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