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<br />Sep~embcr 1993 CYVCC -Sediment Yield Demonstration - Black C ~ 4 <br />Hydrograph Response Shape. A unit hydrograph was chosen for each drainage azea or sub-area <br />model to predict the runoff response. The hydrograph responses available in the SEDCAD+ <br />model are slow, medium, and fast. A slow response corresponds to a forested area, or an azea <br />with a number of obstructions, whereas a fast response corresponds to an unvegetated or poorly <br />protected area. For the demonstration a medium runoff response was chosen. <br />Drainage Basin Area. The drainage areas were determined by digitizing directly from 1"=500' <br />scale maps supplied by CYVCC. <br />Time of Concentration, Tc. Time of concentration was calculated by using the SCS Upland <br />Method as presented in Bazfield, et al., (1981) page 100. All hydraulic lengths, drainage heights, <br />and slopes were measured directly from the 1"=500' scale maps. <br />Curve Number CN. The runoff curve number (CN) is a factor relating the amount of rainfall <br />to the amount of runoff for a given area. It is dependent upon the infiltration characteristics of <br />site-specific soil types and the type and amount of vegetation. <br />Prior to disturbance both areas encompassed the following soil types (Ref: Soils Map, Southfield <br />Mine Permit ): <br />Soil Name <br />Winevada-Splitro Complex <br />Aaberg Silty Clay <br />Cochetopa Loam <br />Hydrologic Soil Grouo <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 <br />vegetation survey conducted by CYCC. <br />Curve numbers were obtained from Procedures for Determining Peak Flows in Colorado and <br />Technical Release No. 55, (USDA-SCS, 1980). Based on soil type and vegetative cover, curve <br />numbers were determined using Figure S-3, found in Procedures for Determining Peak Flows in <br />Colorado, for the following conditions. <br />Block C, <br />Soil Type: Predominately C <br />Ground Cover: 73 %, <br />Cover Type: Herbaceous <br />CN : 76 <br />Pastureland Reference Area <br />Soil Type: Predominately C ' <br />Ground Cover: 51 % <br />Cover Type: Herbaceous j <br />CN : 81 <br />W~T Ol~au~' <br />II~~"(~ <br />~"_)~ <br />U~~ <br />~, ~N ~b~e <br />~~~ <br />~,~ ~~~.~ <br />•~, ~ <br />f ell / a-'w <br />,~~, ~G <br />C!i-~~ ' <br />~i~`" rs~ <br />x,~ ~~ l <br />C;-tk~, ~,ass~~v1;~ l <br />Lf/v I><Yy <br />TERRAMATR/X • P.O. Box 7740!8 • Steamboat Springs, Colorado 80477 • (J03) 879-6260 <br />