The objective of this thesis is to understand and predict future vulnerability of coastal infrastructure to extreme surges and waves generated by hurricanes and tsunamis, which has been manifested during a number of events in the past few decades such as the 2005 Hurricane Katrina and the 2011 Tohoku Tsunami. Towards this objective, a systematic numerical investigation has been made with three aspects.

1) In order to numerically investigate the hydrodynamic characteristics of extreme surges and waves when they impinge bridges, this dissertation develops a numerical solver for incompressible flow on unstructured mesh (SIFUM). The interface between air and water is resolved by a volume of fluid (VOF) method. With the aid of SIFUM, a systematic investigation has been conducted to study hydrodynamic phenomena and their loads on a bridge deck generated by solitary waves, focused waves, and a combination of waves and currents, and effectiveness of mitigation plans is also evaluated.

2) A new hybrid modeling system has been developed by coupling FVCOM and SIFUM to integrally simulate in high-fidelity propagation of surges and waves in open seas towards seashore and their impact at coastal infrastructure, which involves multiphysics phenomena at distinct scales and is difficult to be handled by any existing models. The system is intensively validated, followed by applications to propagation of tsunami and its impact on a bridge deck at a simplified beach as well as on a model beachfront house locating at Rockaway beach, New York.

3) For the purpose to evaluate vulnerability of offshore and ashore infrastructure in the metro New York region, a hindcast of storm surges and waves during the superstorm Hurricane Sandy 2012 is made, and its results match the measurement data. Then, a numerical investigation of storm surges and waves has been made considering sea-level rise and change in hurricane routes. On this basis, an analysis on vulnerability of coastal infrastructure is made with regard to extreme surges and waves, scours, hydrodynamic loads at bridges in the region.

The work in this thesis improves our understanding of relevant hydrodynamic phenomena, future vulnerability of coastal bridges, and effectiveness of potential mitigation measures. In addition, it advances our capability for modeling multiscale and multiphysics coastal ocean flows to a new level, providing an unprecedented and urgently needed platform to numerically study many emerging problems.