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 (Hb) and near‐bank water depth (Hw) 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 Hb/Hw. For large Hb/Hw(> = 2), we observe a cantilever‐shape bank profile. For small Hb/Hw(<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 Hb/Hwand the geometrical contribution to bank retreat from bank collapse (Cbc). Our research quantifies the role of Hb/Hwon bank collapse, bank retreat rate, and the overall Cbc. The stability of river banks is a key process in river morphodynamics and of great relevance to river engineering and management. Progress in this area of research is hampered by the difficulty in obtaining measurements. Bank collapse is in fact a fast and often massive event, which is difficult to capture in the field. Using sediment collected next to a channel experiencing bank retreat, we set up a laboratory experiment to study and quantify the effect of the geometry of the channel on bank stability. Our experiments resulted in different types of bank collapse and indicate that the ratio between the height of the exposed part of the bank (bank height) and the height of the submerged part of the bank (near‐bank water depth) controls bank stability. The role of the ratio between bank height and near‐bank water depth has been long neglected, but our study shows that it plays a major role on bank collapse and retreat and so on the morphodynamic evolution of the entire channel. A laboratory experiment is designed to reproduce flow‐induced bank erosion and consequent bank collapseThe normalized bank retreat rate first increases and then decreases with increasing ratio between bank height and near‐bank water depthOur experiments show that bank retreat is dominated by bank collapse rather than flow‐induced bank erosion