Debris flows as a factor of hillslope evolution controlled by a continuous or a pulse process?

Abstract

Flood effectiveness observations imply that two families of processes describe the formation of debris flow volume. One is related to the rainfall–erosion relationship, and can be seen as a gradual process, and one is related to additional geological/geotechnical events, those named hereafter extraordinary events. In order to discuss the hypothesis of coexistence of two modes of volume formation, some methodologies are applied. Firstly, classical approaches consisting in relating volume to catchments characteristics are considered. These approaches raise questions about the quality of the data rather than providing answers concerning the controlling processes. Secondly, we consider statistical approaches (cumulative number of events distribution and cluster analysis) and these suggest the possibility of having two distinct families of processes. However the quantitative evaluation of the threshold differs from the one that could be obtained from the first approach, but they all agree in the sense of the coexistence of two families of events. Thirdly, a conceptual model is built exploring how and why debris flow volume in alpine catchments changes with time. Depending on the initial condition (sediment production), the model shows that large debris flows (i.e. with important volume) are observed in the beginning period, before a steady-state is reached. During this second period debris flow volume such as is observed in the beginning period is not observed again. Integrating the results of the three approaches, two case studies are presented showing: (1) the possibility to observe in a catchment large volumes that will never happen again due to a drastic decrease in the sediment availability, supporting its difference from gradual erosion processes; (2) that following a rejuvenation of the sediment storage (by a rock avalanche) the magnitude–frequency relationship of a torrent can be differentiated into two phases, the beginning one with large and frequent debris flow and a later one with debris flow less intense and frequent, supporting the results of the conceptual model. Although the results obtained cannot identify a clear threshold between the two families of processes, they show that some debris flows can be seen as pulse of sediment differing from that expected from gradual erosion.

A probabilistic sediment cascade model of sediment transfer in the Illgraben

“…[]. A similarly shaped distribution of debris flows was also found by Bardou and Jaboyedoff [] for historical debris flows in Switzerland. This raises the question of what is controlling the shape of the debris flow distribution.…”

“…Several studies have investigated the production and transfer of sediment through the Illgraben [e.g., Bardou et al ., ; Bardou and Delaloye , ; McArdell et al ., ; Bardou and Jaboyedoff , ; Schlunegger et al ., ; Berger et al ., ; Schürch et al ., ]. In a previous study, we used digital photogrammetry to produce a record of erosion and deposition in the upper catchment between 1963 and 2005 [ Bennett et al ., ].…”

“…In addition to the record of sediment discharge, the probability distribution of landslide volumes for the catchment has been estimated [ Bennett et al ., ], and there are estimates of erosion and storage of sediment on the hillslopes and in the channel [ Berger et al ., ; Bennett et al ., ], as well as all necessary climate data. The hillslope‐channel cascade approach has been qualitatively described in the Illgraben by Bardou and Jaboyedoff [], including important debris flow generating mechanisms [ Bardou and Delaloye , ; Badoux et al ., ]. Importantly, previous studies in the catchment enable the independent estimation of the majority of model parameters such that calibration of the model does not involve extensive fine tuning.…”

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

“…, ), it seems logical to assume that debris‐flow magnitudes may also show heavy‐tailed behavior. Indeed, Bardou and Jaboyedoff () demonstrated a heavy‐tailed magnitude distribution for a compilation of historical debris flows from the Swiss Alps, while Bennett et al. () compiled observations from the Illgraben (Switzerland) that also show heavy‐tailed behavior.…”

Assessing sediment dynamics from geomorphological maps: Bruchi torrential system, Swiss Alps

“…• ), presence of water and connection of sediment supply zones to a main channel or a stream are the fundamental parameters acting on debris flow triggering (Zimmermann et al, 1997;McArdell et al, 2007;Bardou and Jaboyedoff, 2008). This paper presents applied geomorphological mapping, at various scales and with various legends, for a torrential system located on the right side of the upper Rhone river valley (Switzerland).…”