THE HUD LANDSLIDE MAPPING PROGRAM (1970-1975)

Raymond C. Wilson

Study of the landslide hazard in the Western Region, by people in the Branch of Engineering Geology, began about the time the Menlo Park facility opened and continued at a modest pace through the 1960s. Between 1970 and 1975 the Federal Department of Housing and Urban Development (HUD) sponsored the San Francisco Bay Region Environment and Resources Planning Study, a joint project with the Western Regional Office of the USGS. Its purpose was to develop geologic-hazard maps for urban planning in the rapidly growing Bay area. This unprecedented undertaking resulted in scores of maps on a wide range of earth-science phenomena, including geology, landslides, groundwater resources, seismic hazards, and water quality.

In particular, the program funded the preparation of reconnaissance-level bedrock geology and landslide hazard maps, initially at 1:62,500 scale. These were followed by special reports on landslide mapping and the correlation between rainfall and historic landslide activity. Perhaps the outstanding landslide products of the HUD program were fifty-six 7.5-minute USGS quadrangle maps, "Preliminary photointerpretation maps of landslide and other surficial deposits in the southeastern San Francisco Bay region", prepared by Tor Nilsen and released in 1975.

Nilsen's work was appreciated during the severe winters of 1981-82 and 1982-83, when many older, deep-seated bedrock landslides shown on his maps were reactivated by heavy rainfall. In all, some 80,000 landslides were recorded on HUD-funded USGS maps. By combining the maps of geology, landslides, and ground slope, in 1972 Earl Brabb and colleagues Earl Pampeyan and Manuel Bonilla were able to prepare perhaps the first large-area map showing the susceptibility, or likelihood, of future landslide activity, for locations in San Mateo County. Suitably updated, their method has since been fully automated for GIS application to other Bay area counties.

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