Narrow Results

At 14:46 local time on March 11, 2011, a magnitude 9.0 earthquake occurred off the coast of northeast Japan. This earthquake generated a tsunami that struck Japan as well as various locations around the Pacific Ocean. With the participation of about 300 researchers from throughout Japan, joint research groups conducted a tsunami survey along a 2,000 km stretch of the Japanese coast. More than 5,200 locations have been surveyed to date, generating the largest tsunami survey dataset in the world. The inundation height and run-up height were surveyed by laser, GPS, and other instruments, and the tidal correction has been accurately adjusted using a tidal database and a numerical simulation for Tohoku, an area where tide gauges were destroyed by the tsunami. Based on the survey dataset, the regional and local scale analyses were conducted to understand the basic characteristics of this event. Maximum run-up heights greater than 10 m are distributed along 500 km of coast in direct distance. The affected area of this event was several times larger than historically recorded in Tohoku. The mean inundation height in the southern Sanriku region is 10–15 m and there are several peaks of inundation along the coast from the northern to middle part of Sanriku.

Bridge superstructures were washed away in the 2011 Tohoku earthquake tsunami. To determine whether and when bridge superstructures are washed away, it is essential to evaluate the temporal change in horizontal and vertical tsunami forces. In this study, tsunami force acting on bridge superstructures and its estimation formulae are investigated from hydraulic model experiments and numerical simulations. From experimental results, it is found that it can be determined from a comparison between the temporal change in horizontal tsunami force and that in static friction force taking into account vertical tsunami force whether and when a bridge superstructure move. Furthermore, a drag force equation with the projection area taking into account water surface elevation and a Morison equation in which inertia force is added to the drag force equation are proposed for horizontal tsunami force, and an estimation formula composed of buoyancy force and downward lift force is proposed for vertical tsunami force. From a comparison with numerical results, it was revealed that the estimation formulae can predict the temporal change in the horizontal and vertical tsunami forces except for short wave period, suggesting that it can be evaluated using the estimation formulae whether and when a bridge superstructures moves.

On 16 September 2015, a Mw 8.3 earthquake occurred off the coast of Illapel, Central Chile, generating a tsunami that caused moderate damage along the coast of Central Chile. To investigate tsunami damage and evacuation responses, we conducted a post-tsunami field survey between La Serena and Concón. The average tsunami inundation height in the survey area was 3–4m; however, in the innermost corner of Coquimbo Bay, which was shielded from the tsunami source by the Coquimbo Peninsula, the tsunami inundation height was 6.0–6.2m, decreasing gradually along the coast away from the corner. This phenomenon was also confirmed around Tongoy Bay, which is geographically similar to Coquimbo Bay. Based on our results, we hypothesize that the tsunami was diffracted and refracted by the peninsula, and then converged on the innermost corner of the bay, located behind the peninsula. Interviews with local residents confirmed that most people evacuated immediately after feeling strong seismic motion, but before the tsunami warning and evacuation orders were issued. However, several people did not evacuate, either because they believed that they were located in a safe place, or because they wanted to protect their property.

Energy dissipation mechamism is the key to study tsunami hazard mitigation. Numerical method is adopted to study the interaction between bores and square cylinders. The model solves the three-dimensional Navier–Stokes equations with Large-Eddy Simulation turbulence model. The Volume-of-fluid (VOF) method is used to track the complex free surface. We focus the investigation on the effect of cylinder height on the flow field. The results show that the turbulence diffusion is the main mechanism for energy dissipation. The flow patterns are significantly different within and beyond the cylinder array. The taller cylinders cause smaller velocity magnitude in the downstream area. In addition, a larger value of velocity magnitude and vorticity near the bottom is identified in the tall-cylinder case. These unique featuers make different dissipation rates.

Study of boulder transport by tsunamis is challenging because boulder size, shape, and composition vary greatly; furthermore, flow conditions, topography, and initial conditions are generally unknown. To investigate the mechanism of boulder pickup, experiments of tsunami-like flow past spherical boulders partially buried in a sediment bed are conducted. The experiments are performed in a large centrifuge facility to reduce scale effects and the corresponding dynamic similitude is discussed. The traditional approach to determine boulder pickup is adapted for the case of a half-buried spherical boulder. The adapted model predicts that the boulders are transported, but does not accurately predict the timing of pick up. To investigate the difference in pickup timing, two physical phenomena are discussed: pore-water-pressure dissipation in the soil, and the impact of the free-surface flow on hydrodynamic forces. For a spherical shaped boulder, vertical forces (i.e. buoyant and lift forces) are critical for the initiation of boulder pickup. It was found that spherical boulders that are three-quarter buried in the soil are not transported, even when exposed to flow conditions that would otherwise predict transport.

Experimental and numerical studies were conducted to assess the stability of the breakwater of Fujairah Port on the east coast of the United Arab Emirates against tsunamis. Experiments were conducted on a 1:50 scale and numerical analysis was done with the formulas of tsunami-breakwater interaction. The experimental observations corroborated the numerical results. The breakwater was safe up to 3m of tsunami height, but when the tsunami height increased from 3m to 6m, the percentage damage to the leeside (portside) of the breakwater increased compared to the seaside. The leeside of the breakwater was deformed by tsunami overtopping and seepage — the breakwater cross-section reshaped from trapezoidal to quasi-triangular at 6m tsunami impact, however, the breakwater was not breached. Rubbles of the breakwater were transported due to sliding, rolling, and saltation by overtopping and seepage. For 6m tsunami height, rubbles from the seaside rolled over up with saltation and transported to the leeside. A large number of rubbles were transported landward from the leeside of the breakwater but a few were displaced offshore from the seaside.

The recently developed Consistent Particle Method (CPM) is used to model breaking waves in tsunami and violent sloshing waves in a moving tank. Solving the Navier-Stokes equations in a semi-implicit time stepping scheme, the CPM eliminates the use of kernel function which is somewhat arbitrarily defined and used in other particle methods. It is demonstrated that the method is applicable to large amplitude free surface wave problems that involve breaking phenomenon. Tsunami wave impact on a fixed structure is modeled using CPM. The simulated results show fairly good agreement to the actual nonlinear wave motions including overturning and breaking of waves. Large amplitude sloshing waves in a moving tank are investigated with CPM. Experiment was conducted in the laboratory to verify the CPM solutions. The hydrodynamic pressure computed by the CPM agrees well with the experimental results.