Urban landslides represent the primary geologic hazard in urbanized California hillside and coastal sectors in terms of public and private losses. They have resulted in billions of dollars in damage losses as well as corrective and legal costs. Nevertheless, they have been shown to be one of the geologic hazards most amenable to avoidance and reduction by land-use planning.
In California, the evaluation of landslides for land-use planning purposes is being conducted at three principal levels; (1) on a regional basis, (2) on a tract or community basis, and (3) on a site basis. A high demand exists for level 1 studies by governmental surveys to supply sufficient basic data for proper regional planning and in preparation for detailed studies in levels 2 and 3. At each level, landslides and other geologic hazards are evaluated in proper scale and perspective so that they can be dealt with judiciously and economically in land-use planning. Estimated reduction of damaging failures attainable by investigations at the three levels combined is 95 to 99 percent.
The integration of geotechnical and nontechnical data can be achieved by producing a planning map that weighs all geotechnical data and attempts to answer the question, “For what use is the land best suited?”
A point system for landslide risk in a planning area can generally be established, based on the following factors: (1) adequacy of landslide database, (2) landslide stability ratings, (3) records of landslides in area, (4) geologic-engineering codes and standards, (5) implementation and enforcement of the codes and standards, and (6) performance records.
Land-use planning in high landslide-risk areas is generally accomplished by selection of a land use such as open space for the landslide-susceptible part and development of the nonlandslide part, at least until feasible corrective measures can be assured. Case histories in the Seal Cove-Moss Beach area of northern California and the Palos Verdes area of southern California serve to show how different the planning of these landslide areas might have been had the fund of geotechnical information, land-use planning techniques, and climate of public understanding approached the level evolving today.