A linear relationship between wave power and erosion determines salt-marsh resilience to violent storms and hurricanes

Abstract

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...

Establishing a sediment budget in the newly created “Kleine Noordwaard” wetland area in the Rhine–Meuse delta

“…According to the approach of Fagherazzi and Wiberg (2009), which estimates the maximum wave height and accompanying bed shear stress, wind waves do not exceed a height of 8 cm for wind speeds up to 15 m s −1 under these conditions of water depth and fetch length. In accordance with Leonardi et al (2016), who have shown that bank deterioration is lin-early related to wave energy, the low rate of cut bank retreat in the study area can be attributed to the low wave height. Higher wind waves and an inherent larger rate of cut bank retreat can only occur at the boundary of the terrestrial zone in the northeast of the area and at the southwest edge of the island.…”

Establishing a sediment budget in the newly created Kleine Noordwaard wetland area in the Rhine-Meuse delta

“…This is 30 probably the major reason for the low rates of cut bank retreat in the majority of the study area. In accordance with Leonardi et al (2016) who have shown that bank deterioration is linearly related to wave energy the low rate of cut bank retreat in the study area can be attributed to the low wave height. Higher wind waves and an inherent larger rate of cut bank retreat can only occur at the boundary of the terrestrial zone in the northeast of the area and at the southwest edge of the island.…”

Revisiting salt marsh resilience to sea level rise: Are ponds responsible for permanent land loss?

“…Marshes with large tidal ranges, even if more able to accrete vertically [ Kirwan et al , ], are not immune to this type of loss [ Leonardi and Fagherazzi , ]. Marsh edge retreat is generally gradual [ Leonardi et al , ], and it occurs where fetches are large [ Schwimmer , ], for example, at the marsh boundary with bays and lagoons [ Marani et al , ; Mariotti and Fagherazzi , ]. As such, wave‐induced erosion and drowning are put into two different categories: the former is classified as edge loss and the latter as interior loss [ Mariotti and Carr , ].…”