A Review on Bank Retreat: Mechanisms, Observations, and Modeling
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Abstract:
Abstract Bank retreat plays a fundamental role in fluvial and estuarine dynamics. It affects the cross‐sectional evolution of channels, provides a source of sediment, and modulates the diversity of habitats. Understanding and predicting the geomorphological response of fluvial/tidal channels to external driving forces underpins the robust management of water courses and the protection of wetlands. Here, we review bank retreat with respect to mechanisms, observations, and modeling, covering both rivers and (previously neglected) tidal channels. Our review encompasses both experimental and in situ observations of failure mechanisms and bank retreat rates, modeling approaches and numerical methods to simulate bank erosion. We identify that external forces, despite their distinct characteristics, may have similar effects on bank stability in both river and tidal channels, leading to the same failure mode. We review existing data and empirical functions for bank retreat rate across a range of spatial and temporal scales, and highlight the necessity to account for both hydraulic and geotechnical controls. Based on time scale considerations, we propose a new hierarchy of modeling styles that accounts for bank retreat, leading to clear recommendations for enhancing existing modeling approaches. Finally, we discuss systematically the feedbacks between bank retreat and morphodynamics, and suggest that to move this agenda forward will require a better understanding of multifactor‐driven bank retreat across a range of temporal scales, with particular attention to the differences (and similarities) between riverine and estuarine environments, and the role of feedbacks exerted by the collapsed bank soil.Keywords:
Beach morphodynamics
Bank erosion
Bank
Temporal scales
Bank failure
Bank erosion consists of two processes: basal erosion due to fluvial hydraulic force and bank failure under the influence of gravity. Because bank resistance force varies with the degree of saturation of bank material, the probability of bank failure is the probability of the driving force of bank failure being greater than the bank resistance force. The degree of saturation of bank material increases with river stage; therefore, the frequency of bank failure is correlated to the frequency of flooding. Consequently, the rate of bank erosion is due to both basal erosion and bank failure, and bank failure is a probabilistic phenomenon. In this paper, for cohesive bank material experiencing planar bank failure, a deterministic approach was adopted for basal erosion analysis, whereas the probability of bank failure was included in the analysis of bank failure. A method for calculating the rate of bank erosion was derived that integrates both basal erosion and bank failure processes, and accounts for the effects of hydraulic force, bank geometry, bank material properties, and probability of bank failure.
Bank erosion
Bank failure
Bank
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The paper presents a study on non-cohesive and cohesive homogenous bank failure processes,in-fluence of bank failure mass on bank re-collapse as well as the interaction between bank failure and bedevolution through a series of experiments carried out in a 180° bend rectangular flume. The results revealthe iteration cycle between bank erosion and bed deformation;basal erosion due to fluvial hydraulic force,bank failure under the influence of gravity,failure block staying at bank toe temporarily or being remobi-lized by the flow,exchange between bank failure mass and bed material,bed-material load being reworkedor transported either as bed load or as suspended load,and bed deformation. Same as bank failure,themixing of bank failure mass and bed material is more serious near the curved flow apex. Moreover,non-co-hesive bank failure tends to occur near the water surface,while cohesive bank failure near the bank toe.For non-cohesive material,the bank erosion amount and residual amount of bank failure mass on the bedincrease with the near-bank velocity or bed movability. But for cohesive material,only bank erosion amountobeys the above rule. In addition,the research indicates that the mechanism and patterns of bank collapseand interaction between bank failure and bed evolution,which could provide a theoretical basis for further study.
Bank erosion
Bank failure
Bank
Flume
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