The role of wave and current forcing in the process of barrier island overwash

Pirrello, Mark A. (1992) The role of wave and current forcing in the process of barrier island overwash. Masters thesis, University of Florida.

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Abstract

With the rapid growth and development of barrier islands, understanding the long-term stability of these islands is an integral part of future coastal planning. The overwash process is the largest influence on the long-term stability of these islands and thus a corresponding understanding is of major importance. A laboratory experiment was undertaken to physically model the wave and current forcing as they pertain to the overwash process. The physical model was subjected to various storm conditions common to the occurrence of the overwash. Combinations of wave height, wave period, and overwash depth were tested in an attempt to isolate the significant parameters. Water surface gradients were also applied to observe their influence on the overwash process. Wave height, current, and bed profile measurements were taken at different locations throughout the tank. In addition, wave height transformation modeling and mean current prediction were performed and compared to the laboratory results in an attempt to model the overwash process through computer simulations. The experimental results demonstrate that the water surface gradient is the mechanism for transporting large quantities of sand on to and over barrier islands. In addition, two other conclusions were drawn about the overwash process: 1) the overwash depth plays an important role in determining the overwash velocity and hence the amount of sand deposited on the barrier island. 2) There seems to exist a correlation between the strength of the return flow and bar formation. It was also determined that modeling the wave height transformation during the overwash process is possible if the model is expressly written for the overwash process and not for nonoverwashing cases. The method utilized to predict the mean currents during overwash was not able to predict their strength but was able to substantiate the correlation between return flow and bar formation. As a result of overwash, the increased shoreward mass transport and reduced return flow in the water column are able to initiate and sustain a shoreward sediment transport. Finally, it was concluded that in all likelihood only "significant" overwash events affect the longterm stability of the barrier islands.

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