| Bulking debris flows (BDFs) are generated by infiltration excess (Horton) overland flow, eroding sufficient sediment on steep, sparsely-vegetated hillslopes to achieve debris-flow rheology. BDFs erode sediment from hillslopes and previously unchanneled hollows, entering the perennial stream network and scouring alluvium along their path to junctions with larger streams, where they typically deposit. Although BDFs have been documented in arid or burned sites throughout the western United States for over 100 years, their initiation is poorly understood, as are their geomorphic and ecological effects.;I developed a numerical model of BDF initiation from overland flow to test the hypothesis that BDFs can be generated without mass failure or episodic addition of sediment. The model incorporated erosion of sediment as bedload, transfer of bedload to suspended load, and alteration of fluid properties based on increased suspended load, which in turn increased bedload and suspended sediment transport. The predicted transition to debris-flow sediment concentrations and the pattern of hillslope erosion showed good agreement with field observations.;Debris-flow generation in arid landscapes that experience progressive sediment bulking should lead to measurably different slope--area trends of channel-head locations and topography than humid landscapes by shallow landslide debris flows (SLDFs). I hypothesized that the slope--area trend of BDF channel heads would follow the underlying hillslope topography, rather than previously described theoretical relationships, and this expectation was confirmed by field surveys. I also hypothesized that the lack of bedrock erosion by BDFs would result in a different topographic signature than the profile previously attributed to SLDFs. Surprisingly, survey results revealed longitudinal profiles for BDFs similar to SLDFs, but they did not correspond with the observed extent of BDF erosion.;I hypothesized that BDF disturbance would reduce riparian vegetation and shading, leading to increased stream temperatures and adverse effects on aquatic biota. Field work demonstrated that BDF effects on stream temperature lasted for 7-40 years and were found to be greater than for wildfire disturbance alone. Stream temperature was well correlated with solar radiation, but reach heating was complicated by site-specific cooling, which appears to be related to the size of the local, alluvial aquifer. |
