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Understanding the interactions between dune systems and beaches is critical to determining the short-term shoreline response and the long-term resilience. In this study, almost 15 years of monthly beach/dune measurements were analyzed for three different profiles at Vougot Beach, France to understand and predict shoreline changes from intra-annual to multi-annual time-scales. Four migration modes: advance/retreat (translation modes) and steepening/flattening (rotation modes) were identified through a centroid analysis. The analysis showed that translation and rotation can occur simultaneously, with long-term trends of beach retreat and profile steepening (lower beach retreating and upper beach advancing), which was interrupted by two energetic wave events causing profile flattening (lower beach advancing and upper beach retreating). These two observations are evidence of how the sediment contribution resulting from the dune erosion events temporarily caused a large advance in the shoreline position. A recent modeling approach that accounts for different time-scales is applied to predict the shoreline changes, showing significant improvements in comparison to a traditional shoreline equilibrium model when time-scales related with the dune erosion and recovery are considered. The results showed that the dune system affects the beach profile evolution both spatially, with different impacts at different elevations along the cross-shore profile, and temporally, by periodically redistributing the sediment in the system. [ABSTRACT FROM AUTHOR]

Overhanging riverbank stability is a key feature of bank retreat. Although different mechanisms of bank collapse have been studied by many researchers, prediction of cantilever failure patterns and understanding of three‐dimensional failure surfaces continue to puzzle scientists. To address this problem, we conducted laboratory experiments and numerical simulations to investigate the stability of overhanging riverbank. In the laboratory experiments, we analyzed the characteristics of failure surface using a three‐dimensional laser scanner. Results show that with decreased ratio between bank height and near‐bank water depth, there is a transition from a 'tensile failures followed by toppling failure' pattern to a 'shear failure' pattern. Flow infiltration in our experiments leads to a sharp decrease in soil shear strength and consequently the occurrence of shear failure. For toppling failure, the observed upper retreat distance is much greater than the lower undermining depth, thus challenging the widely adopted assumption of a constant failure surface along the endpoint of the cantilever. Also, we found that failure surface distance can be characterized by twice the width of the overhanging block. As for the failure surface angle, a negative linear correlation is proposed with respect to bank retreat distance, implying a distinct distribution of bank stability along the arched‐shape bank line induced by bank collapse events. Overall, our study deepens understanding of overhanging stability, providing a new perspective on bank retreat process, critical for the morphodynamics of rivers and estuaries. Plain Language Summary: Overhanging bank stability is a key process of river morphological dynamics, which plays an effective role in bank retreat and soil loss in multi‐layered riverbanks. Therefore, it significantly affects river engineering and management. Because of the difficulty in obtaining measurements, predicting large‐scale bank collapse and reducing economic losses are still hard to tackle. To this end, we conducted laboratory and numerical studies to better understand the mechanism of cantilever failure. Both laboratory and numerical results indicate that an increase in bank height or decrease in water depth causes the shift of bank failure mechanism from shear to toppling failure. By using a three‐dimensional laser scanner, we analyzed the characteristics of failure surface which imply a distinct distribution of bank stability along the collapse‐induced arched‐shape bank line. Overall, this study provides a new perspective for the understanding of overhanging stability, which should be considered for better prediction and guiding realistic protection. Key Points: Shear‐ or toppling‐type cantilever failure is related to the ratio between bank height and near‐bank water depthThe distance of the toppling failure surface to the bank line is around twice the width of the overhanging block3D structures of failure surfaces indicate a negative linear correlation between failure surface angle and bank retreat distance [ABSTRACT FROM AUTHOR]

Reliable predictions and accompanying uncertainty estimates of coastal evolution on decadal to centennial time scales are increasingly sought. So far, most coastal change projections rely on a single, deterministic realization of the unknown future wave climate, often derived from a global climate model. Yet, deterministic projections do not account for the stochastic nature of future wave conditions across a variety of temporal scales (e.g., daily, weekly, seasonally, and interannually). Here, we present an ensemble Kalman filter shoreline change model to predict coastal erosion and uncertainty due to waves at a variety of time scales. We compare shoreline change projections, simulated with and without ensemble wave forcing conditions by applying ensemble wave time series produced by a computationally efficient statistical downscaling method. We demonstrate a sizable (site‐dependent) increase in model uncertainty compared with the unrealistic case of model projections based on a single, deterministic realization (e.g., a single time series) of the wave forcing. We support model‐derived uncertainty estimates with a novel mathematical analysis of ensembles of idealized process models. Here, the developed ensemble modeling approach is applied to a well‐monitored beach in Tairua, New Zealand. However, the model and uncertainty quantification techniques derived here are generally applicable to a variety of coastal settings around the world. Key Points: We assess uncertainty of ensemble Kalman filter data‐assimilated shoreline change predictions using the spread of model trajectoriesWe demonstrate an increase in modeled shoreline uncertainty due to ensemble wave conditions compared with single realization simulationsUncertainty in shoreline predictions can be quantified using a novel, straightforward relationship, derived analytically [ABSTRACT FROM AUTHOR]

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/Hw and the geometrical contribution to bank retreat from bank collapse (Cbc). Our research quantifies the role of Hb/Hw on bank collapse, bank retreat rate, and the overall Cbc. 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

The process of completing and communicating rigorous, high‐impact science depends heavily on obtaining quality peer review prior to publication. As AGU journals strive continually to publish excellent work, we recognize the demand that this places on reviewers' time, especially as most of us seem to receive ever‐increasing numbers of requests to review manuscripts. JGR‐Earth Surface typically strives to obtain three reviews per manuscript, to best ensure the quality of the science that we publish. We thank the members of the earth‐surface community who volunteered their time to complete these reviews for JGR‐Earth Surface in 2018: a total of 874 reviews provided by 650 scientists. We greatly appreciate your time, your thoughtful and constructive review comments, and especially your dedication to this all‐important part of producing high‐quality science. Key Points: The Editors thank the 2018 peer reviewers [ABSTRACT FROM AUTHOR]

We set up a two‐dimensional modeling framework to explore the development and the equilibrium configuration of a tidal channel. We consider two types of morphodynamic modeling systems coupling different flow and sedimentation models, to study the feedback mechanisms controlling the morphodynamics of tidal channels. The simulations indicate that the tidal channel first experiences a pseudo equilibrium state, then diverges from it, and finally approaches a dynamic equilibrium. The reason for this behavior is demonstrated analytically using linear perturbation analysis. We identify distinct positive and negative feedbacks that lead the tidal system to a dynamic equilibrium. One of the feedback mechanisms is controlled by the flow momentum balance, while the other one depends on the overall sediment balance. The dynamic equilibrium configurations obtained in the numerical simulations compare favorably with the theoretical equilibrium. Deviations from theory can be explained analyzing the assumptions in the flow and sedimentation models. Moreover, the minimum width‐to‐depth relationship of a tidal channel cross section is derived and is found to be in agreement with field measurements. Key Points: Tidal channel formation is simulated through a simplified two‐dimensional modelWe identify positive and negative feedbacks leading to dynamic equilibriumA width‐to‐depth relationship for tidal channels is derived and is consistent with observations [ABSTRACT FROM AUTHOR]

Subtidal flows driven by density gradients affect the tide‐averaged sediment transport in estuaries and, therefore, can influence their long‐term morphodynamic evolution. The three‐dimensional Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport modeling system is applied to numerically analyze the effects of baroclinicity and Earth's rotation on the long‐term morphodynamic evolution of idealized funnel‐shaped estuaries. The morphodynamic evolution in all the analyzed cases reproduced structures identified in many tide‐dominated estuaries: a meandering region in the fluvial‐tidal transition zone, a tidal maximum area close to the head, and a turbidity maxima region in the brackish zone. As the morphology of the estuaries evolved, the tidal propagation (including its asymmetry), the salinity gradient, and the strength of subtidal flows changed, which reflects the strong bathymetric control of these systems. The comparison with barotropic simulations showed that the three‐dimensional structure of the flow (induced by density gradients) has leading order effects on the morphodynamic evolution. Density gradient‐driven subtidal flows (1) promote near‐bed flood dominance and, consequently, the import of sediment into the estuary, (2) accelerate the morphodynamic evolution of the upper/middle estuary, (3) promote a more concave shape of the upper estuary and reduce the ebb‐tidal delta volume, and (4) produce an asymmetric bathymetry and inhibit the formation of alternate bars that would form under barotropic conditions. This latter effect is the consequence of the combined effect of Earth's rotation and baroclinicity. Key Points: Density‐driven flows affect the long‐term morphodynamic evolution of estuaries by enhancing near‐bed flood dominanceThe combined effect of baroclinicity and Coriolis acceleration tends to promote cross‐channel bathymetric asymmetryThree‐dimensional process‐based numerical models accounting for salinity variations resemble bathymetric and hydrodynamic features observed in estuaries [ABSTRACT FROM AUTHOR]