Murphy's Hole Area - Big Creek to Charlie's Hump
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.
Chehalis River near Big Creek
Figure 1: A view of the headwaters of a small creek south of Browns Creek. At least seven debris slides failed in these headwaters, most failed off of roads. At least two of these landslides transformed into debris flows and probably reached Chehalis River. (Click on photo for larger image)
Figure 2: A view of the western valley wall of the Chehalis River and into the headwaters of a small creek. Two debris slides occurred on the valley wall and at least two occurred in the headwaters. (Click on photo for larger image)
Chehalis River at Murphys Hole
Figure 3: A view of Murphy's Hole. A large landslide flowed into the Chehalis River; however, the landslide did not block the Chehalis River. Numerous small debris slides transformed into debris flows along the surrounding streams. (Click on photo for larger image)
Figure 4: A closer view of the landslide. The landslide was larger than most other landslides recorded during this event. It is difficult to determine how deep this landslide is, but it does appear that the landslide was fairly shallow and contained bedrock underneath it. (Click on photo for larger image)
Figure 5: A view of the Murphy’s Hole. The Chehalis River at this point contains a lot of sediment. The sediment input could be from landslides further upstream or erosion from the storm. (Click on photo for larger image)
Chehalis River from Hull Creek to Charlie's Hump
Figure 6: A view of a debris slide that probably reached the Chehalis River.
Figure 7: A closer view of the debris slide in Figure 6. The road appears to have pirated some of the material as it came down. Another small landslide is located to the far right of the photo.
Figure 8: A view of the Chehalis River. At the height of the flooding, the Chehalis River incorporated trees and debris. As the debris flowed down, some of the logs became snagged on trees, creating a small logjam. As the waters receded, these logs became emplaced on the gravel.
Figure 9: A view near the confluence of Chehalis River and Crim Creek. Three debris slides were recorded in this area, at least one transforming into a debris flow.
Figure 10: A view of a meander bend of the Chehalis River. As the floodwaters flowed down the river, it eroded into the meander bend, creating an unstable bank.