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• the pit and diverted away from the structure. In these cases, a conservative procedure was used; the strutture was analyzed for the largest anticipated disturbed drainage area that will eontri bu to runoff to the structure. IC was estimated that this condition will occur if ell of the proposed disturbed area is stripped of vegetation and topsoil et the same time. In addition, the structure was conservatively designed to include adequate capacity for sediment storage end to treat the runoff from a 10-year, 24-hour precipitation event. Finally, the design conservatively assumed the pond is 700 percent full to the emergency spillway during the 25-year, 26 -hour precipitation event and provides for maintenance of adequate freeboard to prevent overtopping of the embankment. SCS Curve Number end "k" Factor. The curve numbers area based on information presented in Attachment 13-1. The "k" factors were determined with the aid of the USDA-ARS's revised USLE software package and ere shown in Attachment 13-1. The curve numbers and "k" factors were then "weighted" for each subwatershed as shown on Teblea 13-SA-2 end 13-SA-3. TABLE 73-3A-2 weighted CM Celcul ations for Pond 009 watershed Vegetation Hydrologic Vegetation Cover Type Soil Grouo Cover (X) CN Aree (Ac) CN x Area Aspen B 30-70 48 34.58 1,659.84 Mixed Brush B 30-70 48 30.57 1,467.36 Nixed Brush C 30-70 57 2.24 127.68 Mixed Brush D 30-70 63 1.93 121.59 Sagebrush B 30-70 51 61.22 3,122.22 Sagebrush C 30-70 63 5.99 377.37 Meadow B 0-70 69 0.55 37.97 Steep Mountain Brush B 0-70 b7 160.84 10,776.28 Steep Mountain Brush C 0-70 77 13.21 1,017.17 Steep Mountain Brush D 0-70 83 2.08 172.64 Nestern Nheatgrass D 30-70 B9 5.13 456.57 Topsoil Removed C - 91 120.21 10.939.11 438.55 30,275.80 • Yeigh[ed Average CN = 30.275.80 = 69.0 438.55 13-SA-3