Two separate events occurred at this location: a slide that failed on Dec. 3rd and flowed over State Route 6; and another event along the Chehalis River, probably during the main flooding on Dec. 3rd through 5th. The landslide along State Route 6 is located about 1.5 miles east of the Doty-Dryad Road. The landslide failed in mature timber and formed into a debris flow and flowed over State Route 6, blocking both lanes. The event along the Chehalis River blew out a bridge at Chandler Road.
Map 1: The location map of the landslide located on SR-6 and the bridge washed out at Chandler Road. Click on the map to view a larger image.
State Route 6 Landslide
Figure 1: Looking up at the initiation point of the debris flow from State Route 6. The initiation landslide mass was thin soil over an impenetrable and unaffected substrate. This landslide started as a shallow landslide, probably as slump or soil slip. Instead of stopping after its initial movement, it continued downhill. As it gained momentum and strength, it changed into a different type of landslide, a debris flow. Debris flows can be one of the most destructive types of landslides. Luckily, this landslide caused only minor amounts of damage to State Route 6. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)
Chehalis River Washout
When the Chehalis River flooded, it saturated soils and eroded into the bank, removing the stabilizing slope. As the flood waters 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 2: A view of the bridge site linking State Route 6 with Chandler Road. When the floodwaters reached this bridge, it probably acted like a dam, blocking debris and holding back some of the flow of the water. As the pressure increased, the bridge contorted and twisted, much of which can still be viewed on the road. Finally, the bridge gave way, flowing downstream a short distance and releasing a surge of water and debris. Click on the photo to view a larger image. (photo by Kelsay Davis)
Figure 3: A view of the remains bridge linking State Route 6 with Chandler Road, which was washed a short distance downstream of the initial location. The bridge was completely intact, and was not moved very far downstream. Click on the photo to view a larger image. (photo by Isabelle Sarikhan)