ELEVATED WATER LEVELS ALONG THE FLORIDA GULF OF MEXICO BARRIER COASTS INDUCED BY THE 2004 HURRICANE IMPACTS: MORPHOLOGICAL EVIDENCE AND PREDICTION

Significantly elevated water level during hurricane impact plays a crucial part in controlling barrier-beach erosion and overwash. Accurately measuring and predicting the maximum water level is essential in understanding and assessing hurricane-induced morphological changes. Three factors contribute to the elevated water level during hurricane impact: storm surge, wave setup, and swash runup. Storm surge can be measured by numerous tide gages along the coast and is relatively well documented. A number of well-established models are available to predict storm surges. Wave setup and swash runup are difficult to measure and typically not monitored in a long term, and therefore poorly documented. Several empirical models have been developed to predict the maximum wave setup and swash runup using offshore deep-water wave conditions. None of the existing models are adequately verified for the application along the Gulf coast.

Maximum water level can be deduced from the maximum elevation of significant morphological change. In this study, beach/dune surveys were conducted at numerous locations along the Florida Gulf barrier coasts immediately before and after the impacts of Hurricanes Frances, Ivan, and Jeanne. Maximum storm water levels were obtained by comparing pre- and post-storm beach/dune profiles. The highest elevation of significant beach/dune change (e.g., level of back-beach erosion, upper limit of foreshore slope change, dune scarping in some case, and berm buildup) corresponds to the highest water-level elevation, and is therefore used in this study to define the maximum storm-water level. Comparing the “measured” highest water level and the measured storm surge from existing tide gages, the former is considerably higher than the later. The difference is due to wave setup and swash runup. Present data indicate that wave setup and swash runup constitute a large portion, up to 50%, of the elevated water level. Two existing empirical formulas yielded reasonable estimates of the wave setup and swash runup for the Florida Gulf coast.