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Storm Surge: Influence of Bathymetric Fluctuations and Barrier Islands on Coastal Water Levels

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Abstract

The impact of hurricanes on coastal communities has been highlighted in recent years. As researchers, our goal is to better understand the physics and mechanisms that create and drive storm surge from hurricanes. To achieve this goal, both analytic and complex numerical modeling techniques, as well as historical data, are used to test the effects that selected parameters have on the total surge levels. The focus of this work is on bathymetric properties, and how they can affect the water levels at the coast. An automated, 2D coupled wave-surge modeling system was developed using the SWAN and ADCIRC models. The system is calibrated using historical data as well as both analytic and complex numerical modeling techniques. As further validation, the coupled model system is used to perform flood level predictions along the Mississippi coast. The forcing from the momentum flux due to wave breaking is an important component to the modeling system. As such, time is taken to explore and validate the methodology used to include this physical process. The SWAN model is used to compute the flux in radiation stresses. Once adopted, the system is employed to test the sensitivity of the surge levels at the coast to variations in the bottom contours. A suite of bottom perturbations is tested for different sloped bottom profiles in the 1D cases. The domain for the 2D tests is the Gulf of Mexico, with the bathymetry varied offshore of Mississippi, Alabama and the Florida panhandle. The accuracy in surge predictions can be retained with a RMS difference of less than 4.6% of the unperturbed value when the bottom variation is less than 60% of the water depth. The significance of variations decreases in depths greater than 30 m. Outside the 30 m depth contour highly resolved bathymetric data is not required to accurately compute the surge levels at the coast. In the extreme case of a perturbation that breaks the surface, 100% or greater of the water depth, the variation from the surge calculated on the undisturbed profile is more significant. For the 1D cases, an idealized profile is used as the model domain. The 2D simulations employs a bathymetric data sets that are representative of the Mississippi coast both with and without barrier islands. If the island is not overtopped, the surge at the coast is lowered with the presence of barrier islands. For a wind speed of 50 m/sec, the island should be at least 3.5 m above the mean water level to reduce the chances of overtopping. In addition to the height of the island being important, the seaward facing slope of the island should be steeper than 1:100 in order to be an effective block to surge levels. The work presented in this dissertation has helped to gain a more complete understanding of the role bathymetry plays in coastal storm surge. With the modeling system developed, we can more readily simulate storms. This ability allows the researcher to test parameters and evaluate their significance in a more efficient manner.
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... The results show that despite large local differences between the bathymetries, there is no significant influence on wave runup. Weaver (2008) and Weaver and Slinn (2010) investigated the influence of bathymetry changes on the model-based prediction of storm surge water levels in three study areas. They found that a change in local bathymetry by 620% led to a change in the storm surge water level at the coastline by 65%. ...
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