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• HYDROLOGIC ANALYSIS <br />A detailed analysis was conducted for existing drainage conditions to identify stone <br />runoff peak flows, volumes, and sediment ]oads using the industry-accepted model called <br />SEDCAD. Output from this model was used to size the various surface water management <br />facilities required during mining and for post-reclamation. <br />A detailed analysis was also conducted for the key mining period approximately yeaz 6 to <br />7. This period represents a disturbed area with all major anticipated operating activities <br />occurring simultaneously, including active mining, backfilling of overburden, and reclamation of <br />the final mine surface. These results were used to size surface water management facilities <br />- within the active mine area. <br />To assess storm water runoff for post-reclamation conditions, a qualitative analysis was <br />perfonned, based on the results of existing drainage conditions analysis. <br />This section of the plan presents: <br />^ Drainage model used for hydrologic analyses, including a discussion of input <br />variables such as existing watersheds on the site, design precipitation basis, and soil <br />pazameters. <br />^ Drainage evaluation results - in terms of peak flows, volumes, and sediment loadings <br />• for existing conditions, conditions during the key period of block years 6 to 7, and <br />post-reclamation conditions. <br />SEDCAD Model <br />The SEDCAD computer model (Warner 1998) was used to generate individual sub-basin <br />storm runoff hydrographs and sediment-graphs. The model uses the SCS TR 55 emulation to <br />generate runoff hydrographs for various storm events, which are then routed using the <br />Muskingum method to combine sub-basin hydrographs. The model uses the universal soil loss <br />equation (RUBLE) methodology to calculate sediment-graphs, which are then routed through the <br />arroyos. Input variables are described below for watersheds, design storms and soil pazameters. <br />Watershed Delineation. The project area was first divided into ten major arroyos, labeled <br />"A" through "J" (see Figure 1) using the Project azea 2-foot contour interval mapping. A major <br />arroyo was defined as a drainageway that crosses the property boundary, where potential impacts <br />are measured (that is, changes in peak runoff, volume, or sediment loads). These major arroyos <br />were further divided into 50 sub-watersheds ranging in size from 14 to 177 acres, with an <br />average size of 72 acres. This additional division of azeas provided reasonable discretization to <br />estimate storm runoff sediment load amounts. A summary of the sub-basin chazacteristics is <br />provided in Table 1. <br />• <br />SWMP BC June 28 2002.doc, OG/28/02 <br />