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<br />. <br /> <br />. <br /> <br />. <br /> <br />002241 <br /> <br />probability of a flood flow rate for each month was also <br />developed for each tributary using the techniques previously <br />referenced in the ~~t~o~~!~~y report. Scientifically <br />acceptable statistical procedures were used to combine these <br />two separate events. The Technical Subcommittee was <br />satisfied that the resulting flow rate represented a <br />realistic one-in-one hundred year river flow. <br /> <br />RECOMMENDATION <br /> <br />The Task Force recommends the acceptance of the <br />procedure as described in the previously referenced <br />Methodology report. <br /> <br />IV. ISSUE <br /> <br />Determine the duration of the one-in-one hundred year <br />river flow event and the resulting river water surface <br />elevations upstream and downstream of the tributary. <br /> <br />DISCUSSION <br /> <br />The duration of the flow resulting from Hoover Dam <br />releases is relatively constant and as such provides a river <br />water surface level that is also relatively constant and easy <br />to determine. To determine the total volume and duration of <br />tributary flood events, it was again necessary to use <br />accepted techniques of statistical correlation as were <br />discussed in Issue I. The one-in-one hundred year flow rate <br />(or 40,000 cfs) as statistically determined represents some <br />combination of less than a 100-year event for each of the dam <br />releases and the tributary flow. Since the dam release <br />represents a fairly constant situation, it was necessary to <br />judgmentally assign some level of flow to the tributary <br />contribution, then identify the volume and duration of flow <br />associated with that flow event. <br /> <br />After consideration of the desert terrain and localized <br />weather patterns, it was decided to assume that the one-in- <br />one hundred year flow rate would be a combination of the 100- <br />year event on the tributary combined with a base flow <br />existing in the river. At this point, it was possible to use <br />the process to develop a hydrograph (discharge over time) for <br />the tributary event. The river flow rates determined from <br />this process varied from slightly greater than 35,000 cfs to <br />nearly 50,000 cfs at various locations along the river. For <br />river reaches determined to have one-in-one hundred year <br />events less than 40,000 cfs, the steady state model was used <br />to determine water surface levels for the 40,000 cfs flow <br />rate. <br /> <br />V-4 <br />