Potential San Francisco Bay Landslides During El Niño

The U.S. Geological Survey produced special landslide hazard maps of the San Francisco Bay Area for the California State Office of Emergency Services (OES) and the National Weather Service (NWS), in light of the 1997-98 El Niño Season.

• Now, these maps are available on-line.

The USGS has experience with catastrophic events associated with the winter storms of 1982 and 1983.

New USGS maps are geared to reduce the loss of life and property from storm-induced landslides and debris flows (a.k.a. mudslides).

Landslide maps identify areas where the greatest threat to property exists from the movement of deep-seated landslides (see computer simulation from 1996-97 for example). The areas potentially subject to slope failure generally coincide with the location of previous landslides and specific geologic materials.

Now, fly-by movies are available:

• One fly-by depicts an animation of a slow-moving bedrock landslide, from a cross-sectional view. An instructional introduction is included in the animation.
• The other two animations show, as red patches, areas of mapped landslides in the East Bay Hills and in Marin County. These areas are part of the 85,000 landslides that have occurred sometime in the past throughout the Bay region.

Debris flow/mudslide maps show areas where life-threatening debris flows would likely occur in the event of an intense winter rainstorm such as that of January, 1982 (see map and illustration for summary). During this rainstorm, 14 people were buried beneath rushing debris and lost their lives. A computer simulation of a 1996-97 winter season debris flow is included as an example.

During this year's rainy season, the USGS, OES, and NWS maintain daily contact. Using short-term predicted rainfall as well as real-time rain gauge data, OES hopes to identify potential "hot-spots"; i.e., potential debris flow hazard areas, identified by the USGS, that happen to be experiencing heavy rainfall.