Mapping and quantifying sediment transfer between the front of rapidly moving rock glaciers and torrential gullies

The sedimentary connection which may occur between the front of active rock glaciers and torrential channels is not well understood, despite its potential impact on the torrential activity characterizing the concerned catchments. In this study, DEMs of difference (DoDs) covering various time intervals between 2013 and 2016 were obtained from LiDAR-derived multitemporal DEMs for three rapidly moving rock glaciers located in the western Swiss Alps. The DoDs were used to map and quantify sediment transfer activity between the front of these rock glaciers and the corresponding underlying torrential gullies. Sediment transfer rates ranging between 1500 m3/y and 7800 m3/y have been calculated, depending on the sites. Sediment eroded from the fronts generally accumulated in the upper sectors of the torrential gullies where they were occasionally mobilized within small to medium sized debris flow events. A clear relation between the motion rates of the rock glaciers and the sediment transfer rates calculated at their fronts could be highlighted. Along with the size of the frontal areas, rock glacier creep rates influence thus directly sediment availability in the headwaters of the studied torrents. The frequency-magnitude of debris flow events varied between sites and was mainly related to the concordance of local factors such as topography, water availability, sediment availability or sediment type

Erosion and sediment transfer processes at the front of rapidly moving rock glaciers: Systematic observations with automatic cameras in the western Swiss Alps

When connected to torrential channels, the fronts of active rock glaciers constitute important sediment sources for gravitational transfer processes. In this study, a 2013– 16 time series of in situ webcam images from the western Swiss Alps was analyzed to characterize the erosion processes responsible for sediment transfer at the front of three rapidly moving rock glaciers and their temporal behavior. The main erosion processes comprised rock fall, debris slide, superficial flow and concentrated flow. These processes were induced by (i) changes of the frontal slope angle produced by rock glacier advance, and (ii) increases in water content of the sediments at the rock glacier front due to melt processes and rainfall. Erosion almost ceased during winter, when the front was frozen and snow‐covered. The onset of snowmelt triggered an active period of high‐frequency erosion events. After the melt period, sediment transfer continued as occasional rock falls, while other erosion processes occurred only during or following rainfall events. Intense regressive erosion phases that triggered debris flows were rare and occurred when enhanced snowmelt and/or recurring rainfall induced substantial groundwater flow on the debris slopes directly below the rock glacier fronts

Scenario building and runout modelling for debris flow hazards in pro-/periglacial catchments with scarce past event data: application of a multi-methods approach for the Dar catchment (western Swiss Alps)

In high mountain areas, the disposition (susceptibility of occurrence) for debris flows is increasing in steep terrain, as – due to climate change – rapid glacier retreat and permafrost degradation is favouring higher availability of loose sediments. The probability of occurrence and magnitude of pro- and periglacial debris flows is increasing, too, as triggering events such as heavy thunderstorms, long-lasting rainfalls, intense snow melt or rain-on-snow events are likely to occur more often and more intensely in future decades. Hazard assessment for debris flows originating from pro- and periglacial areas is thus crucial but remains challenging, as records of past events on which local magnitude-frequency relationships and debris flow scenarios can be based on are often scarce or inexistent. In this study, we present a multi-methods approach for debris flow hazard scenario building and runout modelling in pro- and periglacial catchments with scarce past event data. Scenario building for the debris flow initiation zone reposes on (i) the definition of meteorological and hydrological triggering scenarios using data on extreme point rainfall and precipitation-runoff modelling, and (ii) the definition of bed load scenarios from empirical approaches and field surveys. Numerical runout modelling and hazard assessment for the resulting debris flow scenarios is carried out using RAMMS-DF, which was calibrated to the studied catchment (Le Dar, western Swiss Alps) based on the area of debris flow deposits from the single major event recorded there in summer 2005. The developed approach is among the first to propose systematic scenario building for pro- and periglacial debris flows triggered by precipitation dependent events