Toshinori Ogasawara

CHARACTERISTICS OF THE 2011 TOHOKU EARTHQUAKE AND TSUNAMI AND ITS IMPACT ON THE NORTHERN IWATE COAST

On March 11, 2011, at 14:46 local time, a powerful earthquake occurred off the Sanriku coast of northeastern Japan. It was immediately apparent that this was the strongest earthquake on the historical record, and the resulting series of tsunami ravaged the Tohoku region. In Iwate Prefecture, the tsunami height generally exceeded the design tsunami level for safety, which had been determined on the basis of the 1896 Meiji-Sanriku tsunami, the 1933 Showa-Sanriku tsunami, and the 1960 Chile tsunami. As of January 10, the number of dead and missing in Iwate rose to 6,035. Over 24,000 buildings had been destroyed. After the 1933 Showa-Sanriku, disaster prevention facilities were built along the coast. However, many tsunami seawalls and water gates suffered enormous damage from the impact of the 2011 Tohoku Tsunami. This paper sketches out the characteristics and scope of the damage in the coastal areas of Iwate Prefecture, with a particular focus on the northern coast of Iwate.

Mass Flux and Vertical Distribution of Currents Caused by Strong Winds in a Wave Tank

Abstract The velocity fields of wind-driven currents under strong winds were measured in a wind-wave tank with a double bottom. The tank has the characteristics to satisfy partially the continuity of the mass flux and to reduce return-flow effects on the currents. The lower part of the double-bottom tank functions as a duct to circulate the currents, allowing the measurement of the return-flow velocity. The velocity measurements were made on the currents just below the mean water level by using a high-resolution particle image velocimetry (PIV) system and tracking floats and on the currents inside of the duct by using a normal PIV system. Thus, accurate data of the vertical distribution and mass flux of the currents driven by strong winds were obtained from the PIV data. As a result, it is found that the flux in the surface layer with the thickness of 2 times the significant wave height amounts to about 30% of the total mass flux of the currents driven by strong winds with a reference wind speed of 12.0 m...

Numerical analysis of the characteristics of waves propagating in arbitrary ice-covered sea

Abstract Safe exploration and transportation of natural energy resources in polar and subpolar seas such as the Sea of Okhotsk and the Arctic Ocean requires an understanding of the characteristics of ice-coupled wave propagation. Using a time-domain solution involving both the boundary element method and the finite-element method, a numerical procedure is developed to analyze the wave properties for arbitrary ice conditions. This is done by applying a distinction index to discrete nodes representing the dynamic boundary conditions. The numerical results agree well with experimental data for different floe lengths, thicknesses and elastic moduli, obtained by using model ice plates. The elastic deformation of the ice floe depends strongly on the flexural rigidity of individual ice plates.

Applicability of Numerical Simulations for Ductility Design of Breakwaters against Mega Tsunami

The design to strengthen coastal defence structures with their ductility is going to be employed. This study applies three different numerical models; CADMAS-SURF2D-OpenFOAM and MPS, to verify their accuracy compared with the hydraulic experiments of breakwaters under tsunami load and to clarify their applicability for ductility design. In case that tsunamis overflow crown heights of breakwaters extremely, it was suggested that tsunami velocity behind breakwaters could not be simulated accurately by CADMAS-SURF2D, while OpenFOAM and MPS had good agreement with experimental results. It was also found that numerical results may lead to underestimate necessary weights of armor blocks.