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Castle Harbor Causeway Replacement Feasibility Study

Computer analyses of waves and hydrodynamics were performed by Applied Coastal for this project in support of a feasibility study, directed by the Bermuda Ministry of Works and Engineering, conducted to determine the best option for replacing the causeway structure that is the only automobile thoroughfare between the main island and the airport. In 2003 the Castle Harbour causeway, which dates back to the 19th century, was heavily damaged and made impassible when Bermuda suffered a direct hit by the Category 3 hurricane Fabian. Though the causeway was repaired and is in service at this time, the government is studying alternatives for replacing this ailing structure to improve the storm survivability of this vital route. For the first phase of the project, Applied Coastal performed an analysis of extreme winds and waves in Castle Harbour to determine conditions at the Causeway. A 65-year-long meteorological record recorded by the US Navy at the Bermuda Airport was used to determine extreme wind conditions. The STWAVE numerical model was utilized in the determination of extreme wave heights and periods during likely hurricane conditions. These wave conditions were used as design guidance for the new crossing structure. It was considered possible that a large wave energy flux from the open ocean through the southern opening of the Harbour would cause waves within the Harbour basin to be larger than would result from wind generation alone. The model results indicate that, even though there are deep channels between the Harbour and the open ocean, the many small islands that span the southern basin opening effectively block waves from outside the Harbor. Therefore damage to the causeway structure results from locally generated wind-waves from within the Harbour basin. For the second phase of the project, a study of hydrodynamics in Castle Harbour was performed. Tide gauges were deployed for three months at locations necessary to calibrate a hydrodynamic mode of the basin. An Acoustic Doppler Current Profiler (ADCP) survey was performed during the tide gauge deployment to measure precisely the flow of water through the causeway arch-spans and at the inlets of the Harbour, all at one-hour intervals for the duration of a tide cycle. In addition to the ADCP, stationary acoustic current meters were deployed for two weeks to measure flow velocities in the causeway arch-spans. The calibrated hydrodynamic model developed using these data will be used to determine flushing impacts and changes to tidal currents that would result from the proposed causeway replacement alternatives.

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