By:
SAN FRANCISCO BAY LANDSLIDE MAPPING TEAM*
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Summary
The U.S. Geological Survey (USGS) has initiated a program to provide digital data and plottable map files outlining areas of potential landslide activity in the ten-county San Francisco Bay region. The effort commenced in September 1997 and targeted a December completion to make the information available in advance of major storms anticipated from the 1997-1998 El Nino event. The San Francisco Bay Landslide Team's experience with catastrophic slope failures in prior winter storms, combined with the availabilty and first-hand knowledge of digital topographic, geologic, and climatologic data for the entire San Francisco Bay region, provides the basis for identifying areas potentially vulnerable to slope failure.
Three types of damaging landslide activity are addressed here: slides, earth flows, and debris flows. The nomenclature of slope failure is complex (Varnes, 1978). For present purposes, we use the term slide to include both slumps and translational slides, and earth flow to represent flows of clayey earth (Wentworth and others, 1997). These kinds of slides move slowly, in contrast to the rapid movement of debris flows (Ellen and others, 1997). Slides and earth flows deform the ground when they move and remain in the landscape as recognizable landslide masses, whereas debris flows run downslope to locations lower in the landscape and form separate, thin deposits that quickly become unrecognizable.
The six-part folio presented here features digital cartographic displays. Included are topography in shaded relief together with the road network and hydrography (Graham and Pike, 1997), mapped distribution of slides and earth flows (Wentworth and others, 1997), likely debris-flow source areas (Ellen and others, 1997), and rainfall thresholds for debris flows (Wilson and Jayko, 1997). Complementing the regional displays is an index to detailed maps that inventory Bay area landslides (Pike, 1997).
These maps and tabular data constitute the fundamental tools needed to assess potential slope-failure hazards in a regional context. Most of the information is mapped by counties at the 1:125,000 scale, which both communicates the degree of accuracy of many of the data and provides data at a level of generalization appropriate for emergency planning - the initial intended use of this Folio. The data are also presented at a scale of 1:275,000 to supply the necessary regional setting.
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