elb's hovel of thoughts

Thursday, December 30, 2004

Power of a tsunami; how Penang could have been hit

One very common comment made by people regarding the tragic event is 'how powerful the water was, being able to lift cars etc'.

Little physics lesson time: The water was powerful simply because of the momentum it had. What is momentum? It is simply the weight of the object multiplied by its velocity. Another way to put it is the constant force of the object at any instant.

How heavy is water? 1 metre x 1 metre x 1 metre of water weighs 1000kg. One ton. According to eyewitnesses in Penang, the waves there were a couple of metres high. We assume a length of 10metres per wave (I have no idea about actual values, but I think it is a reasonable estimate). Hence the volume of water per metre of coastline would be 2m (depth) x 1m (width) x 10m (length) which gives us a total of 20 cubic metres, or 40 000kg of water crashing down for every metre of coastline in a matter of a few seconds.

A tsunami usually hits the shore with shallow water at around 20miles an hour (or 8.89 metres per second, or 32km/h). Hence the water per tidal wave would hit with a total momentum of 1280 tons.km/h, or 355tons.metre/second. Not something to be fooled around!!! The good thing is that it hits over a long period of time, say maybe each surge lasts for 20 seconds. This gives a impulse of 17.75 tons.metre/second^2. Still very powerful!

That force could easily cause all the destruction you see; in fact it would be two and half times as worse for the people at Sri Lanka (they got hit by waves 5 metres high).

As to why Penang could have been hit by the tidal waves (yes, Penang was not hit by a real tsunami, but a weakened version of it called a tidal wave), my theories are:
1) The quake was powerful enough that a tidal wave was formed east of Sumatra. Highly unlikely though, otherwise we'd have reports coming in from states south of Penang
2) The waves that were travelling north of Sumatra got deflected towards Phuket/ Penang. Yes, it is possible for waves to deflect off objects it hits.
3) The radial effect of waves as it passes the top of Sumatra would mean that the waves would be able to reach Penang easily as it is within radius.

What do I think?
Number 1 is most probably happened, according to some media reports. Strangely though; we should have reports coming in from states south of Penang had it happened.
2 might sound far fetched for most of you, but in order for Gurney Drive (which is north east of Penang) to have gotten hit, I think that there had to be a reflection from somewhere north of it.

Probably a combination of reflection and radial effect.


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