2008-12-04_PERMIT FILE - C1994082 (32)

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distributions of soils in the area, sedimentology information, spillway . dimensions, etc. The drainage area(s) for the watershed, as well as, slope lengths and distances for time of concentration estimates were measured on an approved permit topographic map (see Exhibit 13-4B). The design conservatively assumed the pond is 100 percent full to the emergency spillway invert elevation (e.g., principal spillway is inoperable) during the 25-year, 29-hour and 100-year, 29-hour precipitation events and provides for maintenance of adequate freeboard to prevent overtopping of the embankment. SCS Curve Number and "k" Factor. The curve numbers used for the revised SE DCAD demonstration are presented in Attachment 13-12. The "k" factors were determined based on soil survey information and National Resource Conservation Service soil interpretations. The curve numbers and "k" factors were then "weighted" for each sub-watershed as presented in the Hydrology Input Parameter tables found at the back of Addendum 13-3A-A. Precipitation and Storm Type. The precipitation amounts at the Yoast Mine area for the 10-year, 25-year and 100-year precipitation events were obtained from NOAA Precipitation Frequency Atlas - Frequency Atlas of the Western United States - Volume II - Colorado, 1973. A standard SCS Type II storm was utilized £or each analysis. • 10-year, 24-hour event: 1.7 inches • 25-year, 24-hour event: 2.1 inches • 100-year, 24-hour event: 2.5 inches sedimentology. To determine the sediment loading to the pond, the average annual "R" method was used. The amount of sediment storage is determined for a defined length of time and is based on the design storm R, design storm sediment load, average annual R, and the number of storage years (Y). R is the rainfall erosivity factor. The equation for required sediment storage is: Required Storage = (Annual R)(# years)(Sto rm Sediment Load) Storm R The TR-30 SEDCAD4 demonstration used an annual R value of 10 versus the 1996 design which used an R value of 30. This updated R value was obtained from Figure 8A.1, Isolines of R factor for Western U. S., which was taken £rom Design Hydrology and Sedimen tology for Small Catchments (Haan, et. al., 1993 - see attached). A sensitivity analysis was conducted varying the number of storage years (Y) to maximize sediment storage capacity and maintain compliance with the TR-30 13-3A.3 Revised 05/07