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

Lilliwaup to Sund Creek Landslides

Four separate events occurred in this area during the storm event. Two debris avalanches moved across Highway 101 south of Lilliwaup Bay. A small debris avalanche in a small town north of Sund Creek slid into a house, killing one person. A hyperconcentrated flow in Sund Creek flowed over Highway 101, depositing large amounts of mud and debris. Hyperconcentrated flows often are referred to as 'mudflows', especially in the media; however, 'mudflow' is often used as a generic term that does not describe the landslide type. Hyperconcentrated flows are an intermediate stage somewhere between a debris flow (or a landslide with more rock than water) and flood waters with typical sediment loads. Hyperconcentrated flows are basically floods with a significantly higher amount of sediment and debris than a normal flood.

Location of events

Map 1: Location map of the four landslides between Sund Creek and south of Lilliwaup Bay. The hyperconcentrated flow is not mapped except for the area near the bridge. Click on map to view larger image.

Northern Landslides, south of Lilliwaup Bay
These shallow landslides are typical of the types of landslides that were triggered during the storm event. The two landslides occurred in shallow soils on bedrock and other substrate. These soils are usually less than four feet thick and probably lost cohesive strength as they were saturated.

Figure 1: A look at the landslide initiation point. Note the bedrock and shallow soil. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 2: The landslide moved across Highway 101. It was partially cleared to allow emergency and repair crews through. Note the large trees within the landslide debris; their roots had not been lodged deeply into the bedrock, allowing them to move with the soil mass. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 3: The second of the two landslides, which are very similar. Note the spring in the scarp of the landslide; this probably helped to concentrate water underneath the soil, allowing it to flow more easily. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Sund Creek Area
This landslide occurred in shallow soil on bedrock, as has been common during this storm event. The landslide flowed into the residence, burying and suffocating a man who had been sleeping in a room close to the cliff. Landslides kill and injure hundreds of people each year. Better record keeping and education about previous landslide events and common hazards during rainstorms in Washington might have prevented this death.

Figure 4: The landslide occurred behind the house; the scarp is visible behind the tops of the trees.The landslide flowed into the house, destroying it and suffocating a man who had been sleeping in a room near the slope. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 5: A closer look at the house that was damaged by the slide; the landslide probably moved rapidly, giving little or no warning to anyone in the house. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Sund Creek Hyperconcentrated Flow

Figure 6: Looking upstream at Sund Creek from the bridge. Note the large amount of sediment and debris. It is not clear if the sand and gravel mound on the lower right was the result of human activity, but the woody debris was deposited by the hyperconcentrated flow. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 7: A view of the upstream side of the bridge across Sund Creek. The log jam consists of logs that were filtered out of the flow by the bridge and its guardrail. Click on photo to view larger image. (photo by Isabelle Sarikhan)

Figure 8: Looking north on Highway 101 across the bridge crossing Sund Creek. The hyperconcentrated flow overtopped the bridge, flowed out of the channel and spread north onto residential lots and the highway and into the sea. Structural damage to houses appeared to be minimal, because large debris, like the logs on the bridge, had been caught by other obstacles before the flow reached the residences. Click on photo to view 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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