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From deep ocean to beach
Furious journey of the killer tsunamis
By Dr. Janaka Wijetunge
A tsunami is a series of ocean waves generated by a disturbance such as an earthquake occurring below or near the ocean bed. Tsunami waves travel outward from the point of disturbance in all directions just like the ripples created by throwing a stone into a pond.
The purpose of this short article is to increase awareness and knowledge of the way in which tsunami waves move.

Main features of water waves
Sketch-1 identifies the main features of a water wave, as this will help us understand the behaviour of tsunami waves. We see that a wave has a high water level and a low water level: the high point is called the 'crest' and the low point the 'trough'. That part of the wave above the mean or still water level is sometimes called a 'positive wave' and that below the water level a 'negative wave'.

The height of the wave is simply the vertical distance between the crest and the trough of a wave. On the other hand, the length of a wave is the horizontal distance between two nearby crests.

How much time it takes for two successive wave crests (or troughs) to pass a fixed point, say a pole erected on the seabed, is the wave period. If the wave period is, for example, 10 seconds, then a new wave arrives every 10 seconds.

Very little happens to water waves when they travel in deep water. However, as the waves get to shallow coastal waters, the height of the wave is increased but the length of the wave is shortened.

The end result of these is a dramatic increase in the wave steepness, which eventually leads to the breaking of the wave. The broken wave loosely known as the 'bore' then runs up along the beach. Immediately, the water that has moved up the beach slope begins to run down back to the sea under gravity.

All of us have experienced this whenever we stand on the beach: our feet getting wet at regular intervals due to the water running up the beach slope. One other thing to note is that ocean waves transport energy from one point to another but they carry only very little water mass along with them. Furthermore, almost all of the energy carried by the waves is released at the beach, which as we know, often leads to beach erosion.

Types of waves
There are two main types of waves on the sea surface. One is the ordinary sea waves formed by wind action on the water surface; the type of wave that we love to watch whenever we get near a beach. These waves usually have periods (the time for a single wave cycle) less than 30 seconds, and are therefore, known as short period waves.

The second type of waves is caused by the tidal action and has much longer periods, of about 12 hours in the coastal waters of Sri Lanka. These tidal waves too have a crest part or a positive wave and a trough part or a negative wave. But, when you go out to the beach, you do not at once notice the tide as a wave because the wave period is large, and what you see is the slow rise and fall of the water level over a longer time.

The height of the tidal waves around the country is just about half a metre. However, in some other parts of the world, for example in the south coast of Bangladesh, the tidal range can be several metres, so large extents of land known as tidal flats get inundated and exposed alternately with the slow rise and fall of the tide.

Travel speed of tsunami waves
The sudden upward or downward movement of a portion of the seabed momentarily raises or lowers the water surface above, creating tsunami waves. The height of the waves so generated is typically less than a metre, but the wavelength is about hundred thousand times more. The period of tsunami waves typically ranges from 5 to 60 minutes (10 to 30 minutes being more common) whilst the wavelength is about 100 kilometres for a small earthquake and 400 kilometres for a large earthquake.

Although the effect of ordinary sea waves is limited to a few metres below the water surface (say, down to about 50 to 150 metres), the effect of tsunami waves extends several kilometres deep into the ocean, and often across the whole water depth.

The tsunami on December 26, was originated at a location off the north-western coast of Sumatra, about 1500 km south-east of Sri Lanka. There, a section of seabed, about 1000 kilometres long, rose up to 30 metres, as a result of the earthquake of magnitude 9.0 on the Richter scale, pushing the entire water column up and down. The tsunami waves so generated spread out from there at great speeds.

How fast a tsunami wave travels can be estimated using a simple formula. The speed of a tsunami wave in kilometres per hour is roughly equal to eleven times the square root of the water depth in metres. So, in water 4000 m deep, a tsunami will travel at a speed of about 700 kilometres an hour, nearly as fast as a jet plane, but closer to the beach, say in water 10 m deep, it will travel at a speed of only about 35 kilometres an hour, the speed of a slow car. As the average depth of the Indian Ocean is about 3900 m, the tsunami wave that occurred on December 26, at about seven in the morning would have taken, as a rough estimate, a little over 2 hours to reach the east and the south coasts of Sri Lanka.

Discreet crossing
In deep water, tsunami waves are hardly visible. This is because, as mentioned earlier, the height of a tsunami wave is about 1 metre, but the wavelength about 100 kilometres. So, we see that the height of just 1 m is spread out over a distance of 100 km, in other words, the tsunami waves in deep water are extremely flat. It is so flat that you would not feel it at all if you were in a ship or would not see anything unusual on the water surface if you were in an aircraft. This allows tsunami waves to cross a large expanse of ocean water rather discreetly and effectively hiding the enormous amount of energy that they carry to batter the coast down later on.

Also, this is why it is advised to move your boats that are landed on the beach or moored in a harbour out into the sea; that is, of course, if you know that a tsunami is coming.

Violent in shallow water
Tsunami waves devastate shorelines because shallow water near the beach slows them, shortens the wavelength, but increases the height dramatically. For, example, the tsunami wave on December 26 may have been just a metre high in the deep ocean but as it reached shallow water the wave built up quickly, reportedly reaching heights of 5 - 6 metres, and perhaps even more at some places.

The tsunamis entering bays and lagoons, can get even bigger because the sides of the bay or lagoon tend to shorten the length of the wave and push (funnel) it upwards. This, probably, is one reason why some cities located close to bays like Galle suffered particularly severe damage compared to locations where the coastline is straight.

Seabed exposed
One other interesting thing to note is that, often, the initial tsunami wave is a trough (a negative wave) causing a drawdown or a lowering of the sea level. The receding waterline exposes the sea bed down to several hundred metres, leaving fish flopping on the mud and also drawing the curious to the shoreline, and to their deaths because the withdrawing of the sea is inevitably followed by the arrival of the crest (positive part) of the tsunami wave, as has happened in many places around the country.

What we need to understand here is that, when a crest or the high point of a wave comes the water level goes up, so similarly, when a trough or the low point of a wave arrives the water level goes down.

Wall of turbulent water
Do tsunami waves curl over and break like ordinary sea waves when they get near the beach? The answer is, often they do not break in that manner; instead the tsunami waves near the beach appear as a wall of turbulent water (a bore) several metres high (see Sketch-2.). Pushed by the enormous mass of water behind it, this wall of water runs up several hundred metres inland with great destructive power, snapping trees, toppling walls, and smashing houses.

Tsunamis can travel up rivers and streams too. Persons caught in the path of a tsunami have little chance of surviving. They can be easily crushed by debris or they may simply drown as reported by many survivors of the recent tsunami attack. Children and the elderly are particularly at risk, as they have less mobility, strength, and endurance.

The flood of water running up landward may reach a maximum vertical height (run-up height) of 10 - 20 metres or even more above the sea level. The run-up flow velocity can also be significant; Sunday’s tsunami overturned several compartments of a Galle-bound train and pushed a large dredger several hundred metres away, suggesting very high velocities, perhaps more than 5 - 6 metres per second.

Further damage
Then, a few minutes later, the water that ran up inland will begin to run down (recede or drain away) to the sea causing further damage. Meanwhile, the trough of the tsunami wave arrives, often exposing large patches of the seabed. But, as mentioned earlier, a tsunami is not a single wave but a series of waves of different size, so the water will rush in again soon with the next wave crest arriving as before, causing additional damage.

Often the most destructive is not the first or the second wave, but the third or the fourth. This tragic cycle of destruction may repeat several times before the hazard finally passes.

(The writer is a Senior Lecturer at the Faculty of Engineering,
University of Peradeniya)

What we can do for the future
A few things that we as a nation can do are: develop a reliable early warning system so the damage prone areas can be evacuated, remove structures that are easily damaged by tsunami attack from low coastal areas, design those structures that must be in damage prone areas to withstand the potential flooding and surge, and build barriers that might consist of structures (rather expensive, so a question mark on the economic feasibility) or groves of trees to limit surge damage in key areas. The best advice in the meantime is "move quickly to higher ground' if a tsunami is coming.

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