MODELING RELATIVE ALLUVIAL FAN FLOOD HAZARDS FROM SURFICIAL GEOLOGIC MAPS

The threat to lives and property from post-fire floods and debris flows has been repeatedly demonstrated over the past 10 years following California wildfires. The California Geological Survey began assessing post-fire debris flow hazards in the early 1970s and continues to do so as requested by local and state emergency assistance organizations. We have observed that most of the high-hazard areas are at the urban/wildland interface, which are often on alluvial fans that were formed by repeated floods and debris flows, where flow paths are distributary, uncertain, and flow magnitudes are highly variable.

Alluvial fans are facing increasing development pressures because of their relatively planar slopes, good surface drainage characteristics, and often excellent views. Unfortunately, this leads to increased risks to lives and property on them.

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. 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, are designed to assist landowners, land-use planners, developers, regulators, and the public in assessing alluvial fan flooding hazards in the pre-project design phase.