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

Cedar Flats Area

On December 3rd, warm rains rapidly melted snow on the ground, saturating soils that began to slide. Three landslides on the tributary to Swift Creek triggered three debris flows, carrying debris and sediment into Swift Creek and creating a hyperconcentrated flow. By 8:30am, debris appeared to have clogged the culverts where Swift Creek flows under Cedar Flats Road, blowing out the road and stranding several hundred residences. Hyperconcentrated flows often are referred to as 'mudflows', especially in the media; however, 'mudflow' is often 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.

 

Map 1: Cedar Flats Road washout is indicated by the red star. The locations are approximate for landslides, debris flows, and hyperconcentrated flow. Click on the map for a larger image.

 

Figure 1: A view of Cedar Flats Road at the washout; note the size of the culverts. The remains of three culverts were found during the investigation. If the culverts had clogged, water would have infiltrated into the road prism, eventually saturating the fill, causing it to burst. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 2: Two of the three culverts that failed out of the Cedar Flats Road. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

Baker Road Debris Flows

 

Figure 3: A view of the initiation point for the northern debris flow. The landslide is typical of many landslides during this event, as it moved shallow soils above impermeable and unaffected substrate, probably Crescent basalt. The slide moved this shallow soil and debris into the stream channel that formed into a debris flow. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

Figure 4: Looking downstream from the initiation point of the landslide; note the classical levee (deposition) where the landslide banked the corner in the upper center of the image, where the channel turns to the right and disappears from view. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 5: A view of the stream channel heavily scoured by the debris flow. The debris flow scoured to bedrock and heavily eroded the sides of the channel. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 6: The debris flow that was flowing down the small channel joins into the main channel. At this point, the landslide had grown in size and speed and flowed over the right side of the channel. The landslide had to turn more than 90 degrees to join the main channel, probably temporarily stalling the landslide before it continued downhill. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 7: A view of the main stream channel and the debris flow. Around the bend is an old logging railroad crossing that was washed out by the debris flow. Note that the scouring from the landslide reached the tree roots. Click on the photo for a larger image.

 

Figure 8: A closer look at scarring on the trees from the mass of rock and woody debris that was flowing in the debris flow. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

Figure 9: When the debris flow reached the old logging railroad crossing, it stalled. Water, rock, and debris built behind the crossing until the pressure pushed through the old railroad prism. The impounded material burst with the release of pressure, rushing downstream in a torrent of water, rock, and debris. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 10: The old logging railroad crossing at Baker Road. The cement culvert (3 foot) on the right side of the photo was the original stream channel before the washout. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

B-8000 Road Landslide

 

Figure 11: Looking up from the B-8000 road at the shallow landslide that initiated a debris flow on a tributary of Swift Creek. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 12: A view at the landslide crossed the B-8000 road; the landslide moved across the road and picked up strength as it flowed down the bank. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

Figure 13: Looking up at the B-8000 road; note that the landslide had scoured to bedrock shortly after flowing into the road bank. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 14: The stream channel was scoured to bedrock and the sides of the channels were heavily eroded. The local resident giving the tour stated that this stream was heavily vegetated and the side channels had been expanded by the debris flow. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 15: Looking down channel at the debris flow. Note the fallen trees in the center of the photo that have been scarred or broken by the debris flow; this helps give us an approximate height for the landslide. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 16: The left channel debris flow is the B-8000 debris flow and occurred first; however, another channel to the upper right of the photo also contains a debris flow that failed after the B-8000 debris flow moved through this area. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 17: A view of the debris avalanche on the right side channel. The slide plane appeared to be on bedrock and the slide mostly blocked the stream channel. This slide produced a debris flow that flowed over the B-8000 debris flow. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 18: Looking up at the debris avalanche and the beginning area of the debris flow. Note the scouring on the stream banks and the lone old growth stump in the middle of the stream channel. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 19: The debris flow quickly picked up strength and was able to scour to bedrock shortly after initiating from the debris avalanche. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 20: Looking down towards where the channels join (Figure 16); the debris flow had enough material to top the banks of the channel, slopping over into the forest floor. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

Swift Creek Hyperconcentrated Flow

 

Figure 21: Small landslides dotted the Swift Creek valley sides as the hyperconcentrated flow moved towards Cedar Flats Road. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

Figure 22: All of the shallow landslides observed in this valley occurred on impenetrable substrate topped with shallow soils. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 23: A view of a sediment fan. As the hyperconcentrated flow moved through the Swift Creek Valley, it appeared to create temporary logjams, allowing the debris to settle out of the water behind the dam. When the dams were breached, a torrent of water would once again scour the channel, back cutting into the deposited debris or picking up new debris and material and repeating the process downstream. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 24: When the hyperconcentrated flow reached Munson Drive, it acted as a dam. The waters spread out, filling the valley flow and depositing about 13 inches of sand behind the road. Click on the photo for a larger image. (photo by Isabelle Sarikhan)

 

Figure 25: Swift Creek flowed over Munson Road, from the extent of the traffic cones to behind the origin of where the photo was taken. Click on the photo for 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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