Coupling a channel width evolution model and a multi-grain size sediment transport model: a simplified approach to predict the response of a river reach to a catastrophic sediment release

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, the sediment grain size distribution can span several orders of magnitude and evolves during transport.

The role played by the grain size distribution on morphodynamics depends on transport modes and on grain size interactions. On one hand, fine sediments that tend to be transported in suspension and thus higher in the water column than coarse sediments contribute to floodplain formation and maintenance. On the other hand, coarse sediments that tend to be immobile or transported as bed load contribute to armouring of the channel bed surface that prevents its degradation and in turn leads to channel widening.

Assuming a single effective grain size may limit accurate forecasting of morphodynamic and sedimentological changes in rivers systems during landslide-induced sediment cascades. Modelling the response of a river reach in 3D, meaning that both morphodynamics (2D) and stratigraphy (1D) are resolved may be challenging due to computational time and computer memory. To cope with these limitations, we propose a 2.5D numerical model as a simplified approach. It incorporates: i) a multi-grain size sediment transport model with the ability to capture the transport of suspended and bedload material as well as the dispersion rate and sediment sorting patterns of various grain sizes such as armouring and downstream fining (threshold of motion and explicit grain-size specific entrainment and deposition rates), ii) an explicit transfer of sediment from the river channel to adjacent floodplains (based on the vertical distribution in the water column), iii) freely evolving channel width and slope, and iv) an algorithm to handle channel and floodplain sedimentary records (stratigraphic layers).

We conducted numerical simulations on a constricted river reach that consists of a straight channel with a floodplain on both sides. Numerical simulations reveal: i) 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.

These preliminary 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. The combination of empirical data with numerical simulations will allow us to predict for the first time the full signal (all grain sizes) of sediment cascades preserved in stratigraphy in response to an extreme event at the scale of a catchment. 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.