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
storm data.