Spatio-temporal dynamics of sediment transfer systems in landslide-prone Alpine catchments
Authors: Clapuyt François , Vanacker Veerle , Christl Marcus , Van Oost Kristof , Schlunegger Fritz 
Published: September 2019

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.