Oregon Department of Forestry Storm Impacts and Landslides of 1996: Final Report

Acknowledgments The authors would like to acknowledge all the parties that supported the design, coordination, implementation, analysis and budget of this project. Foremost, we would like to thank the field crew members: They met the daily challenges of collecting the unprecedented data set presented in this report, working on dangerous terrain and traversing long distances to find landslides in remote locations. were instrumental in various phases of data collection, mapping, management, and analyses. Mary Gorton designed the cover. supplementary budget support to the Forest Practices storm study budget. Finally, this study would never have been completed without the commitment from Jim Brown, Charlie Stone and Ted Lorensen that re-prioritized agency resources to make this project possible. Executive Summary The Storms of 1996 During the months of February and November 1996, two very large storms affected most of Western Oregon and parts of Northeast Oregon. The February storm was a high intensity, long duration rainfall event that affected the northern portion of the state. The November storm was a shorter duration and higher intensity rainfall event than the February storm, and affected an area south of the February storm. Both the storms resulted in large numbers of landslides, debris torrents, and altered stream channels. Study Design With oversight from a team of experts in the landslide and natural resource field, the Oregon Department of Forestry implemented a 3-year monitoring project to evaluate the effects of these storms. This project was primarily a ground-based study. However, remote sensing techniques (aerial photographs in particular) have commonly been used both to predict where landslides will occur and to inventory where landslides and channel impacts have occurred. Therefore one goal of the project was to determine the accuracy and precision of remote sensing data in identifying landslides, channel impacts, and landslide-prone areas. A second goal of the project was to determine landslide frequency and channel impacts, particularly as they relate to forest practices. Specific forest practices that were considered include harvest practices that may have caused ground disturbance (i.e. yarding, site preparation), treatment of slash, road construction, and road drainage. Past research and monitoring has related landslide frequency to stand structure (particularly dominant tree species and age). Therefore a related goal of this project was to examine relationships between storm impacts and forest stand structure adjacent to landslide initiation points and along stream channels. Eight study areas were monitored over a two-year period. Six areas …

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