Search

Microfibers often dominate sediment microplastic samples, but little is known about their ecological effects on benthic organisms and functions. Polyethylene terephthalate) (PET) microfibers were added to 36 sediment chambers at six concentrations (0–0.5 g kg–1sediment) to assess the effects on microphytobenthos (MPB), a key deposit-feeding bivalve, Macomona liliana, and sediment nutrient pools. MPB photosynthesis was promoted in 18 chambers through a 12 h light/dark cycle. Another 18 chambers were maintained under dark conditions to inhibit photosynthesis. After 35 days of MPB growth and stabilization, four M. lilianawere added to each chamber for a further 40 days. MPB biomass and composition were examined alongside M. lilianabiochemical and behavioral properties and porewater dissolved inorganic nutrient concentrations. Increasing microfibers resulted in lower MPB biomass, fewer diatom-associated fatty acids (FAs), and an increase in cyanobacteria. The changes in MPB coincided with up to 75% lower energy reserves and reduced burrowing activity in M. liliana. In the light, nitrate + nitrate (NOx) was significantly elevated and related to M. lilianaand MPB biochemical properties. Ammoniu (NH4+) concentrations increased but were variable in both the light and the dark. Our results suggest that increasing microfiber concentrations influence the interactions between M. lilianaand MPB and affect biogeochemical processing in coastal marine sediments.

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. The Editors thank the 2018 peer reviewers

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. 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

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. 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

Video measurements of runup were collected at low tide along several profiles covering an alongshore distance of 500 m. The morphology displayed a complex shape with a shoreline sandwave in the lower beach face of about 250 m long mirrored in the inner sandbar. Wave conditions were stationary and moderate (offshore height of 2 m and peak period of nearly 13 s) but yet dissipative. Runup energy was dominated by infragravity frequencies. Alongshore variations in runup (by a factor up to 3) observed both in the incident and infragravity bands were much higher than reported previously (e.g., Guedes et al., 2012, https://doi.org/10.1016/j.csr.2012.08.022; Ruggiero et al., 2004, https://doi.org/10.1029/2003JC002160) while the alongshore variations in other environmental parameters (e.g., foreshore beach slope) appear to be much lower. Our data suggest that the beach morphology in the inner surf zone plays a crucial role by inducing rapid and significant modification in the incident wave pattern and the alongshore coherence length scales were consistent with the typical alongshore length scale of the morphology. The topic of this work is to present new field data focusing an alongshore variation of the runup. The runup is the time‐varying position of the water's edge on the foreshore of the beach. This parameter is crucial in better forecasting coastal hazards (e.g., coastal flooding and erosion) and coastal structure defense design. Alongshore variations in runup (by a factor up to 3) are observed in presence of a shoreline sandwave mirrored in the inner sandbarAlongshore variations in runup are essentially driven by rapid and significant modification in the incident wave pattern rather than by foreshore beach slope β2σvariationObserved alongshore coherence length scales are consistent with the typical length scale of the morphology

The JGR‐Earth Surface Editors thank all the peer reviewers from 2017

La morte di Pio XI (febbraio 1939), i cui rapporti con il governo fascista si erano drasticamente incrinati nel suo ultimo anno di vita, aveva suscitato interesse e apprensione nel Regime italiano, che da tempo teneva sotto osservazione molti cardinali per conoscere il profilo del futuro pontefice, adoperando a tale scopo tutti i canali a sua disposizione, non solo l’Ambasciata presso la Santa Sede ma anche i rapporti degli informatori della Polizia Politica infiltrati all’interno dei Sacri Palazzi. Tra i candidati alla tiara primeggiava il cardinale Eugenio Pacelli, sul quale tuttavia le fonti erano discordanti nell’attribuirgli un atteggiamento favorevole o contrario al Fascismo. Nell’incertezza il Regime si avvalse della singolare collaborazione del docente universitario Guido Manacorda, già intermediario personale tra Mussolini e Hitler, cattolico fervente e ben introdotto in Vaticano. Dalle sue carte, ancora inedite, emerge l’estremo tentativo del Regime italiano di carpire i segreti del conclave ed eventualmente di influenzare persino la scelta del futuro successore di Pietro.

Bifurcating rivers shape natural landscapes by distributing water and sediments on fluvial plains and in deltas. Symmetrical bifurcations were often found to be unstable so that one branch downstream of the bifurcation enlarged while the other dwindled. A unified theory able to predict bifurcation stability in both gravel bed and sand bed rivers is still lacking. Here we develop a new theory for the stability of bifurcations for the entire range of gravel bed to sand bed rivers. The theory indicates opposite behavior of gravel bed and sand bed rivers: we predict that symmetrical bifurcations are inherently stable for intermediate Shields stresses but are inherently unstable for the low and high Shields stresses found in the majority of rivers on Earth. In the latter conditions asymmetrical bifurcations are stable. These predictions are corroborated by observations and have ramifications for many environmental problems in fluviodeltaic settings. We show a unified model for stability of gravel and sand bed river bifurcationsWe predict that the majority of symmetrical river bifurcations on Earth are unstableOur results are favorably supported by observations

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

Using a numerical model, we show that the surf zone of embayed beaches systematically flushes out more floating material (simulated using passive tracers) than on open beaches, with most exits occurring through the headland rips. For obliquely incident waves, a headland rip acts as a persistent conduit for transporting floating material out of the surf zone and into the inner shelf region. Wave angle and embayment size determine which headland rip (upwave or downwave) flushes out more the surf zone material. For narrow embayed beaches, passive drifters exit the surf zone through the upwave headland rip. For wider embayed beaches, the longshore current has enough room to develop and is further deflected against the downwave headland where most drifters exit the surf zone. Our results indicate that wave‐exposed rugged coasts strongly enhance exchange of floating matter (e.g., pollutants and nutrients) at the ocean/continent interface. Rip currents on embayed beaches flush more than on open beachesSurfzone flushing rate increases with wave obliquity and embayment narrownessHeadland rips can be persistent conduit for moving drifters out of the surf zone

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. Changes in the shoreline position are the result of a delicate balance between hydrodynamic (e.g., waves, currents, and water levels) and morphology (e.g., dunes, beachface). Dunes are a natural barrier to storms and much of their ability to stand erosive events is related to their recovery after a storm. At the same time, the beachface often responds to incoming waves by either changing its slope (rotation mode) or by advancing/retreating (translation mode). Using 15 years of profile measurements at Vougot beach, France, we identified the different modes and observed if, when and how they occur. Our analysis shows that the evolution of the beach system was controlled by the sediment exchanges and redistribution between the dune and the intertidal beach. Different elevations of the intertidal beach profile showed distinct behavior and different time‐scales of change. Our work provides evidence that a multi‐temporal scale approach is necessary to predict sediment redistribution along the beach profile. A centroid analysis and shoreline modeling identified the influence of the dune system on shoreline evolution over a range of time‐scalesDifferent beach migration modes (steepening/flattening and advance/retreat) were identified using a centroid analysisOur analysis showed that time‐scales related to dune erosion and recovery play an important role in shoreline modeling performance A centroid analysis and shoreline modeling identified the influence of the dune system on shoreline evolution over a range of time‐scales Different beach migration modes (steepening/flattening and advance/retreat) were identified using a centroid analysis Our analysis showed that time‐scales related to dune erosion and recovery play an important role in shoreline modeling performance

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. 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. 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 Shear‐ or toppling‐type cantilever failure is related to the ratio between bank height and near‐bank water depth The distance of the toppling failure surface to the bank line is around twice the width of the overhanging block 3D structures of failure surfaces indicate a negative linear correlation between failure surface angle and bank retreat distance

The use of power law forms to describe hydraulic geometry is a classic subject with a history of over 70 years. Two distinct forms of power laws have been proposed: at‐a‐station hydraulic geometry (AHG) and downstream hydraulic geometry (DHG). Although the utility of these semiempirical expressions is widely recognized, they remain poorly understood in terms of the mechanisms underlying the differences between AHG and DHG, as well as the variability among different systems. In this study, we attempt to address these basic issues. Two hypotheses are proposed: (a) the different geomorphic relationships represented by AHG and DHG result from the control of lateral adjustment of the bank and flow turbulence over short and long timescales, respectively; and (b) the systematic variability of the AHG and DHG exponents is related to the description of the frictional resistance. These two hypotheses are embedded in our theoretical models and lead to explicit functional forms for AHG and DHG. The verification of our hypotheses is based on a large data set consisting of over 550 b‐f‐mexponents and 120 power law hydraulic relations. The analysis highlights the role of uncertainties in data acquisition and theoretical/statistical explanations. In addition, the theoretical expressions of AHG also provide an explanation of at‐many‐stations hydraulic geometry (AMHG) in a physical sense. Overall, our work provides new insights into the fundamental theory of power laws and hydraulic geometry. Theoretical expressions of both at‐a‐station hydraulic geometry (AHG) and downstream hydraulic geometry (DHG) are derivedThe theoretical expressions of AHG provides a physical explanation for at‐many‐stations hydraulic geometryThe different behaviors of AHG and DHG are attributed to the lateral adjustment of the bank and flow turbulence, respectively Theoretical expressions of both at‐a‐station hydraulic geometry (AHG) and downstream hydraulic geometry (DHG) are derived The theoretical expressions of AHG provides a physical explanation for at‐many‐stations hydraulic geometry The different behaviors of AHG and DHG are attributed to the lateral adjustment of the bank and flow turbulence, respectively

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. 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 We assess uncertainty of ensemble Kalman filter data‐assimilated shoreline change predictions using the spread of model trajectories We demonstrate an increase in modeled shoreline uncertainty due to ensemble wave conditions compared with single realization simulations Uncertainty in shoreline predictions can be quantified using a novel, straightforward relationship, derived analytically

Meander migration results from the interaction between inner bank accretion and outer bank erosion/collapse. This interaction has been usually treated as a long‐term average of a sequence of erosion events determined by flow hydrographs. Little attention has been paid to the role that individual bank collapse events play on meander evolution. To fill this gap, we developed a numerical model of river meandering that describes explicitly bank collapse. Results show that as bend curvature increases due to meander migration and elongation, the initially scattered locations of bank collapse events converge toward the channel section where bed shear stress attains a maximum. Simulations illustrate the observed catch‐up behavior between inner and outer banks, driven by intermittent bank collapse events. Moreover, bank collapse is found to speed up short‐term meander migration and, consistent with field observations, meanders turn out to evolve toward a state characterized by constant channel width. Meanders are one of the most ubiquitous morphological features observed in natural rivers. They consist of a series of alternating bends that, seen from above, display one of the most striking morphological patterns in nature. The migration of meandering rivers is attributed to outer bank retreat, due to erosion, and inner bank accretion, due to deposition. In this work, we propose a morphodynamic model to investigate the effects that repeated bank collapse events have on meander evolution. Our results show that the migration of meander bends can be described through a catch‐up behavior, driven by outer bank collapse and subsequent inner bank accretion, that on average ensures a nearly constant channel width. Meander bend migration is described through a catch‐up behavior, driven by intermittent bank collapse eventsAs the river bend evolves increasing its apex curvature, bank collapse tends to occur at the location of maximum shear stressIndividual bank collapse events speed up the short‐term migration rate of meandering rivers Meander bend migration is described through a catch‐up behavior, driven by intermittent bank collapse events As the river bend evolves increasing its apex curvature, bank collapse tends to occur at the location of maximum shear stress Individual bank collapse events speed up the short‐term migration rate of meandering rivers

Wave‐induced flooding is a major coastal hazard for the low‐lying atolls of the Pacific. These flooding events are expected to increase over time, which may cause significant coastal damage in some locations. Coastal flooding analysis (forensic or forecasted) is particularly challenging in these small islands due to the co‐occurrence of several swells and local seas propagating in a complex configuration of archipelagos. Therefore, assessing the contribution of swells and wind seas on the flooding hazards that threaten the atoll islands requires the spectral characterization of the wave climate, since integrated wave parameters do not accurately represent the wave conditions in these environments. On the other hand, the relative short records of wave conditions, represent only a small fraction of the possible range of combinations that could produce a wave‐induced flooding event. For these reasons, we propose the analysis of all the spectral energy arriving toward a study site, by isolating and parameterizing each swell train. Then, taking into account the link with large‐scale climatic patterns (i.e., El Niño Southern Oscillation), we present a new multi‐modal seas emulator capable of generating infinitely long time series of synthetic individual swell trains and seas. This new climate‐based emulator allows a better understanding of swell behavior in the Pacific, and the generation of multimodal wave conditions to populate the historical records as a key point to perform robust coastal flood risk assessments considering climate variability. Coastal flooding events generated by waves are a key issue for low‐lying Pacific atolls. Those events are expected to increase over time due to climate change and in order to be prepared, it is important to study where those waves are generated. As those atolls are located in the middle of a large ocean, where concurrent waves approach from every direction, we need to use the wave spectrum which characterizes the distribution of energy at different directions and frequencies. We propose the analysis of Majuro, capital of the Marshall Islands, aggregating all the spectral wave energy surrounding the atoll into a "super‐point". Then, we identify the local generated waves (i.e., seas) and the individual far field generated waves (i.e., swell trains), and find its link with large scale climatic patterns (i.e., El Niño Southern Oscillation). Knowing this link, we develop a climate‐based emulator capable of generating infinitely long time series of synthetic swell trains and seas that maintain the same properties as the historical records. This new emulator allows a better understanding of swell behaviour, and the generation of multimodal wave conditions to populate the historical records in order to perform robust coastal flood risk assessments considering climate variability A climate‐based emulator of swell trains and local seas capable to generate multimodal wave conditions is presentedAn algorithm to isolate and parameterize swell trains is developed and waves are linked to large‐scale climate patternsThe validation of the reconstructed spectra against the hindcast confirms the capability to reproduce the historical ocean waves behavior A climate‐based emulator of swell trains and local seas capable to generate multimodal wave conditions is presented An algorithm to isolate and parameterize swell trains is developed and waves are linked to large‐scale climate patterns The validation of the reconstructed spectra against the hindcast confirms the capability to reproduce the historical ocean waves behavior

The processes underlying sediment transport mechanics are still debated. Here, we present novel ensemble particle bedload measurements, obtained with high‐frequency imaging of particle velocities and activity over various bedform migration stages. We show that the bedload particle flux quickly responds to the unsteady non‐uniform flow over the bed. Mean particle velocity linearly increases over the dune slopes towards the crest. In contrast, particle activity exhibits a power law increase over the dune slopes, with the top layer almost saturating in the saltation regime. Furthermore, the spatial and temporal variability of particle motion over dune slopes is high. For lower flow, spatial and temporal variability from small‐scale topography, such as superposed bedforms, spurs, and irregular crestlines, are shown to be important for quantifying sediment dynamics. With increasing flow, however, the cross‐stream particle velocities increase significantly, influenced by the large‐scale dune topography (transverse slopes). Overall, variability in the particle flux increases with both flow strength and frequency of turbulent events. For the mean particle flux over the dune slopes, however, differences between neighboring dunes are large, which are attributed to dune topography variations and flow acceleration. The presented insights are essential to validate numerical simulations of sediment dynamics in three‐dimensional flow conditions over dune beds. Rivers convey water and sediment, often resulting in bedforms covering riverbeds. Yet, our understanding on how individual sediment particles contribute to the formation of various bedforms remains rudimentary. This major knowledge gap is mainly due to a complete lack of direct particle measurements over bedforms. Here, we present the collection of a novel data set of not only bedforms, but also individual particles that contribute to the formation of the bedforms, for a riverbed in controlled laboratory conditions. Using cameras to visualize and quantify the transport of sediment, we can appreciate and quantify the complexity of sediment transport and the limitation of present theories. Particle activity of the bedload layer saturates, whilst particle velocity continues to increase over dune stoss slopesBedload particle flux responds to small‐scale topographic features for lower flows, while it is independent for higher flowsSediment transport peaks over the upper section of the dune stoss slopes Particle activity of the bedload layer saturates, whilst particle velocity continues to increase over dune stoss slopes Bedload particle flux responds to small‐scale topographic features for lower flows, while it is independent for higher flows Sediment transport peaks over the upper section of the dune stoss slopes

Shorelines respond to a number of “drivers” operating on a variety of time‐scales. For some time‐scales (e.g., seasonal), the driver‐shoreline relationship is often evident; however, at longer time‐scales (e.g., multiannual), the shoreline changes may be superimposed on changes at shorter time‐scales and thus are difficult to identify. Here, we predict shoreline evolution from storm events to decadal time‐scales, using a novel approach based on the Complete Ensemble Empirical Mode Decomposition. This approach identifies and links the primary time‐scales in the model drivers (large‐scale sea level pressure [SLP] and/or waves) with the same time‐scales in the shoreline position. The multiscale approach reproduced shoreline changes at two beaches more skillfully than a common shoreline model when SLP and wave information were used in combination. In addition, the analysis can be applied to climate indices, providing the opportunity to link longer time‐scales with climate patterns (e.g., El Niño Southern Oscillation). Beaches are changing constantly, advancing or retreating depending for instance, on the climate and ocean conditions. Beach retreat and advance may occur in cycles (seasonally, annually, or over several decades) or because of particular events such as storms. All these changes are superimposed and difficult to disentangle. Therefore, the same beach can look completely different in summer or winter, and the changes are not the same year after year. Therefore, predicting the beach state over the following months, years, or decades is a daunting task. Here, we introduce a new approach to the prediction of shoreline changes and test it at two beaches (one in New Zealand and the other in Australia). The new approach relates changes in shoreline position with “drivers” (waves and atmospheric patterns) decomposed into time‐scales (e.g., seasonal, annual, and bi‐annual) and uses these connections to predict shoreline changes. A novel modeling approach to shoreline prediction is presented and compared with an established model at two study sitesA decomposition of different time‐scales (from storm to climate anomalies) in shoreline and drivers is used to predict shoreline changeThe addition of sea level pressure information to wave bulk parameters as model inputs improves shoreline predictions A novel modeling approach to shoreline prediction is presented and compared with an established model at two study sites A decomposition of different time‐scales (from storm to climate anomalies) in shoreline and drivers is used to predict shoreline change The addition of sea level pressure information to wave bulk parameters as model inputs improves shoreline predictions

The description of the geomorphic characteristics in power law forms has been the subject of research, over the past 70 years, and has become the cornerstone of regime theory. However, just why power functions should represent such geomorphic relationships remains poorly understood. Hence, differences in the values of the regime exponents observed for different river systems remain largely unexplained. To address this issue, we derived generic forms of the power law relationships without postulating any power functions of the discharge. The theoretical approach accurately captures the systematic variations of the regime exponents shown by a number of large data sets from previous research. We also explain how frictional resistance is responsible for the systematic variability of regime exponents. Overall, our study provides a robust mechanism to describe the variations of the exponents, along with a deductive explanation of the power laws at the core of fluvial hydraulic geometry. We analytically deduce why power functions of water discharge describe the geometric scales of fluvial channel cross sectionsWe demonstrate how frictional resistance may explain the governing relationships of hydraulic geometryOur theoretical approach captures the systematic variations of the power law exponents observed in natural river channels

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

Video measurements of wave runup were collected during extreme storm conditions characterized by energetic long swells (peak period of 16.4 s and offshore height up to 6.4 m) impinging on steep foreshore beach slopes (0.05–0.08). These conditions induced highly dissipative and saturated conditions over the low‐sloping surf zone while the swash zone was associated with moderately reflective conditions (Iribarren parameters up to 0.87). Our data support previous observations on highly dissipative beaches showing that runup elevation (estimated from the variance of the energy spectrum) can be scaled using offshore wave height alone. The data is consistent with the hypothesis of runup saturation at low frequencies (down to 0.035 Hz) and a hyperbolic‐tangent fit provides the best statistical predictor of runup elevations. The best statistical predictor of runup is an hyperbolic‐tangent fitIt provides evidence of saturation at infragravity frequenciesAccounting for wave period is critical in explaining runup variability