Thursday, July 12, 2018

Long Runout Landslide in Skagit Valley

Perkins, Reid and Schmidt (2017, Geology) compared landslides associated with glacial terraces in three valleys of the Cascade Range, the Cedar, North Fork Stillaguamish and Skagit valleys. Although the terraces have similar topographic appearances, the underlying glacial related sediments at each terrace differed and thus the mechanics and scale of the slides are differed. The terrace reviewed in the Skagit Valley is called Burpee Hill and is located west of Concrete.

Lidar derived figure from Perkins, Reid and Schmidt (2017) of the slides on the south side of Burpee Hill

Oates (2016) investigated the slides at Burpee Hill mapped the geology units and thicknesses of the units. She noted that the run out distances of these slides was truncated by the river. However, the deposit of one of the slides still remains on the opposite bank of the Skagit. 

The slide run out from the upper slopes of the slide area to the distal end of the deposit is over 6,000 feet. The slide deposit is a location of numerous homes in part because the slide deposit has created an elevated area above the flood levels of the Skagit River.

Regardless of the different mechanisms and style of slide initiation, all three terraces reviewed by Perkins, Reid and Schmidt (2017) are capable of generating long run out landslides. The hazard areas posed by these long run outs is a challenging policy issues at many government levels.  

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