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<br />A-2 <br /> <br /> <br />consequence of the availability of sediment supply. A geologic assessment of historic flow <br />deposits and fan geomorphology will enhance the hydraulic analysis. These assessments may <br />include the identification of source areas, percentage of silt and clay in undisturbed deposits, <br />depositional features, drainage pattern, estimates of maximum flow depths and velocities on fan <br />surfaces, aggradational or incisional fan geomorphology, and soil condition (unconsolidated or <br />well-compacted soil). <br /> <br />Flow Deposits <br /> <br />Deposited materials on fan surfaces as reported by the counties range from sand and silt <br />to boulders. The location, type, and size of deposits may reveal the magnitude of historic events, <br />and they are good indicators for predicting future events. Some alluvial fans are characterized by <br />mass wasting due to head cutting and bed and bank erosion, large volume of sediment deposits, <br />debris flow, and/or mudflow, Fan slope and runoff are important factors of flow deposits. <br />Consideration should be given to fan slope and modification of the fan surface by development. <br /> <br />Flood Control Measures <br /> <br />A few counties including Madera and Orange have built dams, debris barriers, debris <br />dams, debris and flood detention basins, concrete-lined channels, levees and floodwalls, <br />diversions, and/or bypasses to mitigate flood hazards from alluvial fan flooding. Ordinarily, the <br />performance requirements and physical and environmental constraints on these structural <br />measures is site- and application-specific as required by Federal, State, and local authorities, <br />Mitigation measures for an alluvial fan should be site- and application-specific for mitigating the <br />associate hazards, The range of flood management and mitigation measures available can be <br />analyzed, taking into account the identified hazards, existing structures, anticipated future <br />development, cost versus benefit to locality, and public acceptance. <br /> <br />Flooding Records <br /> <br />The best place to start investigating an alluvial fan is to check available documents or <br />records of flooding. If debris flows or erosions have occurred on the fan recently, the fan is <br />definitely an active fan, Then, aerial photographs from different years can be compared for <br />changes in channel geometry and other depositional features on the fan surfaces. Unfortunately, <br />most alluvial fans are in unincorporated, undeveloped areas of these counties. Thus, a majority <br />of historical flooding records are not available, and no specific studies have been done on most <br />of those alluvial fans. Geomorphic characteristics must be applied to determine the status and <br />relative age of an alluvial fan, whether it is active or inactive. More field observations and <br />measurements are needed for a more reliable prediction of potential flood hazards on these <br />alluvial fans. <br />