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<br />. <br /> <br />. <br /> <br />. <br /> <br />002238 <br /> <br />CHAPTER V <br /> <br />DRAFT <br />August 30, 1989 <br /> <br />RECOMMENDATIONS ON THE DEVELOPMENT OF SPECIFIC DESIGN <br />CRITERIA FOR THE CREATION OF THE FLOODWAY BOUNDARIES. <br /> <br />The boundaries of the Floodway are determined by the <br />size of the river flow that must be accommodated. The river <br />flow is determined by the combination of releases from the <br />mainstream dams (Hoover, Davis, and Parker) and side inflow <br />from tributaries and washes which represent a realistic one- <br />in-one hundred year river flow or 40,000 cfs, whichever is <br />greater. These requirements are stated in Section 5(b) (1) <br />of the Colorado River Floodway Protection Act and the <br />accompanying legislative history. <br /> <br />The vast volume of reservoir storage space at and above <br />Hoover Dam and river runoff above Hoover Dam dominate the <br />river flow patterns. Tributary runoff below Davis Dam occurs <br />from normally dry washes as a result of localized, intense <br />thunderstorms. While these flows may be of significant size <br />locally, they are usually of short duration and will <br />dissipate within the river channel over a relatively short <br />distance and time span. Historic river flow records are <br />quite good regarding normal flows in the river and releases <br />from the dams. However, there are very few good records on <br />rainfall events or flood events in the normally dry tributary <br />area. <br /> <br />A major problem faced by the Task Force was to develop a <br />technically sound process to evaluate tr ibutary flood flows <br />considering the lack of available data. A smaller Technical <br />Subcommittee was formed to develop a recommended process to <br />determine tributary flood flows and recommended specific <br />design criteria for the creation of the Floodway boundaries. <br />It was composed of Task Force members and personnel from the <br />Bureau of Reclamation and the Corps of Engineers and private <br />consultants retained by entities represented on the Task <br />Force. The final recommended design criteria and the <br />recommended tributary flood flow determination process <br />developed by the Subcommittee were documented in a draft <br />report of May 20, 1988 entitled Methodology for Determining <br />the Floodway Flows and Boundaries on the Colorado River. <br />This report was accepted by the full Task Force at its <br />October 25, 1988 meeting. The final version of this <br />Methodology report is included as Appendix F. <br /> <br />V-l <br />