Zhao, Kun, Coco, Giovanni, Gong, Zheng, Darby, Stephen E., Lanzoni, Stefano, Xu, Fan, Zhang, Kaili, and Townend, Ian
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. As one of the most persistent and dominant processes that shape rivers, estuaries, and coastal regions, bank retreat is of great relevance to the management of water courses and the protection of wetlands. Here, we review bank retreat processes with respect to both rivers and tidal channels, including observations of erosion/failure processes and bank retreat rate, as well as methods proposed for numerical modeling. We highlight that different hydrodynamic forces associated with fluvial and estuarine settings may nevertheless have similar effects on bank stability. Using data from the literature we show that observed bank retreat rates are not only controlled by flow strength and soil composition (e.g., discharge, vegetation cover, and critical shear stress for erosion), but are also related to other factors, such as channel width and bank geometry. We summarize recent developments in bank retreat modeling, discuss their advantages and challenges, and propose some recommendations for their future use. Overall, this review deepens our understanding of bank erosion/collapse processes, and how they shape both fluvial and estuarine landscapes. Bank retreat is a key control on river and estuary morphodynamicsMulti‐variable predictors of bank retreat from the literature are systematically reviewedNumerical approaches to bank retreat are critically appraised, resulting in a series of recommendations Bank retreat is a key control on river and estuary morphodynamics Multi‐variable predictors of bank retreat from the literature are systematically reviewed Numerical approaches to bank retreat are critically appraised, resulting in a series of recommendations