In recent years, studies on tsunami mitigation and the tsunami emergency management have been frequently conducted. Further, many of tsunami awareness system have been developed to minimize the loss of life, injuries or assets damages. However essentially, civil engineering should focus on the characteristic of built environment such as the seashore’s structure, resident structure or literally, the tsunami evacuation building structure to reduce the damages. In other words, scientists should conduct more study on the interactions between tsunami waves and coastal structures. Therefore, in order to gain more understanding, the present study performs numerical analysis on three-dimensional models and conducts hydraulic model experiments as its verifications. This study investigates the impact of tsunami load to several dimensions of cylindrical structures as the idealization model tests of seashore structures, and follows by the investigation of the proper resistant barrier to tsunami counter measure.
The development of three-dimensional numerical analysis based on continuity equation and momentum equation. Volume of Fluid (VOF) method, which was introduced by Hirt & Nichols in 1981, is applied to distinguish the air and water zone, furthermore the staggered grid mesh is used to discretise the governing equations. In the propagation of tsunami wave bore, the difficulty on providing time history of velocities can be solved by introducing analytical equation that was derived by Fukui in 1963. Aside of numerical simulation, series of physical experimental are conducted as verifications. The experimental are carried out in an open channel flume, with slope variations and bore heights changes to understand the tsunami propagation at the complex sea bottom area. Cylindrical structure with variation on height and diameter is attached on the dike at downstream area to investigate the interaction between tsunami and structure. Tsunami bore wave is propagated by instantly pulling the division gate, which separated the deeper water in the upper stream area with downstream quiescent water. Tsunami bore wave physical quantities are measured by sets of measurement apparatus, i.e. strain gauges, current meter, wave gauges and pressure gauges. And as results, the wave forces, fluid velocities, water level and wave pressures can be recorded in particular locations. In order to investigate the proper resistant barrier, this study conducts series of cases with and without protective structure for tsunami waves, i.e. vertical seawalls, flaring seawalls and cylindrical weir.
This study confirms that the analytical equation derived by Fukui can be used in numerical simulations of tsunami bore wave propagation. The development of three-dimensional numerical simulation of tsunami bore wave acting on cylindrical structure shows good agreement with experimental ones in term of water surface elevation, fluid velocities, initial wave force, sustained wave force and wave pressures both in the propagation and inundation areas. In case of bore propagation over the dike, the highest pressure occurred at the bottom section of the cylinder, which has the similarities to case of vertical seawall. The front face receives the highest wave pressure, which gradually decreases along the circumferential direction. Among the protective structures, which are applied in this study, the cylindrical weir has the advantages on delaying the arriving time of tsunami waves on the downstream structure. In addition, cylindrical weir successfully reduces the fluid velocity at the bottom section of structure, which may provides solution for mitigating scouring problems. The reduction of wave velocities at the bottom section leads to the reduction of wave pressure and forces acting on the cylindrical structure at this area. However, the overtopped wave causes complex and irregular wave motion to the middle and bottom section of cylindrical structure. In addition, the cylindrical weir attributes, i.e. diameters and heights, and the level of the overtopped wave significantly affect the pressure and force magnitude.
The results of the present study provide useful basic materials to produce better strategy for hard countermeasure against tsunami disaster. However, further study should be carried to obtain advance understanding of the interaction between tsunami bore wave and coastal structure, such as the consideration on the actualization of tsunami wave in the form on small unbroken wave riding on the long period of tsunami wave. And also the further study on the scouring effect in case of cylindrical weir is used.
The present study produces the following significant outcomes to add to the body of knowledge on civil engineering, particularly on coastal structures, and public safety:
1. The development of three-dimensional numerical simulation and generation of tsunami bore wave.
The numerical simulation, which is based on the continuity and momentum equations, is confirmed to have good agreement with the experimental results in the wave physical quantities both at the propagation and inundation area. Further, the verification for the cases with multiple slope strongly indicates that this model can be apply for other cases for accurate representation of the natural phenomenon of tsunami bore wave.
2. Providing time histories of fluid velocities on the generation of tsunami bore wave.
The introduction of analytical equation, which was derived by Fukui in 1981, offer good agreement in the comparison with experimental ones in term of bore wave propagation. Therefore, the difficulties on measuring fluid velocities on the actual phenomena can be solve by using this equation.
3. Investigating the effect of several protective structures against tsunami bore wave.
Cylindrical weir, which is required by building codes in DGSM (Dangerous Goods Safety Management), has advantages to delay the arriving time of tsunami to the structures and reduce the velocities, pressures and forces of wave at the bottom section. However, the recommendation on this protective structure to overcome the scouring problem need further study.
Tsunami and structure interactions, bore type tsunami, protective structure, three-dimensional numerical simulation, time histories of tsunami velocities.