The factors that make alluvial fans desirable for development - 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. Over the past two decades, alluvial fans in southern California have experienced high development pressures and repetitive losses due to the unique flood hazards associated with alluvial fans. In 2007 California's Alluvial Fan Task Force (AFTF) was charged by the Governor to review the state of knowledge regarding alluvial fan floodplains and develop recommendations that would be specific to alluvial fan floodplain management.

As a technical consultant to the AFTF, the California Geological Survey (CGS) developed an approach integrating surficial geologic maps with geologic assessments for first-order identification of the areal extent and relative magnitude of alluvial fan flooding hazards. Maps incorporating these relative hazard designations, supplemented with the delineation of debris flow hazard areas and potential channel avulsion sites, can assist landowners, land-use planners, developers, and regulators in identifying the most hazardous areas prone to alluvial fan flooding, particularly during the planning process.

In order to recognize the necessity for geologic assessment, CGS developed a tool to assist in identifying areas underlain by alluvial fans. The alluvial fan identification tool consists of a Geographic Information System database that includes regional-scale advisory maps that identify areas underlain by Quaternary-age sediments. For alluvial fan areas, CGS developed geologic data that may be used for first-order hazard identification. CGS Special Report 217 provides this data in a GIS-based compilation of high-resolution geologic maps of Quaternary- age and older deposits covering approximately 20,000 square miles of southern California. Presented in a common seamless format, Special Report 217 merges the existing geologic mapping by various authors, normalizes the geologic unit descriptions, and differentiates Quaternary-age materials, including alluvial fan deposits, at a scale of 1:100,000. The geologic data are also presented as PDF maps at a scale of 1:100,000.

Jeremy T. Lancaster
California Geological Survey
888 South Figueroa Street, Los Angeles, CA 90017
Jeremy.Lancaster@conservation.ca.gov

Jeremy Lancaster is an engineering geologist (PG, CEG) with the California Geological Survey (Los Angeles), and serves as a technical specialist on flooding and debris flow processes as related to the geomorphology of alluvial fan systems. Recently he served as a technical consultant on the California Alluvial Fan Task Force, and assisted in developing chapters and appendices for the document titled, "The Integrated Approach For Sustainable Development On Alluvial Fans." Additionally, he has served as a panel specialist on the topic of alluvial fan flooding and debris flows for the Floodplain Management Association, Maricopa County Flood Control District (AZ), and most recently the Episodic Streams Symposium. Mr. Lancaster has published several abstracts on the topic of assessing alluvial fan flood hazards using surficial geologic maps coupled with engineering geologic investigations, and currently has three publications in press/preparation on the topic.