PREFERRED DIRECTIONS OF FLOW ON ALLUVIAL FANS

In this paper, modifications of the method used to delineate flood-plains on active alluvial fans for the national flood insurance program are suggested, given new data regarding the deviations of channels from the medial radial line at the apex of the fan. In the original methodology, it was assumed that every point on an alluvial fan contour had an equal probability of being impacted during a flood event; that is, a uniform probability distribution was assumed. Data now available suggest that a normal distribution better describes the probability of points on an alluvial fan contour being hit during a flood event. Results from an example of the application of the modified methodology are compared with the results obtained using the original methodology.