Erosion mechanism of point bar retreat under the protection of a flexible mattress

•The impacts of flexible mattress protection structure on fluvial erosion and channel evolution were investigated.•Point bar failure under the mattress protection is an episodic event reaching the static equilibrium state.•A mechanical model was proposed for point bar retreat covered by the flexible mattress. 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.