Wednesday, April 16, 2014

A Few Deep-Seated Bedrock Landslides in Whatcom County

Shortly after the Oso/Hazel slide John Stark with the Bellingham Herald contacted me and asked if Whatcom County has any comparable landslide risks. In terms of likely imminent threat, there are no sites in Whatcom County that are directly comparable to the Hazel Slide in terms of size and pending risk. The Clay Banks (nooksack-river-temporarily-blocked-by-landslide, nooksack-river-blocking-landslide-notes, further-update-on-nooksack-river/clay-banks-via-DT, and more-on-clay-banksnooksack-landslide) has been the location of the largest slides along the Nooksack River, but its height is less than the Oso Slide and has more cohesion and is at least partially compacted.
That said, the steep mountain slopes of the more easterly part of Whatcom County hold plenty of potential smaller but still very hazardous landslide and debris flow risks. 
And then there is the fact that even bedrock mountain slopes do fail, and the Northwest Cascades of Whatcom County have had plenty of deep-seated bedrock landslides. While the risk of a large catastrophic bedrock failure event taking place anytime soon is low, the extent of the slide deposition area does warrant some long term planning considerations.
So a bit of LiDAR to illustrate a just a few.  

South end of Sumas Mountain

Sumas Mountain has very large deep seated bedrock failures on all sides. The one above is perched just above confluences of the three forks of the Nooksack River. Clearly there has been some major shifting and block movement, but thus far no valley filling collapse. The bedrock is Chuckanut Formation sandstone and siltstone.

Large slide west of Kendal

This slide has been well know as it shape is clear even without LiDAR on topographic maps with its spectacular lobe protruding out onto the former glacial river outwash plain clearing showing it is a post ice-age landslide. This slide is on the east side of Sumas Mountain within Chilliwack metasedimentary rocks.

There are numerous landslides on this particular Mountain. The deposit of landslide debris is clearly post glacial and covers the lowest youngest part of the North Fork Nooksack River valley. This slide in Chuckanut Formation rock is one over several that have come off of the aptly named Slide Mountain.

Slide west of Glacier, Washington

This slide is also within Chuckanut Formation. Unlike the previous shown landslides, this one does not show up on the geologic maps. Its lower end blends together with a much large landslide deposit from Church Mountain. The North Fork Nooksack flood plan is incised into the valley floor with a elevated terrace covering the southern half of the valley floor. That terrace is a huge landslide deposit from 2,000 year old Church Mountain (fieldtrips/the-church-mountain-landslide).

The above shown deep-seated bedrock failures are all pre historic. But there are few examples of deep-seated historic bedrock failures in recent years in Whatcom County.

One of the more pressing problem slides even stars in a movie: WWULandslide/landslideCamTimeLapse.

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