The Evolution of Sediment Sources Over a Sequence of Postfire Sediment‐Laden Flows Revealed Through Repeat High‐Resolution Change Detection

Abstract

Postfire debris flows are particularly complex to study because they do not form discrete initiation locations and commonly involve multiple simultaneously operating erosional processes. Although recent work has begun to elucidate a more mechanistic understanding of postfire debris flows, there is still a paucity of detailed sediment budgets characterizing these events. In this study, we seek to understand how postfire sediment sources and erosional processes change over multiple storm cycles. To do this, we performed repeat high-resolution change detection in a headwater catchment burned by the 2018 Holy Fire in the Santa Ana Mountains, California, USA. This included terrestrial laser scanning in a zero-order catchment (0.95 ha) and unmanned aerial vehicle structure from motion of a headwater channel network (up to 6.5 ha). During the initial storm events that produced runoff-generated debris flows, we found that the evacuation of dry ravel and prefire colluvium accounted for half of the eroded material. These initial flows also acted to clear out much of the material stored within downstream headwater channel networks. In subsequent storm events of equal or greater rainfall intensity, total erosion from the study site was subdued, and the relative importance of shallow hillslope erosion from interrill and rill erosion was increased, as has been noted in similar studies in the region. Overall, this suggests that channel sediment supplies may be more rapidly depleted than hillslope sources, which may drive a trend of decreasing sediment fluxes over time from burned headwater catchments subject to repeated runoff events.

Movement of Sediment Through a Burned Landscape: Sediment Volume Observations and Model Comparisons in the San Gabriel Mountains, California, USA

“…We filled in this data gap with TLS data (Figure 3) on a small hillslope. The TLS data show a complex pattern of dry‐ravel, dispersed raindrop‐driven and overland flow erosion, concentrated rill erosion, and sediment deposition (Figure 3), similar to observations in other steep burned sites (e.g., Guilinger et al., 2020). At the TLS site, the erosion was from a combination of disperse interrill erosion, dry ravel, and rill erosion (Table 2).…”

“…The transition from hillslopes to channels occurs at channel heads (Dietrich & Dunne, 1993). Following wildfire, channel heads can migrate upstream (Wohl, 2013; Wohl & Scott, 2017), as colluvial hollows become sites for gully rejuvenation (Hyde et al., 2007, 2014) scouring into defined channels (Guilinger et al., 2020; Rengers, Tucker, Moody, et al., 2016; Rengers et al., 2018). Gullies are incisional channels subject to ephemeral flow that migrate upstream through headcut erosion (Rengers & Tucker, 2015), but unlike rills they are not rapidly infilled and can persist on a landscape for decades or longer (Rengers, Lunacek, & Tucker, 2016; Rengers & Tucker, 2014; Rengers et al., 2016).…”

Watershed Sediment Yield Following the 2018 Carr Fire, Whiskeytown National Recreation Area, Northern California

“…These were measured during an above-average precipitation water year, and occurred despite the presence of snow in the upper catchments having limited post-fire erosion for much of the wet season, though erosion rates also might have been enhanced by rain-on-snow precipitation in some storms. The high proportion of sediment flux from Brandy and Boulder Creeks in the first few post-fire storms (Figures 5a and 6a), even though these were not the most intense storms that year (Figure 3), was consistent with findings by Guilinger et al (2020) that sediment yield can decrease substantially after the early post-fire storms as hillslope sediment supply is depleted and soil and vegetation begin to recover. However, we did not have sufficient data resolution to examine more closely the progression of sediment yield with individual storm sequence.…”

“…2D) and enlarging the reservoir deltas. This relative importance of wet sediment transport rather than dry contrasted substantially with post-fire sedimentary processes in the San Gabriel Mountains of southern California (DiBiase & Lamb, 2020;Guilinger et al, 2020).…”

Postwildfire Soil‐Hydraulic Recovery and the Persistence of Debris Flow Hazards

“…Postwildfire erosion can incise hillslopes on the order of several centimeters (Staley et al., 2014), thereby thinning or removing the fire‐affected soil horizon during rainstorms. New work has shown that high‐resolution change detection technologies can be useful to monitor inter‐rill and rill erosion and that much of this erosion takes place during the first few storms following the fire (Guilinger et al., 2020). Incorporating this process into our simulation framework would require remeshing the finite‐element domain as a function of time – a technical hurdle that is beyond the scope of this study.…”