Laboratory Experiments of Bank Collapse: The Role of Bank Height and Near-Bank Water Depth

We set up a laboratory experiment to reproduce flow-induced bank erosion and bank collapse and to study the role of bank height (H_b) and near-bank water depth (H_w) on bank stability. Five laboratory experiments were conducted in a plexiglass-walled soil tank, using silt collected from natural tidal channel banks (D₅₀ = 75 μm). During each experiment, the bank was subject to a steady and uniform flow. We measured the variations in total soil stress, pore water pressure (when negative, called matric suction), and water content inside the bank and flow velocity and suspended-sediment concentration upstream and downstream of the bank. Results show that the experiments can reproduce four failure types commonly observed in nature including toppling, tensile and shear failures, and erosion and failure driven by loss of matric suction. The patterns of bank failure can be related to H_b/H_w. For large H_b/H_w (> = 2), we observe a cantilever-shape bank profile. For small H_b/H_w (<2), we first observe cracks on the bank top, followed by shear failures along a vertical or inclined surface separating the cantilever block up from the bank top. When accounting for our results in the context of previous experimental studies, we find a transition point characterized by a maximum normalized bank retreat rate. For toppling failures, we also find a positive correlation between the ratio H_b/H_w and the geometrical contribution to bank retreat from bank collapse (C_bc). Our research quantifies the role of H_b/H_w on bank collapse, bank retreat rate, and the overall C_bc.