Biomorphodynamics of river banks in vegetated channels with self-formed width

Laboratory and field studies investigating the mutual interaction between riparian vegetation dynamics and river morphodynamics have revealed that riparian vegetation may play an important role in the evolution of channel beds and river banks. In order to disentangle this still debated question, field and modeling techniques have helped to explore and better understand the time and spatial scales of such processes. Simple morphodynamic models for river evolution have typically used a constant discharge to describe in-channel processes and basic relationships for river bank dynamics. In order to overcome these limits we propose a longitudinally integrated dynamical model that describes the bank pull - bar push mechanisms in channels with symmetric cross section. Different hydrographs (constant, periodic and stochastic discharge) are applied to investigate channel width and vegetation biomass evolution trajectories and equilibrium values. Results show the interplay of riparian vegetation and water flow in controlling channel width evolution and the trajectories of channel adjustment to flow perturbations. These results also highlight the limit of adopting a constant discharge when describing mutual flow and vegetation processes affecting channel evolution. In addition, under stochastic forcing, the model shows the existence of a range of flood frequencies for which the cooperation between the hydrologic time scales and that characterizing vegetation colonization induces a regular pattern in channel width time variations (coherence resonance). Finally, model application to real case studies confirm the possibility to use the model to interpret long-term river evolutionary trajectories in realistic applications.