2010-12-17_REVISION - C1981019 (122)

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• 1.4 HYDROGRAPH RESPONSE SHAPE A unit hydrograph is chosen for each drainage area or sub -area modeled to predict the runoff response. The hydrograph responses available in the SEDCAD4 model are slow, medium, and fast. A slow response corresponds to a forested area or an area with a number of obstructions. A fast response corresponds to an un- vegetated or poorly protected area. Medium hydrograph responses were identified as representative for mine areas. The internal convolution increment is 0.05 hours, and values are saved at the user - specified interval of 0.1 hours or greater. A convolution increment of 0.1 was specified. It should be noted that a time of concentration less than 0.125 hours bypasses the unit hydrograph technique and instantaneous run -off is assumed. As explained in the SEDCAD4 Users Manual (Schwab and Warner, 1998), the time of concentration restriction of 0.125 hours is dictated by a combination of array size restrictions, minimum internal convolution interval, and the user - specified time increment for saving convolution values. 1.5 DRAINAGE BASIN AREA Drainage basin areas were determined for each pond from scaled drawings. The initial (6 -month operational scenario) condition was used as the worst -case representation for all ponds. The condition reflects topsoil stripping on both the Collom Lite and Little Collom X pit areas, facilities pad, and 6- month spoil pile footprint. Topsoil stockpiles are assumed to be surrounded by perimeter (or "ring ") ditches, that will be of sufficient volume to contain all runoff, and therefore are not counted towards the total drainage area. Watershed areas were delineated to divide disturbed from undisturbed land, in order to avoid averaging widely disparate curve numbers. 1.6 TIME OF CONCENTRATION, Tc The time of concentration was calculated using the NRCS upland method (Natural Resources Conservation Service, 1986). The hydraulic lengths, drainage heights and associated slopes were determined from scaled drawings. The values calculated for each structure are shown on a sub - watershed basis in the SEDCAD4 printouts. 1.7 MUSKINGUM ROUTING PARAMETERS. K, X The Muskingum Routing Parameters were also calculated using the NRCS upland method. The hydraulic lengths, drainage heights and associated slopes were determined from scaled drawings. The values calculated between each structure and/or sub - watershed are shown in the SEDCAD4 printouts. 1.8 CURVE NUMBER. CN The runoff curve number is a factor relating the amount of rainfall to the amount of runoff for a given area. Curve numbers were designated on a land use basis as determined from Vegetation Map 4C. Table 1, PartA provides a summary of the various land uses and their corresponding curve numbers used in the initial modeling. All sagebrush areas were assumed to be in fair condition, though some instances of good condition do occur in the watershed. After the initial modeling effort provided in Exhibit 7 -23, Part A was completed, the Colowyo Mine modified the curve numbers to be used for future modeling. These changes were revised under TR -73. Additional support for the use of the revised curve numbers is provided in Volume 2D, Introductory text, "Methodologies and Assumptions for Sediment Pond Design Evaluations. The revised curve numbers are presented in Table 1, Part B below and will be employed for all future design work. All of the initial SEDCAD modeling presented in Exhibit 7 -23, Part A is based on the curve numbers shown in Table 1, Part A. Future modeling will use curve numbers presented in Table 1, Part B. Exh. 7 -23 -2 Revision Date: 12/15/10 Revision No.: PR -03