Salt marsh erosion rates and boundary features in a shallow Bay

Abstract

Herein, we investigate the relationship between wind waves, salt marsh erosion rates, and the planar shape of marsh boundaries by using aerial images and the numerical model Coupled‐Ocean‐Atmosphere‐Wave‐Sediment‐Transport Modeling System (COAWST). Using Barnegat Bay, New Jersey, as a test site, we found that salt marsh erosion rates maintain a similar trend in time. We also found a significant relationship between salt marsh erosion rates and the shape of marsh boundaries which could be used as a geomorphic indicator of the degradation level of the marsh. Slowly eroding salt marshes are irregularly shaped with fractal dimension higher than rapidly deteriorating marshes. Moreover, for low‐wave energy conditions, there is a high probability of isolated and significantly larger than average failures of marsh portions causing a long‐tailed distribution of localized erosion rates. Finally, we confirm the existence of a significant relationship between salt marsh erosion rate and wind waves exposure. Results suggest that variations in time in the morphology of salt marsh boundaries could be used to infer changes in frequency and magnitude of external agents.

Dynamics of salt marsh margins are related to their three‐dimensional functional form

“…20-year time period of change assessed here, ramps are much more dynamic than ridge-runnels which, in turn, are more dynamic than cliffs. This concurs with previous work that has identified a linear relationship between marsh edge retreat and wave power (Marani et al, 2011;Leonardi et al, 2016). Geomorphic context and energy exposure therefore appear to strongly influence the potential for lateral changes observed at marsh margins (Leonardi and Fagherazzi, 2014).…”

“…The vertical faces of cliffs do not tend to allow the establishment of vegetation or other biofilms to increase the stability of the sediment (van De Koppel et al, 2005). Fronting mudflats may also widen through marsh retreat, either enhancing energy dissipation and reducing erosion (Mariotti and Fagherazzi, 2010) or increasing fetch, thereby enhancing wave power and, consequently, erosion (Marani et al, 2011;Mariotti and Fagherazzi, 2013;Leonardi et al, 2016). Once water levels overtop the marsh platform, forces on the face decrease (Tonelli et al, 2010) and the cliff begins to act to both reflect and shoal waves, with a proportion of the wave energy propagating beyond the margin, depending on wave-height:water-depth ratios (Möller and Spencer, 2002).…”

Seagrass Impact on Sediment Exchange Between Tidal Flats and Salt Marsh, and The Sediment Budget of Shallow Bays

“…Specifically, as the seagrass is removed the mean shear stress increases for both unvegetated (Figure 3a) and vegetated areas (Figure 3b), even if differences in previously vegetated areas are more evident (Figure 3b). A uniformly distributed input sediment concentration represents potential riverine inputs during flood conditions, or large resuspension events during storms; such situations are the major contributors of inorganic sediments to salt marsh systems (e.g., Fagherazzi & Priestas, 2010;Falcini et al, 2012;Leonardi et al, 2016Leonardi et al, , 2017. This effect also depends on plant density and tends to decrease for less dense meadows ( Figure S4).…”

Salt Marsh Loss Affects Tides and the Sediment Budget in Shallow Bays

“…The marsh coverage data came from the Center for Remote Sensing and Spatial Analysis's geographic information systems database. Different salt marsh loss scenarios are tested, which represent a uniform erosion of the marsh areas (Figure ); these are simplified cases as some marshes within the bay eroded faster than others (Leonardi, Defne, et al, ; Leonardi, Ganju, & Fagherazzi, ). Loss percentage ranges from 25% to 100% (when all vegetated areas are removed).…”

“…Around half of the interior shoreline is eroding less than 0.5 m/year or is not eroding at all; the other half is eroding at around 0.5–2 m/year, and only 2% of the marsh has erosion rates exceeding 2 m/year (Leonardi, Defne, et al, ; Leonardi, Ganju, & Fagherazzi, ). The highest erosion rates are found in the marshes surrounding Great Bay (Leonardi, Defne, et al, ). Spartina alterniflora and Spartina patens are the dominant species in tidal wetlands of the estuary (Kennish, ).…”