LANDSLIDE OCCURRENCE ON MOUNTAINOUS TERRAIN IN NORTH-CENTRAL IDAHO: A PROGRESS REPORT, 1974-75

In January of 1974, the Pacific Northwest was hit by a devastating rain-on-snow storm that caused widespread flooding and considerable landslide activity. Since early reports suggested extensive landslide damage through north and central Idaho, a 5-year study was designed to inventory the extent of damages from the January 1974 and subsequent storm events and to evaluate site factors associated with each landslide over 10 cubic yards in size. The Clearwater National Forest was chosen for the study area because of its varied landscapes and bedrock. Topography in the Clearwater National Forest is characterized by rugged high relief landforms with steep unstable slopes. Unstable slopes in conjunction with large volume water imputs from rain on snow and/or snowpack melt, hazardous bedrock conditions and man's activity has generated 441 measured landslides over the past 2 years. Debris avalanches were most prevalent in 1974 while in 1975 debris slides predominated. Indications are that nearly 50 percent of these slides are associated with road construction practices during the early and mid 1960's. Road repair and slide stabilization are direct costs attributable to landslides. Road repair costs averaged approximately $130,000 annually. If all critical landslides were stabilized it is estimated that the costs would be about $1,100,000 per year. In addition, approximately 25 percent or 154,000 tons of landslide debris was deposited in and along active stream channels annually. This study indicates that additional expenditures of time and money for detailed road location studies and more expensive road design could have reduced total transportation costs by diminishing annual road maintenance and periodic reconstruction costs. In addition, environmental impacts could have been reduced as well. The Clearwater National Forest is now applying geotechnical skills to road and slope stability surveys. These skills are proving beneficial in the design of sound forest transportation systems.

  • Supplemental Notes:
    • This article is an excerpt from the Proceedings of the Fourteenth Annual Engineering Geology and Soils Engineering Symposium that was sponsored by Idaho Transportation Department, Division of Highways; University of Idaho, Department of Geology and Department of Civil Engineering; Idaho State University, Department of Geology and Department of Engineering; and Boise State University, Department of Geology, and Department of Physical Science and Engineering. This symposium was held at the Rodeway Inn, Boise, Idaho, and was hosted by Boise State University.
  • Corporate Authors:

    Idaho Department of Highways

    P.O Box 7129
    Boise, ID  USA  83707
  • Authors:
    • Day, N F
    • Megahan, W F
  • Publication Date: 1976-4-9

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Filing Info

  • Accession Number: 00142676
  • Record Type: Publication
  • Files: TRIS, STATEDOT
  • Created Date: Feb 1 1977 12:00AM