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Open-File Report 01-146

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

By Douglas M. Morton and Rachel M. Hauser

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

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.

First posted May 14, 2001

  • Illustrations HTML
    55 photographs in .tif format, generally six inches in the longest dimension and 300 dots per inch (dpi). Individual image file sizes range from 5-10 MB.

For additional information, contact:
Geology, Minerals, Energy, and Geophysics Science Center
U.S. Geological Survey
345 Middlefield Road, MS 901
Menlo Park, CA 94025-3591
http://geomaps.wr.usgs.gov/gmeg/

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Suggested citation:

Morton, Douglas M. and Hauser, Rachel M., 2001, A Debris Avalanche at Forest Falls, San Bernardino County, California, July 11, 1999: U.S. Geological Survey Open-File Report 01-146, 67 pp., https://pubs.usgs.gov/of/2001/0146/.



Contents

Abstract

Acknowledgments

Introduction

Debris Flows and Avalanche, July 1999

Debris Avalanche of July 11, 1999

Conclusions

References

Illustrations


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