Initiation Zone Characterization of Massive Debris Flows on Mount Hood, Oregon, November, 2006

In November, 2006, a storm generated a minimum of 13.4 inches of precipitation in six days, triggering major debris flows in many of the drainages on Mount Hood, Oregon. This subduction-zone volcano is located in the Cascade Range 80 miles east of Portland. An initial reconnaissance survey determined that the November storm triggered debris flows on all sides of the volcano. Of the eleven primary drainages surveyed, seven experienced powerful torrents; these include the White River, Salmon River, Clark Creek, Newton Creek, Eliot Creek, Ladd Creek and Sandy River basins. Five of these flows caused major damage to bridges and roadways. Highway 35, a key access route on Mount Hood, was closed in two places. Further investigation and analysis revealed two separate mechanisms for flow initiation. Debris flows have been caused either by landslides, or through coalescence of multiple small debris flows into a major channel. Estimates for initial landslide volumes range from 2,200 cubic meters to 120,000 cubic meters. Unconsolidated regolith was sampled in the initiation areas for subsequent particle size analysis and bulk density determinations. Average bulk density measurements for landslide induced debris flows range from 1.36 – 1.76 g/cm3. Physical characterization of source area materials will be used to assess factors controlling debris flow initiation. Preliminary findings indicate that drainages with abundance of non-vegetated, unconsolidated sediment and over steepened slopes may be more susceptible to debris flow processes. This project seeks to answer the question of why some drainage systems spawned debris flows, while others experienced only flooding. Recognition of common factors linked to debris flows at Mount Hood will be applicable to identification of hazard prone areas on other volcanoes in similar climactic and tectonic settings. The primary outcome of the project will be a debris flow hazard map derived from the November 2006 storm event.