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 of a clearcut on east valley wall near Wildwood. In aerial reconnaissance landslide investigations, landslides in clearcuts (0 – 5 years old) and young stands (5 to 15 years old) are easier to observe than in submature (15 to 50 years old) to mature (50+ years old) timber. Although the image is a bit out of focus, it appears there is a debris flow in the small stream in the right side of the clearcut. (click on photo for larger image)
Figure 2: A view of at least 12 debris slides on the east valley wall near Wildwood. Some of these landslides continued as small debris flows into the valley floor. Also, note the water in the fields, the flood waters nearly covered the whole Boistfort Valley floor. (click on photo for larger image)
Figure 3: A view of the South Fork of the Chehalis River. The flooding brought timber and mud from the hills and deposited them along the river channel and in the farm fields. (click on photo for larger image)
Figure 4: A view of a small log jam along the South Fork of the Chehalis River. The floodwaters were laden with sediment and debris. As the floodwaters retreated, mud and debris was deposited in areas around the Boistfort Valley. (click on photo for larger image)
Figure 5: A closer view of the log jam and the South Fork of the Chehalis River. Note the steep channel banks along the edges of the meander bends. When the Chehalis River flooded, it saturated soils and eroded into the banks, 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 sediment rich waters. (click on photo for larger image)
Figure 6: A view of Newland Creek on the west valley side near Wildwood. Eight small landslides were observed in this area, although most appear not to have reached Newland Creek. (click on photo for larger image)