Avulsions and the spatio-temporal evolution of debris-flow fans

Abstract

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

“…Deposition of sediment by repeated debris flows results in the formation of debris‐flow fans where channels emerge from confined catchments into areas of local accommodation (Beaty, ; Blair & McPherson, ; De Haas et al, ; De Haas, Kleinhans, et al, ; Harvey, ; Hooke, ; Ventra & Nichols, ). Debris‐flow fans owe their characteristic semiconical form to shifts of the active channel and locus of deposition in space and time, termed avulsions (e.g., De Haas et al, ; De Haas, Densmore, et al, ; Schumm et al, ; Schürch et al, ; Whipple & Dunne, ).…”

“…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, ).…”

“…De Haas, Densmore, et al () analyzed the spatiotemporal patterns of debris‐flow fan evolution based on direct field observations (Imaizumi et al, ; Suwa & Okuda, ; Wasklewicz & Scheinert, ) and reconstructions of debris‐flow deposition (e.g., Bollschweiler et al, ; Dühnforth et al, ; Helsen et al, ; Stoffel et al, ; Schürch et al, ; Zaginaev et al, ). They identified two important controls on debris‐flow avulsion that operate over separate time scales.…”

“…First, on the time scale of individual flows, deposition of debris‐flow lobes may locally plug channels, promoting avulsion in subsequent flows (Whipple & Dunne, ). 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.…”

“…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.…”