INVESTIGATION OF A 2009 DEBRIS FLOW AND THE USE OF EXPLOSIVES FOR LANDSLIDE RISK MITIGATION: RATTLESNAKE RIDGE, KING COUNTY, WA

In January 2009, a major rain-on-snow event in Western Washington triggered a debris flow in eastern King County that traveled approximately 4,500 feet, depositing mud and large woody debris on an alluvial fan and damaging several structures. The landslide occurred in forestlands managed jointly by the Washington State Department of Natural Resources and King County as a “Natural Resource Conservation Area”. It initiated at a former logging road that is part of an extensive “orphaned” road system that has not been used for several decades. Our investigation of the landslide and surrounding area had three objectives: (1) determine the factors that contributed to debris flow initiation, (2) assess the near-term landslide potential at the site and develop a mitigation strategy to prevent further movement, and (3) complete a larger-scale evaluation of the orphaned road system to identify other areas with landslide potential.

The debris flow resulted from the large-magnitude rain-on-snow event but the condition of the orphaned road was found to be a contributing factor. The movement began as a shallow debris avalanche triggered by surface flow over the road and sub-surface saturation of the road prism. The debris flow mobilized once the slide mass entered the head of a high-gradient stream channel approximately 200 feet downslope from the road. Following the event, we identified a high likelihood of further slope movement at the site due to the presence of over-steepened, unconsolidated road fill material. Fill removal using heavy equipment was considered but the poor condition of the road restricted access. Therefore, the fill material was detonated and dispersed across the slope using explosives approximately one month after the initial landslide occurred. While the blasting was conducted with approximately four feet of snow on the ground, it was effective in reducing the short-term risk to downslope private properties. Once the area was snow-free, a larger-scale investigation of the orphaned road system was carried out. The investigation revealed that nearly half of the road system exhibited some evidence of instability. Five additional sites along the same road were identified as having high landslide potential and fill removal using explosives is proposed for this fall.