Landslide Reconnaissance Following the December 3, 2007 Storm - Lewis County
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Landslide Reconnaissance Following the December 3, 2007 Storm - Lewis County 
 

Pe Ell

Three landslide events were document southwest of Pe Ell, all originating out of forest-managed lands and into urban areas. Two of the events were debris flows, originating out of hollows, both blocking two lanes of State Route 6, and one flowing into a residential yard. The largest of the three landslides occurred west of Meyers Road, flowing over State Route 6 and destroying three structures and a vehicle and impacting utilities. The three landslides failed during the rapid snowmelt and intense rains on December 3rd, 2007.

Map 1: A location map of the landslides investigated southwest of Pe Ell on SR-6. Click on the map to view a larger image.

5500 block of Pe Ell landslides
These two landslides occurred in clear cuts and initiated out of convergent hollows. The landslide masses were composed of shallow soils over an impenetrable substrate, which acted as the failure plane. As the landslides left the hollow, they began to increase in momentum, channelizing and flowing like a stream.  Debris flows, like these, typically contain more rock and sediment than water.

Figure 1: Looking towards the initiation point of the southwest debris flow, which partially blocked State Route 6. As the landslide flowed downhill, it pushed down trees and shrubs, which probably slowed it down significantly before reaching the road. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 2: A closer view of the initiation point of the southwest landslide. The initiation landslide material composed of very thin soil overlaying bedrock that most likely acted as the sliding plane for the landslide. The removal of the tree canopy can aid in a more rapid melting of the snow and greater overland flow. Because this soil is so thin, it would not have taken much water to trigger this landslide. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 3: The northeast debris flow, which flowed across State Route 6 and into the property of 5503 State Route 6. This landslide mass was 6 to 8 feet thick, which was thicker than most landslides investigated. During our investigation, we noticed a small stream at the toe of the landslide. This landslide might have originated on a small spring or in a place that concentrated groundwater. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 4: A view of the northeast debris flow, looking downhill from the initiation point. When the landslide failed, it grew in momentum and channelized into a debris flow, but there was more debris than the channel could hold. As a result, the landslide sloshed down the channel, spilling debris over the channel as it went. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 5: The run-out of the northeast debris flow and the debris deposited in the residence’s yard at 5503 State Route 6. Landslides will continue to move until they find stability. As this landslide moved downhill, it gained speed and combined with the enormous weight of the rock and water, it continued to flow over the road and into the yard across the way. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Pe Ell Landslide
The Pe Ell landslide was one of the largest landslides investigated during our reconnaissance. The landslide initiated out of a recently harvested hillside and flowed over State Route 6 and into a field, completely blocking State Route 6. The landslide contained numerous springs and an investigation of the toe of the landslide indicated a significant amount of water came down with the landslide. It is unclear if this water was from surface flow or subsurface flow. The water deposit was relatively free of sediment, but contained some woody debris. The landslide completely covered State Route 6 and damaged three residential structures, a truck and utility poles.

Figure 6: Looking from the toe of the landslide towards the headscarp. The landslide contained numerous springs, two of which can be viewed to the right of the yellow tractor. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 7: A closer look at the springs on the landslide. Springs are common on deep-seated landslides. As the landslide moves, it contorts the material, creating cracks, folds, and weakness planes. As the subsurface water flows into these structures, they are often forced out into the surface because of contorted material. Click on the photo to view a larger image. (photo by Kelsay Davis)

Figure 8: One of the buildings damaged by the landslide. The landslide pushed the house off its foundation, moving it at least 6 feet. Note the truck in the center of the photograph. As the landslide flowed towards the house, it picked up any objects in front of it (like a bulldozer). When the landslide reached the truck, it pushed it along with the landslide, forcing it into the house. Click on the photo to view a larger image. (photo by Kelsay Davis)

Figure 9: Looking at the residence from the opposite side of the landslide. Landslides contain a lot of weight and because so, carry a lot of force. This house is fairly light compared to the landslide mass and was easily pushed by the landslide. Luckily, the landslide stopped before completely destroying the house. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 10: A view of the extent of the landslide. If the landslide continued past this point, the house probably would have broken apart from the stress and strain of the landslide pushing it. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 11: A view of a smaller house located on the same property. The landslide did not appear to push this house like the other house. It may have been well bolted to the foundation, helping to hold back the landslide. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 12: A closer look at the smaller structure. The house is twisted and contorted from the force of the landslide striking it. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 13: The garage was also damaged during the landslide event. As the mass moved around the second house, it struck the foundation of the garage, destroying it. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

Figure 14: A closer look at the garage foundation. The landslide struck the foundation to the left of this photo, destroying the foundation. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)

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 Contacts

Stephen Slaughter
Geology & Earth Resources Division, Hazards Geologist
Washington State Department of Natural Resources
360-902-1498
Fax 360-902-1785
stephen.slaughter@dnr.wa.gov

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