SHACK 9 DEBRIS SLIDE CASE HISTORY, ELECTRON, WASHINGTON

A case history is presented for a debris slide on timberland that damaged the water supply flume for a hydroelectric project near Electron, Washington. The debris slide occurred on the Puyallup River valley wall slopes during a rainstorm event on Memorial Day weekend of 2006 at a time when the flume was shut down for maintenance. Slopes in the vicinity of the landslide are convergent and exceed 70 percent locally. The soils consist of relatively thin Quaternary glacial deposits overlying Tertiary age volcaniclastic rocks. Harvest of trees on the plateau at the crest of the steep valley wall slopes in the late 1990's may have contributed to windthrow of trees on the steep slopes. Windthrown conifer trees and associated slide debris damaged an approximate 100-foot long section of the flume. The slide debris mobilized nearly 400 feet downslope and delivered sediment directly to the Puyallup River. A slope stability reconnaissance was completed to evaluate the risks for flume worker safety, potential impacts to the flume and potential for subsequent sediment delivery to the river. Windthrown trees located on the steep main scarp and slide body, as well as remaining standing trees along the slide margins pose a risk for sediment delivery to the river, potential additional damage to the flume and to worker safety. A debris containment system consisting of three cabled fence lines was installed downslope of the flume within the slide body to reduce the potential for slide debris delivery to the Puyallup River. An additional cable fence line was installed upslope of the flume to reduce the risk of personal injury to workers from falling debris during reconstruction. This example shows the challenges and complexity, from a technical and regulatory perspective, of long-term management of forested areas on or adjacent to steep slopes where structures or improvements may be at risk to slope movement.