Unravelling earth flow dynamics with 3-D time series derived from UAV-SfM models

Abstract

Abstract. Accurately assessing geo-hazards and quantifying landslide risks in mountainous environments are gaining importance in the context of the ongoing global warming. For an in-depth understanding of slope failure mechanisms, accurate monitoring of the mass movement topography at high spatial and temporal resolutions remains essential. The choice of the acquisition framework for high-resolution topographic reconstructions will mainly result from the trade-off between the spatial resolution needed and the extent of the study area. Recent advances in the development of unmanned aerial vehicle (UAV)-based image acquisition combined with the structure-from-motion (SfM) algorithm for three-dimensional (3-D) reconstruction make the UAV-SfM framework a competitive alternative to other high-resolution topographic techniques.In this study, we aim at gaining in-depth knowledge of the Schimbrig earthflow located in the foothills of the Central Swiss Alps by monitoring ground surface displacements at very high spatial and temporal resolution using the efficiency of the UAV-SfM framework. We produced distinct topographic datasets for three acquisition dates between 2013 and 2015 in order to conduct a comprehensive 3-D analysis of the landslide. Therefore, we computed (1) the sediment budget of the hillslope, and (2) the horizontal and (3) the three-dimensional surface displacements. The multitemporal UAV-SfM based topographic reconstructions allowed us to quantify rates of sediment redistribution and surface movements. Our data show that the Schimbrig earthflow is very active, with mean annual horizontal displacement ranging between 6 and 9 m. Combination and careful interpretation of high-resolution topographic analyses reveal the internal mechanisms of the earthflow and its complex rotational structure. In addition to variation in horizontal surface movements through time, we interestingly showed that the configuration of nested rotational units changes through time. Although there are major changes in the internal structure of the earthflow in the 2013-2015 period, the sediment budget of the drainage basin is nearly in equilibrium. As a consequence, our data show that the time lag between sediment mobilization by landslides and enhanced sediment fluxes in the river network can be considerable.

Monitoring the crisis of a rock glacier with repeated UAV surveys

“…These distances are not vertical (i.e. as in DEM differencing), as they account for the local surface roughness and locally variable slope, thus providing a more realistic thickness or surface change (Clapuyt et al, 2017;Lague et al, 2013). Additionally to the distance calculation, M3C2 provides spatially variable confidence intervals that help to assess the areas in which changes are statistically significant (i.e.…”

“…The rate of mass transfer can be variable through time and space and depends on multiple factors, such as differential sediment evacuation or accumulation rates, 3-D deformation through either compressive or extensive flow regimes and changes in the rheological behaviour due to permafrost thawing or warming of ice (Kääb et al, 2007). Rock glacier velocities have increased considerably over the last four decades in places such as the European Alps (Delaloye et al, 2010;Roer et al, 2008), Alaska (Daanen et al, 2012) and the Tien Shan mountain range (Sorg et al, 2015). Since the last decade, and due to the high density of systematic in situ measurements, several rock glaciers in the European Alps have been identified as destabilised and significant changes in their morphology have been observed (Bodin et al, 2017;Lambiel et al, 2017;Roer et al, 2008).…”

Spatio-temporal dynamics of sediment transfer systems in landslide-prone Alpine catchments

“…Within the Entle River catchment, an earthflow named Schimbrig has been particularly active over the last 150 years, acting as a sediment factory by excavating and mobilizing sediments because of its deep rotational structure (Clapuyt et al, 2017;Lopez-Saez et al, 2017;Savi et al, 2013;Schwab et al, 2008). It is located in the first-order Schimbrig catchment, the latter draining successively into the Rossloch River and the Kleine Entle, before entering the trunk river, i.e.…”

“…The earthflow occurs on the hillslopes of the Schimbrig ridge and is not directly connected to the Schimbrig stream, except during short episodes when superimposed debris flows occur (Schwab et al, 2008). The Schimbrig earthflow consists of a fine-grained matrix of silt and mud, with centimetric to decimetric large clasts (Clapuyt et al, 2017). The internal structure of the earthflow is complex with nested rotational units (Clapuyt et al, 2017).…”

“…Because landslides are a dominant source of sediments in mountainous environments (Korup et al, 2010;Vanacker et al, 2003), one can expect that the magnitude and frequency of landsliding (e.g. Crozier and Glade, 1999;Hovius et al, 1997;Malamud et al, 2004) will directly impact the bulk sediment flux of a drainage basin. Nevertheless, the contribution of landslides to the overall sediment budget is still poorly constrained: landslides stochastically supply sediment to the river network, and their geomorphic efficiency varies according to the mechanisms and scales of sediment connectivity (Benda and Dunne, 1997;Bennett et al, 2014).…”