2009-06-23_PERMIT FILE - C1981019A

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site specific conditions. Typically, it would be assumed that the bold, "current" values would be used for all future design work beyond the May 2009 date. The non-bold, "historic" values were used in all existing designs. None of the current structures modeled with the "old" curve numbers will be remodeled at this time. Table 7 provides a scanned image of published curve numbers typical for disturbed areas; these values will not be used directly but are only provided to support the changes to Table 1. Table 8 provides a similar listing from the National Engineering Handbook (NEH-4) (USDA-SCS, 1985); these values are included within the drop down menus of the SEDCAD model. Values in Table 8 will not be used directly but are only provided to support the proposed changes to Table 1. Table 9 lists the location and name of all existing designs as of the date of this change. TABLE 1 SCS CURVE NUMBERS COVER COVER TYPE HYDROLOGIC PERCENT CURVE CURVE CONDITION VEG. NUMBER, NUMBER, COVER HISTORIC CURRENT Undisturbed Mountain Shrub Good 85 41 57 Oak-Aspen Good to Fair 50 - 85 41 57 Sagebrush Good 85 47 47 Sagebrush Fair 50 63 63 Disturbed Topsoil Stripped Bare N/A 91 85 Reclaimed Topsoiled and Bare N/A 86 83 Seeded 1 to 2 Year Fair - Good 25-45 61 77 Growth 3+ Year Growth Fair - Good 70 56 74 Sources: SCS TR-55 Urban Hydrology for Small Watersheds June, 1986 Also see Tables 7 and 8 for additional support of curve numbers provided. Curve numbers presented in the historic column represent previously agreed upon values between DRMS and Colowyo. Other values within the "current" column represent typical values that may be used for site specific conditions based on the judgment of the design engineer. All current curve numbers are based on hydrologic Soil Group C. 1.9 BASE FLOW A base flow of 0.15 cfs was used for the Streeter and Prospect Pond designs. A base flow of 0.03 cfs was used for the West Pit Pond design. Base flows were not used in any other pond design calculations. Pit water will not be discharged through outfalls that have not been permitted for pit water discharge under an approved CDPS permit. Additionally, for those outfalls approved for pit water discharge but where this discharge was not included in the pond design as a base flow, pit pumpage will not occur during any storm runoff or snow melt less than or equal to the 25-year, 24-hour storm event. 2.0 SEDIMENTATION CALCULATIONS The sedimentation calculations for each pond on the Colowyo Mine were performed using the SEDCAD4 Exhibit 7 Page 3 Revision Date: 6/19/09 Revision No.: TR-73