Numerical modelling of channel adjustment in alluvial meandering rivers with riparian vegetation

Riparian vegetation is increasingly seen as an effective and environmentally acceptable tool in river management and river engineering, which necessitates a better understanding of the geomorphological impact of vegetation. This study examines the potential of a computational approach to address these issues, based on undertaking controlled experiments using a newly developed numerical model. The new model, MRIPA, combines an existing two-dimensional flow and sediment transport model with a geotechnical bank-stability analysis, and is further extended to account for the hydraulic and geotechnical effects of riparian vegetation. This coupled fluvial-geotechnical model is then calibrated and evaluated, before being applied to simulate a range of vegetation scenarios to investigate the influence of vegetation density and biophysical characteristics of vegetation on geomorphological change. Results show that riparian vegetation can have a considerable impact on the morphological evolution of river channels. The most pronounced effect is on channel planform evolution, with up to 60% reduction in floodplain area loss, depending on the species and density of the simulated vegetation. Bed topography is also affected, albeit to a lesser extent. Throughout the scenarios investigated herein, the impact of simulated woody species on bed topography change and bank retreat is consistently higher than the impact of simulated herbaceous species. Finally, and critically, it is found that the impact of vegetation is spatially variable, as the local effects of vegetation can propagate downstream due to the interactions between channel planform, flow and bed response. This spatial variability of the effects of vegetation highlights the complexity and intricacy of fluvial systems.