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..._,-._--_.-"',...~........._.;........ 22 ALLUVIAL FAN FLOODING general case, where a probability density function may be defined at each point of interest rather than for a cross section, would unnecessarily complicate the drawing of Figure 1-5.) The alluvial fan flooding approach (Figure l-5b) is a more realistic way of depicting the floodplain boundaries of even a river system to a large flood because the riverine flooding approach (Figure 1-5a) relies on maximizing conveyance in a preflood cross section to determine which surfaces are flooded and ignores the possibility that a surface can be flooded due to the redirection of flows at an upstream point. Thus, a key difference between alluvial fan flooding and riverine flooding is the implicit assumption of spatial consistency of the depth discharge relationship between adjacent surfaces. Based on physical processes, the alluvial fan flooding approach is the superior way for viewing how floods occur in general, and the riverine flooding approach is a special case where nothing out of the ordinary is going on. The question raised by Figure 1-5 is whether the presence of uncertainty in flood processes by itself constitutes alluvial fan flooding because failure to deal with channel changes by modeling scour, fill, and lateral movement (as the riverine flooding approach fails to do) results in grossly inaccurate delineation of flood hazard boundaries. A previous National Research Council committee (NRC, 1983) concerned with the effects of process uncertainty in alluvial rivers was faced with the question of whether flood studies should use riverbed mobility models rather than fixed-bed models. Its conclusion was that the uncertainty introduced by ignoring the effects of sediment degradation/aggradation was no greater than the additional uncertainty introduced by. the use of mathematical techniques. That committee's conclusion was that evaluating the effect of parameter uncertainty (i.e., the variation in channel roughness, geometry, and slope) was a suitable way to deal directly with process uncertainty in alluvial rivers. In other words, when executing the riverine flood paradigm it is often better to identity the potential impacts of error upon predicting the behavior of real floods than to strive for a more realistic approach in the hope that this error will go away. Simple approaches often yield satisfactory results. For example, the Flood Insurance Study for the City of Palmdale (FEMA, 1987, p. 8) recognizes this and contains the following counsel: Average depths of flooding were assigned based on standard hydraulic calculations through irregular cross sections. In many cases, the assigned average depth is not representative of the true degree of flood hazard. This situation occurs where the average depths are based on a wide cross section which encompasses one or more low flow drainage courses. The actual depth of flooding and, consequently, the true flood hazard will be greater adjacent to the drainage course, The intensity of flood and sedimentation processes on many actively accumulating parts of alluvial fans is much greater and the frequency, magnitude, and suddenness of channel changes are more severe, however, than envisioned by the 1983 committee. In these more complex cases, the problem of channel location and flow distribution can be guided by the kind of process-based understanding, field evidence, and analysis techniques outlined in Chapters 2 and 3. .'