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• represent existing conditions prior to any mining activity. Assumptions for postmining <br />conditions that are reflected by the postmining input parameters include; 1) atl <br />disturbed areas have been regraded, topsoiled, and reseeded according to criteria outlined <br />in the PAP; 2) all temporary structures (culverts, check dams, etc.), roads, and <br />facilities have been removed and reclaimed; and 3) vegetation has attained at least 70 <br />percent cover and is reflective of conditions expected at bond release. <br />The drainage areas above both ponds (005 and 006) were divided into subwater sheds, and a <br />junction and associated "null" structure were assigned to the outlet of each subwatershed. <br />Junctions were located along a main branch that corresponds to the main or longest channel <br />in each basin (005 and 006 Culch). Each subwatershed was assigned a separate branch, <br />along which a "null" structure was located in order to provide model outputs for each <br />subwatershed. Tables 17-23 and 17-24 present a listing of subwater sheds and corresponding <br />junctions and areas used to modes both drainages for premini ng and postmining conditions, <br />respectively. <br />Watershed areas and topographic measurements used to determine the various input <br />• parameters for both the pre- and postmining runs were obtained from 1:400 scale maps of <br />the Seneca II-W Mine (see Exhibits 17-2 and 17-3j. The postmining topography map, Exhibit <br />20-2, was used for calculating input parameters that reflect postmining basin <br />characteristics (average watershed slope, time of concentration, etc.). Two storm depths, <br />1.60 inches (10-year, 24-hour storm) and 2.6 inches (100-year, 24-hour storm) were used in <br />combination with a Type II distribution to model both pre- and postmining conditions. <br />These storm values were obtained from the SCS document titled Procedures for Determining <br />Peak Flows in Colorado (Wilkes and King, 1980). <br />Hydrology input parameters include watershed area, curve number, time of concentration, <br />and Muskingum routing parameters. Curve numbers for premining conditions were selected by <br />assigning curve numbers to vegetation types (mixed brush, aspen, etc.), and corresponding <br />soils (i.e., hydrologic soil Group D) mapped in the Seneca II-W Mine vicinity (see Exhibit <br />10-1, Tab 10, and Exhibit 9-1, Tab 9). Curve numbers were assigned to each type according <br />to criteria developed for selecting curve numbers as outlined by the SCS and presented in <br />design reports for each proposed sediment pond (see Attachments 13-3 through 13-5, Tab 13, <br />Facilities). For postmining conditions, a curve number of 61 was selected assuming that <br />. vegetation has reestablished to a hydrologic cover greater than 70 percent on replaced <br />topsoil (clay loam to sandy loam, hydrologic soil Group 8). Curve numbers for each <br />63 <br />