November 20, 2009
This morning there were only a few reported landslides, one that occurred near Port Angeles that unfortunately resulted in the death of Neal Richards of WSDOT, who was struck by a branch while working on clearing the landslide. The only other reported landslides so far was by Jefferson County, who said they have received reports of mudslides. As the morning goes on, we should be able to gather more information on these landslides.
May 22, 2009
One of the more interesting landslides I have ran across was a debris flow triggered during the December 3rd, 2007 storm west of Pe Ell, Lewis County. Kelsay and I were conducting a reconnaissance of SW Washington to try and find out just how bad the landslides were. We pulled up through Pe Ell to find two debris flows that had come across State Route 7 and surrounded a house.
We parked and I decided to hike up the western debris flow as Kelsay went to look at the eastern debris flow. Unfortunately, I didn’t see the ditch that was covered in mud and quickly went up to my hips in mud (and since I do all my field work in a skirt, that was about as awful as it got!).
Past falling in the mud, the hike up was fairly easy and the scarp was amazing. A thin layer of soil and dirt has slid off of a hollow area (not to be confused with the DNR Forest Practices definition of a bedrock hollow), that is a volcanic tuff.
The soil is between 6 inches and two feet, depending on which side of the scarp you are at. The convergent topography with intense precipitation probably greatly contributed to the landslide moving. The rainfall in this area was probably between 16-20 inches during the storm (the majority falling on December 3rd). Intense rain + shallow soils + impermeable substrate = landslide. Actually, that is the formula we saw again and again for almost all landslides during the December 3rd storm.
The other interesting thing to point out, the area was recently harvested. The lack of canopy coverage can increase the rate that rainfall will reach the ground (from a timed delay to no delay). On weak storms and wet winters, this could increase landslide activity, but we haven’t seen it very many compelling cases around Washington State (but there is a nice study from Canada). However, during the December 3rd storm, the intense precipitation and lack of canopy might (and I will go out on a limb and say almost certainly) have increased landslide activity. To what extent and what increase, that remains to be seen.
May 1, 2009
This is one of the more odd landslides to occur this year in Washington State. On April 3rd, at about 2:30am, in the small community of Glendale on south Whidbey Island, residences were evacuated by the fire department. A beaver dam had clogged the Glendale Road culvert, created a pond about 20 acres in size and the torrential rainstorm was continuing to add water behind the dam. Road crews tried to pump out the creek to relieve stress on the road. By 7am, the road gave way, washing out a 100-150ft section of the road and sending a torrent of mud, woody debris, and water 6-10 feet high into the community below. Up to twelve homes were damaged by the dam burst mudflow.
The community of Glendale is located at the mouth of Glendale Creek. The area is probably prone to periodic flooding from the creek.
There is a nice deep-seated landslide just south of the community which is clear in the LiDAR image. The dam-burst traveled about 4,700 feet downstream with an elevation drop of about 210 feet, or a slope at about 4%. That is a fairly low slope to move at, so this event must have had a good deal of force behind it. Most of the debris that came into the town appears to be fine grained or large woody debris. Woody debris, though bulky, is fairly light in weight. Any coarser material was probably deposited upstream.
Who ever would suspect something so cute and cuddly.
A bit of beaver dam information from Wikipedia:
“Beaver dams are created as a protection against predators, such as coyotes, wolves and bears, and to provide easy access to food during winter. Beavers always work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Because of this, destroying a beaver dam without removing the beavers is difficult, especially if the dam is downstream of an active lodge. Beavers can rebuild such primary dams overnight, though they may not defend secondary dams as vigorously. (Beavers may create a series of dams along a river.)”
Maybe someone should see if the beavers rebuilt after this disaster.