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Larry Routten and Erica Crosby -7- November 3, 1993 <br />estimate, given the short time constraints. The number used with this revised design is slightly <br />lower; however, it has been researched in detail, and it is still considered to be a conservative <br />estimate. <br />Time of Concentration <br />The SEDCAD software derives the Time of Concentration parameter using channel relief, <br />channel length, and channel condition as input variables. These variables were derived <br />manually from the USGS Quadrangle map as follows: <br />Flow Path 1 <br />This path was measured to be approximately 2,200 feet in length, covering a representative <br />flow path with approximately 580 feet of vertical relief. This was measured from the 7,500- <br />foot contour line to the point at which the Quadrangle shows a defined water course at <br />approximately the 6,920-foot contour line. <br />The representative flow condition over this path is assumed to be non-channelized flow over <br />forested ground. <br />(Represented by the number 1 in SEDCAD nomenclature) <br />Flow Path 2 <br />This path was measured to be approximately 10,000 feet in length, covering a representative <br />flow path with approximately 820 feet of vertical relief. This was measured from the <br />6,920-foot contour line to approximately the 6,100-foot contour line. <br />The representative flow path condition over this path is assumed to be channelized flow, <br />flowing bank full. <br />(Represented by the number 9 in SEDCAD nomenclature) <br />All lengths were derived using an ASI-brand map wheel. The SEDCAD table used to describe <br />various flow conditions is attached (Exhibit CI. A general description of the algorithm by which <br />SEDCAD uses the above variables to derive the Time of Concentration is attached (Exhibit D). <br />Peak Flow <br />Using the above-referenced inputs, theoretical peak flows were derived for the 10-year, <br />24-hour; the 25-year, 24-hour; and the 100-year, 24-hour flows using the SEDCAD software. <br />In general, the SEDCAD software derives peak and total runoff estimates using acurve-number- <br />based runoff prediction method. Full documentation regarding the SEDCAD model (Warner and <br />Schwab, 1992) is available at the DMG office. The U.S. Department of Agriculture (1986) also <br />provides an excellent description of acurve-number runoff model. <br />