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Post-fire changes to the transport regime of dry ravel, which describes the transport of individual particles downslope, are poorly constrained on a regional level but critical to understand as ravel may contribute to elevated sediment fluxes and associated debris-flow activity observed post-fire in the western United States. In this study, we evaluated post-fire variability in dry ravel travel distance exceedance probabilities and disentrainment rates through a series of field experiments simulating ravel with particles collected in situ. We conducted experiments between March 2021 and March 2022 on soil-mantled hillslopes in the Diablo Range of central coastal California following the Santa Clara Unit Lightning Complex fire of August 2020 with the goal of identifying a regime of "bounded" (light-tailed) or "runaway" (heavy-tailed or nonlocal) motion for different particle sizes between 3 and 35 mm. We conducted this study on both grassy south-facing slopes and oak woodland north-facing slopes. We tracked the post-fire evolution of particle transport regimes by fitting a probabilistic Lomax distribution model to the empirical travel distance exceedance probabilities of different particle sizes on a range of experimental slopes. Our experimental results indicated that a general transition from more runaway to more bounded transport occurred for our largest experimental particles (median intermediate axis of 28 mm) on south-facing slopes as vegetation recovered within the first year post-fire, while small and medium particles (median intermediate axes of 6 and 13 mm respectively) on south- or north-facing slopes and large particles on north-facing slopes did not experience notable changes in transport behavior. After the first year, seasonal variation in vegetation characteristics, such as grass density, appeared to control particle motion. [ABSTRACT FROM AUTHOR]

Beach loss is a growing global challenge that threatens the safety of coastal communities, the health of coastal ecosystems, recreational amenities, and regional economies dependent on tourism. Spatial gradients in longshore sediment transport, or divergence of drift (DoD), is a primary driver of beach width change over multi-annual time scales, but the response of any particular beach can be challenging to characterize and predict. Here we present a new method to characterize DoD using a non-uniform segmentation of the coastline informed by the spatial distribution of longshore transport potential, including the location of physical barriers identified using satellite imagery, and both maxima and reversals in longshore transport potential derived from nearshore wave data. The method demonstrates improved capacity to predict beach width trends at sandy beaches compared to methods that rely on a uniform segmentation of the coastline. In an application to southern California where satellite data documents two decades of beach width change, the non-uniform segmentation method correctly predicts the sign of beach trends at 93% of transects within a littoral cell and achieves a linear correlation between DoD and beach width trends exceeding 0.8. Moreover, we find that a minimum of five years of data are required to establish consistently strong correlations between DoD and beach width changes. Conversely, use of a uniform segmentation is shown to be unreliable for estimating beach width trends due to strong sensitivities to shoreline segment size. This work shows the potential to leverage nearshore wave data and satellite-based beach width data for improved characterization of shoreline dynamics relevant to beach erosion management. • A new shoreline modeling method is presented for estimating divergence of drift. • Segment boundaries are identified using longshore transport characteristics. • Strong correlations were shown between divergence of drift and beach width trends. • A littoral Peclet number characterizes the dominant longshore transport mechanism. [ABSTRACT FROM AUTHOR]