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.salt marsh | resilience | hurricanes | wind waves | erosion T he potential of salt marshes to serve as natural buffers against violent storms seems even more important in view of significant threats imposed by climate change, such as increased storminess and higher hurricane activity registered in the past decades (1-12). Recent research results show that salt marshes reduce wave energy during storms and possibly, mitigate storm surges (13-15). These results triggered a flurry of planned coastal restorations centered on the concept of "living shorelines" (14), which use vegetated surfaces to reduce the impact of hurricanes (13-16). However, little is known about the endurance of salt marshes against wave action and whether such ecosystems can survive extreme events.Most marsh erosion occurs at its seaward boundary, where the effect of waves is concentrated (2, 3). Wave erosion constitutes one of the main contributions to salt marsh deterioration, and even very small waves can cause failure of large salt marsh blocks (2,7,17). Despite the complexity of the problem, some studies have identified a correlation between wave energy and lateral rates of marsh erosion (18,19). Erosion of marsh edges by wave action is caused by many different mechanisms, such as the indentation of V-shaped notches into linear stretches of shoreline or cliff undercutting when lower sediment layers are eroded more rapidly than the overhanging root mats (2,17,19). Varying resistance to wave erosion can be caused by biotic and abiotic factors, such as geotechnical characteristics of the sediments (7, 20), vegetation characteristics (21), height of the marsh scarp, an...

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.

Bars and subaqueous levees often form at river mouths due to high sediment availability. Once these deposits emerge and develop into islands, they become important elements of the coastal landscape, hosting rich ecosystems. Sea level rise and sediment starvation are jeopardizing these landforms, motivating a thorough analysis of the mechanisms responsible for their formation and evolution. Here we present recent studies on the dynamics of mouth bars and subaqueous levees. The review encompasses both hydrodynamic and morphological results. We first analyze the hydrodynamics of the water jet exiting a river mouth. We then show how this dynamics coupled to sediment transport leads to the formation of mouth bars and levees. Specifically, we discuss the role of sediment eddy diffusivity and potential vorticity on sediment redistribution and related deposits. The effect of waves, tides, sediment characteristics, and vegetation on river mouth deposits is included in our analysis, thus accounting for the inherent complexity of the coastal environment where these landforms are common. Based on the results presented herein, we discuss in detail how river mouth deposits can be used to build new land or restore deltaic shorelines threatened by erosion.

[1] Mouth bars are morphological units important for deltas, estuaries, or rivers debouching into the sea. Several processes affect the formation of these deposits. This paper focuses on the role of tides on shaping mouth bars, presenting both hydrodynamic and morphodynamic results. The effect of tides is analyzed in two end-member configurations: a river with a small tidal discharge compared to the fluvial discharge (fluvial dominated) and a river with a very large tidal discharge (tidal dominated). Mouth bar formation is analyzed using the coupled hydrodynamic and morphodynamic model Delft3D. The presence of tides influences the hydrodynamics of the jet exiting the river mouth and causes an increase in the averaged jet spreading. At low tide the lower water depth in the basin promotes a drawdown water profile in the river and an accelerated flow near the mouth. The resulting velocity field is characterized by residual currents affecting growth and final shape of the mouth bar. Simulations indicate that mouth deposits are characterized by the presence of two channels for negligible tidal discharge, whereas three principal channels are present in the tidal-dominated case, with a central channel typical of tidal inlets. On the basis of our numerical analyses, we present a robust criterion for the occurrence of mouth deposits with three channels. Trifurcations form when the tidal discharge is large with respect to the fluvial one and the tidal amplitude is small compared to the water depth. Finally, predicted mouth bar morphologies are compared with good agreement to river mouths in the Gulf of Mexico, USA.

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.