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<br />Transition Section <br /> <br />Deposition/Scour Evaluation <br /> <br />o Water Surface Profile <br /> <br />HEC-2 <br /> <br />Profile with Deposition/Scour <br /> <br />o Dike Height Determination <br /> <br />HYDROLOGY <br /> <br />The hydrology utilized in this evaluation is consistent with ltle revised Flood Insiurance Study <br />hydrology recently developed for the 1991 Flood Control Master Plan Update of the Las Vegas <br />Valley undertaken by the Clark County Regional Flood Control District (CCRFCD). The Flood <br />Insurance Study hydrologic model for the Range Wash Basin was based on a computer model <br />developed by Boyle as part of their Facility Planning Study. Because the construction of the <br />We,st Range Wash Diversion Dike is immanent, the FIS reco\~nizes the existence of the dike. <br />Those portions of the Range Wash basin below the diversion dike are included in the FIS model <br />of the lower basin. The diverted flows discharge into a detention basin and do not impact any <br />flocldplains other than the obvious reduction of flow in the 10wElr Range Wash basin. No model <br />was developed which included the upper basin. Thl~refore, the 1991 Facility Planning Study <br />model was used to determine design inflows to the diversion dike. The modeling methodology <br />ancl basic assumptions are consistent with those in the Flood Insurance Study. A 100-year <br />peak discharge of 6,584 cfs was used in this analysis. Other recurrence intervals were also <br />simulated for use in this analysis including the 2, !i, 10, 25, and 50-year events. Peal< flows and <br />total hydrographs have been developed for each. (See "Hydrology Section" Design Notebook). <br /> <br />AVERAGE ANNUAL SEDIMENT YIELD <br /> <br />The average annual sediment yield at the apex of the fan is the starting point for all subsequent <br />sediment transport evaluations of the Range Wash systems. Thle MOU indicates that the <br />average annual yield used as the basis for the 2, 5, 10, 25, 50,. and 100-year event yield since <br />no specific estimates were available within the Range Wash Basin which related yield to a <br />recorded discharge. It was concluded that a reliable estimate of average annual sEidiment yield <br />could be developed and used for the development of specific Elven! yields. For this evaluation <br />the PSIA Methodology was used to estimate sediment yield at the diike location. The details of <br />this methodology are presented in the 1991 Facilities Planninfl Stu,jy and are included in the <br />"Ave. Annual Yield Ref." section of the Design Notebook. The average annual yield value <br />calculated for the West Tributary of Range Wash was 16 AF (0.4 Af/Acre/Yr). This was <br />compared to regional estimates developed by the Corps of Engineers in their Las Vegas Valley <br />Feasibility Study. There was good correlation with the COE range of 0.3 to 0.5. CCll1l,equently, <br />an average annual sediment yield at the dike location of 16 acre.feet per year was used. <br /> <br />EVENT SPECIFIC YIELD <br /> <br />The approach utilized in developing event specific yields differed somewhat from that proposed <br />in the MOU. A defensible relationship between the 100-year event yield and the average annual <br />sediment yield could not be identified after a review of the available literature. Various values of <br />the mlationship between 100-year event yield and the aVElrage annual yield were identified in the <br />