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I I I I I I I I I I I I I I I I I I I 14 07/16/99 SA Risk-Based Analysis The U.S Army Corps of Engineers has provided a framework that may be used to analyze flood- hazards on alluvial fans using the principles of risk-based analysis in Guidelines for Risk and Uncertainty Analysis in Water Resources Planning (1992). This method uses the total probability equation that will be discussed in detail in Section 5B. The degree of uncertainty associated with a prediction of a given flood scenario is assessed by bringing to bear evidence derived from geomorphologic and other studies. This method tracks the effects of the error associated with a calculation to provide a confidence band in ensuing predictions of flood-hazard severity. 5B FAN Program Assumptions: flooding in rectangular channel, critical depth, erosion of rectangular channel banks until the change in width per the change in depth equals -200, the probability density function of a discharge occurring at the apex is log- Pearson Type III, the frequency of flood events for various recurrence intervals, i.e., 2-year through 500-year, can be adequately defined, equal probability along contour arcs (random flow paths), (also provides for multiple channels at normal depth assuming total width is 3.8 times the single channel width) Limitations: fluvial (as opposed to debris flow) fonned fan, unstable flow paths Recommended Applications: highly active conical fans The FAN computcr program provides one method of analyzing the flood-hazards on alluvial fans. The methodology used by the FAN program defines the risk of inundation at any particular location by applying the definition of the base flood through the theorem of total probability. The methodology itself is broader than the use within the FAN program. Let H be a random variable denoting the occurrence of flooding at a particular location. That is: I if the location is inundated H= o if the location is not inundated