Performance and mechanisms of a novel breakwater for protecting coastal box girder bridges from freak waves

Coastal bridges are vulnerable to extreme wave attacks, and the potential for damage from high-energy freak waves is increasing under climate changing scenarios. To mitigate the impact on these structures, a novel breakwater design is proposed, combining a floating structure anchored to a traditional submerged breakwater. Using a two-dimensional flume developed in OpenFOAM, simulations of freak waves and wave-structure interactions were conducted. The protective capabilities of the combined breakwater were assessed by analyzing the changes in force on the bridges, and the wave dissipation mechanisms were explored through wavelet transform results and flow field analysis. The study reveals that the combined breakwater effectively reduces wave loads compared to conventional submerged breakwaters, due to the conversion of high-frequency waves into low-frequency waves facilitated by the floating structure. This interaction, along with the submerged structure, enhances the overall stability and service life of the breakwater. The stability and load reduction performance are influenced by the initial draft and length-to-height ratio of the floating structure, with optimal performance achieved at specific configurations. The efficacy of the proposed breakwater is somewhat limited by long-period and highly nonlinear waves, while water depth has minimal impact. This combined breakwater design addresses the limitations of both submerged and floating breakwaters, offering valuable insights for the design of coastal bridge protection systems against freak waves.