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There are still numerous uncertainties over the influence of anthropogenic interventions on salt marsh dynamics. This study uses the Ribble Estuary as a test case and an integrated approach of numerical modeling and paleoenvironmental analysis to investigate the contribution of embankment construction to long‐term marsh accretion. Accretion rates derived using optically stimulated luminescence dating (OSL) were combined with a multi‐proxy paleoenvironmental investigation on sediment cores extracted from the salt marsh, the mobile seafloor of the central Irish Sea and the river catchment area. These analyses provided a first evolutionary perspective on the Ribble Estuary preceding any management interventions. The paleoenvironmental analyses were then compared to simulations conducted using the hydrodynamic model Delft3D to investigate the effects of embankment construction on estuarine hydrodynamics and morphodynamics of the salt marsh over the period constrained by the OSL. The numerical simulations showed that embankments were responsible for an overall intensification of the ebb currents in the system which promoted sediment export. The paleoenvironmental analyses showed that the marsh has been accreting at a rate of 4.61 to 0.86 cm yr−1 over the last ca. 190 years and that the high sedimentation rate was caused by a naturally high rate of sediment supply. The model‐data integration showed that the effects of the embankment construction on sediment transport did not compromise the long‐term resilience of the salt marsh because of the high rates of sediment supply and the river dredging which enhanced the flood dominance of the tide near the tidal flat. Plain Language Summary: Salt marshes are important ecosystems and can help protect the coastline from flooding. Sediment availability is important to help the establishment and maintenance of salt marshes. Large quantities of sediments can help salt marshes to expand and grow in elevation. Instead, the removal of sediments can cause marsh erosion. Management interventions along a coastline can influence the movement of sediments and determine how much sediments can reach (or not reach) the marsh platform. This study found that in the Ribble Estuary, a site renowned for its high rate of sediment supply, the construction of embankments promoted sediment export out of the estuary, but the high sedimentation rates caused by a natural high rate of sediment supply from the ocean, aided by river dredging, allowed long‐term marsh accretion despite the embankments presence, meaning that the marsh continued to grow in elevation over time. Key Points: The construction of embankments along an estuary can reduce salt marsh accretion by promoting sediment exportEmbankments do not necessarily compromise the long‐term resilience of salt marshes when high rates of sediment supply are presentChannel dredging can promote sediment import and reduce the effects of embankment construction on sediment transport [ABSTRACT FROM AUTHOR]

Salt marshes are valuable ecosystems that must trap sediments and accrete in order to counteract the deleterious effect of sea level rise. Previous studies have shown that the capacity of marshes to build up vertically depends on both autogenous and exogenous processes including ecogeomorphic feedbacks and sediment supply from in‐land and coastal ocean. There have been numerous efforts to quantify the role played by the sediments coming from marsh edge erosion on the resistance of salt marshes to sea level rise. However, the majority of existing studies investigating the interplay between lateral and vertical dynamics use simplified modeling approaches, and they do not consider that marsh retreat can affect the regional‐scale hydrodynamics and sediment retention in back‐barrier basins. In this study, we evaluated the fate of the sediments originating from marsh lateral loss by using high‐resolution numerical model simulations of Jamaica Bay, a small lagoonal estuary located in New York City. Our findings show that up to 42% of the sediment released during marsh edge erosion deposits on the shallow areas of the basin and over the vegetated marsh platforms, contributing positively to the sediment budget of the remaining salt marshes. Furthermore, we demonstrate that with the present‐day sediment supply from the ocean, the system cannot keep pace with sea level rise even accounting for the sediment liberated in the bay through marsh degradation. Our study highlights the relevance of multiple sediment sources for the maintenance of the marsh complex. Key Points: The sediment eroded from marsh edges helps to build the remaining marshesMarsh erosion has a twofold effect: more sediment available for the remaining marshes but decrease in the trapping capacity of the lagoonManagement actions should consider the effects associated with wetland edge erosion at basin scale [ABSTRACT FROM AUTHOR]

Salt marshes are dynamic systems able to laterally expand, contract, and vertically accrete in response to sea level rise. Here, we present the grand challenges that need to be addressed to fully characterize marsh morphodynamics. The review focuses on physical processes and quantitative models. Without predictive models, it is impossible to determine the future marsh evolution under accelerated sea level rise. In these models, one of the challenges is to resolve both horizontal and vertical dynamics within the same framework. Vertically, the marsh has to accumulate enough material to contrast rising water levels. Horizontally, marsh erosion at the ocean side must be compensated by landward expansion in forests, lawns, and agricultural fields. The dynamics of the marsh‐upland boundary are still not fully understood and will require more research in the upcoming years. The complexity of marsh vegetation is seldom captured in predictive models of marsh evolution. More research is needed to understand the effects of each species or species assemblages on hydrodynamics and sediment transport. Here, we further advocate that a sediment budget resolving all sediment fluxes in a marsh complex is the most important metric of marsh resilience. Characterization of these fluxes will enable to connect salt marshes to other landforms and to unravel feedbacks controlling the evolution of the entire coastal system. Current models of marsh evolution rely on sparse data sets collected at few locations. Novel remote sensing techniques will provide high‐resolution spatial data that will inform a new generation of computer models. Key Points: Vertical and horizontal dynamics must be combined to understand salt marsh evolutionA sediment budget is the key variable to measure marsh resilience to sea level riseRemote sensing will provide spatial data sets to inform a new generation of numerical models of salt marsh evolution [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]

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]

A process-based numerical model is applied to investigate sediment transport dynamics and sediment budget in tide-dominated estuaries under different salt marsh erosion scenarios. Using a typical funnel-shaped estuary (Ribble Estuary, UK) as a study site, it is found that the remobilization of sediments within the estuary is increased as a result of the tidal inundation of the eroded salt marsh. The landward export of the finest sediment is also intensified. The relationship between salt marsh erosion and net landward export of sediments has been found to be non-linear—with the first 30% salt marsh erosion causing most of the predicted export. The presence of vegetation also influences the sediment budget. Results suggest that vegetation reduces the amount of sediment being transported upstream. Again, the trapping effect of salt marsh in terms of plant density is non-linear. Whilst a vegetated surface with a stem density of 64 plants/m2 decreased the net landward export of very fine sand by around 50%, a further increase in stem density from 64 to 512 plants/m2 had a relatively small effect. [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]

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]

AbstractIn the original article, Xiaorong Li’s given and family names were transposed. It is correct as shown here. The original article has been corrected. [ABSTRACT FROM AUTHOR]

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]