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

Thomas Everett,Robert Twilley,Qin Chen,Arash Karimpour

doi:10.1007/s12237-018-0454-z

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 long-term 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.

Highlights

The Mississippi River Delta consists of extensive coastal wetlands within deltaic estuaries on Louisiana’s coastline
Wave power values calculated from the SWAN model results are tested against the observed wave power at the wave gauge deployment site
Swell energy is less affected by local winds; the highest values of the Pb,swell and Pb,sea occur simultaneously

Summary

Introduction

The Mississippi River Delta consists of extensive coastal wetlands within deltaic estuaries on Louisiana’s coastline. Communicated by Arnoldo Valle-Levinson wetlands provide significant defense mechanism against hurricane storm surges, limiting their propagation inland, reducing threats to infrastructure and the lives of both people and wildlife (Hu et al 2015; Wamsley et al 2010). Such services of coastal wetlands are currently under great strain from a variety of coastal proccesses including relative sea level rise, saltwater intrusion, storms, and sediment deprivation. Chronic wetland losses have converted vegetated lands into open waters and increased wind fetch This increase in wind fetch generates larger waves, which in turn leads to greater erosive forces that impact the soil stability of wetlands

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