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I I I I I I I I I I I I I I I I I I' il 3 07/16/99 assessments of flood risk or in the reliable mitigation of the hazard. Counter to active alluvial fan flooding hazards, an inactive alluvial fan flooding hazard is characterized by relatively stable flow paths. However, areas ofinactive alluvial fan flooding, as with active alluvial fan flooding, may be subject to sediment deposition and erosion, but to a degree that does not cause flow path instability and uncertainty. An alluvial fan may exhibit both active alluvial fan flooding and inactive alluvial fan flooding hazards. The hazards may vary spatially or vary at the same location contingent on the level of flow discharge. Spatially, for example, upstream inactive portions of the alluvial fan may distribute flood flow to active areas at the distal part of the alluvial fan. Hazards may vary at the same location, for example, with a flow path that may be stable for lower flows, but become unstable at higher flows. An example of an alluvial fan that cxhibits both active and inactive alluvial fan flooding is depicted in Figure I. In this example, the area between the topographic apex and the hydrographic apex (apex definitions will be discussed below) would be considered inactive alluvialfanflooding since this reach is characterized by a stable, entrenched channel which can convey the lOO-year flood discharge without overbank flooding. The area below the hydrographic apex would be considered active alluvial fan flooding since this area characterized by flow path uncertainty and abrupt deposition and ensuing erosion of sediment as the channel loses its competence to carry material eroded from a steeper, entrenched upstream sourcc arca.