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:\'urrnrhrr 199% CYCC'-.ti~•dinr~nr ))cld Drnnm.arurirnr-rblirrc• 3 .-1 Hydru~raph Response Shape. A unit hydrograph was chosen for each drainage area or sub-area model to • predict thr runoff response. The hydrograph responses available in the SEDCAD+ model are slow, medium, and fast. A slow response corresponds to a forested area or an area with a number of obstructions, where as a Last rcsponsc corresponds to an unve,~etated or poorly protected area. For the demonstration, a medium runoff response was chosen. Drainage Basin Area. Representative one acre drainage basin areas were modeled for [he reclamation block and reference area being evaluated. Represen[auve one acre unmined drainage areas were modeled to exhibit overland (low throughout die length of [he defined watershed. Representative one acre mined drainage areas were divided into tour sub-watersheds due to the presence of contour fwrows throughout the reclamation blocks. Within these areas, overland Flow lengths are shortened with subsequent Flow paths consisting of overland drainage, contour furrow Clow, and then combined Flow along grassed waterways representative of areas of concentrated runoff. Time of Concentration. Time of concentration (tc) within each representative one acre drainage was calculated with SEDCAD+ using [he SCS Upland Method, as presented in Barfield, e[ al. (1981), page 100. Representative one acre areas were used to derive hydraulic lengths and drainage paths. Average slopes for Blocks North and South were measured directly from I" = 500' scale maps. These slopes were used as inputs for calculating both time of concentration and Muskingum routing parameters where appropriate. Slopes of the contour furrows in the reclaimed blocks were assumed at 0.5 %. Curve Number. The runoff curve number (CN) is a factor relating the amount of rainfall to the amount of runoff for a given area. It is dependent upon the infiltration characteristics of site-specific soil types and the type and amount of vegetation. Prior to disturbance, the mined areas encompassed the following soil types • (Ref: Soils Map, CYCC Permit No. C-84-062): SOIL NAME HYDROLOGIC SOIL GROUP Buckskin Loam C Routt Loam C Binco Silty Clay Loam D Unnamed Loam - Aaberg Silty Loam D Splitro Sandy Loam C Parachute -Merino Complex C Hydrologic Soil Groups based on Routt County SCS designations. '~ Soil groups for the reclaimed blocks are assumed to be a mixture of the groups which were originally stripped. Ground cover values for the Mine 3 North, Mine 3 South, Dryland Reference, Low Sage Reference, and the Pastureland Reference Areas were obtained from the 1995, 1996, and 1997 vegetation surveys conducted by CYCC. Curve numbers were obtained from the SEDCAD+ selection tables based on soil type and vegetative condition. Curve numbers were determined for the following conditions: Block North Hydrologic Soil Group: Predominantly D Land Use Description: Pasture/Range Condition: Good • Cover Type: Grass CN: 80 r\bon d rc I \m i ne3\scdkcdm inc J.doc