A morphodynamic model of river meandering involving floodplain heterogeneity, vegetation dynamics, and bank collapse

Abstract:

Meanders are one of the most ubiquitous phenomena observed in natural rivers, characterized by a series of regular sinuous curves, bends, loops, turns, or windings. Meander migration results from the synergy between inner bank accretion and outer bank erosion/collapse. Despite the evidence of such morphodynamic behavior, little attention has been paid to the role that floodplain heterogeneity, vegetation dynamics, and bank collapse play on meander evolution. We develop a numerical model to simulate river meandering that involves floodplain heterogeneity, vegetation dynamics, and bank collapse. The stress-strain analysis is used to simulate bank collapse. Floodplain heterogeneity is expressed by clay content and varies due to the cutoff-induced scroll bars and oxbow lakes, which determines the erodibility coefficient and the critical shear stress for bank erosion, and soil cohesion. Vegetation dynamics are simulated as a function of the nearest distance to river bank, and the variation in biomass follows a logistic law and an exponential decay for the increase and decrease, respectively, consequent to river migration. The deposition coefficient for bank accretion and the critical tensile strength depend on the variation in biomass. Results show that as bend curvatures increase, the initially scattered bank collapse events converge towards the section where the bed shear stress attains a maximum. We suggest the presence of a catch-up behavior between inner and outer banks, driven by the intermittent bank collapse. Bank collapse significantly speeds up meander migration, and meanders seem to evolve towards a state where migration rates reach a maximum, agreeing well with field observations. The inclusion of floodplain heterogeneity and vegetation dynamics significantly affects meander evolution and the emergence of planform complexity, which in turn, generate more complicate distribution of clay content and biomass. Our study suggests that the interactions between bank collapse, vegetation dynamics, and soil heterogeneity plays a significant role on bend migration, and therefore on the evolution of meanders.