Field and Numerical Investigation of Wave Power and Shoreline Retreat in Terrebonne Bay, Southern Louisiana

Abstract

The marshland in southern Louisiana is a vital resource that many people and wildlife depend on. Terrebonne Bay was chosen for this research because it has experienced one of the largest wetland loss rates among Louisiana estuaries. Investigations of wave power and shoreline retreat rate could provide decision makers with a better understanding of the contributing wave force responsible for local shoreline retreat. A strong relationship has been shown to exist between wave power and shoreline retreat [Schwimmer (2001) and Marani et al. (2011)]. Whether or not this strong relationship exists in Terrebonne Bay is the subject of this research. In situ measurements over a period of 12 months are presented. Shoreline surveys were completed along with the deployment of wave gauges to determine the wave power directly in front of the marsh edge. The shoreline surveys were carried out roughly every three months and a continuous wave record was obtained between two consecutive surveys. These results are then directly compared to historical records as well as predictive models. Following the in-situ measurements, large-scale retreat rate and wave power are examined. Aerial photographs of Terrebonne Bay are used to obtain long-term retreat rates at selected sites between the years of 1998 and 2010. Numerical models were then utilized to obtain hindcasts of wave power at these sites. Historic wind and bathymetry data were used to obtain the water level in the bay (Delft3D). Next a wave model (SWAN) was implemented to obtain the wave power. These results are presented along with the in-situ measurements in Terrebonne Bay. A simple linear relationship between wave power and shoreline retreat was not observed in Terrebonne Bay. Although retreat rates increase as the wave power increases in general, large scatter in the data suggests that the spatial variability of parameters such as scarp height, marshland elevation, and soil properties should be included in future analysis. The temporal variability in yearly wave power also suggests that different formulations other than a yearly average wave power should be used when comparing to retreat rate values.