Nozomu Yoneyama

Effects of the Offshore Barrier Against the 2011 Off the Pacific Coast of Tohoku Earthquake Tsunami and Lessons Learned

In this study, the effectiveness of an offshore breakwater for the 2011 off the Pacific Coast of Tohoku Earthquake Tsunami was examined by two-dimensional (2D), quasi three-dimensional (quasi-3D) and three-dimensional (3D) numerical models. First, both 3D numerical models were applied to the behavior of tsunami inundation for Kamaishi Bay in Iwate Prefecture where an offshore deep-water breakwater was installed against an assumed tsunami before 2011. The numerical results indicate 20 % error of maximum inundation height compared with the post-event tsunami survey on the land. It is found that the offshore breakwater significantly reduced the tsunami height on the land. The reduction of tsunami height on the land gave about 30 % tax revenue in comparison with similar locations with or without breakwater. Based on the results the construction and or rebuilding of damaged offshore breakwaters can be considered as a viable option against tsunami particularly in vulnerable areas.

Numerical Analysis for Evacuation Possibility From Small Underground Space in Urban Flood

This study aims to precisely predict the inundation process occurring in small underground spaces including staircases by means of a numerical simulation. A three dimensional numerical simulation model with the volume of fluid (VOF) method is applied to a staircase and a small underground space under an urban flood condition. The simulated staircase is a full-sized hydraulic model, and the small underground space is a hydraulic model at a 1/15 scale. It is found that the numerical simulation model can be applied to the staircase during urban floods, and the computation result involving the small underground space is in good agreement with the experimental result. Furthermore, we examine the evacuation possibility from an underground space by comparing the computed water depths with the indexes. As a result, we can obtain a lot of information about the evacuation possibility from the studied underground space.

Underground Inundation Analysis by Integrated Urban Flood Model

Recent urban flood induces inundation into underground space and causes extensive damage. An inundation flow model is developed which can treat inundation in both surface and underground space. We combine a horizontally 2-D inundation flow model based on unstructured meshes with an underground inundation model by use of pond model. A runoff model based on the kinematic wave model is also incorporated. This model enables us to predict underground inundation by imposing rainfall condition in an urban river basin. This model is applied to Sannomiya area and Sannomiya underground mall in Kobe City, Japan. As a result, it is found that the underground mall there is likely to be dangerous by inundation due to heavy rainfall such as that observed in Tokyo in September 2005. It is also found that setting of step at each entrance is one of the effective measures to reduce the risk at underground inundation.

Multiscale coupled three-dimensional model analysis of the tsunami flow characteristics around the Kamaishi Bay offshore breakwater and comparisons to a shallow water model

ABSTRACT A two-way coupled two-dimensional (2DH) shallow water to three-dimensional (3D) Navier–Stokes equation model, named 2-way Coupled Long Wave to Navier–Stokes 3D (2CLOWNS-3D), is applied to the 2011 Tohoku-oki Earthquake Tsunami in Kamaishi Bay, Japan. 2CLOWNS-3D simulates the entire evolution of the tsunami from its source to inundation at the coast, in which the 3D model component is used to model the flow through the submerged opening of a large-scale offshore tsunami breakwater. 2CLOWNS-3D is compared with a 2DH model simulation in order to identify where it becomes beneficial. It is found that flow rates through the submerged breakwater opening, and thus the resulting inundation heights, are similar between 2DH and 2CLOWNS-3D model simulations when an appropriate momentum dispersion coefficient is applied to the former. However, significant differences between the model simulations are identified in relation to; hydrodynamic forces acting on the submerged caissons, velocities in the large-scale jet structure emanating from the breakwater, and bed shear stresses in the vicinity of the breakwater. Furthermore, the characteristics of the large-scale jet structure simulated by the 3D model are reasonable when employing the realizable k-ε turbulence closure. Our results demonstrate the practical merit of 2CLOWNS-3D for simulating complex geophysical scale flows.

Three-dimensional numerical analysis to predict behavior of driftage carried by tsunami

This study aims to develop a three-dimensional (3D) numerical analysis code for the prediction of driftage behavior during a tsunami. The main features of this code are as follows: (1) it can simulate the six degree-of-freedom motion of driftage in a 3D flow field; (2) it can consider the interaction between fluid flow and driftage motion; and (3) it can compute the impact of the collision with a wall based on the Lagrangian equation of impulsive motion. In this code, we assume that the fluid pressure and viscosity cause driftage motion and that driftage motion affects fluid flow through deformation of the boundary between the fluid and itself. The code was applied to a hydraulic experiment carried out by subjecting a wooden body to an abrupt flow of water. The obtained numerical solution of driftage motion agreed well with the experimental result. It is concluded that our code can be used to successfully predict the behavior of driftage carried by a tsunami.

NUMERICAL ANALYSIS FOR HIGH TURBIDITY WATER FLOWING INTO DISCHARGE TUNNEL OF HYDRO-POWER PLANT

It is purpose of this report to present a prediction method, which is developed to analyze behavior of high turbidity water flowing into a discharge tunnel of hydro-power plant. The features of this method are (1) to treat a density current by applying a numerical method for incompressible flow with density variation and (2) to apply turbidity area transportation equation and sediment transportation equation for turbidity behavior simulation. The prediction method is verified by comparison with a turbid water behavior flowing into a discharge tunnel in hydraulic model experiments. As a result, the computed flow velocity and the behavior of turbidity water agree well with experimental results