FLOOD02285 - Laserfiche WebLink

Home Browse Search

INTRODUCTION 23 IMPLICATIONS OF ALLUVIAL FAN FLOODING FOR FLOODPLAIN MANAGEMENT The nature of alluvial fan flooding as currently defined by the NFIP has implications for both floodplain management and the mitigation of flood hazards. Consider the situation in Figure I -5, which has been redrawn in Figure I -6 to illustrate the implications of using fill to raise the level of surface I, This surface is already above the computed base flood elevation, but it is clear that if one accounts for some degree of error, surface I could be completely inundated because it is protected only by a small berm in the natural topography. If ordinary riverine flooding is the appropriate paradigm to apply to the site (Figure 1-6a), filling surface I has no effect whatsoever on the computed bankfull flood elevation for surface 2 (the main channel). This would therefore be an acceptable strategy to reassure the concerned parties that the area is protected from flooding. Viewed from the perspective of alluvial fan flooding (Figure 1-6b), however, where surfaces I, 2, and 3 each have a chance of conveying all or part of the flood, protecting surface I in this manner would not be an allowable mitigation strategy. The reason is that eliminating surface I as a potential flow path, increases (theoretically) the frequency with which surfaces 2 and 3 get flooded. This is equivalent to increasing the base flood elevation for the n-year event and could therefore be an infraction ofNFIP regulations (e.g., section 65.12). Similar actions that are generally considered to be good floodplain management practice would also come under question. For example, reinforcing a levee reduces the uncertainty about its failure potential during a flood and better protects areas behind it. But eliminating the flow path through a breach in the previously substandard levee could increase both the computed stage-frequency curve and the chance for other failures further downstream. These conclusions illustrate the disconnect between the alluvial fan flooding and the riverine flooding paradigms in the context of floodplain management. For risk assessment under alluvial fan flooding, existing channels cannot be relied on to convey the 100-year peak flow, so their role is ignored. For riverine floodplain management, however, the channels are significant. They convey the smaller flood events, they indicate how floods have occurred in the past, and they define where future facilities may be located. The default assumption of a uniform risk (FEMA, 1995) or complete uncertainty across an alluvial fan is a formalized guess that allows one to delineate risk on the Flood Insurance Rate Map using a straightforward technique. A FIRM showing alluvial fan flooding hazards mapped in this manner is an expression of uncertainty or the absence of knowledge about floods, however, rather than an indication of how one might actually occur. Unlike a riverine FIRM, an alluvia] fan flooding FIRM is oflimited use for mitigation and management of flood hazards. By making a conservative trade- off in favor of what might happen, this type of FIRM ignores the importance of what has happened. If the uniform-risk FIRM is interpreted literally, then it can be argued using formal mathematics of the kind that underlies the existing FEMA procedure that any mitigation effort, short of complete channelization, increases the flood risk on another part of the fan. Thus floodplain managers are left with the peculiar responsibility of preserving uncertainty.