COMPARISON OF WAVE ATTENUATION BY SALT MARSHES DURING TWO TROPICAL STORMS

Abstract:

Field observations during extreme events are valuable for assessing the effectiveness of salt marshes for coastal protection. An array of wave gages was rapidly deployed in upper Terrebonne Bay, Louisiana when Tropical Storm Cristobal (2020) was entering the Gulf of Mexico. Prior to the deployment, a cross-shore wave model that predicts wave height variation over an inundated wetland was utilized to determine the length of the transect and the layout of wave gages based on the storm forecast provided by the National Hurricane Center. Although Cristobal eventually shifted from its projected path eastward and resulted in smaller wave height and inundation depth than what was predicted, storm-induced waves were observed at the study site. To quantify the capacity of wetland vegetation at wave energy dissipation, wind sea and swell were separated and the variations of these two types of energy along the cross-shore transect were examined. A drag coefficient inferred from the field data collected during Tropical Storm Lee (2011) at the same site was used in the wave model to determine the wave energy dissipation due to vegetation. The wave energy dissipation caused by wave breaking at the marsh edge was separated from that caused by the vegetation drag. A comparison of the storm characteristics, storm surges, and energy dissipation rates during the two tropical storms shows that vegetation submergence and the ratio of wave height to inundation depth are two key controlling factors of wave energy reduction in wetlands.