Department of Water Resources Home

Flood Management

Alluvial Fan Maps

In 2004, following a tragic post-fire debris flow in San Bernardino County, the Governor signed Assembly Bill 2141. The bill directed the Department of Water Resources (DWR) to seek federal funding for the establishment of a stakeholder-driven Alluvial Fan Task Force (AFTF) to develop a Model Ordinance and planning tools to mitigate flood hazards associated with alluvial fan flooding. This type of flooding is prevalent after wildfires where areas remain vulnerable five years after an incident.

Alluvial fan floodplains are found throughout California. However, they are most prevalent in San Bernardino, Riverside, Los Angeles, Ventura, Santa Barbara, San Luis Obispo, Kern, Imperial, Orange, and San Diego counties. Growth forecasts predict that communities located where alluvial fans are present will accommodate up to 60 percent of the new development occurring in southern California in the 21st century.

In March 2007, DWR announced a partnership with California State University, San Bernardino's (CSUSB) Water Resources Institute to develop the AFTF under the direction of DWR staff. The AFTF is charged with reviewing the state of knowledge of alluvial fans; examining the flood risks; and developing local planning tools, including a "Model Ordinance" and a set of "Design Guidelines for Development on Alluvial Fans" aimed at reducing losses to human, built and natural resources resulting from the natural hazard of flooding on alluvial fans.

The Department has obtained funding for Alluvial Fan Mapping activities for implementing recommendations made by the Alluvial Fan Task Force.