2007-10-30_PERMIT FILE - C1982057 (2)

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To determine the sediment loading, the Average Annual "R" method was utilized to quantify sediment storage for a set number of storage years. Historically the SEDCAD demonstrations at the II-W Mine used an Annual R factor of 30. This value is suited for barren soils which generate large volumes of sediment, not reclaimed landscapes that are revegetated. In addition, the storage year value was set at one (1) year. The revised as-built SEDCAD demonstration uses an Annual R-factor of 10. This value was obtained from Figure 8A.1, Isolines of R factor for Western U.S. (see attached), which was taken from Design Hydrology and Sedimentology for Small Catchments (Haan, et al., 1993). A sensitivity analysis was conducted varying the number of storage years to maximize the sediment storage capacity while assuring that the pond could store the sediment yield from the design storm and maintain compliance with the applicable rules and regulations. The number of storage years was increased in the sedimentology sub-routine until the effluent settable solids concentration in pond discharge exceeded the water quality criterion (0.5 milliliters per liter [ml/1]), or the sediment elevation was within 2-feet of the top of the CMP riser (as per recommendations of the SEDCAD Design Manual and User's Guide [Warner, et al., . 1998 ]) Particle Size Distribution: The particle size distribution was based on sieve and hydrometer analysis of various soil types in the II-W Mine permit area. The specific gravity and submerged bulk specific gravity of the soil are estimated to be 2.50 and 1.17 respectively. Soil Erodibility Factor: The soil erodibility factor (k) is based on soil type. Soil types were determined as part of the baseline soil survey (see Tab 9). Approved k factors are presented in Attachment 13-5B, Table 2, Soil Erodibility Factor. The values in this table were agreed upon by representatives of CDRMS and SCC in conjunction with Permit Revision No. 2. Length-Slope Factor: The length-slope factor is described as the typical length-slope for the subwatershed and the distance from the point of origin of overland flow to a point where the slope decreases such that deposition occurs or until the flow enters a defined channel. The length-slope for Pond 005 was determined to be between 300 and 400 feet, and the average slope was determined to be 22.2 percent. Control Factor: The control factor (C) is used to account for vegetative or mulch cover. Approved control factors for postmine land conditions are presented in Attachment 13-5B, Table 3, Control Factor. The values in this TR-59 13-3C-5 Revised 05/07