Coastal Engineering Journal

The Indian Ocean Tsunami occurred on 26 December 2004, causing serious damage in the Maldives, which is 2,000 km distant from the epicenter. A post-tsunami survey was carried out from 31 January to 4 February 2005. Tsunami height distribution, tsunami behavior in atoll and characteristics of tsunami disaster in atoll island were discussed through survey results. The height of tsunami traces in the Maldives ranged from 0.6 to 3.4 m. The trace height was not small even where there was a developed reef and even at island inside an atoll. The tsunami behavior appeared to be complex in an atoll. Several minutes were necessary for tsunami inflow to an atoll. Inertial force was small in the run-up process in some islands, but was not so in some islands. Because the Maldives consists of low-lying islands, a solid structure and artificial ground is required to improve the safety level of the Maldives.

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.

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.

Tsunami waves are considered the most dangerous natural hazard affecting the population of the world living near the coastal belts. With the increasing intensity of economic exploitation of coasts there is also an increase in socio-economic consequences resulting from the hazardous action of tsunami waves generated from submarine seismic activity and other causes. On 26 December 2004, the countries within the vicinity of East Indian Ocean experienced the most devastating tsunami in recorded history. This tsunami was triggered by an earthquake of magnitude 9.0 on the Richter scale at 3.4°N, 95.7°E off the coast of Sumatra in the Indonesian Archipelago at 06:29 hrs IST (00:59 hrs GMT).

As of now (September, 2005), the only Tsunami Warning System (TWS) that is in existence is the one for the Pacific Ocean, which began in the late 1940s. Following the recent disastrous tsunami of 26 December 2004 in the Indian Ocean, the nations around the Indian Ocean rim are now working together to establish a tsunami warning system which should become operational in the near future. One of the most basic information that an Indian Ocean tsunami warning center should have at its disposal, is information on tsunami travel times to various coastal locations surrounding the Indian Ocean rim, as well to several island locations. Devoid of this information, no ETA's (expected times of arrival) can be included in the real-time tsunami warnings.

The importance of ETA for tsunami warning system motivated the computation of arrival times comprising 250 representative coastal locations from 35 countries, showing the feasibility of developing a TWS in a relatively short time-span. Numerical accuracy in computating arrival times for this energetic event has been verified from in situ tide gauge data and satellite track data from Jason-I and Topex/Poseidon in the Indian Ocean and also from coastal stations off South Africa. The expected outcome of this work is to develop a widely distributed tsunami travel time (TTT) atlas which can serve as a valuable information database to reduce warning time in the event of tsunamis in the Indian Ocean and promote awareness among the population dwelling in the littoral belts of the South-Asian countries.

This paper reports the results of field surveys of the 2004 Indian Ocean Tsunami in Southern Thailand and Northern Sumatra. After outlining the general state of the tsunami strike, tsunami height and distribution in Sec. 3, aspects of inundated flow such as current velocity (water particle velocity), fluid force, relation between inundation depth and the degree of damage to reinforced concrete buildings, shape and height of surge front, inundated flow with floating bodies, effects of coastal forest on reducing tsunami energy and force are discussed. Damage estimation problems are also discussed. A simplified theory for predicting the moving velocity of floating bodies with inundated flow is presented.