How waves shape salt marshes

Leonardi, Nicoletta; Fagherazzi, Sergio

doi:10.1130/g35751.1

Cited by 83 publications

(95 citation statements)

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“…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).…”

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“…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)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Recent research results show that salt marshes reduce wave energy during storms and possibly, mitigate storm surges (13)(14)(15).…”

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

Leonardi

Ganju

Fagherazzi

2015

Proc. Natl. Acad. Sci. U.S.A.

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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...

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

Leonardi

Ganju

Fagherazzi

2015

Proc. Natl. Acad. Sci. U.S.A.

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239

179

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“…The marshes in Shell Bay display jagged outlines, indicative of low wave and tidal current stress (Leonardi and Fagherazzi, 2014). The Stour Estuary marshes (S2) 6 km upstream of the mesotidal Stour mouth are subject to a spring tidal range of 3.8 m and fluviotidal currents due to their estuarine fringing position (sensu Allen, 2000) and therefore display more linear boundaries.…”

Section: Test Sitesmentioning

confidence: 99%

Unsupervised detection of salt marsh platforms: a topographic method

2018

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Abstract. Salt marshes filter pollutants, protect coastlines against storm surges, and sequester carbon, yet are under threat from sea level rise and anthropogenic modification. The sustained existence of the salt marsh ecosystem depends on the topographic evolution of marsh platforms. Quantifying marsh platform topography is vital for improving the management of these valuable landscapes. The determination of platform boundaries currently relies on supervised classification methods requiring near-infrared data to detect vegetation, or demands labourintensive field surveys and digitisation. We propose a novel, unsupervised method to reproducibly isolate salt marsh scarps and platforms from a digital elevation model (DEM), referred to as Topographic Identification of Platforms (TIP). Field observations and numerical models show that salt marshes mature into subhorizontal platforms delineated by subvertical scarps. Based on this premise, we identify scarps as lines of local maxima on a slope raster, then fill landmasses from the scarps upward, thus isolating mature marsh platforms. We test the TIP method using lidar-derived DEMs from six salt marshes in England with varying tidal ranges and geometries, for which topographic platforms were manually isolated from tidal flats. Agreement between manual and unsupervised classification exceeds 94 % for DEM resolutions of 1 m, with all but one site maintaining an accuracy superior to 90 % for resolutions up to 3 m. For resolutions of 1 m, platforms detected with the TIP method are comparable in surface area to digitised platforms and have similar elevation distributions. We also find that our method allows for the accurate detection of local block failures as small as 3 times the DEM resolution. Detailed inspection reveals that although tidal creeks were digitised as part of the marsh platform, unsupervised classification categorises them as part of the tidal flat, causing an increase in false negatives and overall platform perimeter. This suggests our method may benefit from combination with existing creek detection algorithms. Fallen blocks and high tidal flat portions, associated with potential pioneer zones, can also lead to differences between our method and supervised mapping. Although pioneer zones prove difficult to classify using a topographic method, we suggest that these transition areas should be considered when analysing erosion and accretion processes, particularly in the case of incipient marsh platforms. Ultimately, we have shown that unsupervised classification of marsh platforms from high-resolution topography is possible and sufficient to monitor and analyse topographic evolution.

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“…However, many natural forces are also erosive, including waves formed by winds over long stretches of water (Pye 1995;Schwimmer 2001;Leonardi et al 2016), but these waves can be larger when water becomes deepened by sealevel rise, excessive dredging, or other reasons. Tidal currents can induce meander patterns of erosion and redeposition (Biggs 1982;Kearney et al 1988;Kleinhans et al 2009) and wind-driven wave action can also have strong effects (Fagherazzi and Wiberg 2008;Leonardi and Fagherazzi 2014;Priestas et al 2015). The interaction of sea-level rise and sediment deposition rates can also cause the widening of creeks (Fagherazzi et al 2012).…”

Section: Communicated By Carles Ibanez Martimentioning

confidence: 99%

Long-Term Erosional Trends Along Channelized Salt Marsh Edges

Browne

2017

Estuaries and Coasts

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Salt marshes provide important habitats for many species in the estuaries along the east and Gulf coasts of North America. With many species dependent on these coastal marshlands and extensive documentation that these marshlands are disappearing, a clear understanding of the mechanisms causing loss is critical. Much of the salt marsh was lost to reclamation and construction before these activities were curtailed circa 1970; however, losses due to other causes have continued and multiple hypothesized causes have been proposed, not all mutually exclusive. Yet it remains unclear whether there are legacy effects from the reclamation projects. When the edges of salt marshes are cut into, and gentle vegetated slopes are replaced by sharp edges adjoining deep water of 2 m or more, erosion could accelerate and could continue for many years. One method that may help shed light on the relative importance of the various causes of salt marsh erosion would be to compare the erosion rates of specific edges within a marshland that are exposed to particular conditions. We therefore used several sets of aerial photography spanning 84 years to track the changes at specific edge locations along marsh edges and then make comparisons between anthropogenically created edges and naturally created edges, including comparisons within use and width categories of navigational channels. Erosion rates were found to remain significantly higher on channelized edges than along otherwise similar wetland edges even several decades after modification. Likely reasons include the continued exposure of underlying layers that lack reinforcing plant root systems, vertical edges that are more vulnerable to undermining from wave action, and increased erosion related to altered tidal flows.

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How waves shape salt marshes

Cited by 83 publications

References 26 publications

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

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

Unsupervised detection of salt marsh platforms: a topographic method

Long-Term Erosional Trends Along Channelized Salt Marsh Edges

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