Department of Civil Engineering , The University of Tokyo
Coastal Engineering Laboratory



About Coastal Engineering Lab.

Numerical calculation of wave tansformation in shallow water based on Boussinesq equation

Wave transformation in shallow water is important to nearshore sediment transport and coastal topography changes. Clarification of the mechanism of wave transformation is thus of great importance in coastal engineering. Among many wave theories, those based on the Boussinesq equations have advantages such as that they incorporate the nonlinear interactions among refraction, diffraction, reflection and shoaling, and may also represent the interaction of waves and currents which must otherwise be treated with the concept of radiation stress. This study modifies Peregrine's theory by including breaking, bottom friction and undertow.

Mathematical model of wave deformation and sediment transport in runup area

The sediment transport in wave runup zone is one of the most important problems in coastal engineering. In this study we use Boussinesq equation, introduce a new explaining shoreline changes, and try to explicate wave runup and down in wave runup zone and a mechanism of sediment transport resulting from them. The result of the experiment in the laboratory done before modeling them mathematically shows this model qualitatively. In addition we propose the formula of sediment transport rate which can be applied to the sediment transport in runup area, and build a mathematical model simulatimg from wave deformation to coastal topographic changes in this area.

Study on the wave transformation and the sediment transport under the standing wave field

In front of a vertical wall that is built for waterfront development and shore protection, the partial standing wave field may be formed. In order to forecast topography changes near such a wall, it is important to predict the sediment transport rate under partial standing wave actions where wave amplitude changes rapidly in space. Most of the well-known formulas on sediment transport rate are valid only under progressive waves. This study is aimed to provide a new formula which can be applied not only to the progressive wave field but also to standing wave field. We approach the movement of sediments from the Lagrangian instead of Eulerian point of view.

Development of the sediment transport rate formula in mixed grain size

The problems of beach erosion resulting from sediment transport is very important in engineering. In especially the phenomena called sheet flow, the layer of movable sediment with high concentration in the beach causes a great deal of sediment transport rate, so sheet flow has an strong influence on beach erosion. As Dibajnia・Watanabe's equation(1994) is one of the sediment transport formula in sheet flow, but this was derived from the experiment with sediment in the single size, it isn't general. Therefore in this study we make it a purpose to extend the usable range of this formula by applying it to the result of the experiment with sediment in various grain size and specific gravity, and furthermore to be able to apply it to the sediment transport rate in mixed grain size.

Development of the numerical model of overtopping

Overtopping is important to harbor tranquility in addition to wave transformation processes such as refraction and diffraction. So far, the effects of overtopping on waves in the harbor are calculated through multplying the incident wave height by a transmission coefficient or employing other emprical methods. None of these methods is reliable because they oversimplify the physical phenomenon of overtopping as essentially a linear phenomenon. In this study, the Couchy-Poisson wave theory is used to represent the overtopping induced waves in the harbor and an efficient numerical model is proposed.

Study on the mechanism of the large-scale blue tide

The blue tide in Tokyo bay causes serious damages to fishery. Large-scale blue tide which occurs decadally is particularly disastrous. In this study, we first studied the mechanism of the large-scale blue tide. According to the observation in 1994, it is clear that, only when the large-scale blue tide occurs, upwelling of water mass over the coastal trenches which are created by dredging for reclamation. By this fact, we concluded that the upwelling of water mass in these trenches is an indicator of the scale of blue tide. We also developed a 3-D numerical model for circulation in Bays and applied it to the simulation of 1994 Blue Tide. This simulation showed that when a large-scale blue tide occurs, the upwelling of water mass over the flat bed area is of large scale as well as the water mass in the trenches, which altogether constitute the mechanism to fhe large-scale blue tide.

Numerical simulation of storm surge in the Thai bay

Althogh typhoon invation to the Thai bay is rare, study of the damages by a potential storm surge to Bangkok, which is close to the coastline, is important not only because the capital city has a huge population and concentrated social and economic properties but also because it is low-leveled above the sea. With the sea level rise in the future owing to the grobal warming the danger will further increase. In this study, we try to simulate the storm surge in Thai bay under the present topographical and meteorological conditions, and predict the effects of a future storm surge when the climate changes.