Foundation Engineering responded to a landslide emergency along Kalama River Road in Cowlitz County on December 9, 2015.
Heavy rain brought floods and landslides to southwest Washington, and the city of Kalama took a big hit. - KGW8
The slides along Kalama River Road all appeared to be debris flows and shallow failures on steep slopes. This type of slide is most common during intense, shorter duration precipitation. The weather during the days prior was ideal for debris flow events. Several days of rain saturated the slopes, followed by intense rainfall during the night of December 8th resulting in dozens of shallow slope failures and debris flows effecting Kalama River Road and similar roads in steep-sided valleys. This probably explains why we observed more damage in the western portion of the valley where the slopes are generally steeper and rock shallower.
Debris flows typically consist of a shallow failure on a steep slope into a drainage. Soil, trees and rocks mixed with water form a rapidly moving mass that can be very dangerous. The event typically scours the channel of soil and debris reducing the risk of further events. However, there may be more than one slope or channel feeding into a valley, so there can be multiple events in the same valley. Debris flow are an inherent risk in steep mountain valleys.
To reduce the risk, we recommend reducing exposure with the following:
1. Minimize the time spent near steep side valleys during and for 24 hours following intense rainfall
2. Watch the slopes and listen for the sound of a failure and always have an escape route
3. Avoid operations at night and during intense rainfall