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The most ominous sign of a tsunami


A powerful tsunami can inundate whole regions and sweep away entire towns.
A powerful tsunami can inundate whole regions and sweep away entire towns.

Mark Robinson, Meteorologist, The Weather Network

November 13, 2012 — The most ominous sign is the water pulling back away from the beach. That's the signal to run for higher ground. Fast. A tsunami is on the way.

Tsunamis can be very difficult to predict.
Tsunamis can be very difficult to predict.

What is a tsunami exactly and why are they so hard to predict?

A tsunami is a type of ocean wave that has been generated by a large-scale process such as a large earthquake. 

They are nearly impossible to predict, as the events that generate them are themselves difficult or impossible to predict.  

This is partly what makes tsunamis so dangerous. Without an ability to accurately predict when a tsunami is going to occur, it’s difficult to get warnings out to coastal communities fast enough. 

Known as a "shallow water wave," a type that has a far longer wave length than height, tsunamis can reach incredible speeds (up to 1,000 km/h) but have barely a discernible ripple as they move across the ocean.

It is only when the wave reaches shallow water and begins to slow down (due to frictional effects of the ocean floor) that the wave height begins to rise.  If the energy that created the wave was large enough, heights may reach tens of metres. When the 2004 Indian Ocean tsunami came ashore, there's evidence that the wave may have hit heights of 30 m.

Tsunamis do not look like the classic “surfer wave”, but are instead more like a onrushing sudden tide. Cresting can occur, but generally only just before it hits land. In the 2011 Japan tsunami, some video showed the waves cresting, but most of it looked as if the water was just rising and rising until it spilled over the seawalls.

As the tsunami waters recede, they can carry debris across oceans, which is why British Columbians sometimes find Japanese debris on their shores.
As the tsunami waters recede, they can carry debris across oceans, which is why British Columbians sometimes find Japanese debris on their shores.

This is the real danger of the tsunami; the amount of water that moves inland and sweeps out everything in front of it. 

As the wave sweeps inland, the tremendous force of moving water can tear buildings from their foundations and even destroy whole towns. 

In the 2011 Japanese tsunami, there were many terrifying scenes of whole houses being broken away from their foundations and sweeping past incredulous cameramen. 

It looked like nothing could stop the water and in most cases, that’s exactly what happened.

Eventually, as the wave reaches as far inland as it can and begins to subside, the water reverses direction and begins to flow back out to sea. 

Human habitation is built above the sea level and so as the tsunami ends, the water will all drain back out into the harbours and eventually back out into the open ocean, carrying with it all the debris that was caught up as the wave swept inland. 

This is the reason people in British Columbia sometimes find debris from Japan littering the beaches along their coast.

The Japanese tsunami was probably the best-recorded and most documented tsunami event ever. Given the lead-time of the warnings, helicopters were already in the sky before the first waves hit. 

Many people in the path of the wave rushed to what they thought were safe areas and many brought their cameras with them. 

One of the longest videos shows the entire life cycle of the event as the waves strike the town and nearly completely obliterate it. 

Despite the tragedy and horror of the event, the video is valuable research material for designing barriers and social engineering plans for dealing with these waves.

Water is unstoppable when it rushes in with the force and volume of a tsunami, so when the sea recedes suddenly, knowledge and understanding are your best weapons.

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