San Francisco Bay Region landslide folio part E - Map of debris-flow source areas in the San Francisco Bay region, California

The map was produced using a Geographic Information System (GIS) from a 30- meter-square grid of topographic elevations in the region. The grid of elevations, called a digital elevation model (DEM), permits calculation of hillslope steepness and curvature for each 30-m cell. The potential debris-flow sources shown on the map were determined using the values of steepness and curvature defined in table 1. The parameters in table 1 were determined from values of steepness and curvature at mapped sources of historical debris flows in two calibration areas, one near Montara Mountain in San Mateo County (Wentworth, 1986) and one in Marin County (plate 6 of Ellen, Cannon, and Reneau, 1988). Typical plots describing these measures are shown in figure 2. The values in table 1 were selected by eye from four sets of such plots (two different types of geologic materials in each of the two calibration areas), with the aim of including most of the range of values associated with the mapped sources. Specifically, we used the values where the "sources" curves (fig. 2) attained 50 percent of their maximum value and where the "ratio" curves attained about 50 percent of the value at which they reach a crude plateau. In the meticulously mapped calibration areas, the red zones defined by table 1 include 82 percent of the mapped debris-flow sources. In the bay region as a whole, only 53 percent of the black dots of the map lie within the red zones. This low percentage probably results at least in part from inaccurate location of the black dots due to small-scale base maps used for some of the region-wide mapping.