Thursday, March 24, 2016

Notes on Recent Wenatchee Heights Landslide

Dave Petley noted the landslide south of Wenatchee chelan-county-landslide and some good photographs are provided by photos.

The landslide is within an area mapped with multiple landslides. A massive slide complex that extends from the Columbia River all the way to the mountain crest thousands of feet above is located just to the east.

Part of the Wenatchee 1:100,000 Quadrangle (Tabor and others)
The recent slide is located a bit west of the W in Wenatchee Hill on the map
Note the slide movement arrows on the map including one just east of Qs on the map where the recent slide has moved.

Waitt describes the landslide complex within in the Wenatchee map pamphlet. The general idea is that massive blocks of earth have moved down the slope in an old landslide complex. More recent secondary slides are superimposed on this slide. The bright green (Tw) on the map is designated the Wenatchee Formation. There is some uncertainty in this area of the map regarding that designation in part due to the lithologies being disrupted and displaced by the large scale slides.

The Wenatchee Formation is an alluvial sedimentary formation that is approximately 35 million years old. Hence it predates the Columbia River Basalt Group. The formation is preserved so to speak within a large graben like drop down structural block. The formation contains clay units and is not particularly hard; hence, slides within the clay units may be the problem.

This landslide area is very different than many other landslide areas in Washington State. For one thing this area was not glaciated nor did the huge ice age floods reach this slide location. That means the Wenatchee Formation underlying the area has had time to become deeply weathered and clay formations may have become weathered and softened at considerable depth.

There is a topographic indication that the recent slide is within a larger old slide. The topography also suggests that the slide is very old as streams have incised into the old slide and the slide slopes do not appear sharp edges - that is they have been weathered. The best DEM I have available shows some lumpy ground indicative of a slide along with a shape of a slide, but also shows incised drainages and lack of sharp slide scarp edges.

Two old slide areas are in central area of image

The creek valley on the west side of the old slide has down cut and created a steep slope fronting the old slide and may have set up the right geometry. Some long term deep weathering may have added to the instability. And then this past winter has been very wet. Another factor may be the development of orchards on the old slide area contributing water. Roads redirecting water may also have been a factor. These factors may have taken awhile to weaken the clay below. The images and descriptions suggest that the slide area is smaller than the whole old slide feature and may be a relatively shallow secondary feature with little relation to the old slide.

Lots of "mays" in the paragraph above. The slide is complex and figuring it out and what to do will take some time. Chelan County has hired a geotech firm and issued warnings and closed the road. The slide was reported to still be moving.


Geophil said...

Gazing at the Wenatchee Heights 7.5 min. topo map it appears that Zimmerman Pond may be sag pond located near the head of an older rotational slump.

Dan McShane said...
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