Autogenic knickpoints initiation related to downstream river width dynamic: Experimental approach.
Résumé
Upward propagation of knickpoints is known to reflect landscape disequilibrium in response to changes in boundary conditions such as tectonics, climate or base-level. However, it is suggested that some knickpoints may also form autogenically, solely from river system intrinsic process. Genesis and dynamics of such autogenic knickpoints were explored here through basin-scaled experimental modeling. The experiment consists on a 1.00 x 0.55 m box filled with silica grains, where one of the short sides goes down steadily as constant base-level fall while homogenous precipitation is applied on top of the surface. The experimental topography is digitized on a 5 min step basis to produce 1 mm squared grid DEMs thereafter used to extract hydraulic information such as water depth, water discharge and shear stress with the hydrodynamic model Floodos (Davy et al., 2017).We present here results from three experiments performed with similar precipitation rate but different rates of base-level fall. For the three experiments, knickpoints regularly initiate near catchment"s outlet and propagate through landscapes throughout the duration of experiments, despite steady boundary conditions. We show that their initiation near the outlet occurs from cycles in river narrowing/widening. River narrowing leads to an increase in shear stress and a knickpoint initiation. Once the knickpoint propagate upward, the river widens and the shear stress decreases, down to a new cycle of river narrowing, increasing shear stress and knickpoint initiation. We also show that the propagation rate of such autogenic knickpoints is not consistent with a stream power-based model, as it does not decrease monotonously through the experimental landscape. We propose a new model of knickpoints generation and propagation related to downstream river width dynamic that highlights the need to better consider and understand autogenic processes in landscapes and surface process models.Davy, P., Croissant, T., Lague, D., 2017. A precipiton method to calculate river hydrodynamics, with applications to flood prediction, landscape evolution models, and braiding instabilities: A Precipiton Method for River Dynamics. Journal of Geophysical Research: Earth Surface 122, 1491-1512.