Root House 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 of Foster Creek, which drains into the South Fork of the Chehalis River. A large debris flow moved down this creek, taking out trees and scouring the stream channels. (Click on photo for larger image)
Figure 2: A closer view of the headwaters of Foster Creek. Three debris slides were recorded along the valley walls of this creek. (Click on photo for larger image)
Figure 3: A view of a debris slide on the valley side of Root House Creek. (Click on photo for larger image)
Figure 4: A view of Slide Creek, numerous debris slides dot the valley walls of this creek. The landslides in this photo appear to have failed in different timber ages, from clearcut (0-5 years), young stands (5-15 years), to submature timber (15-50 years). (Click on photo for larger image)