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999 <br />~oc I~ae I Z~11/o ~ <br />S~C ~~ EXHIBIT 15, <br />• POND DRAINAGE DESIGN'S <br />SEDCAD+ MODELING <br />The software package SEDCAD+ Version 3.1 (Sediment, Erosion, Discharge by Computer-Aided <br />Design) (Warner and Schwab, 1992) was utilized to establish a range ofwatershed-specific flow <br />conditions and to evaluate sediment loss at the site. This model takes site-specific information on <br />disturbance, vegetation, soil runoff chazaeteristics, topography, channels, and impoundments and <br />couples it with site specific textural information, erodibility factors, cover and erosion control <br />practice factors to generate runoff volumes, peak flows and estimates of soil loss associated with <br />specific storm events. <br />SEDCAD+ utilizes composite runoff hydrographs generated by the Soil Conservation Service's <br />(SCS's) TR-55 in a distributed pazameter simulation model. Runoff is predicted within <br />subwatersheds and routed to develop composite hydrographs. Watersheds were divided into units <br />based on drainage patterns and variations in vegetation or disturbance. This information is combined <br />with site-specific sedimentology parameters to estimate sediment runoff from discrete storm events. <br />A Type II rainfall hydrograph with Antecedent Moisture Condition II was used to evaluate the 10- <br />. year, 24-hour storm of 2.9 inches, the 25-yeaz, 24-hour storm of 3.4 inches and the 100-year, 24-hour <br />storm event of 4.2 inches to reflect a range of anticipated runoff within [he project area (U.S. <br />Department of Commerce, 1973). Four major vegetation community types were characterized in the <br />analysis: grassland, oak-pinyon, mountain shrub, and ponderosa pine. Soils a[ [he site range from <br />sandy loams to clays, which are considered to be soils with a hydrologic response ranging from B to <br />D. Mapping has identified some soils to the series level and others to the Association level. The <br />model employed an average hydrologic response classification derived from the component soil <br />series. Most soils have a moderately fine texture; depth ranges from shallow to moderate. The soils <br />yield a moderately high runoffpotential (USDA NRCS, 1996). The hydrologic response values for <br />the soils are in Table 1. <br />Curve numbers for the existing conditions of the site were assigned using average values of cover <br />data generated from the baseline collection in Table 2. Water is routed from the top of the <br />watershed to the bottom by the longest route using the time ofconcentra[ion value. The SCS Upland <br />Curve method totals the flow time for overland flow and channel flow. Channel routing was <br />incorporated in azeas where natural channels existed or permanent diversions were constructed. <br />SEDCAD+ was used to predict storm-related sediment loading using the Revised Universal Soil <br />Loss Equation (RUBLE). Runoff and sedimentation in the primary analysis area were modeled to <br />show the impacts of a storm of 10% probability storm, a 10-year 24-hour of 2.9 inches (U.S. <br />Department of Commerce, 1973) on the site in its maximally disturbed condition. Watersheds <br />within drainages in which mining disturbance either had occurred or was projected to occur were <br />. specified further to allow modeling. <br />1 (revised 4/20/01) <br />