Two separate events occurred at this location: a slide that failed on Dec. 3rd, over State Route 6; and a later event along the Chehalis River, probably around Dec. 7th. The landslide along State Route 6 is located about 1 mile east of River Road. The landslide moved translationally over bedrock and flowed over the road, blocking it. The Chehalis River landslides were observed about a quarter of a mile west River Road.
Map 1: The location map for the landslides located on SR-6. Click on the map to view a larger image.
State Route 6 Landslides
Figure 1: A view of a small translational landslide that flowed across State Route 6, blocking one lane. The landslide mass was composed of shallow soils above impermeable and unaffected substrate (bedrock). As the vegetation grew, it was unable to penetrate deep enough into the bedrock to provide stability. As the warm rain rapidly melted the snow, shallow subsurface water flows increased, creating a slipping plane between the bedrock and the soil. A critical level of water was reached and the soil lost its cohesive strength, flowing downhill until it is able to stabilize itself. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Figure 2: A view of the impermeable substrate (bedrock) which was the failure plane for the landslide. A common theme among most of the landslides investigated for this storm was association with thin soils on impermeable substrate (bedrock). Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Figure 3: A view of a small shallow landslide located about 100 feet east of the main landslide. After a intense rainstorm, like the December 3rd storm, hundreds of small slides usually occur on steep slopes. It is difficult to document all of these landslides due to sheer number and lack of damage or danger they present to structures and infrastructure. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Chehalis River Landslides
When the Chehalis River flooded, it saturated soils and eroded into the bank, removing the stabilizing slope. As the floodwaters receded, the saturated soils began to dewater, creating increased pressure on the banks. Coupled with the high erosion, large sections of the banks along the Chehalis River flowed into the river, adding to an already sediment rich waters.
Figure 4: A shallow slump along the banks of the Chehalis River; note the amount of erosion along the banks. The erosion removes lateral strength to the soils, causing them to flow downhill for stability. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Figure 5: Erosion along the banks of the Chehalis River; note the scarred trees, which gives a minimum height of the floodwaters. As the floodwater flowed through this area it carried debris, mostly wood and any buoyant object in its path. As this debris flows with the floodwaters, it bumps into standing trees and structures and can incorporate them into the flow. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)