Search

Parallel tidal channel systems, characterized by commonly cross-shore orientation and regular spacing, represent a distinct class of tidal channel networks in coastal environments worldwide. Intriguingly, these cross-shore oriented channel systems can develop in environments dominated by alongshore tidal currents, for which the mechanisms remain elusive. Here, we combine remote sensing imagery analysis and morphodynamic simulations to demonstrate that the deflection of alongshore tidal currents at transitions in bed elevation determines the characteristic orientation of the parallel tidal channels. Numerical results reveal that sharp changes in bed elevation lead to nearly 90-degree intersection angles, while smoother transitions in bed profiles result in less perpendicular channel alignments. These findings shed light on the potential manipulation of tidal channel patterns in coastal wetlands, thus equipping coastal managers with a broader range of strategies for the sustainable management of these vital ecosystems in the face of climate change and sea level rise. Cross-shore channels can counterintuitively form at coasts dominated by alongshore currents. Sharp changes in bed elevation around the mean sea level lead to nearly 90-degree intersection angles between the channel and the shoreline. [ABSTRACT FROM AUTHOR]

Wave climatologies from historical and projected simulations of the ACCESS1.0, MIROC5 and CNRM-CM5 Global Circulation Models (GCM) were sourced from the Coordinated Ocean Wave Climate Project (COWCLIP) and downscaled using the SWAN wave model. Biases between GCM's historical simulations and a regional hindcast were assessed, and the two best-performing models (ACCESS1.0, MIROC5) had their projections analysed. A general increase in wave height and period was observed along the south/west, together with a decrease in $H_s$ H s along the north/east coasts. The projected near-term (NEA21C) period shows mostly a $H_s$ H s increase, whilst for the long-term (END21C) period, increased and decreased $H_s$ H s are present. The areas of statistically significant changes are larger in the END21C than in the NEA21C period. The wave direction change is counter-clockwise along the west and clockwise along the east coasts. This study is a first assessment of historical and projected GCM-forced waves along New Zealand and the database we generated can be of great value for renewable energy research, risk assessment and the mitigation of future coastal hazards. [ABSTRACT FROM AUTHOR]

Changes in upstream land-use have significantly transformed downstream coastal ecosystems around the globe. Restoration of coastal ecosystems often focuses on local-scale processes, thereby overlooking landscape-scale interactions that can ultimately determine restoration outcomes. Here we use an idealized bio-morphodynamic model, based on estuaries in New Zealand, to investigate the effects of both increased sediment inputs caused by upstream deforestation following European settlement and mangrove removal on estuarine morphology. Our results show that coastal mangrove removal initiatives, guided by knowledge on local-scale bio-morphodynamic feedbacks, cannot mitigate estuarine mud-infilling and restore antecedent sandy ecosystems. Unexpectedly, removal of mangroves enhances estuary-scale sediment trapping due to altered sedimentation patterns. Only reductions in upstream sediment supply can limit estuarine muddification. Our study demonstrates that bio-morphodynamic feedbacks can have contrasting effects at local and estuary scales. Consequently, human interventions like vegetation removal can lead to counterintuitive responses in estuarine landscape behavior that impede restoration efforts, highlighting that more holistic management approaches are needed. Upstream land-use changes are transforming coastal environments around the globe. Mangrove removal aims at restoring estuarine ecosystems but counterintuitively enhances sediment trapping. More holistic management approaches are needed. [ABSTRACT FROM AUTHOR]

The process of shoreline change is inherently complex, and reliable predictions of shoreline position remain a key challenge in coastal research. Predicting shoreline evolution could potentially benefit from deep learning (DL), which is a recently developed and widely successful data-driven methodology. However, so far its implementation for shoreline time series data has been limited. The aim of this contribution is to investigate the potential of DL algorithms to predict interannual shoreline position derived from camera system observations at a New Zealand study site. We investigate the application of convolutional neural networks (CNNs) and hybrid CNN-LSTM (Long Short-Term Memory) networks. We compare our results with two established models: a shoreline equilibrium model and a model that addresses timescales in shoreline drivers. Using a systematic search and different measures of fitness, we found DL models that outperformed the reference models when simulating the variability and distribution of the observations. Overall, these results indicate that DL models have potential to improve accuracy and reliability over current models. [ABSTRACT FROM AUTHOR]

Topographic and bathymetric data are essential for accurate predictions of flooding in estuaries because water depth and elevation data are fundamental components of the shallow-water hydrodynamic equations used in models for storm surges and tides. Where lidar or in situ acoustic surveys are unavailable, recent efforts have centred on using satellite-derived bathymetry (SDB) and satellite-derived topography (SDT). This work is aimed at (1) determining the accuracy of SDT and (2) assessing the suitability of the SDT and SDB for extreme water level modelling of estuaries. The SDT was created by extracting the waterline as it tracks over the topography with changing tides. The method was applied to four different estuaries in Aotearoa / New Zealand: Whitianga, Maketū, Ōhiwa and Tauranga harbours. Results show that the waterline method provides similar topography to the lidar with a root-mean-square error equal to 0.2 m, and it is slightly improved when two correction methods are applied to the topography derivations: the removal of statistical bias (0.02 m improvement) and hydrodynamic modelling correction of waterline elevation (0.01 m improvement). The use of SDT in numerical simulations of surge levels was assessed for Tauranga Harbour in eight different simulation scenarios. Each scenario explored different ways of incorporating the SDT to replace the topographic data collected using non-satellite survey methods. In addition, one of these scenarios combined SDT (for intertidal zones) and SDB (for subtidal bathymetry), so only satellite information is used in surge modelling. The latter SDB is derived using the well-known ratio–log method. For Tauranga Harbour, using SDT and SDB in hydrodynamic models does not result in significant differences in predicting high water levels when compared with the scenario modelled using surveyed bathymetry. [ABSTRACT FROM AUTHOR]

The process of shoreline change is inherently complex and reliable predictions of shoreline position remain a key challenge in coastal research. Predicting shoreline evolution could potentially benefit from Deep Learning (DL), which is a recently developed and widely successful data-driven methodology. However, so far its implementation for shoreline time series data has been limited. The aim of this contribution is to investigate the potential of DL algorithms to predict interannual shoreline position derived from camera system observations at a New Zealand study site. We investigate the application of Convolutional Neural Networks (CNNs) and hybrid CNN - Long Short-Term Memory networks. We compare our results with two established models, a shoreline equilibrium model, and a model that addresses time scales in shoreline drivers. Using a systematic search and different measures of fitness we found DL models that outperformed the reference models when simulating the variability and distribution of the observations. Overall, these results indicate that DL models have potential to improve accuracy and reliability over current models. [ABSTRACT FROM AUTHOR]

We applied machine learning to improve the accuracy of present predictors of wave setup. Namely, we used an evolutionary-based genetic programming model and a previously published dataset, which includes various beach and wave conditions. Here, we present two new wave setup predictors: a simple predictor, which is a function of wave height, wavelength, and foreshore beach slope, and a fitter, but more complex predictor, which is also a function of sediment diameter. The results show that the new predictors outperform existing formulas. We conclude that machine learning models are capable of improving predictive capability (when compared to existing predictors) and also of providing a physically sound description of wave setup. [ABSTRACT FROM AUTHOR]

Tropical cyclones are associated with extreme winds, waves, and storm surge, being among most destructive natural phenomena. Developing capability for a rapid impact estimate is crucial for coastal applications and risk preparedness. When predicting waves characteristics associated to tropical cyclones, the traditional approach involves a two‐step procedure (a) a Holland‐type wind vortex model and (b) numerical simulations using a wave generation model, using buoy and satellite measurements for validation. In this work, we take advantage of the increasing amount of remote sensing observational data and propose a new empirical model to estimate the wind wave footprint of tropical cyclones. For this purpose, we construct a dataset with over a million satellite observations of waves triggered by tropical cyclones assuming a circular shape of the TC influence area and defining composites of significant wave height as a function of representative parameters of the track characteristics like the minimum pressure, its forward velocity, and its latitude. The validation against buoy data confirms the usefulness of the model for a first and rapid estimation of the wave footprint, although an underestimation of the most extreme events is observed due to the relatively small number of observations recorded. Due to its efficiency, the model can be applied for rapid estimations of wave footprints in operational systems, reconstruction of historical or synthetic events and risk assessments. [ABSTRACT FROM AUTHOR]

We applied machine learning to improve the accuracy of present predictors of wave setup. Namely, we used an evolutionary-based genetic programming model and a previously published dataset, which includes various beach and wave conditions. Here, we present two new wave setup predictors, a simple predictor, which is a function of wave height, wavelength, and beach slope, and a fitter, but more complex predictor, which is also a function of sediment diameter. The results show that the new predictors outperform existing formulas. Therefore, we conclude that machine learning models are capable of not only improving prediction capability (when compared to classical predictors) but also of providing physically sound descriptions of the processes modelled. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Bathymetric data are essential for accurate predictions of flooding in estuaries, because water depth is a fundamental component in the shallow-water hydrodynamic equations used in numerical models. Where LiDAR or acoustic in-situ surveys are unavailable, recent efforts have centred on the use of satellite images to estimate bathymetry (SDB). This work is aimed at (1) determining the accuracy of SDB, and (2) assessing the suitability of the SDB for surge/tidal modelling of estuaries. The SDB is created by extracting the waterline as it tracks over the bathymetry with changing tides, and is applied to 4 different estuaries in New Zealand: Whitianga, Maketū, Ōhiwa and Tauranga Harbour. Results show that the waterline method provides similar bathymetries to the LiDAR with root-mean squared error equal to 0.2 m, and it is slightly improved when two proposed correction methods are applied to the bathymetry derivations: the removing of statistical bias (by 2 cm) and hydrodynamic modelling correction (by 1 cm). Finally, the use of SDB in numerical simulations of surge levels is assessed for Tauranga Harbour with 4 different scenarios that explore the use of SDB in comparison to bathymetry data collected using non-satellite survey methods. One of these includes the well-known Stumpf-ratio method to extract the SDB of subtidal regions (so that only satellite information is used). The use of the satellite derived bathymetry in hydrodynamic models does not result in significant differences in terms of water levels, when compared with the scenario modelled using surveyed bathymetry. [ABSTRACT FROM AUTHOR]

Researchers have extensively investigated the back-barrier islands morphodynamics using numerical methods. However, the influence of rocky mouth islands, which may be submerged by sea-level rise, has been rarely explored. Using the Dongshan Bay in southern China as a reference, this study numerically explores the long-term morphodynamic effect of geological constraints (e.g. rocky islands) for back-barrier basins. Model results indicate that the spatial configuration of mouth islands can considerably affect the morphological development of tidal basins. The presence of mouth islands narrows the inlet cross-sectional area, increasing flow velocity and residual current, resulting in more sediment suspension and transport. Meanwhile, mouth islands tend to increase erosion in the tidal basin and sedimentation in the ebb-delta area. Furthermore, the spatial distribution of mouth islands can also affect tidal basin evolution: the basin-side mouth islands tend to cause more basin erosion with higher tidal currents and more sediment transport. In contrast, the delta-side ones may increase relative sediment deposition in the basin. Finally, larger tidal prisms are observed with more mouth islands and with basin-side mouth islands, suggesting that the number and location of islands can affect the relationship between the tidal prism and inlet cross-sectional area. This modelling study furthers the understanding of barrier basin morphodynamics affected by rocky mouth islands and informs management strategies under a changing environment. [ABSTRACT FROM AUTHOR]

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]

Understanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand‐dominated layers (SDLs) and mud‐dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding. [ABSTRACT FROM AUTHOR]

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‐m exponents 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. Key Points: 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 [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 performed a three‐dimensional high‐resolution Large Eddy Simulation using a sinusoidal ripple‐like bedform geometry with spacing and amplitude consistent with equilibrium conditions under the prescribed oscillatory forcing. The simulation results are validated qualitatively and quantitatively with previous laboratory experiments and numerical simulations. Our numerical simulations allow to analyze the presence of "ribs", strong flow rotational features in the spanwise direction developing perpendicular to the ripple crest. We show that ribs tend to appear on the upslope and crest of ripples and display a dominant spacing, ranging from 0.5λ to λ, during wave phases associated to flow acceleration. When the vortex is transported over the downstream ripple during the flow deceleration, ribs appear for the second time over the upslope of the downstream ripple, with the dominant spacing about 1/8 ripple wavelength. The presence of flow three‐dimensionality corresponds to a large variability of the shear stress over ripples in the spanwise direction, which could ultimately favor the development of three‐dimensional geometry. The three‐dimensional vortex dynamics are evaluated using the concept of vortex strength. Peak vortex strength is observed at the maximum free stream velocity, but the largest variation of the vortex strength and vortex size in the spanwise direction occurs prior to the flow reversal. The variability of the vortex structures might provide a favorable setting for the development of ribs and might relate to the spacing of ribs. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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 Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Summary: Sediment‐stabilizing and ‐destabilizing organisms, i.e. microphytobenthos (biofilms) and macrozoobenthos (bioturbators), affect the erodibility of muddy sediments, potentially altering large‐scale estuarine morphology. Using a novel eco‐morphodynamic model of an idealized estuary, we investigate eco‐engineering effects of microphytobenthos and two macrozoobenthic bioturbators. Local mud erodibility is based on species pattern predicted through hydrodynamics, soil mud content, competition and grazing. Mud resuspension and export is enhanced under bioturbation and prevented under biostabilization through respective exposure and protection of the supra‐ and intertidal. Bioturbation decreases mud thickness and bed elevations, which increases net mud fluxes. Microphytobenthos reduces erosion, leading to a local mud increase of intertidal sediments. In multi‐species scenarios, an effective mud‐prone bioturbator strongly alters morphology, exceeding that of a more abundant sand‐prone moderate species, showing that morphological change depends on species traits as opposed to abundance. Altering their habitat, the effective mud‐prone bioturbator facilitates expansion of the sand‐prone moderate bioturbator. Grazing and species competition favor species distributions of dominant bioturbators. Consequently, eco‐engineering affects habitat conditions while species interactions determine species dominance. Our results show that eco‐engineering species determine the mud content of the estuary, which suggests large effects on the morphology of estuaries with aggravating habitat degradation. [ABSTRACT FROM AUTHOR]

The landscape setting of estuaries varies widely and is an important aspect of determining how they evolve. This paper focusses on alluvial estuaries in river valleys and how they respond to sea level rise. We examine the implications of marine transgression, as a response to sea level rise, where the estuary moves upwards and landwards to maintain its position in the tidal frame (so‐called stratigraphic rollover). Here we encapsulate such kinematic movement of the estuary morphology using a 'morphokinematic' model, to assess the potential response to sea level rise and sediment supply. The model of the estuary form includes a single convergent channel, intertidal and surrounding floodplains (the valley) and allows the relative importance of the space available for deposition of sediments, the accommodation space, to be investigated as a function of rates of sea level rise and sediment supply. The transgression of the system is determined using a sediment mass balance, taking account of any supply from the river and marine environment. Model results confirm that the transgression distance, measured as the distance the entity moves landward, varies in proportion to the change in accommodation space, which mainly depends on the floodplain area. As the size of the floodplain reduces, the transgression distance is less and the system becomes much more sensitive to changes in the rate of sea level rise or changes in sediment supply. The greater demand for sediment when a floodplain is present results in greater cannibalization of the estuary form (i.e. greater landward movement) to meet the sediment demand. When the floodplain is disconnected from the estuary, the synergistic relationship is lost and the accommodation space increases. The capacity for restoration will depend on the availability of sediment and the prevailing rate of sea level rise. [ABSTRACT FROM AUTHOR]

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). Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Abstract: When modelling urban expansion dynamics, cellular automata models focus mostly on the physical environments and cell neighbours, but ignore the 'human' aspect of the allocation of urban expansion cells. This limitation is overcome here using an intelligent self-adapting multiscale agent-based model. To simulate the urban expansion of Auckland, New Zealand, a total of 15 urban expansion drivers/constraints were considered over two periods (2000–2005, 2005–2010). The modelling takes into consideration both a macro-scale agent (government) and micro-scale agents (residents of three income levels), and their multi-level interactions. In order to achieve reliable simulation results, ABM was coupled with an artificial neural network to reveal the learning process and heterogeneity of the multi-sub-residential agents. The ANN-ABM accurately simulated the urban expansion of Auckland at both the global and local scales, with kappa simulation value at 0.48 and 0.55, respectively. The validated simulation result shows that the intelligent and self-adapting ANN-ABM approach is more accurate than an ABM with a general type of agent model (kappa simulation = 0.42) at the global scale, and more accurate than an ANN-based CA model (kappa simulation = 0.47) at the local scale. Simulation inaccuracy stems mostly from the outdated master land use plan. [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

The spatial distribution of urban areas at the national and regional scales is critical for urban planners and governments to design sustainable and environment-friendly future development plans. The nighttime lights (NTL) data provide an effective way to monitor the urban at different scales however is usually achieved by using empirical threshold-based algorithms. This study proposed a novel Artificial Neural Network (ANN) approach, using moderate resolution imageries as NTL, MODIS NDVI and land surface temperature data, to map urban areas. Both random and maximum dissimilarity distance algorithm sampling methods were considered and compared. The validation of the urban areas extracted from MDA-based ANN against the 2011 US national land cover data showed a reasonable quality (overall accuracy = 97.84; Kappa = 0.74) and achieved more accurate result than the threshold method. This study demonstrates that ANN can provide an effective, rapid, and accurate alternative in extracting urban built-up areas from NTL. [ABSTRACT FROM AUTHOR]

Lava flows can cause substantial and immediate damage to the built environment and affect the economy and society over days through to decades. Lava flow modelling can be undertaken to help stakeholders prepare for and respond to lava flow crises. Traditionally, lava flow modelling is conducted on a digital elevation model, but this type of representation of the surface may not be appropriate for all settings. Indeed, we suggest that in urban areas a digital surface model may more accurately capture all of the obstacles a lava flow would encounter. We use three effusive eruption scenarios in the well-studied Auckland Volcanic Field (New Zealand) to demonstrate the difference between modelling on an elevation model versus on a surface model. The influence of surficial features on lava flow modelling results is quantified using a modified Jaccard coefficient. For the scenario in the most urbanised environment, the Jaccard coefficient is 40%, indicating less than half of the footprints overlap, while for the scenario in the least urbanised environment, the Jaccard coefficient is 90%, indicating substantial overlap. We find that manmade surficial features can influence the hazard posed by lava flows and that a digital surface model may be more applicable in highly modified environments. [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]

Double sandbar systems often characterize the surf zone of wave-dominated beaches and display a variety of poorly explained spatial configurations. Here, we explore the morphodynamic stability of double-barred beaches using a model based on linear stability analysis. Surf zone hydrodynamics are described by coupling depth and wave-averaged conservation of mass and momentum with the wave energy and phase equations, including roller dynamics. A simplified sediment transport formulation relates flow to seabed changes. Under normal wave incidence, an alongshore uniform coast with a cross-shore profile characterized by the presence of two sandbars can be unstable, thereby resulting in the development of crescentic and/or rip channel patterns. Our study demonstrates that sandbar coupling can be either in phase (highs and lows of both sandbars are at the same alongshore position) or out of phase (highs and lows of one sandbar correspond to lows and highs of the other sandbar). In line with observations, results of simulations show a large variability in the possible emerging bottom patterns. Our analysis indicates that modes of which the amplitude of the inner sandbar perturbation is larger than that of the outer sandbar are dominant for large height–depth differences between the two sandbar crests and small offshore wave heights. Patterns related to the outer sandbar dominate for small values of the difference in sandbar depth. For intermediate differences between the two sandbar depths, patterns on both longshore bars appear to be fully coupled (similar growth rates and strongly correlated pattern shapes). For relatively large waves and large depth over the outer sandbar, patterns can also develop close to the shoreline and/or inner surf zone together with patterns in the inner or outer sandbar. [ABSTRACT FROM AUTHOR]

We developed a new hindcast for storm surge at a 0.25° spatial scale for the whole New Zealand area using a statistical downscaling technique that links the mean local atmospheric conditions with the maximum storm surge levels at a daily scale. After validating the hindcast against sea level instrumental records from 17 tidal gauges around New Zealand, the same technique has been applied to obtain storm surge projections until 2,100 using different global climate models. The global climate models have been previously classified according to their ability to reproduce the past climatology in the studied area and seven models have been selected in order to explore their effect on storm surge projections. For the two representative Concentration Pathways studied, the projections indicate that the storm surge associated with the 50 years return period will increase in magnitude in the Southern areas while it will decrease in the Northern region. Even where a decreasing linear trend over the annual maxima is observed in the future time series, sporadic events of higher magnitude than the historical peaks can be. [ABSTRACT FROM AUTHOR]

Beaches around the world continuously adjust to daily and seasonal changes in wave and tide conditions, which are themselves changing over longer time-scales. Different approaches to predict multi-year shoreline evolution have been implemented; however, robust and reliable predictions of shoreline evolution are still problematic even in short-term scenarios (shorter than decadal). Here we show results of a modelling competition, where 19 numerical models (a mix of established shoreline models and machine learning techniques) were tested using data collected for Tairua beach, New Zealand with 18 years of daily averaged alongshore shoreline position and beach rotation (orientation) data obtained from a camera system. In general, traditional shoreline models and machine learning techniques were able to reproduce shoreline changes during the calibration period (1999–2014) for normal conditions but some of the model struggled to predict extreme and fast oscillations. During the forecast period (unseen data, 2014–2017), both approaches showed a decrease in models' capability to predict the shoreline position. This was more evident for some of the machine learning algorithms. A model ensemble performed better than individual models and enables assessment of uncertainties in model architecture. Research-coordinated approaches (e.g., modelling competitions) can fuel advances in predictive capabilities and provide a forum for the discussion about the advantages/disadvantages of available models. [ABSTRACT FROM AUTHOR]

As populations around volcanoes grow, the potential for society to be impacted by lava flows is increasing. While lava flows are known to ignite, bulldoze and/or bury structures, little is known about potential impacts to buried infrastructure. We measure temperature profiles below molten rock to constrain a heat transfer model. Thermomagnetic and palaeomagnetic measurements on soil samples from beneath a 2014 Hawaiian lava flow are then used to verify the model. Finally, we illustrate the model's utility in lava flow hazard assessments by modelling a hypothetical lava flow active for 4 weeks in Auckland (New Zealand). The modelling predicts the upper 1.7 m of dry soil would exceed 100 °C after 1 week, and the upper 3.8 m of soil would exceed 100 °C after 4 weeks. Determining the depth profile of substrate heating has important implications for planning and preparedness (e.g. siting buried infrastructure), volcanic impact and risk assessments, and decision-making before and during lava flow crises (e.g. mitigation measures to be employed). [ABSTRACT FROM AUTHOR]

We review the status of current sea-level observing systems with a focus on the coastal zone. Tide gauges are the major source of coastal sea-level observations monitoring most of the world coastlines, although with limited extent in Africa and part of South America. The longest tide gauge records, however, are unevenly distributed and mostly concentrated along the European and North American coasts. Tide gauges measure relative sea level but the monitoring of vertical land motion through high-precision GNSS, despite being essential to disentangle land and ocean contributions in tide gauge records, is only available in a limited number of stations. (25% of tide gauges have a GNSS station at less than 10 km.) Other data sources are new in situ observing systems fostered by recent progress in GNSS data processing (e.g., GPS reflectometry, GNSS-towed platforms) and coastal altimetry currently measuring sea level as close as 5 km from the coastline. Understanding observed coastal sea level also requires information on various contributing processes, and we provide an overview of some other relevant observing systems, including those on (offshore and coastal) wind waves and water density and mass changes. [ABSTRACT FROM AUTHOR]

Accurate simulations and predictions of urban expansion are critical to manage urbanization and explicitly address the spatiotemporal trends and distributions of urban expansion. Cellular Automata integrated Markov Chain (CA-MC) is one of the most frequently used models for this purpose. However, the urban suitability index (USI) map produced from the conventional CA-MC is either affected by human bias or cannot accurately reflect the possible nonlinear relations between driving factors and urban expansion. To overcome these limitations, a machine learning model (Artificial Neural Network, ANN) was integrated with CA-MC instead of the commonly used Analytical Hierarchy Process (AHP) and Logistic Regression (LR) CA-MC models. The ANN was optimized to create the USI map and then integrated with CA-MC to spatially allocate urban expansion cells. The validated results of kappa and fuzzy kappa simulation indicate that ANN-CA-MC outperformed other variously coupled CA-MC modelling approaches. Based on the ANN-CA-MC model, the urban area in South Auckland is predicted to expand to 1340.55 ha in 2026 at the expense of non-urban areas, mostly grassland and open-bare land. Most of the future expansion will take place within the planned new urban growth zone. [ABSTRACT FROM AUTHOR]

This work presents measurements and analysis of sand particle velocities over a subaqueous dune with median sand diameter of 0.85 mm. Time‐lapse images of the mobile bed and an automated particle image velocimetry (PIV)‐based cross‐correlation method are used to obtain mean velocity of sand particles. This technique is shown to be consistent with measurements obtained with manual tracing. The measurements indicate an increase in mean particle velocity over a dune slope. Three regions are distinguished over the dune slope: (1) region of fluctuating particle velocity, (2) region of increasing particle velocity, and (3) region of maximum particle velocity. The observations are aligned with experimental and numerical modelling studies, indicating fluctuations in flow velocity over a dune stoss slope. We furthermore show that the standard deviation of the mean particle velocity is affected by the slope location and decreases from the lower slope towards the upper slope. The particle velocity variability is discussed in the context of general onset and cessation of sediment transport, the effect of the reattachment zone, sweep‐transport events, and the existence of superimposed bedforms. With this work we bridge the gap between measurements of bedload transport at the particle‐scale and at the bedform‐scale. © 2019 John Wiley & Sons, Ltd. [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]

Understanding marine climate variability is important for coastal planning and marine operations. It is also particularly challenging for complicated settings (e.g., islands) and data‐poor regions. The aim of this work is to establish a relationship between daily synoptic atmospheric patterns, and wave and storm surge conditions around New Zealand waters, based on instrumental and reanalysis data. The daily predictor we developed is able to represent sea and swell wave conditions as well as storm surge variability over different temporal scales. However, when climate variability is analysed on a longer temporal period, based on the 20th century reanalysis, large inhomogeneities are found. This highlights the dangers related to assessing climate variability, especially in data‐poor regions (such as New Zealand), where inhomogeneities could be interpreted as actual changes. This work is conceived to present a general methodology to relate atmospheric conditions to regional wave and water level variables, which are drivers of coastal change. The established relationship allows to explore climate variability at different temporal scales. Our approach is general and leads to a relationship between waves and water levels with atmospheric conditions, which is envisaged to be a useful tool for the broad scientific community studying coastal changes. [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]

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. Plain Language Summary: 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. Key Points: 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 β2σ variationObserved alongshore coherence length scales are consistent with the typical length scale of the morphology [ABSTRACT FROM AUTHOR]

Abstract: Simple stability relationships are practically useful to provide a rapid assessment of coastal and estuarine landforms in response to human interventions and long‐term climate change. In this contribution, we review a variety of simple stability relationships which are based on the analysis of tidal asymmetry (TA). Most of the existing TA‐based stability relationships are derived using the one‐dimensional tidal flow equations assuming a certain regular shape of the tidal channel cross‐sections. To facilitate analytical solutions, specific assumptions inevitably need to be made, for example by linearizing the friction term and dropping some negligible terms in the tidal flow equations. We find that three major types of TA‐based stability relationships have been proposed between three non‐dimensional channel geometric ratios (represented by the ratio of channel widths, ratio of wet surface areas and ratio of storage volumes) and the tide‐related parameter a/h (i.e. the ratio between tidal amplitude and mean water depth). Based on established geometric relations, we use these non‐dimensional ratios to restate the existing relationships so that they are directly comparable. Available datasets are further extended to examine the utility of these TA‐based relationships. Although a certain agreement is shown for these relationships, we also observe a large scatter of data points which are collected in different types of landscape, hydrodynamic and sedimentological settings over the world. We discuss in detail the potential reasons for this large scatter and subsequently elaborate on the limited applicability of the various TA‐based stability relationships for practical use. We highlight the need to delve further into what constitutes equilibrium and what is needed to develop more robust measures to determine the morphological state of these systems. Copyright © 2018 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT Passarella, M.; De Muro, S.; Ruju, A., and Coco, G., 2018. An assessment of swash excursion predictors using field observations. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1036–1040. Coconut Creek (Florida), ISSN 0749-0208. Accurate predictions of swash excursion are fundamental for improving the understanding of swash zone dynamics and for coastal management and hazard applications. Researchers have provided several formulas for predicting the swash excursion on sandy beaches. However, the problem of the universality of these formulas is still open. In fact, it is not clear whether the existing formulations are applicable to a wide range of beaches and wave conditions. This study verifies 13 existing swash (total, incident and infragravity) formulations using 13 published experiments (636 swash measurements). The experiments were carried out on beaches worldwide (dissipative intermediate and reflective) under a wide range of wave conditions (including extreme events). Results show that formulas behave differently with under and over prediction. However, some trends can be recognized, which are indeed critical for coastal hazards and management applications: generally, for large swash events (swash >1.5 m) large scatter are found for all formulas. During extreme conditions (swash>2.5 m) total and incident swash are strongly underestimated, while infragravity swash varies for each formula. The maximum errors and the root mean square errors can exceed 2.5 m and 1 m, respectively. [ABSTRACT FROM AUTHOR]

ABSTRACT Jin, C.; Gong, Z.; Geng, L.; Zhao, K.; Xu, B.B., and Coco. G., 2018. Subsurface processes in salt marsh of central Jiangsu coast(China). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 296–300. Coconut Creek (Florida), ISSN 0749-0208. Studies of tidal flats morphodynamics tend to ignore subsurface processes like bioturbation or underground water dynamics so that changes in the surface elevation are always considered to be the result of sedimentation/erosion on the soil surface. However, at times, subsurface variations can be as large as elevation changes driven by sedimentation and so control the evolution trend of surface elevation. In this study, four observation sites were set up on the salt marsh in the central Jiangsu coast (China). A three-year field observation campaign has been carried out studying seabed elevation changes. The Surface Elevation Table-Marker Horizon (SET-MH) technique was applied to measure surface elevation and net sedimentation. The subsurface variation was derived from the difference between the surface elevation and the sedimentation. Results show that the changes in surface elevation are not equal to the sedimentation and that subsurface process, responsible for such deviation, are strongly affected by storm surge. Storm surges increase the underground water content leading to the observed expansion of the subsurface soil. The expansion results in the rapid increase of surface elevation and it takes almost one year to recover to the surface elevation before the storm season. [ABSTRACT FROM AUTHOR]

ABSTRACT Zhou, Z.; Chen, L.Y.; Townend, I.; Coco, G.; Friedrichs, C., and Zhang C.K., 2018. Revisiting the relationship between tidal asymmetry and basin morphology: A comparison between 1D and 2D models. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 151–155. Coconut Creek (Florida), ISSN 0749-0208. Tidal basin morphology is strongly dependent on the condition of tidal asymmetry which may result in either net import (flood-dominance) or export (ebb-dominance) of sediment. Based on the one-dimensional (1D) shallow water equations, various stability relationships have been developed linking tidal parameters with basin geometry. This study revisits the widely used numerical stability relationship developed by Friedrichs and Aubrey (1988), using a 2D depth-averaged model which is able to describe a more extensive flow field and more realistic bathymetries. Model comparison between 1D and 2D models indicates that excluding or including intertidal areas as a momentum-conveying component can highly affect the tidal velocity distribution and magnitude, particularly when the relative water depth is large (i.e., the ratio between tidal amplitude and mean water depth is small). Meanwhile, the tidal distortion parameters (relative tidal phase 2M2-M4 and tidal amplitude ratio M4/M2) derived from the 2D model appear to be more strongly controlled by the relative water depth (a/h) than that in the 1D model. Further research can be devoted to developing more generic stability relationships which can be applied in a more quantitative manner. [ABSTRACT FROM AUTHOR]

Passarella, M.; De Muro, S.; Ruju, A., and Coco, G., 2018. An assessment of swash excursion predictors using field observations. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1036–1040. Coconut Creek (Florida), ISSN 0749-0208. Accurate predictions of swash excursion are fundamental for improving the understanding of swash zone dynamics and for coastal management and hazard applications. Researchers have provided several formulas for predicting the swash excursion on sandy beaches. However, the problem of the universality of these formulas is still open. In fact, it is not clear whether the existing formulations are applicable to a wide range of beaches and wave conditions. This study verifies 13 existing swash (total, incident and infragravity) formulations using 13 published experiments (636 swash measurements). The experiments were carried out on beaches worldwide (dissipative intermediate and reflective) under a wide range of wave conditions (including extreme events). Results show that formulas behave differently with under and over prediction. However, some trends can be recognized, which are indeed critical for coastal hazards and management applications: generally, for large swash events (swash >1.5 m) large scatter are found for all formulas. During extreme conditions (swash>2.5 m) total and incident swash are strongly underestimated, while infragravity swash varies for each formula. The maximum errors and the root mean square errors can exceed 2.5 m and 1 m, respectively. [ABSTRACT FROM AUTHOR]

Jin, C.; Gong, Z.; Geng, L.; Zhao, K.; Xu, B.B., and Coco. G., 2018. Subsurface processes in salt marsh of central Jiangsu coast(China). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 296–300. Coconut Creek (Florida), ISSN 0749-0208. Studies of tidal flats morphodynamics tend to ignore subsurface processes like bioturbation or underground water dynamics so that changes in the surface elevation are always considered to be the result of sedimentation/erosion on the soil surface. However, at times, subsurface variations can be as large as elevation changes driven by sedimentation and so control the evolution trend of surface elevation. In this study, four observation sites were set up on the salt marsh in the central Jiangsu coast (China). A three-year field observation campaign has been carried out studying seabed elevation changes. The Surface Elevation Table-Marker Horizon (SET-MH) technique was applied to measure surface elevation and net sedimentation. The subsurface variation was derived from the difference between the surface elevation and the sedimentation. Results show that the changes in surface elevation are not equal to the sedimentation and that subsurface process, responsible for such deviation, are strongly affected by storm surge. Storm surges increase the underground water content leading to the observed expansion of the subsurface soil. The expansion results in the rapid increase of surface elevation and it takes almost one year to recover to the surface elevation before the storm season. [ABSTRACT FROM AUTHOR]