Rock Creek Area, east of Walville
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: Five small debris slides have failed in the headwaters of this stream. They are difficult to see in the photo. (Click on photo for larger image)
Figure 2: Another angle of the headwaters of the stream. At least two of the debris slides transformed into debris flows and probably reached Rock Creek, which is below the clouds. (Click on photo for larger image)