Laboratory study on protection of tsunami-induced scour by offshore breakwaters

Devastating tsunami waves can mobilize substantial amount of coastal sediment. Scouring is the primary damage caused by tsunamis. Offshore submerged or emerged breakwaters are coastal structures that are commonly employed to provide protection to valuable coastal beaches from energetic ocean waves. However, the protection capabilities of tsunami scour by submerged or emerged breakwater are less understood compared to tsunami runup and tsunami inundation. A set of laboratory experiments are reported in this study on protection of tsunami-induced scour by submerged or emerged breakwaters on a sandy beach. FLOW-3D is used in this paper to calculate flow field of tsunami wave propagation over the breakwater in order to help us to understand the sediment transport and tsunami scour process. Our experiments show that the submerged breakwater could not effectively reduce the tsunami scouring and only could affect the height and position of deposition sand bar. The emerged breakwater could significantly effectively reduce the tsunami scouring on the sandy beach; meanwhile, local scouring caused by plunging jet occurs mainly on the both sides of the structure. It is also found that for typical tsunamis, the scour depth at shoreward is unlikely to reach its equilibrium stage. Local scouring damage is one of the main factors leading to the destruction of coastal structures during a tsunami event. The most important governing parameters on local scouring around breakwater were defined. The final empirical relations that define the magnitude and position of local scouring around the breakwater were presented. The information reported in this study is useful for local authorities to assess potential tsunami damage of structure and to have a better plan for tsunami preventing and reducing.