Catastrophic sediment release in fluvial systems is largely driven by landsliding, which occurs naturally during large earthquakes (e.g. November 2016 Mw 7.8 Kaikōura earthquake, New Zealand) or climatic events in mountain belts (e.g. October 2020 Alex storm, France). Landslides are geological hazards that occur worldwide and landslide debris, such as sediment of heterogeneous grain sizes, cascade from hillslope to lake or sea where they are preserved in the stratigraphy. Sedimentary records can reveal crucial information on paleo-climate and tectonics and thus contribute to unravelling what happens upstream in catchments. However, environmental signals of such forcing, i.e. changes in sediment production, transport and deposition, are difficult to identify since sediment transport modifies the initial signals. Spatial and temporal scales are key factors that make such signals detectable in sedimentary records. Non-linear sediment transport leads to signal shredding and even to its loss depending on the system length, and signal frequency and amplitude. Landslides cause hazards such as aggradation and the formation of fans and floodplains. Therefore, it is important to develop a method that copes with buffered, incomplete and shredded signals to recognise landslides in sedimentary records and thus to better understand past extreme events that triggered them and their amplitude and frequency.
Despite more frequent and severe extreme events worldwide, only few studies connect landslide and reservoirs and none directly unravels how landslide signals propagate through a river system and appear in sedimentary records, which is the aim of the SCALEES (Signature of sediment CAscades following Landslides triggered by Extreme Events in the Stratigraphy) project . We identify that the lack of numerical models able to fully describe the entrainment, transport and deposition of multiple grain sizes and the related channel morphodynamics is a key bottleneck in progressing on this topic.
The specific research objectives of the SCALEES project were: 1) to assess how considering multi-grain sized sediments affects the magnitude and duration of fluvial morphodynamic response (change in grain size and topography) to post-earthquake increases in sediment supply; 2) to predict the signature (amplitude and grain size) of landslides induced by catastrophic events in lacustrine sedimentary records and to identify parameters that control the landslide signal propagation through a river system, and, 3) to predict the signature of landslides induced by catastrophic events in fluvial sedimentary records, to assess how vegetation affects this signature and to determine the role played by landslide in the dynamics of alluvial fans, floodplains and terraces.
