INITIATION LOCATIONS AND PROCESSES OF ALPINE DEBRIS FLOWS, CENTRAL FRONT RANGE, COLORADO

On 28 July 1999, about 480 alpine debris flows were triggered by a thunderstorm along the Continental Divide in Clear Creek and Summit counties in the central Front Range of Colorado. The thunderstorm produced about 43 mm of rain in four hours, 35 mm of which fell in the first two hours. Several debris flows impacted Interstate Highway 70, U.S. Highway 6, and the Arapahoe Basin ski area. We mapped the debris flows from color aerial photography and inspected many of them in the field. Three processes initiated debris flows. The first, which has been termed the “firehose effect”, initiated 10 percent of the debris flows and occurred where overland flow became concentrated in bedrock channels and scoured debris from talus deposits and the heads of debris fans. The second process, which was responsible for 79 percent of the flows, was the transport of material eroded from poorly-vegetated hillslopes via a system of coalescing rills. The third process initiated 11 percent of the debris flows and involved the disaggregation of shallow landslides on vegetated slopes. In general, all three processes initiated above timberline on steep hillslopes (> 30º) downslope from catchments with small contributing areas (< 8000 m²). We observed a relation between the degree of soil development and the process type that generated debris flows. Published soil maps cover about 30 percent of the debris-flow initiation locations. Of this 30 percent, all firehose effect debris flows (4), and 67 percent of the rill debris flows (79) were initiated on bare rock outcrop or rock talus; all but one of the remaining rill debris flows (38) were initiated on unvegetated, eroded surficial materials. About 60 percent of the shallow landslides (6) occurred in moderately to poorly vegetated loam soils with the remaining 40 percent on rock talus and outcrop. In general, areas with greater soil development were less likely to generate runoff and therefore less likely to generate debris flows by the firehose effect or by rilling. Debris flows generated by runoff processes (rilling and firehose effect) increased in volume as they traveled downslope whereas debris flows initiated by shallow landslides tended to deposit material along their paths. Therefore, runoff generated debris flows are potentially more hazardous than those initiated by shallow landslides.