />Ground Cover: Good <br />Cover Type: Herbaceous <br />CN: 74 <br />Pastureland Reference Area <br />Soil Type: Predominately C <br />Ground Cover: Good <br />Cover Type: Herbaceous <br />CN: 74 <br />c~stieu~can.raEv <br />a~irvwr ~:~rpm tp TenaMalrix lnc • 165 South Union Boulevard, Suite 160 • Lakex~nod. Colorado 80228 • (303) 763-5140 <br />