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Coastal protection is of paramount importance because erosion and flooding affect millions of people living along the coast and can largely influence countries' economy. The implementation of nature‐based solutions for coastal protection, such as sand engines, has become more popular due to these interventions' adaptability to climate change. This study explores synergies between Artificial Intelligence (AI) and hydro‐morphodynamic models for the creation of efficient decision‐making tools for the choice of optimal sand engines configurations. Specifically, we investigate the use of long‐short‐term memory (LSTM) models as predictive tools for the morphological evolution of sand engines. We developed different LSTM models to predict time series of bathymetric changes across the sand engine as well as the time‐decline in the sand engine volume as a function of external forces and intervention size. Finally, a MATLAB framework was developed to return LSTM model results based on users' inputs about sand engine size and external forcings. Plain Language Summary: Sand engines are a type of coastal protection where a large volume of sand is added to the coastline to protect low‐lying areas from erosion and flooding. Sand engines, like other Nature‐based solutions, are gaining popularity due to their potentially lower maintenance costs compared to other concrete‐based coastal protection strategies and a number of co‐benefits. However, there are currently no design guidelines or decision‐making tools for sand engines. Here we address this gap in the state of knowledge and use Artificial Intelligence (AI) techniques to analyze the evolution of sand engines under different waves and external forcings. AI models are also used to predict the volume of sand being transferred by the waves from the location of deposition to the surrounding areas. To facilitate the use of AI models, this study proposes a computer software, sand engine surface, which includes all the AI models developed in this study, predicting the evolution of sand engine and volume of sand being transported. Key Points: This study presents models to predicts the evolution of Sand Engine over time in terms of Volumetric changes and Morphological changesAll our Long Short Term Memory models' results are accessible through Sand Engine Surface ApplicationSand Engine Surface is MATLAB framework providing results about time‐dependent morphological changes of sand engines based on users' inputs [ABSTRACT FROM AUTHOR]

Deltaic tidal flat is an important ecosystem that supports the livelihoods of millions of people globally. Due to the complexity and stochastic nature of tidal flat development, it is a challenging task to predict the tidal flat prograding limit accurately. In this study, we fill out this gap from the perspective of numerical simulation by focusing on the tidal flat extension limit of Nanhui Shoal (NHS), the largest marginal shoal of the Changjiang Estuary. Our results suggest that the tidal flat of NHS extended 29.16 m/y seaward in 1989-2002, had a rapid progradation of 158.08 m/y in 2002-2012 and a retreat rate of 26.88 m/y in 2012-2019, while the fluvial sediment supply decreased 56.78% from 1989-2002 to 2003-2012 and another 20.68% from 2003-2012 to 2013-2019. During the 1989-2002 period, the advancing rate of the -5misobaths followed a Gaussian frequency distribution, in agreement with simulation results of cellular automata. We suggest that in the absence of human perturbations, as the sediment input to NHS declines to the threshold of 0.01×108 t/y, the seaward advance of tidal flats may significantly slow down. However, human interferences generated large uncertainties on the development tendency of the NHS. [ABSTRACT FROM AUTHOR]

As climate-driven risks for the world's coastlines increase, understanding and predicting morphological changes as well as developing efficient systems for coastal forecast has become of the foremost importance for adaptation to climate change. Artificial Intelligence is a powerful technology that has been rapidly evolving recently and can offer new means of analysis for the coastal science field. Yet, the potential of these technologies for coastal geomorphology remains relatively unexplored with respect to other scientific fields. This article investigates the use of Artificial Neural Networks and Bayesian Networks in combination with fully coupled hydrodynamics and morphological models (Delft3D) for predicting morphological changes and sediment transport along coastal systems. Two sets of Artificial Intelligence models were tested, one set relying on localized modeling outputs or localized data sources and another set having reduced dependency from modeling outputs and, once trained, solely relying on boundary conditions and coastline geometry. The first set of models provides regression values greater than 0.95 and 0.86 for training and testing, respectively. The second set of reduced dependency models provides regression values greater than 0.84 and 0.76 for training and testing, respectively. Our results highlight the potential of AI and statistical models for coastal applications. [ABSTRACT FROM AUTHOR]

There is growing interest in the adoption of Engineering with Nature or Nature Based Solutions for coastal protection including large mega-nourishment interventions. However, there are still many unknowns on the variables and design features influencing their functionalities. There are also challenges in the optimization of coastal modelling outputs or information usage in support of decision-making. In this study, more than five hundred numerical simulations with different sandengine designs and different locations along Morecambe Bay (UK) were conducted in Delft3D. Twelve Artificial Neural Networking ensemble models structures were trained on the simulated data to predict the influence of different sand engines on water depth, wave height and sediment transports with good performance. The ensemble models were then packed into a Sand Engine App developed in MATLAB and designed to calculate the impact of different sand engine features on the above variables based on users' inputs of sandengine designs. [ABSTRACT FROM AUTHOR]

Our coasts are facing a growing threat from rising sea levels and detrimental storm events. Nature-based solutions are gaining interest for their potential to provide multiple ecosystem services, including coastal protection. Seagrass can influence coastal sediment transport and wave propagation, however, whether seagrass can be used as an effective intervention for coastal protection is unclear. Using the Delft3D model, this study looks at how seagrass affects coastal hydrodynamics in a macrotidal bay, under idealised scenarios that simulate a rise in sea level and storm wave heights, emulating projected sea conditions under climate change. Through the use of a habitat suitability model, a seagrass patch in Morecambe Bay was simulated, based on a location within the bay that is most suitable for seagrass species Zostera marina. Hydrodynamic simulations were run for a number of scenarios for a domain with and without a seagrass patch. Scenarios included variable boundary wave heights, sea levels and vegetation parameters, to test the influence of seagrass in future climates. Results show that there is a reduction in mean and maximum wave height inside and behind the seagrass patch, with the largest changes in wave dissipation observed during higher boundary wave conditions. In these conditions the maximum wave height reduced by over a half within the patch. Simulations examining the influence of seagrass density and canopy height found that a lower density seagrass patch reduced maximum wave height more substantially than higher density patches. Although alternative hard engineered solutions are able to attenuate wave energy more effectively than seagrass, these results demonstrate seagrass could play an important role in conjunction with salt marsh restoration or existing hard engineering solutions, helping to mitigate the risk of flooding and erosion, whilst providing additional ecosystem services such as carbon sequestration and habitat provision. • Habitat suitability model locates site to test seagrass as a nature-based solution. • Simulations find seagrass to influence a reduction in significant wave height. • Potential for seagrass to be integrated into managed realignment plans. [ABSTRACT FROM AUTHOR]

Sediment budget and sediment availability are direct metrics for evaluating the resilience of coastal bays to sea‐level rise (SLR). Here we use a high‐resolution numerical model of a tidally dominated marsh‐lagoon system to explore feedbacks between SLR and sediment dynamics. SLR augments tidal prism and inundation depth, facilitating sediment deposition on the marsh platform. At the same time, our results indicate that SLR enhances ebb‐dominated currents and increases sediment resuspension, reducing the sediment‐trapping capacity of tidal flats and bays and leading to a negative sediment budget for the entire system. This bimodal distribution of sediments budget trajectories will have a profound impact on the morphology of coastal bays, increasing the difference in elevation between salt marshes and tidal flats and potentially affecting intertidal ecosystems. Our results also clearly indicate that landforms lower with respect to the tidal frame are more affected by SLR than salt marshes. Key Points: SLR increases sediment deposition on the marsh platform but reduces the sediment‐trapping capacity of tidal flats and baysFor a given SLR the increase of bottom sediments resuspended and exported out of the system is more significant for sand than mud fractionLandforms lower with respect to the tidal frame are more affected by SLR than salt marshes [ABSTRACT FROM AUTHOR]

A numerical tool integrating hydrodynamics, vegetation growth, stochastic selection, and Monte‐Carlo simulation is developed to predict channel network evolution in a newly inundated back‐barrier wetland. Focusing on the formation and evolution of tidal channels, model simulations suggest a clear tendency towards specific creek topologies with high probability despite creek evolution being subject to random perturbations. These creek topologies are primarily determined by wetland topography while the influence of vegetation is secondary. Ensemble results of Monte‐Carlo simulations identify threshold points leading to bimodal separation of the emergent creek networks characterized by different complexity and efficiency. This, in turn, assists the recognition of necessary human interventions to encourage the formation of complex and far‐reaching creek networks that can promote exchange of materials between estuaries and salt marshes and, hence, maintain salt marsh ecosystem structure and function. Plain Language Summary: A large part of the world's low‐lying areas is heavily managed to protect lives, assets, and landscapes from marine‐derived natural hazards. However, climate change is putting unprecedented pressure on these managed environments, forcing the adoption of "no active intervention" or "managed realignment" strategies in areas where "hold the line" options cannot be justified due to financial constraints. This leads to the very likely possibility that unmaintained coastal defenses will begin to fail, raising questions as to the potential impacts on low‐lying regions as well as the options available to mitigate impacts through targeted interventions. Of particular interest is the question of whether it is possible to facilitate the shift of managed coastal wetlands towards intertidal environments in which tidal creeks form and evolve to support salt marshes. Combining the use of a numerical model and multiple model runs, we find that although the evolution of tidal creeks is subject to random disturbances, creek networks are likely to form with a clear tendency towards specific types. Grouped results of multiple, random simulations are useful in identifying interventions that are necessary for developing far‐reaching tidal creek networks. Influence of vegetation is found to be secondary compared to the wetland topography in terms of influencing tidal creek evolution. Key Points: A reduced complexity model is developed based on simple rules for projecting tidal creek network developmentThe influence of vegetation in determining probabilities of creek formation is secondary compared to wetland topographyEnsemble prediction of tidal creek evolution is effective for identifying threshold points and targeted interventions [ABSTRACT FROM AUTHOR]

Highlights • Sediments are unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platforms. • Trapping capacity of sediment in different intertidal subdomains decreases exponentially with R, the ratio between advection length and the typical spatial length of channels and tidal flats. • The feedback between SLR and sediment trapping has a twofold effect: an increase in salt marsh resilience in the short term but a net loss of sediment in the long term that might jeopardize the entire intertidal system. Abstract Deposition of fine sediments on a marsh platform favors accretion that counteracts Sea Level Rise. However, it is difficult to assess the sediment trapping capacity of a marsh given the heterogeneity of sediment sources and the geometric complexity of the system, with a network of dendritic and meandering creeks dissecting the intertidal area. Here we use a numerical model to study the sediment trapping capacity of a marsh-dominated estuary, Plum Island Sound, USA, and its variations across the landscape. The results highlight the importance of the timing between sediment inputs and tidal phase and show that sediment discharged from tidal rivers deposit within the rivers themselves or in adjacent marshes. Most sediment is deposited in shallow tidal flats and channels and is unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platform. Trapping capacity of sediment in different intertidal subdomains decreases logarithmically with the ratio between advection length and the typical length of channels and tidal flats. Moreover, sediment deposition on the marsh decreases exponentially with distance from the channels and marsh edge. This decay rate is a function of settling velocity and the maximum value of water depth and velocity on the marsh platform. [ABSTRACT FROM AUTHOR]

Abstract The impact of wind waves and salt marsh erosion on sediment transport dynamics of macro-tidal estuarine systems has been examined using a numerical model. Morphological changes associated with salt marsh erosion facilitate the propagation of waves to the marsh edges, and increase the resuspension and export of sediments from the estuary. Results also highlight the impact of tidally-modulated changes in wave action upon sediment transport dynamics throughout a tidal cycle and from neap to spring tide. In particular, wave action at the landward margin of the estuary is limited by the shallow water depth, and tidally induced increments in water depth only cause a modest increase in wave shear stresses. At the seaward side of the domain, the impact of waves is limited by the deeper water, and tidally induced water level variations have a more significant control on wave shear stresses. The outcomes contribute new insights that are important for the sediment budget of macro-tidal estuaries facing climate change and have implications for the long-term morphological evolution of estuarine wetlands. Highlights • Enhanced sediment export from estuaries caused by wind waves • Enhanced salt marsh erosion caused by wind waves • Salt marsh erosion due to morphological changes of estuaries • Wave impact changes with varying water depth [ABSTRACT FROM AUTHOR]

Salt marshes are ecosystems with significant economic and environmental value. However, the accelerating rate of sea-level rise is a significant threat to these ecosystems. Storms significantly contribute to the sediment budget of salt marshes, playing a critical role in salt marsh survival to sea-level rise. There are, however, uncertainties on the extent to which storms contribute sediments to different areas of marsh platforms (e.g., outer marsh vs marsh interior) and on the sediment sources that storms draw on (e.g., offshore vs nearshore). This study uses field analyses from an eight-month field campaign in the Ribble Estuary, North-West England, to understand storms' influence on the sediment supply to different marsh areas and whether storms can deliver new material onto the salt marsh platform which would otherwise not be sourced in fair-weather conditions. Field data from sediment traps indicate that storm activity caused an increase in inorganic sediment supply to the whole salt marsh platform, especially benefitting the marsh interior. Geochemistry and particle size distribution analysis indicate that the majority of the sediment supplied to the salt marsh platform during the stormy periods was generated by an increase in erosion and resuspension of mudflat and tidal creek sediments, while only a minimal contribution was given by the sediments transported from outside the intertidal system. This suggests that, in the long term, storms will promote salt marsh vertical accretion but might simultaneously reduce the overall larger-scale sediment availability with implications for the marsh lateral retreat. • Storms contribute significantly to salt marsh sediment budget. • There are uncertainties as to what extent this contribution benefits the marsh platform. • Field data showed that storms benefit marsh vertical accretion but limit lateral expansion. [ABSTRACT FROM AUTHOR]

This manuscript reviews the progresses made in the understanding of the dynamic interactions between coastal storms and salt marshes, including the dissipation of extreme water levels and wind waves across marsh surfaces, the geomorphic impact of storms on salt marshes, the preservation of hurricanes signals and deposits into the sedimentary records, and the importance of storms for the long term survival of salt marshes to sea level rise. A review of weaknesses, and strengths of coastal defences incorporating the use of salt marshes including natural, and hybrid infrastructures in comparison to standard built solutions is then presented. Salt marshes are effective in dissipating wave energy, and storm surges, especially when the marsh is highly elevated, and continuous. This buffering action reduces for storms lasting more than one day. Storm surge attenuation rates range from 1.7 to 25 cm/km depending on marsh and storms characteristics. In terms of vegetation properties, the more flexible stems tend to flatten during powerful storms, and to dissipate less energy but they are also more resilient to structural damage, and their flattening helps to protect the marsh surface from erosion, while stiff plants tend to break, and could increase the turbulence level and the scour. From a morphological point of view, salt marshes are generally able to withstand violent storms without collapsing, and violent storms are responsible for only a small portion of the long term marsh erosion. Our considerations highlight the necessity to focus on the indirect long term impact that large storms exerts on the whole marsh complex rather than on sole after-storm periods. The morphological consequences of storms, even if not dramatic, might in fact influence the response of the system to normal weather conditions during following inter-storm periods. For instance, storms can cause tidal flats deepening which in turn promotes wave energy propagation, and exerts a long term detrimental effect for marsh boundaries even during calm weather. On the other hand, when a violent storm causes substantial erosion but sediments are redistributed across nearby areas, the long term impact might not be as severe as if sediments were permanently lost from the system, and the salt marsh could easily recover to the initial state. [ABSTRACT FROM AUTHOR]

The hydrodynamic and residual transport patterns arising from oscillating tidal motion have important consequences for the transport of sediments, and for the evolution of the shoreline, especially under macro- and meso-tidal conditions. For many locations there are significant uncertainties about residual currents and sediment transport characteristics, and their possible influence on the morphological evolution of the coastline and on the character of the bed. Herein we use the coastline of SE England as a test case to investigate possible changes in residual currents, and residual transport patterns from neap to spring tide, the reciprocal interaction between residuals and the character of the bed, and the morphological evolution of the coastline at a century timescale. We found that in the long term the morphology of the system evolves toward a dynamic equilibrium configuration characterized by smaller, and spatially constant residual transport patterns. While the spatial distribution of residual currents maintains a similar trend during both neap and spring tide, during spring tide and for large areas residual currents switch between northerly and southerly directions, and their magnitude is doubled. Residual eddies develop in regions characterized by the presence of sand bars due to the interaction of the tide with the varying topography. Residual transport patterns are also computed for various sediment fractions, and based on the hydrodynamics and sediment availability at the bottom. We found that the distribution of sediments on the bed is significantly correlated with the intensity of residuals. Finally, the majority of long-term morphological changes tend to develop or augment sand banks features, with an increase in elevation and steepening of the bank contours. [ABSTRACT FROM AUTHOR]

Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. It is still unclear how resistant salt marshes are to extreme storms and whether they can survive multiple events without collapsing. Based on a large dataset of salt marsh lateral erosion rates collected around the world, here, we determine the general response of salt marsh boundaries to wave action under normal and extreme weather conditions. As wave energy increases, salt marsh response to wind waves remains linear, and there is not a critical threshold in wave energy above which salt marsh erosion drastically accelerates. We apply our general formulation for salt marsh erosion to historical wave climates at eight salt marsh locations affected by hurricanes in the United States. Based on the analysis of two decades of data, we find that violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. In contrast, moderate storms with a return period of 2.5 mo are those causing the most salt marsh deterioration. Therefore, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans. [ABSTRACT FROM AUTHOR]

The hydrodynamics of distributary channels has tremendous impact on nutrient and dissolved oxygen circulation, transport of sediments, and delta formation and evolution; yet many processes acting at the river–marine interface of a delta are poorly understood. This paper investigates the combined effect of river hydrograph and micro-tides on the hydrodynamics of a delta distributary. As the ratio between river flow to tidal flow increases, tidal flood duration at the distributary mouth decreases, up to the point when flow reversal is absent. Field measurements in a distributary of the Apalachicola Delta, Florida, USA, reveal that, once the flow becomes unidirectional, high-discharge events magnify tidal velocity amplitudes. On the contrary, while the flow is bidirectional, increasing fluvial discharge decreases tidal velocity amplitudes down to a minimum value, reached at the limit between bidirectional and unidirectional flow. Due to the different response of the system to tides, the transition from a bidirectional to a unidirectional flow triggers a change in phase lag between high water and high water slack. In the presence of high riverine flow, tidal dynamics also promote seaward directed Eulerian residual currents. During discharge peaks, these residual currents almost double mean velocity values. Our results show that, even in micro-tidal environments, tides strongly impact distributary hydrodynamics during both high and low fluvial discharge regimes. [ABSTRACT FROM AUTHOR]

We present high-resolution field measurements of five sites along the United States Atlantic Coast, and cellular automata simulations, to investigate the erosion of marsh boundaries by wave action. According to our analysis, when salt marshes are exposed to high wave energy conditions their boundaries erode uniformly. The resulting erosion events follow a Gaussian distribution, yielding a relatively smooth shoreline. On the contrary, when wind waves are weak and the local marsh resistance is strong, jagged marsh boundaries form. In this case, erosion episodes have a long-tailed frequency magnitude distribution with numerous low-magnitude events, but also high-magnitude episodes. The logarithmic frequency magnitude distribution suggests the emergence of a critical state for marsh boundaries, which would make the prediction of failure events impossible. Internal physical processes allowing salt marshes to reach this critical state are geotechnical and biological, and related to the nonhomogeneity of salt marshes whose material discontinuities act as stress raisers. [ABSTRACT FROM AUTHOR]

Salt marshes are important ecosystems but their resilience to sea-level rise and possible increases in storm intensity is largely uncertain. The current paradigm is that a positive sediment budget supports the survival and accretion of salt marshes while sediment deprivation causes marsh degradation. However, few studies have investigated the combined impact of sea-level rise and increased storm intensity on the sediment budget of a salt marsh. This study investigates marsh resilience under the combined impact of various storm surge (0 m, 0.25 m, 0.5 m, 1.0 m, 2.0 m, 3.0 m and 4.0 m) and sea-level (+0 m, +0.3 m, +0.5 m, +0.8 m and +1.0 m) scenarios by using a sediment budget approach and the hydrodynamic model Delft3D. The Ribble Estuary, North-West England, whose salt marshes have been anthropogenically restored and have a high economic and environmental value, has been chosen as test case. We conclude that storm surges can positively contribute to the resilience of the salt marsh and estuarine system by promoting flood dominance and by triggering a net import of sediment. Conversely, sea-level rise can threaten the stability of the marsh by promoting ebb dominance and triggering a net export of sediment. Our results suggest that storm surges have a general tendency to counteract the decrease in sediment budget caused by sea-level rise. The timing of the storm surge relative to high or low tide, the duration of the surge, the change in tidal range and vegetation presence can also cause minor changes in the sediment budget. • Storm surges can positively contribute to the resilience of salt marshes. • Sea-level rise can threaten the stability of salt marshes. • Intense storms can counteract the negative impact of sea-level rise. [ABSTRACT FROM AUTHOR]

There has been a decline in fluvial sediment inputs to the world's deltas but it is still unclear to which extent upstream changes in sediment supply directly relate to variations in wetlands extent. We address this gap by investigating the contribution of marine and fluvial sediment sources to tidal flats in Changjiang Delta. Surprisingly, field measurements show that tidal flats accreted despite a > 50% decline in fluvial suspended sediment discharge due to the Three Gorges Dam. Results show that at a decadal time scale only a minimal portion of fluvial sediments contribute to sediment deposition on tidal flats while the adjacent estuary and ocean zone contributed over 90% of the deposit. We conclude that marine sediments give a substantial contribution to the maintenance of coastal wetlands and that there can be a long-time lag between human interventions and the delta's response to the decline in sediment supply. • Model and field data used to study the influence of marine and fluvial sediments on wetlands. • Marine sediments substantially contribute to the maintenance of wetlands. • Changjiang delta accreted in spite of fluvial sediment reduction. [ABSTRACT FROM AUTHOR]

The impact of tide-induced morphological changes and water level variations on the sediment transport in a tidally dominated system has been investigated using the numerical model Delft3D and South-East England as a test case. The goal of this manuscript is to explore the long-term changes in morphology due to sea level rise and the large-scale morphodynamic equilibrium of the South-East England. Our results suggest that the long term (century scale) tidally-induced morphological evolution of the seabed slows down in time and promotes a vanishing net transport across the large scale system. Century-scale morphologically updated simulations show that both morphological changes and net transport values tend to decrease in time as the system attains a dynamic equilibrium configuration. Results further suggest that the presence of a gradual increase in mean sea level accelerates the initial morphological evolution of the system whose morphological rate of change gradually attains, however, same plateau values as in the absence of sea level rise. Given the same base morphology, increasing water levels enhance residual currents and the net transport near the coastline; and vice-versa, decreasing sea levels minimize both residuals and net transport near the coastline. The areas that are more affected by, water level and morphological changes, are the ones where the net transport is the highest. This manuscript explores and allows extending the idea of morphodynamic equilibrium at a regional scale, larger than the one for which this concept has been generally explored i.e., estuarine scale. [ABSTRACT FROM AUTHOR]

Coastal flood warning and design of coastal protection schemes rely on accurate estimations of water level and waves during hurricanes and violent storms. These estimations frequently use numerical models, which, for computational reasons, neglect the interaction between the hydrodynamic and wave fields. Here, we show that neglecting such interactions, or local effects of atmospheric forcing, causes large uncertainties, which could have financial and operational consequences because flood warnings are potentially missed or protection schemes underdesigned. Using the Severn Estuary, SW England, we show that exclusion of locally generated winds underestimates high water significant wave height by up to 90.1%, high water level by 1.5%, and hazard proxy (water level + 1/2 significant wave height) by 9.1%. The uncertainty in water level and waves is quantified using a system to model tide‐surge‐wave conditions, Delft3D‐FLOW‐WAVE in a series of eight model simulations for four historic storm events. Plain Language Summary: Coastal zones worldwide are subject to combined effects of astronomical tides, meteorological storms surges, waves, and wind during storms and hurricanes, which can lead to flooding, property damage, and casualties. Coastal communities and critical infrastructure rely on accurate water level and wave forecasts to mitigate these combined hazards. Forecasts utilize hydrodynamic numerical models, which need to accurately represent these hazards and how they interact with, and feedback to, each other. This study uses a model, Delft3D‐FLOW‐WAVE, to calculate how tides and waves from four historic storm events combine to contribute to water level, wave height, and hazard proxy (water level + 1/2 wave height) in the Severn Estuary, southwest England. Additional simulations are run to show how local winds can further contribute to the hazard. Results show that including locally generating winds in simulations of water level, wave height, and hazard proxy is most important for accurate representation of physical processes that contribute to coastal hazards. Excluding locally generated winds from numerical model predictions could mean that flood alerts, warnings, and evacuation orders are missed, or coastal protection schemes are underdesigned, potentially leading to more flooding. Key Points: Delft3D‐FLOW‐WAVE is used to quantify uncertainty in water level and waves in a hypertidal estuary due to coupling and forcing processesExclusion of locally generated winds underestimates significant wave height at high water by up to 90.1% in the outer estuaryLocal atmospheric forcing is vital in coastal hazard predictions for timely flood warnings and to avoid underdesigning protection schemes [ABSTRACT FROM AUTHOR]

Wetland shorelines around the world are susceptible to wave erosion. Previous work has suggested that the lateral erosion rate of their cliff-like edges can be predicted as a function of intercepting waves, and yet numerous field studies have shown that other factors, for example, tidal currents or masswasting of differing soil types, induce a wide range of variability. Our objective was to isolate the unique effects of wave heights, wavelengths, and water depths on lateral erosion rates and then synthesize a mechanistic understanding that can be applied globally. We found a potentially universal relationship, where the lateral erosion rates increase exponentially as waves increase in height but decrease exponentially as waves become longer in length. These findings suggest that wetlands and other sheltered coastlines likely experience outsized quantities of erosion, as compared to oceanic-facing coastlines. [ABSTRACT FROM AUTHOR]

Abstract Wave overtopping and subsequent coastal flood hazard is strongly controlled by wind and water levels, and is especially critical in hyper-tidal estuaries where even small changes in wave heights can be catastrophic if they are concurrent with high spring tide. Wave hazard in estuaries is largely attributed to high amplitude shorter period, locally generated wind waves; while low amplitude longer period waves rarely impact low-lying coastal zones up-estuary. Here, the effect of wind and wave properties on up-estuary wave propagation and the sensitivity of significant wave height are investigated numerically along the shoreline of the Severn Estuary, southwest England, as an example. Representative values for wind speed and direction, wave height, period and direction are used to identify key combinations of factors that define the wave hazard generation. High amplitude, short period wind waves are sensitive to opposing winds, with a steepening effect that varies along the estuary shoreline, highlighting the effect of estuarine geometry on wave hazard. Low amplitude, long period wind waves respond with maximum variability in significant wave height to strong winds resulting in their propagation further up-estuary. Our results advance current understanding of the compound interaction between wind and waves, and identify critical conditions maximizing the hazard and hazard variability along the shoreline. The outcomes from this research can help to avoid economic losses from operational downtime in ports and harbors, inform sustainable coastal sea defense design and understand how wave hazard may vary under future climate due to changing storm tracks. Results can also be applied to the design of coastal infrastructure and facilitation of emergency response planning. Highlights • Key combinations of wind wave factors simulated to define wave hazard. • Opposing winds steepen high amplitude waves to increase overtopping hazard. • Stronger winds amplify low amplitude waves and facilitate propagation up-estuary. • Results can inform sea defense design and reduce losses from operational downtime. [ABSTRACT FROM AUTHOR]

Storm surge is often the greatest threat to life and critical infrastructures during hurricanes and violent storms. Millions of people living in low-lying coastal zones and critical infrastructure within this zone rely on accurate storm surge forecast for disaster prevention and flood hazard mitigation. However, variability in residual sea level up-estuary, defined here as observed sea level minus predicted tide, can enhance total water levels; variability in the surge thus needs to be captured accurately to reduce uncertainty in site specific hazard assessment. Delft3D-FLOW is used to investigate surge variability, and the influence of storm surge timing on barotropic tide-surge propagation in a tide-dominant estuary using the Severn Estuary, south-west England, as an example. Model results show maximum surge elevation increases exponentially up-estuary and, for a range of surge timings consistently occurs on the flood tide. In the Severn Estuary, over a distance of 40 km from the most upstream tide gauge at Oldbury, the maximum surge elevation increases by 255%. Up-estuary locations experience short duration, high magnitude surge elevations and greater variability due to shallow-water effects and channel convergence. The results show that surge predictions from forecasting systems at tide gauge locations could under-predict the magnitude and duration of surge contribution to up-estuary water levels. Due to the large tidal range and dynamic nature of hyper-tidal estuaries, local forecasting systems should consider changes in surge elevation and shape with distance up-estuary from nearby tide gauge sites to minimize uncertainties in flood hazard assessment. [ABSTRACT FROM AUTHOR]

The performance of sewer networks has important consequences from an environmental and social point of view. Poor functioning can result in flood risk and pollution at a large scale. Sediment deposits forming in sewer trunks might severely compromise the sewer line by affecting the flow field, reducing cross-sectional areas, and increasing roughness coefficients. In spite of numerous efforts, the morphological features of these depositional environments remain poorly understood. The interface between water and sediment remains inefficiently identified and the estimation of the stock of deposit is frequently inaccurate. In part, this is due to technical issues connected to difficulties in collecting accurate field measurements without disrupting existing morphologies. In this paper, results from an extensive field campaign are presented; during the campaign a new survey methodology based on acoustic techniques has been tested. Furthermore, a new algorithm for the detection of the soil-water interface, and therefore for the correct esteem of sediment stocks is proposed. Finally, results in regard to bed topography, and morphological features at two different field sites are presented and reveal that a large variability in bed forms is present along sewer networks. [ABSTRACT FROM AUTHOR]

The synthesis of two derivatized NiII Schiff-base complexes is reported. Self-assembled monolayers (SAMs) have been obtained by reaction of coupling layers, functionalized glass substrates and the derivatized complex precursors. The self-assembled films have been characterized by contact angle measurements, X-ray photoelectron spectroscopy, and UV–Vis absorption spectroscopy. A structure of these SAMs is proposed on the basis of spectroscopic data and molecular metrical parameters. [Copyright &y& Elsevier]

Global assessments predict the impact of sea-level rise on salt marshes with present-day levels of sediment supply from rivers and the coastal ocean. However, these assessments do not consider that variations in marsh extent and the related reconfiguration of intertidal area affect local sediment dynamics, ultimately controlling the fate of the marshes themselves. We conducted a meta-analysis of six bays along the United States East Coast to show that a reduction in the current salt marsh area decreases the sediment availability in estuarine systems through changes in regional-scale hydrodynamics. This positive feedback between marsh disappearance and the ability of coastal bays to retain sediments reduces the trapping capacity of the whole tidal system and jeopardizes the survival of the remaining marshes. We show that on marsh platforms, the sediment deposition per unit area decreases exponentially with marsh loss. Marsh erosion enlarges tidal prism values and enhances the tendency toward ebb dominance, thus decreasing the overall sediment availability of the system. Our findings highlight that marsh deterioration reduces the sediment stock in back-barrier basins and therefore compromises the resilience of salt marshes. [ABSTRACT FROM AUTHOR]

Wave overtopping and subsequent coastal flood hazard is strongly controlled by wind and water levels, and is especially critical in hyper-tidal estuaries where even small changes in wave heights can be catastrophic if they are concurrent with high spring tide. Wave overtopping imposes serious hazard in heavily populated and industrialized hyper-tidal estuaries, where infrastructure, transport networks and natural resources may be located. Wave hazard in estuaries is largely attributed to high amplitude shorter period, locally generated wind waves; while low amplitude longer period waves rarely impact low-lying coastal zones up-estuary. The effect of wind and wave properties on up-estuary wave propagation and the sensitivity of significant wave height are investigated numerically along the shoreline of the Severn Estuary, southwest England, as an example of hyper-tidal estuaries worldwide. Delft3D-WAVE is used to simulate significant wave height with representative values for wind speed and direction, wave height, period and direction used to identify key combinations of factors that define the wave hazard generation in the Severn Estuary. The model confirms high amplitude, short period wind waves are sensitive to opposing winds, with a steepening effect that varies along the estuary shoreline, highlighting the effect of estuarine geometry on wave hazard. Low amplitude, long period wind waves respond with greatest variability in significant wave height to strong winds resulting in their propagation further up-estuary. Our results advance current understanding of the compound interaction between wind and waves, and identify critical conditions maximizing the hazard and hazard variability along the shoreline. The outcomes from this research can help to avoid economic losses from operational downtime in ports and harbours, inform sustainable coastal sea defence design and understand how wave hazard may vary under future climate due to changing storm tracks. Results can also long-term coastal defence and management strategies, as sustainable coastal management requires confidence in the knowledge of any possible future changes to wave hazard. [ABSTRACT FROM AUTHOR]

Seagrasses are marine flowering plants that provide key ecological services. In recent decades, multiple stressors have caused a worldwide decline in seagrass beds. Changes in bottom friction associated with seagrass loss are expected to influence the ability of estuarine systems to trap sediment inputs through local and regional changes in hydrodynamics. Herein, we document a numerical study using six historical maps of seagrass distribution in Barnegat Bay, USA, to demonstrate that reductions in seagrass coverage destabilize estuarine systems, decreasing flood-dominance in areas affected by seagrass disappearance and increasing bed-shear stress values across the entire back-barrier basin. Furthermore, we reveal how seagrass decline has considerably increased the impact of wind-waves on marsh edges between 1968 and 2009. From a comparison with a numerical experiment without submerged aquatic vegetation, we estimate that up to 40% of the computed wave thrust on marsh boundaries can be reduced by seagrass beds and we find that the location of a seagrass patch in addition to its aerial extent plays a crucial role in this attenuation process. This study highlights the benefits of seagrass meadows in enhancing estuarine resilience and reducing marsh-edge retreat by wind-wave attack, which is recognized as a chief agent in lateral marsh loss. [ABSTRACT FROM AUTHOR]

Abstract: Seagrasses are marine flowering plants that strongly impact their physical and biological surroundings and are therefore frequently referred to as ecological engineers. The effect of seagrasses on coastal bay resilience and sediment transport dynamics is understudied. Here we use six historical maps of seagrass distribution in Barnegat Bay, USA, to investigate the role of these vegetated surfaces on the sediment storage capacity of shallow bays. Analyses are carried out by means of the Coupled‐Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) numerical modeling framework. Results show that a decline in the extent of seagrass meadows reduces the sediment mass potentially stored within bay systems. The presence of seagrass reduces shear stress values across the entire bay, including unvegetated areas, and promotes sediment deposition on tidal flats. On the other hand, the presence of seagrasses decreases suspended sediment concentrations, which in turn reduces the delivery of sediment to marsh platforms. Results highlight the relevance of seagrasses for the long‐term survival of coastal ecosystems, and the complex dynamics regulating the interaction between subtidal and intertidal landscapes. [ABSTRACT FROM AUTHOR]

The energetic, macrotidal shelf off South West England was used to investigate and classify the impact of different tide and wave conditions and their interactions on sand transport at a regional scale using a coupled hydrodynamic, wave and sediment transport model (Delft3D). Residual currents and sediment transport patterns are important for the transport and distribution of littoral and shelf sea sediments, the morphological evolution of the coastal and inner continental shelf zones and coastal planning.Sand transport magnitude and direction across this macrotidal environment are heavily influenced by waves. Median (50% exceedance) and extreme (1% exceedance) wave conditions were simulated during neap and spring tides. Effects such as areas shifting from flood- to ebb-dominant sand transport, reversals in the dominant sand transport path and activation of sand transport at depths of 120m were predicted under extreme forcing relative to tide-only and median wave conditions. Wave-tide interactions cause a significant non-linear enhancement of sand transport magnitude, as determined by the difference between sand transport magnitude in a coupled model and its wave-only and tide-only components. A new continental shelf classification scheme was created based on sand transport magnitude due to wave-forcing, tide-forcing and non-linear wave-tide interactions. Non-linear interactions are the dominant forcing mechanism for sand transport across most of this macrotidal environment during extreme waves at springs. At neaps, non-linear interactions drive a significant proportion of sand transport under median and extreme waves despite negligible tide-only sand transport. This emphasises the critical need to consider wave-tide interactions when considering sand transport in energetic environments globally. The above classification method requires a computationally expensive coupled numerical model, and the next step is to make it more generally applicable based on readily available tide and wave characteristics. A weighted K-nearest neighbour classifier was used to predict wave, tide or non-linear interaction dominance of net sand transport based on readily available tide and wave parameters. Of the parameters tested, tide range, mean significant wave height, mean peak period, mean wave direction, water depth and maximum current speed and direction were identified as predictors. Classifications were predicted with an overall accuracy of 81.3% using all identified predictors and 74% using a subset of tide range, mean significant wave height and water depth, based on 5-fold cross validation. A simplified classification scheme was predicted with 93.1% accuracy for all predictors and 88.9% for the subset. This classification scheme may be used to identify the primary forcing mechanisms and relative magnitudes of net sand transport across the shelf across a range of wave-tide regimes, using only readily available wave and tide data as inputs, with benefits to coastal researchers and planners working in energetic, sandy environments worldwide. [ABSTRACT FROM AUTHOR]