RAPID: Evolution of critical shear stress in the seabed of an urbanized estuary and natural estuary after the passage of Hurricane Ian

Florida Gulf Coast University, Fort Myers FL

Investigators

Abstract

Many contaminants in coastal waters adhere to fine sediments. To predict how sediment-associated contaminants are transported it is necessary to know how the sediment moves. The amount of friction by waves and currents that is necessary to resuspend surface sediments is a key factor for sediment transport. The amount of friction needed depends on the size of the sediment grains and on biological processes at the surface of the bed. Algal mats and bacterial films can make it harder to resuspend sediment. However, a strong physical disturbance such as a hurricane can break down the mats and make resuspension easier. This project will study Florida coastal sediments impacted by Hurricane Ian. Time series samples will show how the sediment bed susceptibility to resuspension evolves over the course of a year. This will enable improved predictions about where sediment and sediment-associated contaminants are transported after strong storms. The study also will examine the difference in the evolution of the bed between an undeveloped area surrounded by a mangrove fringe and a developed area that is bordered by seawalls. Prior studies suggest that mangroves trap fine sediments. Bays surrounded by mangroves have larger sediment grains than bays in developed areas without mangroves. The fine sediments in developed bays are more easily available for resuspension. This can enhance the movement of sediment-associated contaminants, and hurricanes have been shown to mobilize sediment-associated heavy metals. This study will have broader impacts by helping predict whether contaminants are likely to be trapped or transported away from regions that are struck by large storms. The study also will inform coastal development choices regarding implementation of mangrove fringes at the water’s edge. This is timely since many homes were destroyed near the southwest Florida coast and will soon be restored. Understanding the critical shear stress of the seabed is necessary to predict the transport of sediment and sediment-associated contaminants. This is especially important, but not well studied, after intense disturbance to the bed caused by storms. The proposed research assesses the evolution of the critical shear stress of the seabed in two coastal bays in southwest Florida after the passage of Hurricane Ian. The critical shear stress depends not only upon the size of the sediment, but the opposing effects of algal mat and bacterial biofilm production versus bioturbation. A rapid response is essential because the biological development may also be rapid, and the effect of the hurricane will be lost if not measured soon. The study will compare how the evolution of the bed differs in a coastal bay that has had extensive development, including the replacement of fringe mangroves with hard stabilization structures, with a nearby but undeveloped bay with no anthropogenic development. The study will measure the critical stress using a Gust erosion chamber on three replicate cores from each of the two sites on a quarterly basis. The amount of bioturbation will be assessed quarterly as well, using X-radiographs of triplicate cores at each site and beryllium-7 profiles. It is hypothesized that the critical shear stress will initially increase as bacterial and algal films develop, then decrease as macrofauna reestablish within the benthos. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.