The role of riparian vegetation flexibility in a bio‐hydro‐morphodynamic simulation

Abstract

AbstractBio‐hydro‐morphodynamic simulation is an essential tool for analyzing fluvial processes under the influence of riparian vegetation, and has developed over the past decade. However, most bio‐hydro‐morphodynamic models have neglected the flexibility of vegetation. The flow‐induced deformation of flexible vegetation changes the resistance to water flow. Therefore, flexibility affects water flow surrounding vegetation patches, and alters bed shear stress below and surrounding patches. To investigate the effects of vegetation flexibility in a fluvial, bio‐hydro‐morphodynamic simulation, we incorporated a model for predicting flexible vegetation reconfiguration in a bio‐hydro‐morphodynamic model based on Delft3D. We studied effects by considering and neglecting vegetation flexibility within a gravel bed river with alternate bars. The vegetation used in the simulation was Phragmites japonica, a widely existing reedy grass in riparian environments. Our results indicate that vegetation flexibility can impact river hydrodynamics and morphology development processes. When accounting for vegetation flexibility, water depth decreases, averaged Shields stress within a vegetation patch increases, and channel width with sediment transport increases. Erosion and deposition within the channel are additionally reduced. The significance of vegetation flexibility depends on multiple factors (i.e., the stem density growth rate, rigidity, and the river bed material). Given a fast stem density growth rate or a large rigidity, differences between flexible and rigid vegetation cases could be neglected. The difference between flexible and rigid vegetation was maximized when the bed gravel size fell within a certain range. Our results suggest that the impact of vegetation flexibility on river morphology depends on the mobility of river bed material. A very active or inactive bed reduced the importance of vegetation flexibility on river morphology.