the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Post-fire Variability in Sediment Transport by Ravel in the Diablo Range
Abstract. 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.
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RC1: 'Comment on egusphere-2023-2694', Emmanuel Gabet, 09 May 2024
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The authors have run some clever experiments and have collected a wealth of interesting data over several field campaigns. The math involved is above my paygrade so, hopefully, the other reviewer will be able to look at it more critically. I have uploaded a pdf of the manuscript with detailed comments. Here are my more general comments.
1) The arguments about how transport regime changes over time after the fire seem to hinge primarily on the condition and density of the vegetation; however, there’s no actual data presented on this, just qualitative observations. There are established techniques for making these types of measurements at the ground level and not employing them in this study was a missed opportunity considering how important the condition of the ground is with respect to frictional resistance. Perhaps the authors took a bunch of pictures and could use them to provide quantitative information? Moreover, the descriptions that are given regarding the vegetation aren’t very systematic; it would be helpful to provide a table describing the condition of the vegetation and the surface during each field campaign, segregated by slope aspect.
2) Also, considering the importance that vegetation and aspect presumably have in modulating the post-fire dry ravel response and the recovery to ‘background’ conditions, there was little explanation of why or how. Indeed, the last sentence in the paper emphasizes the importance of vegetation and aspect, but this idea isn’t explored in the manuscript. For the revision, I would recommend diving into this concept a bit more, otherwise, it’s not clear why it matters that you’re comparing north- and south-facing slopes.
3) The study bills itself as an examination of ‘post-fire’ variability in dry ravel, which implies that what is being measured is unique to recently burned hillslopes. However, the only way to know whether that’s true or not would be if you compared your results to north- and south-facing unburned hillslopes. In other words, including control sites would have been important for determining how much of the effect that you saw was due to the fire. Although it’s obviously too late to implement this suggestion, consider including control sites in the future, especially if you’re documenting changes over time. I would recommend addressing the absence of control sites, and its implication, somewhere in the Discussion. At a minimum, it might spur future studies to consider this.
Manny
Data sets
Lomax2 Hayden L. Jacobson https://doi.org/10.5281/zenodo.10048974
Model code and software
Lomax2 Hayden L. Jacobson https://doi.org/10.5281/zenodo.10048974
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