Date Awarded

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Harry V. Wang

Abstract

During extreme weather conditions such as hurricanes and nor'easters, both the currents and wind waves generated by the atmospheric forces are important. Although they may act and dominate on different temporal and spatial scales, their interactions and combined effects are without doubt significant. In this dissertation, a major effort has been made to couple an unstructured grid circulation model SELFE (semi-implicit, Eulerian-Lagrangian finite element model) and the WWM II (Wind Wave model II). Moreover, this new coupled model system can be executed in a parallel computational environment. After the coupled model was successfully built, the model was verified with ideal test cases, either through comparisons with analytic solutions or with laboratory experiments. It was further validated by field-measured data during two hurricane events. The SELFE-WWM II model framework described above was used to participate in a SURA testbed project that was recently funded by the NOAA IOOS program. The purpose was to improve the storm surge and inundation modeling skill throughout the Gulf of Mexico as well as along the U.S. East Coast. The coupled tide, surge, and wind wave models in two and three dimensions were tested and compared systematically. Two well-known cases were investigated in detail. One was the event of Hurricane Ike of 2008 in the Gulf of Mexico and the other was the April Nor'easter of 2007 in the Gulf of Maine. For the Gulf of Mexico study, the key scientific issue is the origin of the forerunner. It has long been recognized that the forerunner plays an important role in generating large hurricane-induced storm surge in the Gulf of Mexico. The forerunner is a phenomenon whereby water level throughout the vast coastal region was elevated days before the hurricane makes landfall. The forerunner can contribute significantly to the total water level that results subsequently during the primary surge when the hurricane makes landfall. The 2008 Hurricane Ike, which devastated the Galveston Bay along the Texas Coast, is a good example: 1.4 m out of 4.5 m maximum surge was contributed by the forerunner in the Gulf of Mexico. The consensus from initial results of multiple models indicates that the forerunner occurred as a result of Ekman set-up along the broad Louisiana-Texas (LATEX) shelf by the shore-parallel wind field. By contrast, the primary surge was dominated by the low pressure and the maximum wind along a path perpendicular to the shore as the hurricane made landfall. It was found that the cross-shore Ekman set-up is highly sensitive to the bottom boundary layer (BBL) dynamics, especially to the drag coefficient. Given the fact that the Gulf of Mexico is known to be rich in fluid mud, and near-bed flows generally are very weak under fair-weather conditions, one plausible hypothesis is that, during the stormy condition, the suspended sediment-induced density stratification is likely to be ubiquitously present at the bottom boundary layer. A sediment-transport model and wave-current bottom boundary layer sub-model including the sediment-induced stratification effect were coupled to the unstructured grid circulation and wind wave model (SELFE-WWM II) for simulating the forerunner during Hurricane Ike. The model results demonstrate that the bottom boundary layer dynamics have a significant effect on the velocity veering as well as the Ekman set-up across the shelf. In the Gulf of Maine study, the high-resolution coupled SELFE-WWM II model was applied in the Scituate Harbor, a small, shallow coastal embayment, south of Boston. The key issue for the study was the recurring inundation related to the role played by wind waves during nor'easter events. With limited observation data in the Scituate, the model result from SELFE was compared with that from FVCOM. The major findings are summarized as follows: (1) wind waves generated by the nor'easter can profoundly affect the coastal current by increasing the magnitude and altering its direction, (2) while the mean water level inside the Harbor stays the same, the total transport across the harbor mouth increases when wind waves are included, and (3) the total inundation area, primarily in the northern and southern basins within the Harbor, does increase when wind waves are included. There is a question as to why the inclusion of the wind waves did not cause the mean water level to change inside the Harbor while the inundation area was increased. The plausible explanation is that this lack of impact could be that the Stokes transport was small and the increase of water level by the wave set-up was compensated by the expansion of the inundation area in the shallow region.

DOI

https://dx.doi.org/doi:10.25773/v5-2jj9-2t97

Rights

© The Author

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