Landslide Reconnaissance Following the December 3, 2007 Storm - Chehalis Headwaters, Boistfort Valley Area
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Landslide Reconnaissance Following the December 3, 2007 Storm - Chehalis Headwaters, Boistfort Valley Area 
 

Stillman Creek - Little Mill Creek Area

On December 3rd, an intense rainstorm blew in from the south, stalling over the Willapa Hills. Rain gages operated by Weyerhaeuser in the Willapa Hills recorded 14 to 20 inches of rain in a 48 hour period. This intense rainfall quickly saturated soils and flooded creeks and rivers, sending a surge of water downstream. In areas with shallow soils, water penetrating into the subsurface accumulated on bedrock or impenetrable substrates. At some critical point, the accumulated water reduced the pore pressure and resistive forces of the soil enough for it to initiate downhill, leaving the bedrock or impenetrable substrate relatively unscathed. The landslides moved everything above the bedrock or impenetrable substrate, carrying trees, rock, and mud downhill, often into creeks or rivers. These types of landslides we define as debris slides and debris flows, which made up the majority of the landslides recorded during this event.

Figure 1: A view east of Stillman Creek, from the ridge separating Stillman Creek from Little Mill Creek. A least five debris slides failed on the valley wall and numerous smaller slides failed to the lower left of the photo. At least two of these slides appear to have formed into debris slides, and may have reached Stillman Creek. (click on photo for larger image)

Figure 2: A view of the ridge separating Stillman Creek and Little Mill Creek. A debris flow initiated at the road along the top of this ridge, with at least one other landslide flowing into the stream channel. During the intense rainfall, it is speculated that these roads could have captured surface flow and concentrated this water into the streams and locations along the hillside. The high volume of water probably washed out this road, forming a debris flow downstream. (click on photo for larger image)

Figure 3: A lower view of the ridge separating Stillman Creek and Little Mill Creek. Five landslides have failed on the right of the photo and possibly three more in the foreground of the photo. (click on photo for larger image)

Figure 4: A view of the ridge separating Stillman Creek and Little Mill Creek, near their confluence. Numerous debris slides dot this valley wall, many transforming into debris flows. Most of the landslides appear to have reached Stillman Creek. The landslides failed in a variety of places, from roads, clearcuts and young stands. (click on photo for larger image)

Figure 5: A view towards Round Knob from the east. Two landslides were recorded in this area, one near the center of the photo along the road, dusted with snow, the second where the same road crosses the stream. Timber had recently been harvested in this area and is stacked and nearly ready for hauling. (click on photo for larger image)

 

Figure 6: A view towards Little Mountain from the north. Five landslides were recorded along the mountain sides, most appear to be in clearcuts. Little Mountain is one of the location that the Seattle Times took photos of shortly after the December 3rd storm. (click on photo for larger image)

Figure 7: A view of the ridge separating Stillman Creek from the South Fork of the Chehalis River. A least eight landslides were identified in this photo, all occurring on the cut bank of the road. (click on photo for larger image)

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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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