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in xhich tP is the time to peak in hours, and qP is the peak flow rate in <br />cubic feet per second. The lag time was estimated using an equation developed <br />by Snyder (1938): <br />tL ~L(L Lca)0.3 <br />where Ct is a constant ranging from 1.8 to 2.2 with an average of about 2.0, L <br />is the length in miles of the main channel from the divide to the outlet, and <br />Lca is the length in miles of the main channel from the outlet to a point on <br />the stream nearest the centroid of the watershed. The SCS method also <br />requires the time distribution of rainfall. The 24 hour depths were <br />distributed using the 24 hour distribution curve presented by SCS (1973). Six <br />hour depths were distributed using a similar curve developed from the steepest <br />6 hour portion of the 24 hour curve. <br />To determine depths, stream cross-sections were obtained at each of the <br />locations. All stream sections were assumed to have the same main channel <br />cross-section which was determined from discharge measurement notes at the <br />• gaging stations. The main channel section was then idealized to a trapezoid. <br />The remainder of the points were taken from surveyed cross-sections (1,3 and <br />4) or topographical maps (2 and 5). Exhibit 2.5-8 presents the cross-section <br />at Cross-Section 2. The idealized channel cross-section is denoted by points <br />A, B, C and D. Depths were computed using the Manning equation using the <br />average streambed slope as the slope of the energy grade line. Peak <br />discharges and corresponding depths are presented in Table 2.5-6. <br />As a check, discharges and depths were also calculated for the 10, 50 <br />and 100 year, 24 hour events using methods presented by McCain and Jarrett <br />(1976) . The study region lies in two regions - the Northern Plateau and <br />Mountain. Peak discharges and corresponding depths are also presented in <br />Table 2.5-6. The depth in parentheses represents the depth calculated using <br />equations for the Mountain Region. <br />Mapping of the flood elevations for the 24 hour storm with 100 year <br />recurrence interval are shown on Exhibits 2.5-9 and 2.5-10. Similar analyses <br />of the floods on Trout Creek are not included in this document since mining <br />activities will remain well above the flood plain of Trout Creek. <br />• Water Qee - A listing of all water rights in the study are adjudicated <br />as of January 1988 was obtained for the Division of Water Resources. A total <br />2.5-18 November 9, 1990 <br />