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<br />16 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br /> <br />. Alluvial fan flooding transports large volumes of sediment, the deposition of which may <br />influence both the location and the direction of flowing water during a single flood event. This <br />phenomenon is part of the reason for the "unpredictability" of the flow path. <br /> <br />The word" unpredictable" is troublesome in this context, and should be read only in a <br />relative manner. There is always uncertainty associated with the prediction of how floods occur. <br />Thus, in a sense, all flooding is "unpredictable." In the case of riverine flooding, which presumes <br />a stable-bed condition, however, we can set aside this uncertainty because there are established <br />procedures to predict how such floods occur. In the case of the alluvial fan to which the FAN <br />computer program (FEMA, 1990) is applicable, we simplity the uncertainty by assuming that flow <br />path behavior is random and a flood has no greater or less chance of following an established flow <br />path than it has of cutting through the neighbor's backyard. The implication of the definition is <br />that for alluvial fan flooding the flow path behavior is so indeterminate that we cannot set aside <br />the uncertainty and still achieve a realistic assessment of the flood risk. <br />The existing regulatory definition does not describe an alluvial fan but rather a type of <br />flooding that may also occur in nonalluvial fan areas (see Figure 1-3). For example, sediment <br />movement may significantly affect flood flow behavior in river delta areas, and flow velocities <br />high enough to cause erosion and deposition are common in alluvial river floodplains. After the <br />1993 Missouri River flood, sand deposits of 2 feet depth or greater covered 60,000 acres of <br />adjacent farmland, causing damages in excess of more than $100 million (Interagency Floodplain <br />Management Review Committee, 1994). On the other hand, there are alluvial fans that have well <br />defined channels and may not be subject to alluvial fan flooding as defined above. The choice of <br />the term is responsible, in large measure, for the confusion surrounding the definition issue, As it <br />stands, the existing definition is vague and potentially very inclusive. <br />During the hazard identification process, the question is asked whether an area is subject <br />to alluvial fan flooding. The answer affects both how flood zones are delineated (FEMA, 1995) <br />and the rules that apply. Although the way Special Flood Hazard Areas (SFHAs) are delineated <br />for alluvial fan flooding may differ from that for ordinary flooding, the regulatory importance of <br />alluvial fan flooding is realized during mitigation. Section 65.13 of the NFIP regulations precludes <br />the removal of a SFHA based solely on the elevation of a structure above the estimated flood <br />stage or the placement of fill that creates such a condition. It also sets forth standards for <br />structural flood control measures to remove the zone designation by mitigating the flood hazard. <br />Through discussion with representatives of FEMA and the review of various documents, <br />the committee has identified several regulatory difficulties that originate from using the riverine <br />flooding paradigm in alluvial fan flooding situations: <br /> <br /> <br />k< <br /> <br />~'; <br /> <br />~, <br />'~ <br /> <br />I. Issues related to Letters of Map Amendment (LOMAs). A LaMA is issued by FEMA <br />when a parcel of land is inadvertently included in the SFHA because of the limitations of map <br />scale or topographic data. By certifYing that the finish floor of a structure is higher than the <br />adjacent base flood elevation (BFE), the parcel can be removed from the SFHA. For areas subject <br />to debris flows, extreme deposition, or shifting of flow channels, such an approach may not be <br />appropriate, however, because the hazard still exists. Because LaMA applicants are allowed to <br />contact FEMA directly, the agency has no way of determining whether a LaMA should be <br />granted or not apart from the alluvial fan flooding designation. <br /> <br />'f/ <br /> <br /> <br />"j <br /> <br />;~: <br />.',' <br /> <br />{'->.' <br />,.~. <br />~i <br />,~; <br />