POST-1996 DEBRIS FLOW EROSION AND SEDIMENTATION, TUMALT CREEK DRAINAGE, MULTNOMAH COUNTY, OREGON

In February 1996, several debris flows occurred on the south side of the Columbia River Gorge near Dodson, Oregon. The flow in the Tumalt Creek drainage originated on the north-facing, very steep gorge slopes and discharged onto an alluvial fan surface at about elevation 370 m. The alluvial fan is on the order of 2 km long and is composed of older debris flow deposits from several drainages. At least one of the 1996 debris flows reached the Columbia River. Bonneville School, located on the alluvial fan at elevation 55 m and about 1,500 m northwest of the steep gorge slopes, incurred damage to fencing and partial burial of the school’s parking area with muddy debris from an overflow lobe. The Bonneville School District subsequently retained GRI to evaluate the potential for future debris flows to affect the school. GRI observed that subsequent erosion in the main channel of the debris flow stranded the initial overflow lobe, which provided additional catchment space for subsequent flows and diminished the immediate hazard to the school. GRI provided recommendations for debris flow risk management procedures to the school district. An upstream reconnaissance of the Tumalt Creek drainage in 1996 documented areas with sharp, V-shaped channels, erosion and exposure of older debris flow deposits, and large transported boulders up to 3 m in diameter. Re-examination of these areas in 2002 documented channel migration patterns, redistribution of channel sediment, erosion of channel side slopes, and stream downcutting. Alder regrowth and other vegetation patterns also provide useful markers for the 1996 erosional and depositional surface. Several examples of comparative ground photography from 1996 and 2002 will be presented and reviewed. The school was closed in late 1996 in response to long-term changes in student enrollment patterns, not because of the debris flow hazard.