RUNOFF AND EROSION GENERATED DEBRIS FLOWS FROM RECENTLY BURNED BASINS

Debris-flow initiation in recently burned areas is usually attributed to post-fire decreases in rainfall infiltration rates and corresponding increases in runoff rates. Field studies of 210 recently burned, debris-flow producing basins throughout the western U.S. indicate that 76% of the basins showed evidence of debris flows generated through a process of progressive bulking of storm runoff with sediment eroded from hillslopes and channels. We used detailed field and aerial photographic mapping, observations, and measurements of debris-flow events from recently burned basins in the Cerro Grande Fire in New Mexico, the Sula Complex in Montana, and the Coal Seam Fire in Colorado to examine the process of progressive sediment bulking.

Mapping of the transition from sediment-laden flood to debris flow within channels indicated the presence of a highly variable threshold location at which sufficient eroded material is incorporated, relative to the volume of surface runoff, to generate debris flows that persist down the length of the channel. Above this location, the attainment of debris-flow conditions can be transitory; variations in sedimentologic characteristics of the deposits indicate that the flow fluctuates between debris flow and more dilute flows before persistent debris-flow conditions are achieved. The fluctuation in deposit character is attributed to an episodic sediment input to runoff during the event from a series of stepped plunge pools. The plunge pools, and thus the contribution of sediment to runoff, increase in volume with distance down channels. The episodic sediment contributions appear to be necessary in order to entrain sufficient material, relative to the amount of runoff, to impart debris-flow characteristics to the flow. The contributing area upslope from the transition threshold location, its gradient, and underlying materials, the volume of material available on hillslopes and in channels for entrainment, and the degree of channel confinement, all affect the development of debris flows through this process. Continuous channel incision, and associated bank failures, do not appear to be necessary conditions for the generation of debris flows through this process.