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Storm Impacts & Landslides
Landslides can contribute a significant amount of sediment to streams and can adversely affect fish habitat. Therefore, any widespread practice such as timber harvesting or road building that might alter the natural rate and natural magnitude of landslide occurrence is important from a forest management perspective. In Oregon’s forests, debris slides are the landslide of concern. Debris slides are the most common type of landslide in Oregon’s forests and often transform into debris flows that can travel down stream channels and have significant impacts on stream channels. Even in the Cascades, where large earthflows are also common and can contribute just as much sediment to streams, the shallow, rapid nature of debris slides provides a greater challenge to forest management. Debris slides are more likely to be affected by forest management, particularly timber harvest and road building, because they are shallow failures. In addition, debris slides pose a greater risk to public safety because they tend to be rapid failures.
The Oregon Department of Forestry conducted a study to investigate how forest practices might affect the rate and magnitude of landslides. The study (Storm Impacts and Landslides of 1996: Final Report, June 1999) examined the results of two individual storm events and therefore cannot be extrapolated to predict long-term conditions. The study focused on landslides that delivered sediment to stream channels, primarily debris slides and debris flows, because these landslides have the most significant effects on natural resources and public safety and the study does not capture the total population of landslides that occurred as a result of these storms. Variation in storm characteristics (precipitation intensity and duration) and variation across the landscape to susceptibility of landsliding resulted in a range of hillslope and channel responses. The study focused on areas determined to have the most severe impacts from the 1996 storms, so results are expected to be well beyond the average forestland response to these individual storms.
The "Storm Impacts of 1996" study is the largest ground-based study of landslides and forestry in the Pacific Northwest. The study findings reflect on current forest management practices on different land ownerships in Oregon. Information was collected on the age and condition of these forests and compared quantitatively. This ground-based landslide inventory included eight study areas that covered 11,862 hectares. These areas were comprehensively surveyed and 506 landslides that entered stream channels and 352 debris-flows were identified and studied. The "Storm Impacts of 1996" study is the only forest landslide inventory to collect detailed information on stands of intermediate age (20 to 100 years) and is also the only study to quantitatively compare aerial photo inventories with ground-based inventories.

The "Storm Impacts of 1996" made the following conclusions:

  • Landslide inventories using only aerial photographs without significant on-the-ground surveying do not identify the majority of shallow-rapid type landslides.
  • Coarse-scale digital elevation models underestimate slope steepness, especially in areas with irregular, steep slopes.
  • Ground-based investigation provides the most reliable information on landslide occurrence and their characteristics in the forests of western Oregon.
  • Timber harvesting can affect landslide occurrence on the steepest slopes. In three out of four study areas, higher densities and erosion volumes were found in stands that had been harvested in the previous nine years, as compared to forests that were older than one hundred years.
  • Forested areas between the ages of 10 and 100-years typically had lower landslide densities and erosion than found in the mature forest stands.
  • Landslides from recently harvested and older forests had similar dimensions, including depth, initial volume and debris flow volume.
  • In the locations adjacent to landslides, landowners and loggers complied with the forest practice harvesting rules (as changed in 1983) to minimize ground disturbance and slash accumulations on landslide prone sites.
  • Based on the low numbers of road-associated landslides surveyed in this study and on the smaller sizes of these landslides (as compared with previous studies), current road management practices are reducing the size of road associated landslides, as well as the number of landslides.
  • Stream channel impacts varied greatly by study area. Impacts were not directly related to the number of landslides. Large, up-slope landslides that enter stream tributaries with small stream junction angles and steep channel gradient slopes resulted in the greatest stream channel impacts.
  • When evaluating debris flow or torrent risks to resources based on potential run-out, one should consider the potential for large initiating landslides as well as channel junction angles, stream channel gradients, and the riparian condition along the debris flow/torrent path.

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