Investigating the Risk of Galveston's Proposed Floating Sector Gates Failing Under Reverse Loading Conditions

Abstract

The centre piece of Galveston Bay’s 34 billion dollar flood risk reduction plan is a 3.6 km long storm surge barrier built across the Bay’s main tidal inlet. The barrier, which consists of a series of vertical lift gates and two large floating sector gates, will close when a hurricane approaches in order to reduce the volume of hurricane driven storm surge entering the Bay. However, the rotating wind fields of passing hurricanes can blow offshore directed winds over the Bay which can generate a "reverse head" condition, where water levels on the bay side of the closed surge barrier exceed water levels at the open coast. The resulting reverse load threatens failure of the barrier’s two floating sector gates, which can be pulled from their supporting ball joint sockets. This Master Thesis demonstrates why reverse loading is an important load that must be adequately accounted for in the design of the surge barrier. A model is set up to determine reverse loads generated by a specified hurricane. The model accounts for reverse loading due to reverse head and wave action in the Bay and is comprised of; a parametric hurricane model by Holland (1980), a coupled hydrodynamic flow-wave model by Xu et al. (2023), and load formulations for the reverse heads and wave conditions calculated by the hydrodynamic model. Deterministic application of the model shows that reverse heads/loads are a common occurrence and can be generated by hurricanes landing both West and East of Galveston. Furthermore, hurricanes approaching landfall from oblique Eastern directions can generate a high reverse load before the arrival of a high coastal surge, which threaten more severe consequences as in addition to floating sector gate failure, the following coastal surge can enter Galveston Bay and increase flood risk. Probabilistic application of the model is used to estimate the exceedence probabilities of reverse head/loading magnitudes, for an assumed operation procedure where a decision is made to keep the surge barrier either permanently closed or permanently open depending on assumed surge forecast uncertainties