Effects of riparian and floodplain vegetation on river patterns and flow dynamics

Abstract

Flume experiments and field observations demonstrating the effects of vegetation on river planforms are reported in literature, but numerical studies of these effects at the river scale are lacking. We investigated the effects of vegetation in a 2D morphodynamic model using submodels for flow resistance and colonisation of newly formed deposits. In the vegetated areas, the flow resistance was divided into a resistance exerted by the bed and a resistance exerted by the plants, adopting Baptist’s formulation. In this way the model was able to reproduce the decrease in bed shear stress thereby reducing sediment transport capacity, and the increase in hydraulic resistance thereby reducing flow velocities. Colonisation was reproduced by assigning vegetation to the areas that became dry at low water stages. Bank accretion and erosion were reproduced as drying and wetting of the computational cells at the channel margins. The rate of bank erosion was related to the rate of bed degradation at the adjacent wet cells. The model was applied to a hypothetical case with the same characteristics as the Allier River near Moulins, France. The river was allowed to develop its own geometry starting from a straight and uniform channel. Different vegetation densities were found to produce different river planforms. At the highest density the flow concentrated in a single channel and the river formed incipient meanders. Without floodplain vegetation the river showed a clear braided pattern, with the formation of several bars. At low vegetation density the river showed a transition pattern, i.e. a low braiding degree with distinguishable meanders. The braiding degree was also influenced by overbank flows. Since both the river hydraulic regime and floodplain vegetation depend on changes such as climate, river damming and restoration, it is recommended that the impacts on river morphology and vegetation are assessed jointly in an integrated manner.