Fan‐Surface Evidence for Debris‐Flow Avulsion Controls and Probabilities, Saline Valley, California

Abstract

Debris‐flow fans form by shifts of the active channel, termed avulsions. Field and experimental evidence suggest that debris‐flow avulsions may be induced by depositional lobes that locally plug a channel or superelevation of the channel bed above the surrounding fan surface, by analogy to fluvial fans. To understand debris‐flow avulsion processes, we differentiate between these controls by quantifying the spatial distribution of debris‐flow lobe and channel dimensions, along with channel‐bed superelevation, on nine debris‐flow fans in Saline Valley, California, USA. Channel beds are generally superelevated by 2–5 channel depths above the fan surface, and locally by more than 7 channel depths, thereby substantially exceeding superelevation on fluvial fans. Depositional‐lobe thickness and channel depth decrease with distance from the fan apex, although both are highly variable across the fans. Median channel depths roughly correspond to the 50th–75th percentiles of lobe thicknesses, while minimum channel depths roughly correspond to the 10th–25th percentiles. In contrast, the thicknesses of lobes that have triggered avulsions roughly equal local channel depths and are on average twice as thick as the local median lobe thickness. The spatial correspondence between avulsion locations and thick lobe deposits, and the lack of correlation with channel‐bed superelevation, leads us to infer that avulsions on these fans are mostly caused by thick lobes forming channel plugs. Although results may vary with climatic and tectonic setting, our findings indicate that avulsion hazard assessment on populated fans should include mapping and monitoring of channel depths relative to typical deposit thicknesses on a given fan.

Mechanisms for avulsion on alluvial fans: Insights from high‐frequency topographic data

“…Our finding that channel filling may trigger avulsions that are unpredictable in space and time supports recent observations from debris‐flow fans. Field observations indicate that channel plugging can be a trigger for stochastic avulsion of debris flows on fans (de Haas et al, 2019). Similarly, experimental evidence shows that small debris flows may plug a channel so that subsequent larger flows undergo avulsion; moreover, debris flow composition can increase the likelihood of channel plugging and avulsion (de Haas et al, 2018).…”

Exploring new methods to analyse spatial impact distributions on debris‐flow fans using data from south‐western British Columbia

“…Furthermore, there is no relation between the point at which avulsions initiated and the fan intersection point (Figure 10a), except that most avulsions initiated upstream of this point. A possible interpretation for this finding is that avulsion is driven by sporadic channel plugging mechanisms (de Haas et al, 2019; Leenman & Eaton, 2020), rather than a loss of channel confinement. Pre‐event channel measurements using high resolution topographic surveys would be required to test this hypothesis.…”

“…We expected to see a corresponding decrease in cross‐fan runout exceedance probability for more incised fans. De Haas et al (2019) showed that avulsions might be more common on fans with thick lobes compared to their channel depths. A recent study of debris‐flow behaviour in the Colorado Front Range by Schaefer et al (2021) showed that debris‐flow paths with obstructions, low lateral confinement, and down‐fan decreasing slope angles were more prone to avulsion.…”

“…Until recently, much of our understanding of avulsion processes came from observations of fluvial systems (e.g., Bryant et al, 1995; Hamilton et al, 2013; Leenman & Eaton, 2020; Reitz et al, 2010; Reitz & Jerolmack, 2012; Schumm et al, 1987). Recent experimental and field studies implicate factors that affect debris‐flow avulsions, including flow composition, longitudinal position on the fan, path obstructions, degree of lateral confinement, channel depth, lobe thickness, fan slope, event volume, deposition history, and frequency–magnitude distributions (de Haas et al, 2016, 2019; de Haas, Densmore, et al, 2018; Pederson et al, 2015; Schaefer et al, 2021).…”

Global documentation of overlapping lobate deposits in martian gullies

“…We often observe patterns of channel plugging, backstepping and avulsion, typical for the spatio temporal evolution of fans formed by lobate deposits (Figure 5) (De Haas et al, 2016, 2018, 2019c. Most notably, in Taltal, Los, SU-5, SU-7, SU-10, and NU-1 craters, channels that emplaced an elongated lobate deposit or that were blocked by a lobate deposit were completely buried as a result of channel backfilling by subsequent lobe emplacement, ultimately resulting in avulsion (Figure 5a-l).…”