Quantification of Swell Energy and Its Impact on Wetlands in a Deltaic Estuary

Abstract

The extensive coastal wetlands in Mississippi River Delta represent the seventh largest deltaic floodplain in the world, contributing to many services that sustain the economies of the region. Subsidence, sea level rise, saltwater intrusion, wave action from storms, and sediment depletion have contributed to chronic wetland losses, converting vegetated lands into open waters and increasing wind fetch. Among these factors listed, wave energy plays the largest role in marsh edge erosion in an open bay environment. Degrading barrier islands along the shoreline of this delta allow swell energy to enter protected bay areas, contributing to marsh edge erosion. Locally generated wind waves within enlarged bays also contribute to wetland loss. Quantifying the roles of swell and wind waves in marsh edge erosion is essential to any ecosystem restoration design. In this study, a numerical model is implemented to describe the wave climate of combined swell and wind waves in a deltaic estuary. Terrebonne Bay was chosen as the study area because it has experienced one of the largest reductions in barrier islands and wetland loss rates among Louisiana estuaries. A continuous wave measurement in upper Terrebonne Bay was obtained over the course of a year. A spectral wave model is used to hindcast the wave climate in the estuary. The model results are compared against the in situ wave measurement. The wave power is partitioned into swell and wind sea at different locations in Terrebonne Bay using the model results. An extensive analysis on a valid effective wave power range that directly impacts the marsh edge is performed and presented. Insight into the temporal and spatial variability of wave power is gained. Through differentiating swell and wind sea energies around the bay, improvements of longterm wave power computation for shoreline retreat prediction are made. It is found that the swell energy becomes the primary driver of marsh edge retreat in the southwest part of Terrebonne Bay as the barrier islands are degrading.

Fetch and distance from the bay control accretion and erosion patterns in Terrebonne marshes (Louisiana, USA)

“…Marshes facing open bays are more subject to wind‐waves effects. In Terrebonne, Everett et al (2018) showed that rates of edge retreat are correlated with wave power. Waves resuspend sediments from the bottom by applying shear stress (Booth et al, 2000; Fagherazzi & Priestas, 2010; Green & Coco, 2014) and erode marsh edges at the same time (Leonardi & Fagherazzi, 2014; Marani et al, 2011; Priestas et al, 2015).…”

“…For instance, Valentine et al (2021) found that in Terrebonne Bay, brackish marshes located inland tend to degrade as a result of drowning and collapse, while the saline marshes facing the bay tend to degrade due to edge erosion. Everett et al (2018), by employing a wave spectral model, were able to relate wave power to edge retreat and quantify the contribution of swell and wind waves generated within Terrebonne Bay. Bendoni et al (2019) used an improved version of the XBeach model to assess wave impact and morphological changes of the marsh edge profile.…”

Basin-Scale Model for Predicting Marsh Edge Erosion

“…Delft3D provided water levels and currents for the SWAN model. Following Everett et al (2017), the validated SWAN and Delft3D models were employed in Terrebonne Bay to determine the wind-sea wave power generated by winds blowing over the shallow estuaries and the swell power from entering the estuaries form the Gulf of Mexico. The locations of wave power and retreat rates were correlated to shear strength for the development of marsh edge erosion model.…”

Brackish marshes erode twice as fast as saline marshes in the Mississippi Delta region

“…The barrier islands enclosing the south side of Terrebonne Bay are more fragmented compared to those in Barataria Bay, allowing more swell waves to enter the bay and leading to greater edge retreat. Previous studies have shown the importance of swell within Terrebonne, and how this is related to higher marsh edge retreat rates (Everett et al, 2019). On the other hand, only small areas within Barataria Bay are subject to swell wave energy.…”

“…We determined a priori that these sites were likely largely influenced by wind wave erosion, and that they had no large obvious influence of man-made alterations or fault activity. Calibrations in Barataria Bay were done on the regions with fieldwork presented in this study, while calibrations for Terrebonne Bay were done for areas with minimal swell influence, as described from fieldwork by Everett et al, (2019). The model was calibrated to each of these areas, and the erodibility and asymmetry that achieved the best fit for a given salinity zone (saline and brackish) was used for the entire salinity zone domain.…”