Effects of fluvial instability on the bed morphology in vegetated channels

Flume experiments are conducted to investigate the effect of streambed instability in channels with randomly-distributed vegetation, varying vegetation density and flow conditions, in the absence of upstream sediment supply. The bed morphology is captured with the photogrammetry technique and a Laser Scanner, and its changes with the vegetation and flow conditions are investigated. The results demonstrate that the presence of vegetation contributes in promoting the stability of the streambed and the formation of multiple bars. In runs with low vegetation density, the trajectory of sediment transport is predominantly in the longitudinal direction. However, a slight lateral dispersion of sediments is observed in the run with low flow discharge. By increasing the vegetation density, the bed structures become shorter, with a lower wavelength, than before, but with a similar trend. The analysis of the energy spectra and the high-order generalized structure functions of bed elevation fluctuations demonstrates that the bed surfaces are monofractals and can be described by a single exponent. However, the runs affected also by a lateral dispersion of sediments during the sediment transport phase are characterized by multifractality, which implies that a complex bed morphology at small spatial lags occurs at the end of these runs. The study of the two-dimensional (2D) second-order structure functions demonstrates that the bed is characterized by an anisotropic behavior, with flow-aligned bed structures that reflect the way in which the bed was formed.