Collaborative Research: Characterizing controls on postfire steepland ravel - when does bioturbation go too far for diffusive models?

After a wildfire, soils on hillsides can be destabilized and subject to elevated risk of erosion. One way this erosion occurs is through a process known as dry ravel, in which the incineration of vegetation or disturbance by animals causes sediment particles to bounce, roll, and slide down the hillslope. In many steep landscapes, ravel is the primary form of erosion after fires and can supply material to post-fire hazards like debris flows.

Post-fire erosion and hazard prediction models are urgently needed to inform responses to today’s changing wildfire regime, but they are currently hindered by the inability of commonly used sediment transport laws to account for ravel processes. This project will advance understanding of ravel erosion processes by directly measuring sediment movement on hillslopes that were recently burned by wildfires in California.

The resulting process knowledge will enable better modeling of erosion and hazards in response to disturbances like wildfire, as well as better estimates of long-term erosion due to predicted changes in wildfire conditions. The principal investigators will also collaborate with local educators to develop a new outreach program and materials to provide authentic field research experiences to underprivileged K-12 students from groups historically underrepresented in STEM (science, technology, engineering, and mathematics), while shifting perceptions around outdoor experiences and geoscience careers.

This study will use a novel combination of short-lived radionuclide analysis, high-resolution topographic data and physical measurements to calibrate and conduct the first field test of a convolutional sediment flux model on natural sediment transport. This work will test the hypothesis that, under key conditions in newly burned steeplands, disturbance by burrowing animals can entrain sediment fluxes characterized by heavy-tailed particle travel distance distributions, thereby escaping representation by diffusive, continuum-based models.

This study will 1) pioneer a new methodology for inferring particle histories from ravel deposits; 2) document statistics on natural particle entrainment rates, travel distances and fluxes to investigate their functional relationships with controlling physical variables that evolve over time; 3) demonstrate the utility and identify use-case scenarios for nonlocal sediment transport models; and 4) provide insight on how connectivity between geomorphic and biotic processes informs dynamic responses.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.