San Francisco Bay Region Landslide Folio Part E - Map of debris-flow source areas in the San Francisco Bay region, California
These maps show, for emergency service managers in the San Francisco
Bay region, the threshold rainfall that may be capable of
triggering a level of debris-flow activity likely to threaten
public safety. The maps are products of a continuing series of
studies that began after a catastrophic storm on January 3-5, 1982
triggered 18,000 debris flows in the San Francisco Bay region,
causing 25 deaths and $66 million in property damage. The
threshold rainfall values were estimated by re-evaluating a
previous empirical analysis of data from the 1982 storm, and other
historical rainfall records, that normalized the rainfall intensity
data by dividing by the mean annual precipitation (MAP) of the
corresponding rain gage. The present analysis also takes into
account the rainfall frequency, the mean annual number of days with
non-zero rainfall (#RDs), thereby adjusting for the difference in
rainfall frequency between windward-facing slopes where rainfall is
orographically enhanced and leeward-facing slopes and valleys that
lie within rain shadows where precipitation is reduced. The
debris-flow threshold maps were created by digitally combining an
existing regional map of mean annual precipitation, a newly compiled
data set of #RDs from an analysis of long-term (20-40 years)
records of daily rainfall for 33 rain gages in the region, and the
re-normalized thresholds from the empirical analysis of historical
Wilson, R.C., and Jayko, A.S., 1997, San Francisco Bay Region Landslide Folio Part E - Map of debris-flow source areas in the San Francisco Bay region, California: U.S. Geological Survey Open-File Report 97-0745-F.
Planar coordinates are encoded using coordinate pair
Abscissae (x-coordinates) are specified to the nearest unknown
Ordinates (y-coordinates) are specified to the nearest unknown
Planar coordinates are specified in meters
The horizontal datum used is North American Datum of 1927.
The ellipsoid used is Clarke 1866.
The semi-major axis of the ellipsoid used is 6378206.
The flattening of the ellipsoid used is 1/294.98.
The primary intended use of these maps is to facilitate response
planning for debris-flow related emergencies. The maps are
intended to be used in conjunction with the debris-flow
susceptibility map developed by Ellen and Mark (1997) in this same
This report is preliminary and has not been reviewed for conformity
with U.S. Geological Survey editorial standards or with the North
American Stratigraphic Code. Any use of trade, product, or firm names
is for descriptive purposes only and does not imply endorsement by
the U.S. Government.
This database, identified as "Preliminary Maps Showing Rainfall
Thresholds for Debris-Flow Activity, San Francisco Bay Region,
California," has been approved for release and publication by the
Director of the USGS. Although this database has been subjected to
rigorous review and is substantially complete, the USGS reserves the
right to revise the data pursuant to further analysis and review.
Furthermore, it is released on condition that neither the USGS nor
the United States Government may be held liable for any damages
resulting from its authorized or unauthorized use.
ARC/INFO export file Description of Coverage
sfbr-rt24c.e00 Line coverage showing contours of 24 hour rainfall threshold for debris flows.
sfbr-rt6c.e00 Line coverage showing contours of 6 hour rainfall threshold for debris flows.
sfbr-rtg.e00 Point coverage showing rain gage locations, station identifier, average number of rainy days, and mean annual precipitation.
sfbr-rta.e00 Point coverage showing the location of ALERT STATIONS and station identifiers