Studies of fluvial geomorphology should consider the essential roles played by plant communities, in addition to the usual geological and hydrological factors. Mobile-bed flume experiments were undertaken to investigate the effects of vegetation roots on the protection of sandbars from erosion in fluvial channels. Loose sandbars (i.e., containing only sand) and sandbars covered with taproot and fibrous-root vegetation types were used to assess the influence of vegetation on residual sandbar volume and channel erosion in the case of emergent, partly submerged, and submerged sandbars. Results indicate that vegetation roots effectively increase soil cohesion, reducing flow scouring. Fibrous root systems form a root net around sandbars, preventing morphological damage caused by external erosion at low flow rates. Taproots develop solid erosion-inhibiting structures within sandbars through their strong primary and lateral roots, effectively preventing internal scouring at high flow rates. Relative to loose sandbars, vegetated sandbars were 24 %, 121 %, and 222 % more protected from sediment erosion under emergent, partly submerged, and submerged conditions, respectively. The ratio of effective erosion protection increased with increasing discharge, with vegetation roots playing a key role in stabilizing sandbars, particularly under submerged conditions.

Channel retreat can be responsible for the significant loss of banks, farmland, and wetlands, leading to drastic changes in fluvial sediment and local river regimes. Although current studies focus on the erosion process of natural river channels, the mechanism by which revetments, such as the flexible mattress, influence bank evolution is still unclear. Hence, by conducting a generalized model experiment, this study investigates the point bar failure process under the mattress protection, i.e., the episodic event when the soil reaches the static equilibrium state. Specifically, a scour hole develops at the junction between the soft and hard materials, causing mattress suspension on the bank toe's side wall and resulting in a reduction coefficient for transverse scouring rate ranging from 0.08 to 0.15. Based on the theories of soil mechanics and river dynamics, the critical conditions for point bar instability were deduced, and a mechanical model describing its erosion process under the mattress protection was established. Furthermore, our model calculated the bank morphology and total erosion volume at different periods in the flume experiment, demonstrating a good agreement with the measured data. Additionally, variations in stability coefficient and forces exerted on soil (including shear strength, gravity, and fluid pressure) of the typical sections during point bar retreat were analyzed. Sensitivity analysis of the bank toe stability emphasized the controlling effect of soil mechanical properties and the negative feedback of mattress weight. The results reveal the interaction mechanism between the mattress protection and point bar failure, theoretically guiding the bank erosion strengthening and river management planning.