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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />I. <br />:. <br />:. <br />. <br />. <br />. <br />. <br />. <br />,. <br />'. <br />. <br />~ . <br />'. <br />. <br />. <br />I. <br />. <br />. <br />. <br />. <br /> <br />.- <br /> <br />13 <br /> <br />02/23/00 <br /> <br />SA Risk-Based Analysis <br /> <br />The U.S Army Corps of Engineers has provided a framework that may be used to analyze flood <br />hazards on alluvial fans using the principles of risk-based analysis in Guidelines for Risk and <br />Uncertainty Analysis in Water Resources Planning (1992). This method uses the total probability <br />equation that will be discussed in detail in Section SB. The degree of uncertainty associated with <br />a prediction of a given flood scenario is assessed by bringing to bear evidence derived from <br />geomorphologic and other studies. This method tracks the effects of the error associated with a <br />calculation to provide a confidence band in ensuing predictions of flood-hazard severity. <br /> <br />SB FAN Program <br /> <br />Assumptions: flooding in rectangular channel; critical depth; erosion of rectangular channel <br />banks until the change in width per the change in depth equals -200; the <br />probability density function of a discharge occurring at the apex is log- <br />Pearson Type III; the frequency of flood events for various recurrence <br />intervals, i.e., 2-year through SOO-year, can be adequately defined, equal <br />probability along contour arcs (random flow paths); (also provides for <br />multiple channels at normal depth, assuming total width is 3.8 times the <br />single channel width) <br /> <br />Limitations: fluvial (as opposed to debris flow) formed fan, unstable flow paths <br /> <br />Recommended Applications: highly active conical fans <br /> <br />The FAN computer program provides one method of analyzing the flood hazards on alluvial fans. <br />The methodology used by the FAN program defmes the risk of inundation at any particular location <br />by applying the definition of the base flood through the theorem of total probability. The <br />methodology itself is broader than the use within the FAN program. Let H be a random variable <br />denoting the occurrence of flooding at a particular location. That is: <br /> <br />I if the location is inundated <br /> <br />H= <br /> <br />o if the location is not inundated <br />