Linking bioturbation and wildfire to nonlocal transport in steeplands of the California Bay Area

Dry ravel -- the inertia-based particle motion often induced by biotic activity or wildfire -- is a dominant sediment transport process in steeplands across the world. The contribution of ravel to sediment fluxes is important to constrain in long-term sediment budgets, and because it can be a key component of mass wasting hazards such as post-fire debris flows. Ravel fluxes commonly increase after fire due to the release of sediment stored behind incinerated vegetation. Recent work has also shown that reduced surface roughness on burned hillslopes can contribute to this flux by drastically enhancing particle travel distances. Predominantly long-distance particle motion, often termed nonlocal, is poorly described by existing hillslope flux models that fail to account for the effects of topographic variation and particle momentum. This leads to a fundamental gap in our ability to predict postfire sediment fluxes or how landscapes will respond to changing fire regimes. A transition to long-distance, or nonlocal transport can be effectively represented by a statistical framework for particle travel distance distributions, but has only been observed in controlled experiments to date. The degree and conditions under which real ravel fluxes become nonlocal has yet to be established. We perform the first field calibration of this statistical framework in a heavily bioturbated and recently burned field site at the Blue Oak Ranch Reserve near Mt. Hamilton in central coastal California. This field site, which is a thickly soil mantled oak savannah, was burned in the SCU Complex fire in August 2020. We conducted repeat measurements, topographic analyses and field experiments with in situ particles to examine how key variables including surface roughness, slope and particle size control the evolution of ravel particle travel distances as vegetation regrows after fire. We combine these data with measured grain size distributions and short-lived radionuclide analysis for freshly exhumed and mobile particle deposits, to gain new insights on ravel mechanics and fluxes in postfire landscapes.