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

Clapuyt, François; Vanacker, Veerle; Christl, Marcus; Van Oost, Kristof; Schlunegger, Fritz (2019). Spatio-temporal dynamics of sediment transfer systems in landslide-prone Alpine catchments. Solid Earth, 10(5), pp. 1489-1503. Copernicus Publications 10.5194/se-10-1489-2019

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Tectonic and geomorphic processes drive land- scape evolution over different spatial and temporal scales. In mountainous environments, river incision sets the pace of landscape evolution, and hillslopes respond to channel in- cision by, e.g., gully retreat, bank erosion, and landslides. Sediment produced during stochastic landslide events leads to mobilization of soil and regolith on the slopes that can later be transported by gravity and water to the river net- work during phases of hillslope–channel geomorphic cou- pling. The mechanisms and scales of sediment connectiv- ity mitigate the propagation of sediment pulses throughout the landscape and eventually drive the contribution of land- slides to the overall sediment budget of mountainous catch- ments. However, to constrain the timing of the sediment cas- cade, the inherent stochastic nature of sediment and transport through landsliding requires an integrated approach account- ing for different space scales and timescales. In this paper, we examine the sediment production on hillslopes and evac- uation to the river network of one landslide, i.e. the Schim- brig earthflow, affecting the Entle River catchment located in the foothills of the Central Swiss Alps. We quantified sedi- ment fluxes over annual, decadal, and millennial timescales using respectively unmanned aerial vehicle (UAV)–structure- from-motion (SfM) techniques, classic photogrammetry, and in situ produced cosmogenic radionuclides. At the decadal scale, sediment fluxes quantified for the period 1962–1998 are highly variable and are not directly linked to the inten- sity of sediment redistribution on the hillslope. At the millen- nial scale, landslide occurrence perturbs the regional positive
linear relationship between sediment fluxes and downstream distance as the landslide-affected Schimbrig catchment is characterized by a decrease in sediment fluxes and a strong variability. Importantly, the average decadal sediment flux of the Schimbrig catchment is 2 orders of magnitude higher than millennial sediment fluxes computed over the same spa- tial extent. The discrepancy between decadal and millennial sediment fluxes, combined to the highly variable annual sed- iment evacuation from the hillslopes to the channel network suggest that phases of hillslope–channel geomorphic cou- pling are short and intermittent. During most of the time, the first-order catchments are transport-limited and sediment dynamics in the headwaters are uncoupled from the fluvial systems. In addition, our unique spatio-temporal database of sediment fluxes highlights the transient character of the in- tense geomorphic activity of the Schimbrig catchment in a regional context. Its decadal sediment flux is of the same or- der of magnitude as the background sediment flux going out of the entire Entle River catchment. Over the last 50 years, the Schimbrig catchment, which represents ca. 1 % of the en- tire study area, provides 65 % of the sediments that the entire Entle catchment will supply over the millennial scale. These results suggest that episodic supply of sediment from land- slides during intermittent phases of hillslope–channel geo- morphic coupling are averaged out when considering sedi- ment fluxes at longer timescales and larger spatial scales.

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