USGS Open-File Report of01-146

Western Earth Surface Processes Team

A Debris Avalanche at Forest Falls, San Bernardino County, California, July 11,1999

By Douglas M. Morton¹ and Rachel M. Hauser¹

U.S. Geological Survey Open-File Report 01-146
Version 1.0

2001

U.S. Department of the Interior
Gail A. Norton, Secretary

U.S. Geological Survey
Charles G. Groat, Director

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

¹U.S. Geological Survey, Department of Earth Sciences, University of California, Riverside, CA 92521

Summary

This publication consists of the online version of a CD-ROM publication, U.S. Geological Survey Open-File Report 01-146. The data for this publication total 557 MB on the CD-ROM. For speed of transfer, the main PDF document has been compressed (with a subsequent loss of image quality) from 145 to 18.1 MB. This online version also does not include the Windows autorun and display items, the Acrobat Search index files, or Adobe Acrobat Reader installers. The installer is available using the link provided below. The CD-ROM may be purchased using the information near the bottom of this page.

Absract

The community of Forest Falls, California, is frequently subject to relatively slow moving debris flows. Some 11 debris flow events that were destructive to property have been recorded between 1955 and 1998. On July 11 and 13, 1999, debris flows again occurred, produced by high-intensity, short-duration monsoon rains. Unlike previous debris flow events, the July 11 rainfall generated a high-velocity debris avalanche in Snow Creek, one of the several creeks crossing the composite, debris flow dominated, alluvial fan on which Forest Falls is located. This debris avalanche overshot the bank of the active debris flow channel of Snow Creek, destroying property in the near vicinity and taking a life. The minimum velocity of this avalanche is calculated to have been in the range of 40 to 55 miles per hour. Impact from high-velocity boulders removed trees where the avalanche overshot the channel bank. Further down the fan, the rapidly moving debris fragmented the outer parts of the upslope side of large pine trees and embedded rock fragments into the tree trunks. Unlike the characteristic deposits formed by debris flows, the avalanche spread out down-slope and left no deposit suggestive of a debris avalanche. This summer monsoon-generated debris avalanche is apparently the first recorded for Forest Falls. The best indications of past debris avalanches may be the degree of permanent scars produced by extensive abrasion and splintering of the outer parts of pine trees that were in the path of an avalanche.

photo of house wrecked by debris avalanche

Damage to structures produced down fan on the east side of the 3rd(?) wave of the debris avalanche (from figure 52 of this report).

This CD-ROM contains a Portable Document Format (PDF) file that contains 67 pages with 57 illustrations. For those who would like to use the illustrations in this report for other purposes, we have provided all 57 of them in the "illustrations" folder. The 55 photographs are in .tif format and are generally six inches in the longest dimension and 300 dots per inch (dpi). The two maps (figures 1 and 3) are in .ai8 (Adobe Illustrator(2) 8.0) format. The figure captions are also provided separately as an ASCII file (figure-captions.txt).

This report was produced under the Southern California Areal Mapping Project (SCAMP) which is part of the Western Earth Serface Processes Team at the U.S. Geological Survey

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Availability of this publication

This publication, U.S. Geological Survey Open File Report 01-146, is for sale on CD—ROM from:

U.S. Geological Survey
Information Services, National Mapping Division
Box 25046
Denver Federal Center
Denver, CO 80225-0046
Telephone: (888) ASK-USGS

ISBN: 0-607-97134-7

Current pricing information is available from http://mapping.usgs.gov/esic/prices/.

To contact the senior author, email Douglas M. Morton (scamp@usgs.gov).

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URL of this page: http://geopubs.wr.usgs.gov/open-file/01-146
Maintained by: Carolyn Donlin
Created: 05/14/2001
Last modified: 05/23/20011 (mfd)