Effects of debris‐flow magnitude–frequency distribution on avulsions and fan development

Abstract

Shifts in the active channel on a debris-flow fan, termed avulsions, pose a large threat because new channels can bypass mitigation measures and cause damage to settlements and infrastructure. Recent, but limited, field evidence suggests that avulsion processes and tendency may depend on the flow-size distribution, which is difficult to constrain in the field. Here, we investigate how the flow magnitude-frequency distribution and the associated flow-magnitude sequences affect avulsion on debris-flow fans. We created three experimental fans with contrasting flow-size distributions: (1) a uniform distribution; (2) a steep double-Pareto distribution with many flows around the mean and a limited number of large flows; and (3) a shallow double-Pareto distribution with fewer flows around the mean and more abundant large flows. The fan formed by uniform flows developed through regular sequences of stepwise channelization, backstepping of deposition toward the fan apex, and avulsion over multiple flows. In contrast, the wide range of sizes in the double-Pareto distributions led to distinct avulsion mechanisms and fan evolution. Here, large flows could overtop channels, creating levee breaches that could initiate avulsion immediately or in subsequent events. Moreover, sequences of small-to moderate-sized flows could deposit channel plugs, triggering avulsion in the next large flow. This mechanism was most common on the fan formed by a steep double-Pareto distribution but was rare on the fan formed by a shallow double-Pareto distribution, where large flows were more frequent. We infer that some flow-size distributions are more likely to cause avulsions -especially those that produce abundant sequences of small flows followed by a large flow. Critically, avulsions in our experiments could occur by either large single events or over multiple flows. This observation has important implications for hazard assessment on debris-flow fans, suggesting that attention should be paid to flow history as well as flow size.

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

“…Because of the large length scale required to simulate the 3D evolution of fans within a reasonable time‐frame and spatial extent, similarity‐of‐process models (“analogue models”) have become the established norm in laboratory studies of fans and fan‐deltas (e.g. Bryant et al, 1995; Clarke et al, 2010; Davies & Korup, 2007; de Haas et al 2016; de Haas et al, 2018; Hamilton et al, 2013; Hooke, 1967; Hooke, 1968b; Hooke & Rohrer, 1979; Miller et al, 2019; Piliouras et al, 2017; Reitz & Jerolmack, 2012; Schumm et al, 1987; Van Dijk et al, 2009). A similarity‐of‐process model is one that reproduces key aspects of the morphology of the generic “prototype”; importantly, the processes that shape this morphology in the model can reasonably be assumed to do so in the field.…”

Mechanisms for avulsion on alluvial fans: insights from high-frequency topographic data

“…Because of the large length scale required to simulate the 3D evolution of fans within a reasonable time-frame and spatial extent, similarity-of-process models ("analogue models") have become the established norm in laboratory studies of fans and fan-deltas (e.g. Bryant et al, 1995;Clarke et al, 2010;Davies and Korup, 2007;Van Dijk et al, 2009;de Haas et al, 2016;de Haas, Kruijt and Densmore, 2018;Hamilton et al, 2013;Hooke, 1967Hooke, , 1968bHooke and Rohrer, 1979;Miller et al, 2019;Piliouras et al, 2017;Reitz and Jerolmack, 2012;Schumm et al, 1987). A similarity-of-process model is one that reproduces key aspects of the morphology of the generic "prototype"; importantly, the processes that shape this morphology in the model can reasonably be assumed to do so in the field.…”

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

“…Owing to their moderate surface gradients and relatively low frequency of activity, debris‐flow fans are preferred sites for settlements in mountainous areas (e.g., Jakob, ), and debris flows therefore pose a large threat to people, settlements, and infrastructure (e.g., Dowling & Santi, ; Iverson, ; Wieczorek et al, ). Debris‐flow avulsions can be particularly dangerous, because mitigation measures in the active channel may not be able to reduce risk on other areas of the fan after avulsion (De Haas, Densmore, et al, ; De Haas, Kruijt, et al, ; Pederson et al, ). In addition, debris‐flow fan deposits are archives of past flow processes (e.g., De Haas, Braat, et al,; Dühnforth et al, ; Whipple & Dunne, ) and sediment supply (e.g., Dietrich & Krautblatter, ; Franke et al, ; McDonald et al, ), and they may therefore record sedimentary signals of past climate changes (e.g., D'Arcy et al, ).…”

“…Second, the average locus of debris‐flow deposition gradually shifts toward topographically lower parts on a fan over longer, but poorly constrained, time scales that are on the order of tens of events (De Haas, Densmore, et al, ). Limited observations on natural debris‐flow fans (De Haas, Densmore, et al, ; Suwa & Okuda, ; Suwa et al, ) and physical scale experiments (De Haas, Kruijt, et al, ) show that there is a strong relationship between avulsion occurrence and the sequence of flow sizes. In general, following the formation of a channel plug, small‐ to medium‐sized debris flows leave deposits that step back sequentially toward the fan apex, until an event occurs that is sufficiently large to overtop these deposits and avulse out of the active channel.…”

“…While this is not a new idea (e.g., Whipple & Dunne, ), it has not been well tested because we lack systematic data on the sizes of sediment plugs relative to the channels that they block, the locations at which plugs are most likely to form, and the characteristics of the plugs themselves, including their grain size. Plug deposition is a complex process that depends on debris‐flow runout, volume, grain size, and water content as well as the geometry of the channel (De Haas, Densmore, et al, ; De Haas, Kruijt, et al, ; Whipple & Dunne, ). In the absence of detailed field data on most of these parameters, we hypothesize that whether a channel plug is able to sufficiently block a channel to force avulsion depends on the size of the deposit in relation to the channel dimensions.…”