Narrow Results

We surveyed disaster situations in the west coast of Thailand and the south coast of Sri Lanka where damage due to the Indian Ocean tsunami of December 2004 was severe, and investigated the destructive mechanism of structures due to the tsunami. The results we obtained were as follows:.

(1) If a tsunami with a ground-level inundation depth of more than two meters attacks a lowland extending several hundred meters or more inland, severe disaster will occur. In this case, mortar and brick buildings 30 cm or less in thickness will be destroyed.

(2) The stability of a building can be estimated using the easy stress calculating method by assuming a gatetype Rahmen building model and calculating dynamic water pressure.

(3) Destruction of revetments and seawalls is caused by incident wave pressure or return flow. In this case, the stability of a coastal structure is mainly determined by the balance of water pressure and earth pressure.

A huge tsunami caused by the giant earthquake occurred off Sumatra in Indonesia, on 26th December 2004, attacked the coastal regions of many countries facing the Indian Ocean. This paper describes evaluation methods of structural damage in order to mitigate the damage by huge tsunamis. First, the damage by this tsunami at Patong Beach in Phuket province of Thailand is explained. Next, diagrams to get critical inundation heights which break buildings are shown by using a rigid frame model as functions of inundation heights. Then, how to get the inundation height empirically is explained. Moreover, a numerical model to simulate the inundation height is also used, because the method described previously will give inundation heights of low accuracy as geographical feature becomes complicated. Furthermore, there were some structural collapses (a masonry wall and so on) by tsunami back-flow; the stability of a concrete wall against the back-flow is also discussed.

In order to develop a rational methodology to evaluate building damage caused by tsunamis, first, disaster data collected in Thailand, Sri Lanka, and Japan are introduced as verification data. Next, the applicability of an empirical method of predicting inundation depth ( = the inundation height from ground level) based on analysis of those data and a numerical simulation method of tsunami flood is examined by comparing calculation values with the verification data. Then, the validity of a convenient method of evaluating building strength against the tsunami using the gate-type Rahmen Building Model is checked by comparing calculation results with actual damage and diagrams to get pillar widths or wall thicknesses at the breaking threshold for common sizes of buildings are proposed.

The mechanism of tsunami damage to coastal structures and tolerable limits of tsunami height for structural safety are investigated using data from typical tsunami disasters. Results obtained from our investigation are as follows:.

Tsunami destruction to concrete dikes and revetments is caused by incident tsunami pressure or return flow.

In the case of incident tsunami pressure, tolerable limits of tsunami height for structural safety can be obtained by an equilibrium calculation of incident tsunami pressure, static water pressure, and passive backend ground pressure as shown in Figure 3 and Table 4. In the case of return flow, tolerable limits of tsunami height for structural safety can be obtained by an equilibrium calculation of return flow pressure, static water pressure, and active backend ground pressure as shown in Figure 4 and Tables 5 and 6.

This paper describes a numerical model for the simulation of near shore wave dynamics and bottom topography change. In this paper, the nearshore wave dynamics is simulated by solving the depth integrated Boussinesq approximation equations for nearshore wave transformation together with the equation of continuity with a Crank-Nicholson scheme. The wave runup on beach is simulated by a scheme similar to the Volume Of Fluid (VOF) technique. The wave energy loss due to wave breaking and shear generated turbulence is simulated by a k-ε model, in which the turbulence kinetic energy (TKE) generation is assumed as the sum of those respectively due to wave breaking and horizontal and vertical shear. The bottom topography change model calculates the transport of bed load, the uptake, advection, diffusion and deposition of bed materials under combined actions of waves and currents. Especially, a new numerical scheme based on the cell donator-receiver concept has been introduced for the discretization of terms representing the advective transport of suspended sediment. Also, a new suspended sediment pick-up function, which relates the sediment pick-up rate with bed shear stress and turbulence intensity has been introduced. Additionally, a new numerical scheme, which is also based on the cell donator-receiver concept, is proposed for the evaluation of the contribution of bed load transport to the bottom topography change. This new numerical scheme eliminates the necessity of introducing false diffusion terms to the equation governing the bottom topography change, as done by various other researchers, including Rakha et al (1997), Watanabe et al (1988), and thus enable an accurate prediction of the bottom topography change, especially near coastal structures. The verification of the numerical model against data obtained from various indoor experiments reveals that the model is capable of simulating the wave dynamics, turbulence and bottom topography change under wave actions. Especially, with the new differencing scheme for the bottom topography change equation, the numerical model is capable of simulating the wave induced scouring near coastal structures with acceptable accuracy and relatively short computational time.

The purpose of this research is to provide a detailed investigation on the scour near coastal structures by collecting and analyzing indoor experiment and field data and by a numerical model; and based on it to establish a quantitative method for the evaluation of the maximum scouring depth.