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 the southern walls of Patton Creek. At least three debris slides transformed into debris flows on the far valley wall. On the near hillside, at two debris slides flowed off the edge of the photo.