Application of a coupled model of channel width evolution and multi-grain size sediment transport to predict the full signal (all grain sizes) of sediment cascades preserved in stratigraphy

Catastrophic sediment release in fluvial systems is largely driven by landsliding that occurs naturally in mountain belts during extreme events, such as earthquakes or storms. Sediments are routed through the river system until they are stored either permanently in alluvial fans and lakes or temporarily in floodplains. The river response to such catastrophic sediment release has already been studied with 2D numerical models using a single effective grain size. Yet, in natural systems, sediment grain-size distributions can span several orders of magnitude and evolve during transport.

We present the new multi-grain-size sediment transport and sorting model SEDSCAPE (Sediment Entrainment Deposition and Storage based on the Concepts of Accessibility and Partial Equilibrium). This model was developed to predict how sediments of heterogeneous sizes that originate from landslides triggered by extreme events such as earthquakes and storms propagate through a river system. As modelling the 3D response of a river reach is computationally challenging, we couple SEDSCAPE with STRIMM (Lague, 2010), a model of river width evolution, to provide a simplified 2.5D approach. In turn, we can predict both morphodynamic changes and the full spectrum of sediment fluxes towards the floodplain and at the river outlet, and thus the sedimentary records in these locations.

We conducted numerical simulations of a constricted river reach consisting of a straight channel with floodplains on both sides. A time series of sediment and water discharges was applied to predict the response of a river reach affected by a landsliding event over several months to years. For comparison purposes, similar simulations were conducted with a single grain size.

Numerical simulations reveal: i) how different levels of sediment mobilization (water discharge) control sediment sorting processes and in turn sediment fluxes, ii) how the grain-size specific signals propagate in a river reach and are preserved in the channel and floodplain stratigraphy in response to a catastrophic sediment release, and ii) how the channel width adjusts with stochastic flow conditions and sediment supply.

The comparison between single- and multiple-grain-size simulations highlights the relevance of the multiple-grain-size approach to predict the response of a river reach to a catastrophic sediment release. Indeed, only the multi-grain-size approach is able to capture the hysteresis of transport, and different hysteresis patterns are obtained depending on the grain size as they have heterogeneous levels of mobilization and are not affected by sorting processes in the same way.

These results were obtained in the context of the SCALEES (Signature of sediment CAscades following Landslides triggered by Extreme Events in the Stratigraphy) project funded by the European Union. One key outcome of this project is the development of numerical models that will allow us to predict the full signal (all grain sizes) of sediment cascades preserved in stratigraphy in response to an extreme event. It will also pave the way for inverting the stratigraphic record of landslide-induced sediment cascades for quantitative insights into their response amplitudes and relaxation times.