Export of earthquake-triggered landslides in active mountain ranges: insights from 2D morphodynamic modelling.
Abstract
In active mountain ranges, large earthquakes (Mw > 5-6) trigger numerous landslides that impact river dynamics.
These landslides bring local and sudden sediment piles that will be eroded and transported along the river network
causing downstream changes in river geometry, transport capacity and erosion efficiency. The progressive
removal of landslide materials has implications for downstream hazards management and also for understanding
landscape dynamics at the timescale of the seismic cycle. The export time of landslide-derived sediments after
large-magnitude earthquakes has been studied from suspended load measurements but a full understanding of the
total process, including the coupling between sediment transfer and channel geometry change, still remains an
issue. Note that the transport of small sediment pulses has been studied in the context of river restoration, but the
magnitude of sediment pulses generated by landslides may make the problem different. Here, we study the export
of large volumes (>106 m3) of sediments with the 2D hydro-morphodynamic model, Eros. This model uses a new
hydrodynamic module that resolves a reduced form of the Saint-Venant equations with a particle method. It is coupled
with a sediment transport and lateral and vertical erosion model. Eros accounts for the complex retroactions
between sediment transport and fluvial geometry, with a stochastic description of the floods experienced by the
river. Moreover, it is able to reproduce several features deemed necessary to study the evacuation of large sediment
pulses, such as river regime modification (single-thread to multi-thread), river avulsion and aggradation, floods and
bank erosion. Using a synthetic and simple topography we first present how granulometry, landslide volume and
geometry, channel slope and flood frequency influence 1) the dominance of pulse advection vs. diffusion during its
evacuation, 2) the pulse export time and 3) the remaining volume of sediment in the catchment. The model is then
applied to a high resolution (5-10 m) digital elevation model of the Poerua catchment in New Zealand which has
been impacted by the effect of a large landslide during the last 15 years. We investigate several plausible Alpine
Faults earthquake scenarios to study the propagation of the sediment along a complex river network.We characterize
and quantify the sediment pulse export time and mechanism for this river configuration and show its impact on
the alluvial plain evolution. Our findings have strong implications for the understanding of aggradation rates and
the temporal persistence of induced hazards in the alluvial plain as well as of sediment transfers in active mountain
belts.