Speaker: Benjamin Tan
Affiliation: EPSS, UCLA
Date: Wednesday, June 3, 2026
Time: 12:00 PM
Abstract
Wildfire fundamentally alters hillslope and channel sediment transport by rapidly mobilizing dry ravel, which accumulates in channels immediately following burning and is subsequently flushed during the first post-fire rainfall. Using repeat high-resolution topographic surveys and channel-scale swath analyses within the Palisades burn scar, we examine how first-flush sediment storage and redistribution relate to channel steepness (k_{sn}) and underlying lithologic gradients. The Palisades wildfire provides a natural laboratory to examine these dynamics, as the study area spans a pronounced east–west lithologic gradient associated with systematically increasing channel steepness. We show that post-wildfire sediment transport and hillslope failure can be partitioned into two coupled but distinct processes: (1) dry ravel production, which occurs exclusively in steep, high-k_{sn} lower-order channels, and (2) precipitation-driven mobilization and redistribution of this material into channel trunks during early storm events. Dry ravel does not accumulate ubiquitously across the drainage network, but is instead restricted to channels that exceed a critical steepness threshold, indicating a strong topographic control on post-fire sediment supply. We further demonstrate that dry ravel accumulation in steeper lower order channels is spatially coincident with active scouring of the underlying channel bed, leading to sediment budgets that exceed existing infrastructural design capacities. Following rainfall, dominant patterns of erosion and deposition are negatively correlated with k_{sn}: deposition dominates in low-k_{sn} reaches, while high-k_{sn} channels experience net erosion and sediment export. Together, these observations suggest that first-flush sediment response following wildfire is governed not only by burn severity and rainfall intensity, but also by broader lithologic and topographic controls that regulate transport efficiency, sediment storage, and channel–hillslope coupling.