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Hydrology: The hydrology of the watershed was analyzed using the SEDCAD <br />computer model. SEDCAD is an enhanced PC version of the SEDIMOT II program. <br />Attachment 13-1 contains a listing and description of the SEDCAD input <br />variables. Addendum 13-3-A presents the SEDCAD modeling demonstration for the <br />10-year, 24-hour storm event; Addendums 13-3-B and 13-3-C present SEDCAD <br />modeling demonstrations for the 25-year, 24-hour and 100-year, 24-hour storm <br />events, respectively. <br />Input consists of precipitation amount (inches), watershed area (acres), time <br />of concentration (hours), runoff curve number (CN), representative particle- <br />size distributions of soils in the area, sedimentology information, spillway <br />information, etc. The drainage area for this sediment pond was measured from <br />approved permit maps (see Exhibit 13-2). <br />The structure design was analyzed for the largest anticipated disturbed <br />drainage area that will contribute runoff to the structure. It was estimated <br />that this condition will occur if all of the mining area is disturbed and the <br />area is in different phases of mining and reclamation. In addition, the <br />structure was conservatively designed to include adequate storage capacity for <br />sediment storage and to treat runoff from a 10-year, 24-hour precipitation <br />event. Finally, the design conservatively assumed the pond is 100 percent <br />full to the emergency spillway during the 25-year, 24-hour and 100-year, 24- <br />hour precipitation events and provides for maintenance of adequate freeboard <br />to prevent overtopping of the embankment. <br />Precipitation and Storm Type: The precipitation amounts at the Seneca II-W <br />Mine permit area for the 10-year, 25-year and 100-year storms were obtained <br />from Frequency Atlas of the Western States, Volume II, Colorado (NOAA, 1973). <br />A standard SCS Type II storm was utilized for each analysis. <br />Unit Hydrograph and Surface Condition: The SEDCAD computer model provides <br />three unit hydrograph shapes to choose from in determining the runoff <br />characteristics of a watershed. A "slow" unit hydrograph was utilized for <br />undisturbed areas and a "medium" unit hydrograph was used for the reclaimed <br />areas. <br />Watershed Area: The watershed areas were determined by direct measurement <br />from mine permit maps. <br />Time of concentration: The time of concentration, T~, is defined as the time <br />required for water to flow from the hydraulically most remote point in the <br />watershed to the watershed outlet. <br />TR-59 13-3C-3 Revised 08/07 <br />