Post-seismic sediment evacuation: from single landslide removal towards mountain range and global fluxes
Résumé
In active mountain ranges, earthquakes can trigger widespread landslides that mobilise large volumes of sediment. This sudden sediment delivery can cause downstream changes in river geometry and transport capacity that affect the river efficiency to export this sediment out of the epicentral area. The propagation of sediment from landslide deposits through the fluvial network has implications for the management of hazards downstream and for landscape dynamics over time scales of the seismic cycle. However, a full understanding of the processes and time scales associated with post-seismic landslide removal by bedload transport is still lacking. Here, we propose two numerical approaches to assess the evacuation time of landslide material at different spatial and time scales. First, we explore the river morphodynamic response to a landslide occurrence at the reach-scale using a 2D modelling approach. We use a simplified bedrock channel to study the influence of the volume of sediment pulse (VS) and channel transport capacity (QT) on the export time of landslides. Two regimes are identified: (i) the export time is linearly related to VS/QT when the sediment pulse does not affect significantly river hydrodynamics (low VS/QT) and (ii) the export time is a non-linear function of VS/QT when the pulse undergoes significant morphodynamic modifications during its evacuation (high VS/QT). In the latter case, active narrowing of the river within the landslide deposit is responsible for a significant increase in the transport capacity, resulting in faster evacuation times than predicted by theory. Secondly, we propose a statistical approach to quantify evacuation times of clusters of earthquake-triggered landslides at the scale of mountain ranges. Our approach combines an empirical description of co-seismic landslide clusters with the physical processes involved during the post-seismic phase and focuses on hypothetical Mw 8 earthquakes occurring on the Alpine Fault, New Zealand. In particular, we explore the role of landslide connectivity on the modulation of post-seismic sediment fluxes. We also show how this approach could shed light on sediment fluxes from active mountain ranges over longer timescales (i.e. several seismic cycles) and its potential to be coupled to a 2D landscape evolution model.