MODELING ALLUVIAL FAN FLOOD HAZARDS: A DERIVATIVE OF SURFICIAL GEOLOGIC MAPS

The factors that make alluvial fans desirable – relatively planar slopes, good surface drainage characteristics, and often excellent views – are formed by floods and debris flows that can negatively affect lives and property. The California Geological Survey (CGS) is integrating geologic maps that use the Classification of Surficial Materials developed by the USGS (Matti and Cossette, in preparation) with geologic assessments for a first-order assessment of the areal extent and relative magnitude of alluvial fan flooding hazards. These maps may be used to assist in avoiding hazardous areas and to design for proper flood and debris flow management facilities; they are not intended to satisfy FEMA requirements.

The general distribution and relative hazard of alluvial fan flooding is defined as: high ≈ Late Holocene fan surfaces and historic channels and washes, or whole fan areas subject to historic and future migration of flow paths; medium ≈ Late Pleistocene to Middle Holocene alluvial fan terraces, moderately incised and raised above younger channels and washes; and low ≈ Early to Late Pleistocene relict fans elevated significantly above historically flooded surfaces. Maps incorporating these relative hazard designations, supplemented with the delineation of debris flow hazard areas and potential channel avulsion sites, may assist landowners, land-use planners, developers, and regulators in identifying the most hazardous areas prone to alluvial fan flooding in the pre-project design phase.