Narrow Results

The Indian Ocean Tsunami on 26 December 2004 affected Sri Lanka, which is located 1,700 km from the epicenter. A field investigation was conducted along the southwest coast of Sri Lanka to measure tsunami trace heights, investigate the damage, and obtain correct information on the tsunami. The results of the field survey showed that the tsunami arrived on the southwest coasts two and a half hours or more after the earthquake. The average tsunami height was 5 m along the southwest coast and tsunami trace heights of 10 m were found locally. The tsunami destroyed a number of wooden and brick houses, damaged port and harbor facilities and coastal railways, washed away many vessels, and scoured the foundations of coastal structures. The port and harbor facilities such as the breakwaters, and rigid coastal houses continuing along the coast reduced the tsunami and lessened the damage that it caused.

Hydraulic experiments were conducted to investigate the characteristics of tsunami flooding on flat or declining ground, since it was clarified by the field survey that the severer tsunami damage was caused at the place where the coastal land inclines inland. The experimental results show that the pressure acting on the structure on the sloping bottom was at least twice that of the flat bottom under the experimental conditions.

Dredging to maintain channels and berths produces large volumes of sand that need to be appropriately disposed of or used. The construction of artificial shallows and tidal flats has attracted attention as a way of effectively using dredged sand. However, there is still an issue to be resolved in that dredged sand with its fine sediments is vulnerable to wave action. In this study, the wave-induced topographic change in artificial shallows composed of fine sand was examined experimentally for bottom flow velocity on its surface and pore-water pressure on its surface layer. Experimental results showed that the relative vertical effective normal stress ratio calculated from pore-water pressure was negatively correlated with the signed Shields parameter defined as positive landward. Its relationship was unchanged despite a change in the surface profile of the shallows, implying an acceleration in seaward sediment transport and a reduction in landward sediment transport, regardless of the topographic change in the shallows. Furthermore, a signed Shields parameter modified to consider the effects of pore-water pressure gave a reasonable evaluation of the trend in the sediment transport. From these results, it was demonstrated that the effects of pore-water pressure must be considered when discussing the sediment transport.

This study proposes a new embankment reinforcement using steel sheet piles against tsunami overflow, which has been known as the main cause of the failures of the embankments by the 2011 Tohoku Earthquake Tsunami. Effectiveness of the proposed technique was discussed through a hydraulic experiment. A model of embankment was set in a horizontal open channel, and one or two steel plates are installed into the embankment from the top as vertical walls inside. Temporal variations of the shapes of the embankment and the sheet pile structures were obtained from video images. In most of the cases, the sheet pile structures started to rotate after the erosion of the landward slope of the embankment. However the rotation stopped at about $30^{\circ }$ and $10^{\circ }$ from the initial location with the single- and double-wall cases. Height of the embankment after overflow was less than 20% with no reinforcement, while more than 70% and 95% of the height were kept with the single- and double-wall structures, respectively. The performance of the embankment with the reinforcement was also discussed in terms of tsunami energy reduction with an additional fixed-bed experiment.

To design and construct land structures which can withstand earthquake-induced tsunami, it is most essential to evaluate tsunami wave force exerting on the structures quantitatively. This study, therefore, proposes the estimation method of tsunami wave pressure acting on a land structure using inundation depth and horizontal velocity at the front of the structure, which were calculated by a two-dimensional depth-integrated flow model. The comparison between the numerical and experimental results revealed that the proposed method could reasonably reproduce the vertical distribution of the maximum tsunami wave pressure exerting on a land structure.

To reduce the damage caused when a tsunami overflows coastal dikes, this study performs hydraulic experiments and numerical simulations and proposes a new method for slowing the tsunami by intentionally eroding parts of the ground. The effectiveness of the proposed method is assessed, velocity-reduction mechanism is clarified, and reduced tsunami velocity is predicted. Using crushed expanded polystyrene (EPS) as the ground material, vertical erosion is enhanced and a large depression is formed that changes the flow from supercritical to subcritical, thereby decelerating it. Assuming a constant tsunami overflow rate, the tsunami velocity after deceleration can be predicted with $\sim 20\%$ uncertainty.

In this paper, the tsunami risk in the west coast of Japan is introduced. Especially, the inundation risk in Osaka city is discussed. Three visor gates are installed at the mouths of main rivers in the city. The gates are mainly installed to prevent the storm surge due to typhoons but they are effective to reduce the tsunami height at the mouth point of river. Though the gates are necessary to prevent the tsunami inundation, the stability against tsunami forces is not enough because the gates were constructed in 1960s. Therefore, the removable breakwater is now under consideration to reduce the tsunami force. The hydraulic experiment was carried out in order to investigate the applicability of the removable breakwater. The reduction rate by the breakwater was derived from the experiment.

A tsunami force has unknown features due to its complexity, and makes it difficult to be predicted accurately. As a typical example, in the 2011 off the Pacific coast of Tohoku earthquake with huge tsunami, many breakwaters slid or fell down by the unexpectedly terrible damages caused by the tsunami. For effective design of coastal structures, achieving a correct prediction of the tsunami pressure should be an urgent subject. In the existing studies, the tsunami force F with an overflow is estimated by using a compensation coefficient α based on the hydrostatic pressure as: F = αρgh. The compensation coefficient α is usually set as α = 1.1 for the front of the caisson and 0.9 for the rear of that constantly. However, in recent studies, it was found that Parameter α can be easily and randomly changed depending on the boundary condition, and unfortunately, estimation method of its effective has not been developed yet. To resolve this problem, in this study, hydraulic experiments targeting on tsunamis with their overflows on a breakwater are implemented to examine the variability of the compensation coefficient α. And from the experimental results, a simple and effective estimation method for the compensation coefficient α is newly proposed.