19 December 2015
Florence, Oregon: a fatal landslide triggered by intense rainfall
Posted by Dave Petley
A fatal landslide in Florence, Oregon
At about 3:20 am on Friday morning a fatal landslide occurred near to Florence in Oregon. The landslide, which is described in some detail in an article in the Register-Guard, struck the rear of the house of a local resident:
At about 3:20 a.m. Friday, Dolores Miller heard clawing inside the back door of her Florence-area vacation home and went to let one of her two dogs out. And just then the home was hit by a lethal wall of trees, mud and other debris that cascaded from the rain-drenched adjacent hillside … Miller’s husband “heard a scream and he went to look for her and all the mud was in the house,” said Sean Barrett, fire marshal for Siuslaw Valley Fire and Rescue. The landslide filled the home of Miller and her husband, Gary, and knocked a neighboring home two feet off its foundation on Santa Road, a private road off Mercer Lake Road.
The article also has some images of the landslide, including this excellent picture of the source area:
Florence landslide source via the Register-Guard
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Whilst this image, from KATU2, shows the aftermath at the location of the house:
Florence landslide via KATU2
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A very interesting aspect of this landslide is the fact that it appears to have initiated directly below the road. I suspect that those who are investigating this landslide will be interested to know whether the road was partially located on fill, perhaps to create a sufficiently wide road bench on the bend, and on how water was being managed around the road. But I have no indication that either of these things were a factor. The event certainly illustrates the danger of even quite small slope failures if someone is unlucky enough to be in the path.
This part of the USA is currently suffering very heavy rainfall, so the risk of landslides is perhaps unusually high.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
The top photo is eerily familiar as I have had to deal with a large number of similar slips. Typical conditions comprise a roadway built up a hill is typically cut to fill, when the road crosses the creek it is constructed on an entirely fill embankment with the creek thalweg replaced with a culvert. Depending on the shape of the gully that contains the creek and the grade of the thalweg there is potential for storage of water between the fill embankment and the uphill slope. Failures are typically initiated due to the pond over topping the fill embankment; the over topping being caused by the culvert being blocked with debris or being overwhelmed by the incoming water.