The role of wind waves on salt marsh morphodynamics
Priestas, Anthony Michael
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The stability and survival of salt marshes is typically linked to the competing influences of sea-level rise, subsidence, and sediment accumulation and erosion. However, consideration must also be made for wind waves that regulate the erosion of salt marsh shorelines and resuspend sediments in bordering tidal flats thus providing material for marsh accretion. This thesis examines the mechanisms in which wind waves affect marsh morphology, the mechanisms of salt marsh boundary erosion, in addition to linking the processes responsible for sediment mobilization between tidal flats and adjacent salt marshes. Sediment concentration within an open-coast marsh creek along the Louisiana chenier plain is shown to be related to the local wave climate and channel velocity. Calculations of sediment fluxes during ebb and flood tides indicates that while large volumes of sediment are mobilized into the marsh when wind waves are present, only a small portion is stored during each tidal cycle. In the coastal lagoon setting of Hog Island Bay, Virginia, marsh shoreline erosion rates were estimated from direct surveys and through analysis of aerial photographs. Erosion rates averaged 1.3 m/yr, similar to the 50-year historical average determined from previous work at the same location. Based on a calibrated numerical model for wind waves, the average erosion rate was linked to the energy of the waves attacking the marsh boundary. Additionally, results suggest that the effect of large waves forming during storms on erosion rates is negligible. Variations in erosion rates were linked to shoreline sinuosity (a proxy used to describe the result of wave concentration through erosive gullies), sediment characteristics, faunal activity, and marsh elevation. The culmination of the work leads to the hypothesis that waves have two opposite effects on salt marshes. On one hand they erode marsh boundaries thus reducing marsh area; on the other hand they mobilize large volumes of sediments in nearby tidal flats which may facilitate marsh accretion thus contrasting sea-level rise. In conclusion, wind waves destabilize marshes along the horizontal direction despite their potential vertical stability.