28 March 2013
A new landslide on Whidbey Island
Posted by Dave Petley
Many thanks to the various people, too numerous to mention, who highlighted this event to me
Whidbey Island in Washington State in NW USA is a well-known landslide site, and indeed has appeared on this blog before. Yesterday a new landslide developed on a coastal cliff, destroying one house and threatening a further 30 or so. This appears to be the site before the landslide, as shown on Google Earth:
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There is a really fantastic set of images on the Seattle Times blog, taken from both the air and the ground. This one gives a really good view of the landslide itself:
http://seattletimes.com/html/photogalleries/localnews2020650059/2.html (c) Ted Warren / AP |
An interesting aspect of this is to compare the state of the top of the cliff with that after the landslide. This is a post landslide view, also from the Seattle Times:
http://seattletimes.com/html/photogalleries/localnews2020650059/2.html (c) Ted Warren / AP |
This is a similar view from the Google Earth imagery:
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The length of cliff top lost is actually rather small given the size of the landslide. This, in this case most of the movement is in material already at the bottom of the cliff rather than a large detachment from the top. This may well explain why there are no obvious cracks on the Google Earth imagery.
Washington State Department of Natural Resources Division of Geology and Earth Sciences have a great downloadable geologic map for that area:
Geologic Map GM-68: Geologic Map of the Camano 7.5-minute Quadrangle, Island County, Washington (2009)
http://www.dnr.wa.gov/researchscience/pages/pubmaps.aspx
The area of interest was mapped as a recent landslide.
Not sure I buy the argument, but I’ve heard one geologist suggest the weight of the trees, which increases as they grow, can trigger a slope failure like this. We generally think of vegetation as stabilizing a slope.
[Many thanks for your comment. This is true of small landslides (a few cubic metres or so). For large failures like this, the added weight of trees is trivial compared with the mass of soil. A mature oak tree weighs about 15 tonnes; even a small landslide with proportions of 20 x 20 x 5 metres weighs about 3400 tonnes. D.]
Yes, however, the trivial weight of the trees does not have to push the entire mass of the soil, it merely has to exceed the shear strenghth parallel to the slde surface. The weight of the soil has force vector components both perpendicular to and parallel to the slide surface. A portion of the weight of the soil, along with the trees, is contributing to the force opposing the shear strength. What do you think?
[All of the landslide mass has these two components, so in a sense a tree is not different to a piece of soil with the same mass. For anything other than a very small landslide the mass of the trees is small compared with the mass of the soil, so is unlikely to make a substantive difference. Trees do play a key role in both increasing soil strength and in lowering the water table, so in most cases it is better to leave trees in place than it is to fell them. The exceptions are where the potential landslide is very small, when the combination of the added mass and the turning force associated with wind loading can be significant, and where a landslide is likely and the tree will cause additional damage (for example on railway embankments). D.]
A lot of these shorelines are in active retreat. Review of aerial photos over the last 100 hundred years may show significnat retreat, as observed on similar shorelines in Washington and British Columbia. It appears there may be a swimming pool at the failure scarp on one property, hard to tell from photos. Watering lawns provide or leaking water or sewer lines provide sources of raising the water table and destabilizing the slope.