CHARACTERIZATION AND COMPARISON OF DEBRIS FLOW INITIATION ZONES 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 60 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. Three of these flows caused major damage to bridges and roadways. Highway 35, a key access route, was closed in two places. Bridges were damaged or destroyed along the White River and Eliot Creek. Debris flows were triggered either by landslides, or through coalescence of multiple small debris flows into a major channel. Physical and morphological characterization of source areas was used to assess factors controlling debris flow initiation. Preliminary findings indicate that drainages with direct connection to a glacier, an abundance of non-vegetated, unconsolidated sediment, and steep slopes in a morphologically constricted spot may be more susceptible to debris flow processes within an altitude range of about 5800-6600ft. However, an analysis of drainages using Melton’s ruggedness number in combination with other factors suggest that all major drainages on Mount Hood are capable of producing debris flows. This project seeks to answer the question of why some drainage systems spawned debris flows as a result of the November Storm, 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.