hedgehog

In light of the recent disaster in Asia I have been pondering the "big wave" that did all the damage and am wondering if anyone on here can answer some questions I have, or point me at a good reference on the physics of such things.

In the part of the world where I spend a lot of time a 30 foot wave would not be unusual and a North Sea oil rig, for example, would see waves in excess of 30 feet. Over new year I was on a few coastlines and beaches and saw some waves that must have been more than 30 feet.

With this in mind does the tsumani do so much damage because the land is so low lying or is there some other factor involved? I understand the basic physics of a transverse wave and so can't see how a tsumani differs from what I might consider a "normal" 30 foot wave in those terms. My thoughts are that the fact that hundreds of ships didn't sink support the idea that this was a normal transverse wave, all they did was bob up and down a little.

So, basically, does it do all the damage because it's not the height of the wave that matters but the length of your legs or is there another physicall thing in action in a tsumani that makes it different from a "normal" 30 foot wave arriving on a coastline which can cope with such things?

Strange question I know but I have no clear understanding of this.

ProperCharlie

I have no particular understanding of this, but from the footage I have seen the difference between the tsunami and a "regular" large wave seems to be that the tsunami kept on going, and brought millions of tonnes of water with it. A "regular" big wave appears to break as it nears the shore, topple over and then a certain amount of water travels up the beach before retreating. This thing just seemed to keep going regardless of beach/shore/land beneath it.

Scooby96

Dont forget the see receeded approx 400 yards - billions and billions and billions of gallons, dare I say it even perhaps a 'google' of gallons over and above a 'normal' wave.

A transverse wave isnt the same as a tidal / tsunami wave, is it!?

BigRed

Hedgehog have a look here -:http://www.pmel.noaa.gov/tsunami/Faq/

midget1500

i thought that tsunami's where very fast as well? - upto 500mph in that FAQ

Daz34

Speed drops as the water gets shallower and the wave gets higher I believe

ozzy

Read this - http://www.geophys.washington.edu/ts...teristics.html

It's not necessarily to do with the height of the wave, but the wave length. Take large breakers ridden by surfers. They have short wave lengths that break on or before the beach.

Tsunamis don't break the same, but push huge volumes of water inland. If the coast is low lying then it can reach inland for miles.

Stefan

Nicks VR4

http://wcatwc.gov/physics.htm

As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Since the speed of the tsunami is related to the water depth, as the depth of the water decreases, the speed of the tsunami diminishes. The change of total energy of the tsunami remains constant. Therefore, the speed of the tsunami decreases as it enters shallower water, and the height of the wave grows. Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to be several feet or more in height.

When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. Tsunamis rarely become great, towering breaking waves. Sometimes the tsunami may break far offshore. Or it may form into a bore: a step-like wave with a steep breaking front. A bore can happen if the tsunami moves from deep water into a shallow bay or river. The water level on shore can rise many feet. In extreme cases, water level can rise to more than 50 feet (15 m) for tsunamis of distant origin and over 100 feet (30 m) for tsunami generated near the earthquake's epicenter. The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 1000 feet (305 m) or more, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat. Tsunamis may reach a maximum vertical height onshore above sea level, called a run-up height, of 30 meters (98 ft). A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 525 meter (1722 ft) wave

Nicks VR4

Bascially a normal wave circles over
Tsunami the water flows straight

Brit_in_Japan

Not wanting to scaremonger, but the UK may not be immune from tsunami. I remembered the following story from a couple of months back which suggested that the eastern seaboard of the US and the caribbean nations might be at risk from a tsumani resulting from huge landslide in the Canary islands, prompted by a volcanic eruption there. Makes you wonder whether a tsumami warning system should be created for North Atlantic countries too...

http://news.bbc.co.uk/1/hi/sci/tech/3963563.stm

ajm

It's because tsunami waves carry more energy. They have very long wavelengths which means they are very fast in deep water and they also carry a much larger volume of water than wind generated waves.

v = f * lambda where v is the speed of the wave, f the frequency and lamda the wavelength.

As the wave reaches shallow water the wavelength is reduced hence speed is reduced.

For a given energy, if wave speed decreased, amplitude must increase (the water "bunches up"), hence the wave height increases and its devastating. Couple that with low lying land and you get what happened in Asia.

Fuzz

On a slightly different note but just as disasterous....

I wonder when Yellowstone is going to let go..



Andy

BigRed

Poor old Yogi, doh, thats Jellystone

ajm

Better start preparing for a nuclear winter now!

hedgehog

I think this might be the crux of it ajm, in truth it does behave like a normal transverse wave but because its velocity is so great this is where all the additional energy comes from. Most of the 30 or even 60 foot waves I see have a velocity in the deep ocean of perhaps 10 or a few tens of mph. However the tsunami wave has a velocity of several hundreds of mph under those circumstances. When it reaches shallow water this energy has to go somewhere and in the case of my "big normal wave" because the initial velocity is quite low it doesn't have a lot of energy when compared with a tsunami. The tsunami however has a lot of energy as the frequency is rapidly increasing as the velocity reduces and the wavelength reduces due to the faster moving "free ocean" waves catching up on the rapidly slowing "shallow water" waves.

Als E = hf the energy it has for screwing things up is going to increase. I know that in the case of a sea wave the E = hf equation may not be totally true, but it will be true in principle I would suspect.

I am sort of starting to see how what I might consider a normal sized wave might gain the energy to be very destructive, especially on coasts where the ground is low lying.

ajm

That's pretty much it, all waves behave the same with incresing amplitude with reducing depth, that is why we get surf.... in shallow water the v = f * lambda equation also falls over slightly but the principle remains the same. However the longer wavelength of tsunami allows for greater velocity, for exactly the same reason longer displacement boats can go faster than shorter ones.

Also, when you get working out the kinetic energy of having multiple waves at a shorter wavelength (and hence less volume of water per wave) versus one wave with a very long wavelength the volume of water in the tsunami is all tied into one wave and is all delivered at once, whereas the same energy may be spread across 100 normal wind induced waves of the same amplitude, hence the power delivery is spread by the period of the waves and is not so sudden. It's a bit like turbo versus normally aspirated... overall power the same, but delivery completely different.

carl

I'd also like to know why some of the pictures on the TV are showing areas that are now flooded. Surely the water should have receded as quickly as it came in?

ajm

Presumably this is because the land undulates, i.e. the land may be above sea level but there is a dip. Normal rainfall rate can run off via rivers/tributaries, but if the whole dip is filled it will take much longer to run off.

Same reason rivers burst banks and leave flooding for longer than it takes the river to return to normal level.

carl

Yes, I guess it just can't drain away fast enough

fast bloke

If you can imagine someone shouting really loudly and a car bomb going off 20 feet away, the effect would be similar to a 20 foot wave and a 20 foot tidal wave. Someone shouting as loud as they can and a 20 foot wave will merely cause a small interruption in normal service. A car bomb and a tidal wave have similar properties at any given point to the first two. The problem arises because the total amount of energy in the car bomb and the tsunami are much much greater, so the number of given points that suffer the same effect are infinitely larger. The other difference is that a tsunami wave form might be a mile long, while a standard 20 ft wave might be 10-12 feet long. Imagine the lack of devastasion if the wave had struck and then receded in 2-3 seconds instead of 10-15 minutes. When it hits land, the wave might be travelling at 50-60 MPH (depends on the shape of the land underwater) a normal wave will only cover a few feet before it loses energy. A tsunami with a much longer wave form and much more energy will cover a mile or more with the same devestating energy.

DarkStar66

I know this is a little off topic but I just looked up what were the worst Tsunamis in history in terms of lives lost and the Boxing Day one is by far the worst!

Here is the top three before the current one;

1. Atlantic coast (Morocco, western Europe, West Indies) in 1775 - 60,000 killed
2. Sumatra, Java in 1883 - 36,000 killed
3. Japan, in 1707 and Italy in 1783 - 30,000 killed

This just gives you some perspective on how bad the South Asia Tsunami really was!

Fuzz

no.... just how overpopulated the world is becoming


Andy

Leslie

As I understand it, the Tsunami is not a wall of water moving across the ocean at all. The big wave is caused by a shock wave which was generated at the site of the original earth tremor. The sudden slip of the tectonic plates generates the shock wave which radiates in all directions. The shock wave travels through the water in a similar manner to the sound of a hammer blow on a piece of steel passing through the solid substance. This explains the very high speed of travel which I saw reported of from 400 to 600 MPH. The shock wave exists from the top of the ocean to the ocean bed. The visual effect at sea may be a small local rise in sea level as it passes a particular point.

When the shock wave reaches a coast, it becomes pushed upwards by the shallowing of the sea and it then affects the water at the coast. The water will first move out to sea from the coastline exposing a large part of the land which was previously underwater. The sea level will rise as the water moves out until it meets the approaching shock wave. The energy in the shock wave will then move the whole mass of water inland as was mentioned in an earlier post causing the awful damage we have seen in this case.

Les

hedgehog

I've noticed a problem with my considerations.

I assumed that as the wave comes ashore its velocity reduces and therefore its frequency increases because the waves are closer together. I then thought of Planck and got him involved through E = hf.

In truth, because the velocity is less, I have no evidence that the frequency increases but I do know that the wavelength must be reduced. It could be that the frequency remains constant. Bummer.

However, as others have pointed out if you have a box (say an area of sea) and put X energy in then (approx) X energy must come out. When this X energy is spread over a water column 6,000 feet deep then the density isn't very high but when it is concentrated into a wave 30 feet deep then there is going to be a considerable "density" of energy. So it may be in this case that the energy available to "do work" if you like is not related to something like hf but is related to the cross sectional area of the water column.

Leslie

And the original height of the shock wave before it is compressed by reaching shallow water. Dont forget the loss of energy due to heat loss of course.

Les

Geezer

The wave height can only be as large as the vertical sea floor displacement too. This is why Tsunami only reach about 15m.

The wave rises in shallow water because the front of the wave is travelling slower than the front, so the following water has nowhere to go but up.

Geezer

hedgehog

I don't know what I'm talking about and so this might be ***** but...

I think that the wave height need not be limited by the sea floor displacement and, in fact, depending upon the precise circumstances may not even be proportional to the displacement but will be proportional to the energy released. In the end I would expect that the wave height at shore will be related to the energy in the wave and the precise nature of the coast line.

As you point out the water has nowhere to go but up and I would guess that there will be a physical limitation as to how high you can make a "pile" of unsupported water before it will just not go any higher no matter how much energy you put in. In effect this is why waves break as they become unstable.

Can you believe that something that seems as simple as a wave can have me utterly beat as to the exact physics of how it works? I guess this says more about me than about the wave :-)

astraboy

Thats the reason it gets so high as I understand it. Out to sea, tsunamis are not a problem, its only when they get close to land do they get so lethal.
The reason they just keep on going is because its going so fast in the first place.
If you can imagine two cars, one travelling at 90mph and the other at 70 mph, brake at the same time at a constant rate, How fast do you think the 90mph car will be going when the 70 mph car comes to a stop? Think again if you thought 30 mph. Its actually 70 mph. Quite basically, the faster something goes the more it takes to stop it.

Because the wave is travelling so much faster than the usual breakers that people surf on (the ones which are stopped by the incline of the beach etc) then its momentum prevents it from slowing down and breaking, so it just keeps on going until all its energy is dissapated.

When you sit down and think about it, its quite a frightening notion living next to the sea when it can cause so much destruction.
astraboy.

astraboy

I dont think its as comlicated as that. As I understand it, its litterally a case of "the sea floor rises by x metres, it creates a bubble of higher water x metres high, which then radiates in all directions"
astraboy.

DarkStar66

That's certainly part of the problem but I don't think that the areas effected have a high population density compared to a lot of other places in Asia.

Imagine if the wave had gone North/South instead of instead of East/West. It would probably have washed most of Bangladesh out to sea. On the other hand, try not to imagine it!!!