Tsunamis are waves, but only in as much as they are energy passing through a medium. What makes a tsunami unique is all about breaking wave speed...let me explain.You can take breaking wave speed calculations to a complex level with very shallow water and Boussinesq equations but heres a really useful simple formula you can do in your head. Its good because you can do this for any break in the world. All you need to know is roughly what depth the wave is breaking in and a little about the bathymetry.breaking wave speed in knots = sq root of (10 x water depth)

10 is the gravity acceleration co-efficient, use 9.8 if you want to be more precise.

When you calculate depth you need to use an average depth that the entire wavelength will fit into. So over about 300 metres is fine. For beachbreaks, depth slopes fairly uniformly so you can just add 30% to the depth the actual wave is breaking in and call that your average.

Example: ManlyThe main peaks at Manly break in about 10 metres of water, so adding 30% we come up with an average depth of 13m 10 x 13 = 130sq root of 130 = 11.5kts

The thing to keep in mind is that once a swell becomes a shallow wave, period works a little differently in effecting speeds and it becomes more pronounced on reefs or waves with very deep water behind them. Think of it like this - the atmosphere is a fluid just like the ocean and sound travels within it as energy the same way waves do in shallow water. Someone is standing next to a jet engine and you are 100 metres away when they shout and the jet engine is turned on, the sound from both sources travel at exactly the same speed. The reason you only hear the jet engine is that its amplitude (wave height) is so much greater than the human voice. Frequency (wave period) has nothing to do with it at all. But sound waves don't 'break', so period works uniquely in ocean waves as it effects the avergae depth of the breaking wave, making it effectively deeper and hence faster.

So back to our breaking wave, this means that a 10 second period 1 metre wave breaks at roughly the same speed as a 5 second period 1 metre wave. The reason bigger waves break faster than smaller ones is due to the fact that they break in deeper water. It is only when periods become extreme such as in Tsunamis that they have a marked effect on speed. Because the actual height of Tsunmai waves (amplitude) are generaly so small ( 6 inches) though they break in very shallow water, usually only a metre. This brings their speed down substantially. The long period though reaches back into a few hundred metres of water so the average depth of even a small tsunami is over 100 metres. So using the formula:

speed = sq root of (10 x 100)sq root of 1000 = 32kts.

But instead of this wave breaking way out the back, its now on the shore doing 60kms per hour, and because the wavelength is so long, hundreds of metres of ocean are at the same height travelling at the same speed right on the shore line.

A 20 second period wave has a wavelength of around 600 metres, so your average depth calculation needs to stretch back that far past the spot the wave is actually breaking in to work out effect depth for speed calculations.

So a 10 second 2 metre wave at Manly has an effective break speed of 11.5 kts, a 20 second wave = sq rt of (10 x 20) = 14kts so pretty close

Lets make it 5 metre swell, so Queenscliff Bombie will be breaking. Knowing that there is about 15 metres of water on the bombie we can say the wave is breaking in 15 metres of water. Because the bombie is 700 m from the shore though, 300 metres past that puts you into 50 metres of water, so the effective depth for our calculation is more like 35 metres.

Therefore speed = sq rt (10 x 35)= 19 kts

Reefs and open ocean breaks will always break faster than beachbreaks because that depth gradient is so much larger than a beach break. Add a long period swell to that and the effect will be even further pronouced due to stretching back into deeper water.

Teahupoo is a perfect extreme to demonstrate the effect on reefs and the whole theory in general. Whilst the wave effectively breaks in only a few metres of water, its a fall to about 300 metres once you head seaward from the break, so the effective depth for our calculation is about 150 metres.

Therefore speed = sq rt (10x150)=38kts

Big, long period Teahupoo breaks in effective depth of around 200 metres

Therefore speed = sq rt (10 x 200)= 45kts

This is the reason the wave breaks below sea level, the trough is drawing back off the reef the same way a tsunami does and the appearance of the rest of the ocean being the same height as the crest of the wave is because its travelling so close to the speed it would in the open ocean. Teahupoo is the closest thing you will see to a real life example of tsunami dynamics other than the real thing. Now imagine the classic shots of Teahupoo, this will be a 5 metres swell at 20 seconds and Laird Hamilton will cry after towing into it. Now imagine that same wave only 2 feet high but with a period of an hour; an an incredible amount of water passing the same point.

This theory breaks down of course when there is an ASP event and it will be 14 knot 2 foot slop

Remember though that these speeds we are talking about are only valid for a surfer at takeoff. You will be able to move much faster than this going across the face of the wave. The advancing front moves at this speed initially but as the wave / foam moves shorewards it continues to slow. If you rode the wave straight to the shore like they do at Bondi then you would slow as the depth shallowed.

All swell / wave trains slow as they approach a shoaling coast but, the reduction in speed is translated to an increase in height. What causes the wave to 'break' is the ratio of steepness in height compared to period. When this ratio reaches .8, the wave breaks as by now the top of the wave is travelling faster than the bottom which continues to slow. The big factor in a shallow water equation is the steepness of the shore, so a long continental shelf (Northern Beaches) will attenuate more energy whilst a thin shelf (Margaret River, Hawaii) will attenuate far less. Open Ocean reefs have had almost no time to slow their waves down as those waves do break a lot faster than almost any others.

Thoughs and prayers with the Mentawain people during this difficult time.