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Post-wildfire Generation of Debris-flow Slurry by Rill Erosion on Colluvial Hillslopes
  • Paul Alessio,
  • Thomas Dunne,
  • Kristin Diane Morell
Paul Alessio
University of California Santa Barbara

Corresponding Author:palessio@ucsb.edu

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Thomas Dunne
University of California, Santa Barbara
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Kristin Diane Morell
University of California, Santa Barbara
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Overland flow self-organized into rills and eroded colluvium from steep hillslopes after a wildfire in the Santa Ynez Mountains. During 12-15 minutes of runoff, rill erosion generated a slurry that scoured bouldery alluvium from mountain canyons to form large debris flows that severely impacted the community of Montecito, CA. The timing, volume, and peak discharge of the debris flows, and their capacity to scour boulders from canyons, depended on the mechanism and generation rate of the granular fluid (slurry) responsible for the sediment transport. Field surveys and aerial imagery revealed dense networks of rills on bare, burned hillslopes, and levees of fine-grained sediment lining the rill margins indicated that flows were highly viscous and the water and sediment were already intimately mixed before entering stream channels. We mapped networks of rills and measured their cross-sectional geometry to quantify the influences of lithology, hillslope length, gradient, and planform on the sediment volumes released by the rills and their volumetric contribution to the canyon-scale debris flows in six watersheds. We interpreted the mechanism of slurry generation by rill erosion, as far as field evidence will allow, through the lens of field and laboratory experiments conducted by others under comparable conditions. We used the geometry of rills, and the duration of intense rainfall and modelled surface runoff, to develop an empirical model of the rate of slurry generation by rill erosion and to suggest a mechanism by which the necessary intensive mixing occurred.
Nov 2021Published in Journal of Geophysical Research: Earth Surface volume 126 issue 11. 10.1029/2021JF006108