Erosion dynamics modelling in a coupled catchment–fan system with constant external forcing

Recent alluvial fan models have suggested that deep alluvial fan entrenchment could occur without any change in sediment and water influx. Moreover, other studies have shown that the evolution of a fan could strongly depend on feedback between the fan and the mountain catchment. We evaluate if natural entrenchment still occurs in a coupled catchment–fan system, and we evaluate its possible impact on the evolution of mountain erosion. We use a landscape evolution model where the mountain corresponds to an uplifting block and where fans form over an initial horizontal surface. Our experiments confirm that deep entrenchment at the fan apex can occur without a change in climate or in uplift rate under two conditions: (1) the transport threshold (critical shear stress) is significant and (2) the downstream boundary condition corresponds to an open boundary with fixed elevation, which stops the progradation of alluvial fans. A stable entrenchment occurs when sediments reach this limit. The entrenchment can be explained by a strong nonlinear dependence of the transport law on the slope when the shear stress is close to critical. A fan slope increase occurs when sediments reach the model limit, and this drives entrenchment. Fan entrenchment drives strong erosion in the mountain with intensity and a response time similar to those observed for the initial mountain uplift. These results indicate that determining how much of natural erosion is from autogenic mechanisms is essential in order to link landscape entrenchment with past external changes.