U.S. DEPARTMENT OF THE INTERIOR
U.S. Geological  Survey 





Map showing principal debris-flow source areas
in the San Francisco Bay region, California


By STEPHEN D. ELLEN, ROBERT K. MARK, GERALD F. WIECZOREK,
CARL M. WENTWORTH, DAVID W. RAMSEY, AND THOMAS E. MAY
With digital cartographic assistance from SCOTT E. GRAHAM, GREGG S. BEUKELMAN, 
ANDREW D. BARRON, AND RUSSELL W. GRAYMER1


Fast-moving landslides called debris flows or mudslides can be expected to originate 
largely in the areas shown on this map.  In any one rainstorm 
only a small fraction of these source areas may be activated.  
Hazard from moving mud and debris can extend 
well downslope from source areas.

OPEN-FILE REPORT 97-745 E

1997


This report is preliminary and has not been reviewed for conformity with U.S. 
Geological Survey editorial standards or with the North American Stratigraphic Code.  
Any use of trade, product, or firm names is for descriptive purposes only and does not 
imply endorsement by the U.S. Government.

This database, identified as "Map Showing Principal Debris-Flow Source Areas in the 
San Francisco Bay Region, California,"has been approved for release and publication by 
the Director of the USGS.  Although this database has been subjected to rigorous review 
and is substantially complete, the USGS reserves the right to revise the data pursuant to 
further analysis and review.  Furthermore, it is released on condition that neither the 
USGS nor the United States Government may be held liable for any damages resulting 
from its authorized or unauthorized use.

1Menlo park, ca  94025













MAP SHOWING PRINCIPAL DEBRIS-FLOW SOURCE AREAS 
IN THE SAN FRANCISCO BAY REGION, CALIFORNIA
by
Stephen D. Ellen, Robert K. Mark, Gerald F. Wieczorek,
Carl M. Wentworth, David W. Ramsey, and Thomas E. May
with digital cartographic assistance from Scott E. Graham, Gregg S. Beukelman, 
Andrew D. Barron, and Russell W. Graymer


INTRODUCTION

	This map identifies the principal areas in the San Francisco Bay region that are 
likely to produce debris flows, which are also called "mudslides."  Debris flows that 
occur in the bay region are fast-moving downslope flows of mud that may include rocks, 
vegetation, and other debris.  These flows begin during intense rainfall as shallow 
landslides on steep slopes.  The rapid movement and sudden arrival of debris flows pose a 
hazard to life and property during and immediately following the triggering rainfall.  
	Debris flows in a given storm originate from a number of sources scattered 
throughout steep parts of the landscape, as shown in figure 1 (on map sheet; files sfbr-
df.ps, al-df.ps, etc.).  During subsequent storms, new debris flows originate from 
different sources.  These various sources, however, are similar in topographic form 
because debris-flow initiation requires steep slopes and prefers concave parts of 
hillsides.  These topographic characteristics are used to predict the likely future source 
areas shown on this map.  

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 (on map 
sheet).  

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.  
	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.  

THE DIGITAL DATABASE

	This Open-File Report is a digital map database, and the remainder of this 
pamphlet serves to introduce and describe the digital data.  See Open-File Report 97-
745A for information on how to download the digital data and on the organization of the 
numerous data files involved.  Files are provided for a map of the entire San Francisco 
Bay region and for maps of each of the 10 counties in the region.  There are no paper 
maps included in the report.  The report does, however, include PostScript plot files 
(sfbr-df.ps, al-df.ps, etc.) that contain images of the map sheets with their 
explanations.  

Digital Compilation

	The predicted debris-flow source areas were generated from maps of slope and 
plan curvature, which in turn were generated from a 30-m elevation grid of the region.  
This elevation grid, described by Graham and Pike (1997), is also the source of the 
shaded-relief topographic base used for the map plotfiles.  The culture layer of the base 
map in the plotfiles is from Aitken (1997).
	The debris-flow sources mapped after the storm of January 1982 were digitized 
by hand in the 1980's from scale-stable greenline base maps at 1:24,000 and 
1:62,500 scales, onto which features had been transferred from aerial photographs.  
The locations of many of these mapped sources may be inaccurate because of the sparse 
(200 ft) topographic contours present in the 1:62,500-scale base maps, which 
previously had been enlarged from 1:125,000-scale materials.  





Spatial Resolution

	Uses of this digital map should not violate the spatial resolution of the data.  
Although the digital form of the data removes the constraint imposed by the scale of a 
paper map, the limitations on detail and accuracy inherent in map scale are also present 
in the digital data.  Because part of this database has an effective resolution of 
1:125,000, plotting is appropriate at scales no larger than 1:125,000.  


Database Specifics

	The map databases consist of ARC/INFO (Environmental Systems Research 
Institute, Redlands, California) coverages and GRIDs that are stored in UTM projection 
(table 2).  In coverages, digital tics define a 7.5 minute grid of latitude and longitude 
that correspond with the corners of the 7.5' quadrangles that fall within the region.   
	The content of the map database can be described in terms of the lines, areas, and 
points that compose the map, which are summarized in table 3.  Descriptions of the 
database fields use the terms explained in table 4.

Lines  
	Lines (arcs) are recorded as strings of vectors and are described in the arc 
attribute table (see table 5).  Except for lines in the base materials, which are 
described by Graham and Pike (1997), the lines on this map simply define the 
approximate extent of mapping of the debris-flow sources triggered during the storm of 
January 1982.

Areas
	The principal areas of potential debris-flow sources are shown on the map by a 
GRID coverage, consisting of 30-m cells derived from values of hillslope steepness and 
curvature, as described in table 1.  Cells representing potential sources have a value of 
1; all other cells have no data.  The grid is described in the grid attribute table (see table 
6).  

Points
	Point information, specifically the locations of debris-flow sources triggered by 
the storm of January 1982, is recorded as coordinates and related information and is 
described in the point attribute table (see table 7).  

ACKNOWLEDGMENTS AND RESPONSIBILITY

	The predicted debris-flow source areas were determined by Ellen and Mark by 
analysis of detailed mapping (Wentworth, 1986; Ellen, Cannon, and Reneau, 1988) that 
was meticulously digitized by Ramsey and May.  The mapped debris-flow sources 
triggered by the 1982 storm were digitized by Wieczorek and Mark from mapping 
documented in Ellen and Wieczorek (1988); the boundary of this mapping was digitized 
by Beukelman.  The shaded-relief topographic base was created by Graham using 
relations provided by R.A. Haugerud.  

REFERENCES CITED

Aitken, D.S., 1997, A digital version of the 1970 U.S. Geological Survey topographic 
map of the San Francisco Bay region, three sheets, 1:125,000 scale:  U.S. Geological 
Survey Open-File Report 97-500.  
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.  
Graham, S.E., and Pike, R.J., 1997, Shaded relief map of the San Francisco Bay region, 
scale 1:275,000:  U.S. Geological Survey Open-File Report 97-745 B.
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.

FIGURES AND TABLES

Figure 1 (on map sheet):  Perspective view showing typical debris flows (yellow) 
resulting from a major storm, in relation to potential source areas (red zones) like 
those shown on the map.  In any one storm only a small fraction of the potential source 
areas may be activated.  Most debris flows in the San Francisco Bay region travel only 
several tens of yards or less, but some may travel hundreds of yards downslope from 
their sources and others have traveled as much as a mile or more down stream channels.  
Numbers refer to typical hazardous locations discussed in text.  


Figure 2:  Plots of hillslope steepness (A) and plan curvature (B) at mapped debris-
flow sources in areas of non-granitic bedrock near Montara Mountain.  Values are 
measured from 30-m elevation grid; maximum value of each curve is adjusted to 100.  
Sources, values at mapped debris-flow sources; area, values in area as a whole; ratio, 
the ratio of "sources" to "area."  Erratic behavior of ratio at high values of steepness and 
low values of curvature results from sparsity of data points.  


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 is dominant in determining potential source areas of the map; plan curvature 
excludes only 6.5 percent of hillslopes 20 degrees and steeper]

	Hillslope steepness				Plan curvature
	
	20 degrees and greater			+0.01 and less
						(including all negative values)



Table 2 - Map projection
The map is stored in Universal Transverse Mercator projection

PROJECTION UTM	
UNITS METERS		-on the ground
ZONE 10			-UTM Zone 10  
DATUM NAD27
PARAMETERS			-none



Table 3 - Summary of database components

		LAYER 			TYPE			FILE NAMES

Potential debris-flow source areas		Grid			sfbr-df, etc.

Mapped 1982 debris-flow sources		Points			sfbr-dfs, etc.

Boundary of mapping of 1982 sources	Arcs			sfbr-dfml, etc.


Table 4 - Field definition terms

ITEM NAME		name of the database field (item)
WIDTH		maximum number of digits or characters stored
OUTPUT		output width
TYPE			B-binary integer, F-binary floating point number, 
 			I-ASCII integer, C-ASCII character string
N. DEC. 		number of decimal places maintained for 
			floating point numbers 



Table 5 - Content of the arc attribute tables (sfbr-dfml.aat, etc.)

ITEM NAME		WIDTH	OUTPUT	TYPE	N. DEC	

FNODE#		4		5		B		starting node of arc
									(from node)   
TNODE#		4		5		B		ending node of arc 
									(to node)
LPOLY#		4		5		B		polygon to the left of
									the arc
RPOLY#		4		5		B		polygon to the right 
									of the arc
LENGTH		4		12		F	3	length of arc in 
									meters 
<coverage>#		4		5		B		unique internal 
									control number
<coverage>-ID	4		5		B		unique identification 
									number













Table 6 - Content of the grid attribute tables (sfbr-df.vat, al-df.vat, cc-df.vat, etc.)

ITEM NAME		WIDTH	OUTPUT	TYPE		N. DEC

VALUE			4		10		B			Value of cell 
										in the grid
COUNT		4		10		B			Number of 										cells of that 
										value



Table 7 - Content of the point attribute tables (sfbr-dfs.pat, al-dfs.pat, etc.)

ITEM NAME	WIDTH	OUTPUT	TYPE	N. DEC	
	
AREA			4	12		F	3	not used 
PERIMETER		4	12		F	3	not used
<coverage>#		4	5		B		unique internal control
								number
<coverage>-ID	4	5		B		unique identification number
PTTYPE		35	35		C		point type