Modeling frequent debris flows to design mitigation alternatives

Abstract

Debris flows are a common problem in Western Washington State. One persistent location of debris flows is Slide Ridge. Glacial till deposits erode in debris flows which travel to Lake Chelan, passing through the community of Shrine Beach in Washington State. In the early 1990s an unlined debris channel was constructed from the apex of Slide Ridge to Chelan lake and a large debris basin was constructed on the upslope side of the road crossing. Every 1-2 years there is a flow large enough to fill the basin, pass over the road to continue downstream to Lake Chelan, and the road is left covered in debris. The largest debris flows since 2003 have volumes estimated to be between 803 m3 to 9863 m3. Samples show the sediment is 85% gravels and dominated by angular cobbles. A number of models are being tested for their ability to predict future debris-flow volume, maximum debris-flow height, and runout distance. Results of the modeling will be used to design and evaluate mitigation measures that include the installation of grates, nets, altering the road configuration, and combination of these measures.