MAP UNITS
     The red zones of the map portray the principal areas from which debris flows can be expected during future storms. The black dots represent the debris-flow sources mapped after the catastrophic storm of January 1982. The dots provide an example of the abundance of debris flows that might be expected during a major rainstorm, and they illustrate the approximate nature of this predictive map.

HAZARD
     Because debris flows travel downslope and downstream from source areas, hazards commonly extend beyond the red zones of the map. In the vicinity of the red zones, common areas of hazard are 1) near the base of steep hillsides, 2) near the mouths of steep sidehill drainages, and 3) in and near the mouths of canyons that drain steep terrain. Examples of these hazardous settings are shown in figure 1.

USE OF THE MAP
     The map shows the principal source areas of debris flows from natural hillslopes in the region. Debris flows and other fast-moving landslides may also occur from man-made slopes, such as the cut slopes along highways in hilly terrain, but such sources generally are not shown on the map. Debris flows also are possible from unmodified steep areas outside of the red zones, but we expect that most debris flows will originate in or near the red zones of the map.
     Because the map depicts source areas and not flow paths, and because of its broad areal coverage and small scale, the map is intended to be used as a guide to general areas of debris-flow hazard rather than as a predictor of hazard at specific sites. Appropriate uses include storm-preparedness planning for emergency access and response.

PREPARATION OF THE MAP
     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. Mapped sources of historical debris flows in calibration areas were used to determine the parameters in table 1; the calibration areas we used are near Montara Mountain in San Mateo County (Wentworth, 1986) and in part of Marin County (plate 6 of Ellen and others, 1988). In these meticulously mapped calibration areas, the red zones 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, because small-scale base maps were used to plot some of the locations during region-wide mapping.

Table 1: Hillslope steepness and curvature used to map potential debris-flow source areas [Values as measured on 30-m elevation grids. Plan curvature is curvature of the ground surface in a horizontal plane; it is measured as 1/r, where r is radius of curvature in meters and negative values indicate concave areas such as swales]

Hillslope steepness   Plan curvature

20 degrees and greater +0.01 and less

REFERENCES CITED
Ellen, S.D., Cannon, S.H., and Reneau, S.L., 1988, Distribution of debris flows in Marin County, plate 6, in Ellen, S.D., and Wieczorek, G.F., eds., Landslides, floods, and marine effects of the storm of January 3-5, 1982, in the San Francisco Bay region, California: U.S. Geological Survey Professional Paper 1434, 310 p., plate 6 at 1:24,000 scale.

Wentworth, C.M., 1986, Maps of debris flow features evident after the storms of December 1955 and January 1982, Montara Mountain area, California: U.S. Geological Survey Open-File Map 86-363, 2 sheets at 1:24,000 scale.