Factor: Tsunami height and actual flood height

A tsunami's wave height depends on many parameters. Some are directly linked to the characteristics of the original earthquake like its strength, its focal depth, and the rupture process. They determine the height of the wave on the open sea. When the tsunami runs up on the shore, however, its height will depend on the topography of the seabed near the shore and of the shoreline itself.

Different formations lead to great fluctuations in wave height even on neighbouring stretches of coastline. The Sumatra tsunami showed that the loss potential along a piece of coast cannot be assumed to be uniform.

It was only in certain regions that the waves reached extreme heights. On the west coast of Thailand, for example, they varied from 3—5 m on Phuket to over 10 m in Khao Lak. This extremely high wave only built up on a strip of coast some 20 km long, whilst a height of 5—6 m at most was reached on the rest of the coast.

The risk inspectors repeatedly came across areas where the damage was particularly extreme. This observation was not limited to Thailand but was confirmed in other coastal regions like South India and Sri Lanka too.

It must always be considered in any large-scale observation with a view to estimating the loss potentials of ocean-wide tsunami events. The risk appears to be particularly high in wide bays because if the wave is aligned accordingly, it will be channelled and increase in height.

However, a tsunami's destruction potential is determined not by the absolute height of the wave but by the height it actually reaches in built-up regions.

On Phuket, the houses are nearly all built at least 3—4 m above normal sea level. The flood level was thus much lower than the height of the tsunami itself. It varied between about 0.5 m in Kata Beach to 2 m in Patong and up to 3 m in Kamala.

In Khao Lak the hotel facilities are about 4 m above sea level. Because of the extreme wave heights, however, flooding was over 5 m deep in some places.

Damage

Lower levels of flooding as experienced in Kata Beach did not cause any structural damage to buildings, and even thin, non-loadbearing partition walls suffered hardly any damage. The main losses involved damage to contents and electrical equipment and the corrosion of pipe systems by salt water.

In the hotel grounds, there was moderate damage to wooden fences, beach bars, and gardens. The damage on many beaches was hardly visible a few days or weeks afterwards.

The destruction was much greater in those places where the floods reached 1—2 m. There the damage included collapsed partition walls, but was otherwise the same as in those places affected by floods up to a depth of 0.5 m.

Due to the higher energy of the water, more material was carried along with it and caused additional damage. The water gushed into all the buildings and destroyed the interior decoration. Ground floor rooms in large hotel complexes especially were damaged, and in the outside areas the building automation systems and swimming pools suffered badly.

But there was only slight structural damage even when flood levels reached 2.5 m, and collapsed walls were observed in no more than a few houses — but as the water rose, the ceilings too were damaged.

In Khao Lak, the water reached flood levels of 5 m, swamping small bungalows near the shore and forcing its way up into the first floors of the large apartment blocks of some hotel complexes so that only the second floors and those above were left undamaged.

In some cases, the main buildings of these complexes, though often built on a slightly higher level than the other buildings, were flooded up to first floor. The interiors and the ceilings were badly damaged, and most of the rooms that were flooded presented a picture of complete chaos. Many light-weight partition and outside walls collapsed due to the high levels of water.

The water also swept along large numbers of palm trees and debris from other destroyed buildings, which, coupled with the energy of the water, were sufficient to overthrow less stable structures and destroy them completely.

In fokus

It may be assumed that business will be interrupted for at least one year at all the facilities affected by high flood levels. In many parts of the areas where there was very severe destruction, reconstruction work had still not started two months after the catastrophe.

Even where repair work was pushed ahead on a large scale, there was hardly any likelihood of these facilities opening again in the first year after the catastrophe. Delays have also been caused by the sometimes chaotic conditions in the regions with severe destruction.