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

Abstract

Abstract. Tectonic and geomorphic processes drive landscape evolution over different spatial and temporal scales. In mountainous environments, river incision sets the pace of landscape evolution, and hillslopes respond to channel incision 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 network during phases of hillslope–channel geomorphic coupling. The mechanisms and scales of sediment connectivity mitigate the propagation of sediment pulses throughout the landscape and eventually drive the contribution of landslides to the overall sediment budget of mountainous catchments. However, to constrain the timing of the sediment cascade, the inherent stochastic nature of sediment and transport through landsliding requires an integrated approach accounting for different space scales and timescales. In this paper, we examine the sediment production on hillslopes and evacuation to the river network of one landslide, i.e. the Schimbrig earthflow, affecting the Entle River catchment located in the foothills of the Central Swiss Alps. We quantified sediment 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 intensity of sediment redistribution on the hillslope. At the millennial 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 spatial extent. The discrepancy between decadal and millennial sediment fluxes, combined to the highly variable annual sediment evacuation from the hillslopes to the channel network suggest that phases of hillslope–channel geomorphic coupling 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 intense geomorphic activity of the Schimbrig catchment in a regional context. Its decadal sediment flux is of the same order 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 entire 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 landslides during intermittent phases of hillslope–channel geomorphic coupling are averaged out when considering sediment fluxes at longer timescales and larger spatial scales.

Modelling localized sources of sediment in mountain catchments for provenance studies

“…On the hillslopes, several mass movements like earthflows and landslides are present. The sediment produced by these processes is stored on the hillslopes for potentially long time periods and is only delivered to the river network when short-lived and rare episodes of hillslope-channel coupling take place, consisting mostly of superimposed debris flows (Schwab et al 2008;Clapuyt et al 2019). This mechanism indicates a transport-limited regime on the hillslopes.…”

“…This result is consistent with field observations by Schwab et al (2008), who proposed that in the Kleine Emme the connectivity between landslides on the hillslopes and the river network only takes place occasionally by activation of debris and earth flows. Clapuyt et al (2019) also argued that this coupling is switched on only rarely and for short periods of time, and therefore the contribution of hillslope-generated sediments to the annual sediment load is negligible most of the time. The hypothesis of dominant channel sediment production processes in the Kleine Emme basin is supported by qualitative geomorphological observations, such as the presence of river cut terraces and the lack of significant main river adjustment at the confluence of small tributaries.…”

“…In this work we chose to use a constant value, to explore its role in basin scale modelling. However, Clapuyt et al (2019) showed that episodic hillslope-channel coupling in specific landslide areas in the Kleine Emme basin are key to determining the seasonal sediment load contributions, even if often negligible at the annual scale. This suggests that a better representation of the hillslope-channel balance in the Kleine Emme could be obtained by using a temporally variable λ.…”

Modelling impacts of spatially variable erosion drivers on suspended sediment dynamics

“…Finally, the diverse geomorphology of the basin has been the subject of several studies, and long-term estimates of denudation rates are available (e.g. Schlunegger and Schneider, 2005;Schwab et al, 2008;Dürst Stucki et al, 2012;Van Den Berg et al, 2012;Clapuyt et al, 2019). Measurements of precipitation, air temperature and sunshine duration are available from automatic weather stations located inside or in the vicinity of the basin operated by MeteoSwiss.…”

“…The different topographic connectivity of the south-eastern and north-western regions reflects the different geomorphology of these two parts of the basin. In fact, the south-eastern region of the basin is characterized by a predominantly Last Glacial Maximum landscape with wide valleys and major instabilities, which are in most cases not directly connected to the river network (Schwab et al, 2008;Van Den Berg et al, 2012;Clapuyt et al, 2019). On the other hand, the north-western part of the basin, i.e.…”