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 (D 50 = 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 .Plain Language Summary 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.

Biofilm mediated intertidal sediments exhibit more complex erosional behaviour than abiotic systems. A major feature of intertidal systems is the exposure to repeated cycles of high and low shear created by tidal conditions and also less predictable episodic events, such as storms. There is very little information on how biofilm‐forming communities respond to these conditions. In this study, the effects of both single and repeated‐cycles of shear on the stability of newly developed bio‐sedimentary beds was examined. Cleaned sand, without any potential biostabilization, was used as the control. For the single‐cycle scenario, biofilms were incubated on a non‐cohesive sandy bed under prolonged low shear periods varying between 5 and 22 days, after which erosional stress was applied. No significant biostabilization was observed for the youngest bio‐sedimentary bed (after five days of low shear incubation). After 22 days, microbial communities were characterized by a firmly attached surface biofilm. To cause erosion, greater hydrodynamic stress (0.28 Pa) was required. The erosional behaviour of the underlying sand was also affected in that bedform ripples noted in the control system were no longer observed. Instead, a sudden ‘mass erosion’ took place (0.33 Pa). The one‐cycle scenario indicated that significant biostabilization of sand only occurred after a relative long calm period. Under repeated cycles of stress (five days of low stress followed by high stress event and re‐incubation, repeated for four cycles = 20 days), frequent cyclic disturbance did not degrade the system stability but enhanced biostabilization. The properties of the sub‐surface sediments were also affected where erosion rates were further inhibited. We hypothesize that organic material eroded from the bed acted as a ‘biofilm precursor’ supporting the development of new biofilm growth. A conceptual framework is presented to highlight the dynamics of bio‐sedimentary beds and the effects of growth history under repeated‐cycles. © 2019 John Wiley & Sons, Ltd.

A self-designed "bottom boundary layer hydrodynamic and suspended sediment concentration (SSC) measuring system" was built to observe the hy- large vertical velocity gradient and a "surge" feature. We conclude that the very shallow water stages are transient and may not contribute much to the whole water and sediment transport, while they can play a significant role in the formation and evolution of micro-topographies on tidal flats.

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Fe3O4/BiFeO3(001) superlattices comprising multiferroic BiFeO3 and ferrimagnetic half-metallic Fe3O4 have been investigated using first principles calculations. Two models were simulated: Model (a) contains the interfaces of Fe(A)−BiO and Fe2O4(B)−FeO2; Model (b) contains the interfaces of Fe(A)−FeO2 and Fe2O4(B)−BiO. The magnetization enhances 13% and 8% for models (a) and (b) due to the interfacial bonding between Fe(A)/Fe(B) and Bi atoms, respectively. The much larger enhancement in model (a) is ascribed to the facts that the Fe(A) atoms are surrounded by relatively less O atoms than Fe(B) in model (b), which increases the hybridization between Fe(A) and Bi atoms. The calculated results suggest that the number of oxygen atoms at the interfaces plays an important role on determining the interfacial coupling strength. Meanwhile, the interfacial bonding also affects the spin polarization of the Fe3O4 at the interface.