Modelling and assessment of tsunami-induced vortex flows hazards from the 2011 MW9. 0 Tohoku-oki earthquake in harbors and adjacent area

Tsunamis not only lead to flooding but also cause strong flows within semi-closed basins and give rise to some practical concerns. Vessels and maritime facilities were vulnerable to tsunami current in harbors, which often occurred many hours after the initial tsunami arrival. Due to limitation of field observation and lack of knowledge on the generation of vortex, the study on tsunami over the past sixty years mainly focus on the characteristics of tsunami waves and its inducing flooding. There is few study on the tsunami-induced flow, which results in incomplete understanding of tsunami waves and their related disaster. It is very important and urgent to carry out the research and services on strong current induced by tsunamis. This paper compares the excitation mechanism and simulation methods, and tries to get the balance between accuracy and efficiency for tsunami-induced flow simulation. By analyzing the main physical dissipation mechanism of shallow turbulent coherent structures (TCS), the nonlinear shallow water equations considering the 2D horizontal dissipation mechanism can be used to simulate tsunami-induced vortex flows. Three high resolution harbors models were set up based on the finite volume model Geoclaw. The resolution of the three models is 5 meters. Far-field Tsunami wave and current characteristics from Tohokuoki earthquake are analyzed relied on the tsunami source model estimated by tsunami dart buoy inversion. The simulation results of the tsunami wave and current characteristics are in good agreement with the observations. It is further shown that tsunami-induced flow is more sensitive, and small error for the free surface and the phase can induce larger error for the current flow. The accurate simulation for tsunami-induced flow is more challenging. In present work, the energy distribution characteristics of tsunami wave and current in harbors are investigated. Relative to spatial variability of wave amplitude, tsunami current has much greater spatial sensitivity, and it is strong around the harbor entrance and the head of breakwaters. The tsunami wave attacked the Oarai port within 30 minutes, and induced an enormous vortex flow within the harbor. The numerical model has successfully reproduced the violent vortex flow near Oarai port induced by the tsunami. Finally, the hazard levels for tsunami-induced flow are derived by considering the flow distribution within the harbor. Minimum water depth and the minimum time cycle for vessels evacuation is also derived in the paper. The study provides quantitative guidance on tsunami-induced current in harbors and ports for general public and EMA ( Emergency Management Agency). Hence, it is rewarding to carry out high-resolution, site-specific forecasting skills on tsunami wave and current characteristics in harbors and bays.