The Stillaguamish Valley, upstream from the community of Oso, abounds with landslide scarps that indicates a very long history of landslide activity. One of the larger slides, which was two to three times larger than the recent slide, occurred a mile or so west of the recent slide. It spread across the valley floor and pushed the Stillaguamish River against the south valley slope. Recent river erosion in this area weakened the south valley slope, cracks developed on the hillside and a major slide control project was undertaken to protect the highway and power line.
Landslides occur when the force of gravity exceeds the internal friction of the materials underlying the hillside. This can be caused by the liquefaction of saturated sand layers from jarring by an earthquake, the erosion of the toe of a slope by a river, or by an increase in water in the hillside sediments. Landslides caused by river erosion and the increase of internal water generally develop cracks along the hillside that indicate some movement. These cracks can funnel water from overland runoff into the potential slide area, lubricate a slide plane and allow gravity to move the hillside. Clear-cutting of vegetation on hillsides will generally increase overland runoff and can contribute to the development or reactivation of land slides.
I think it would be prudent to examine the hillside above the recent landslide scarp to determine whether there are any cracks. If they find cracks, it would indicate the sliding isn’t over and the landslide will continue to grow.
It would also be desirable to establish some markers and survey lines across the debris field below the scarp. Monitoring of the position of these markers would determine whether there is continued movement of slide material toward the river. A weakening of the toe of the slope could give gravity another opportunity to move the hillside.
Jack E. Sceva
Stanwood
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