Recent history has provided more exposure to devastating tsunamis than any generation before us. The international media has allowed us to watch events in real time on television and our global tsunami warning systems allow us more time to prepare than the tell-tale signs visible on the shoreline just before impact.
This exposure provides increased awareness of tsunami, but also creates a new challenge; whenever there is a tsunami warning in Australia people tend to expect to see a large wave inundate the coastline. In Australia these inundation waves are rare, however we regularly experience the effects of tsunami in the marine environment.
The effect of tsunami in the marine environment may not be as dramatic, but it is still dangerous. These effects may be unusual currents, strong rip currents and abnormal tides which can be hazardous for anyone in or on the water.
A tsunami is a series of waves travelling across the ocean due to a sudden displacement of a large body of water. This displacement can be caused by events such as undersea earthquakes, undersea landslides, land sliding into the ocean, volcanic eruptions or even asteroid impacts.
Tsunami a unique Wave
Tsunamis are different from wind swell waves and have three overlapping but quite distinct stages.
Generation of tsunami is caused by any force that disturbs a water column. As outlined above, this could be due to an undersea landslide, a volcanic eruption, or an asteroid impact though the most common cause of tsunamis are earthquakes.
Propagation of this displaced water occurs as it moves across the deep ocean. When a tsunami is generated, the waves will spread out in all directions. Tsunami:
- can travel as fast as 950 kilometres per hour through deep water and;
- have extremely long wavelengths in the deep ocean.
Inundation is the stage which can result in some localised overflow onto the immediate foreshore. Less frequently, tsunami can result in more serious inundation of coastal land, serious threat to lives and damage to property, especially in low lying coastal areas.
Tsunami will look and behave differently depending on the shape of the sea floor and coastline. As tsunami move into shallow water, their amplitude may increase. When this occurs it is called shoaling. Shoaling does not occur in every coastal environment. It is more likely to occur in a bay, harbour or lagoon where the wave is funnelled as it moves inland.
How tsunami differ from wind swell waves?
The wavelength of wind swell waves is much shorter than that of tsunami. You can see the next wind swell wave coming to the beach. Normal ocean and wind swell waves can cause motion in the water to depths of 150 metres. These waves may cause motion without inundating normally dry land areas. If you click on the animations below you can see how the motion of a wind swell wave differs from that of a tsunami.
Tsunami have extremely long wavelengths, up to hundreds of kilometres, even as they reach shallow coastal water. The second and third waves can arrive ten minutes to two hours apart. The passage of tsunami involves the movement of water all the way to the seafloor. The energy moving through tsunami may result in them being able to travel long distances inland.
What happens when a tsunami reaches the shore?
If the wave trough arrives before the wave crest there may (but not always) be a drawdown of the coastline which exposes the ocean floor. The distance of drawdown varies depending on the wavelength as it approaches the coast and also the slope of the beach. For example, the drawdown could be hundreds of metres if the beach slope isn't very steep. There can be anywhere between five and twenty minutes before the tsunami arrives.
As tsunami approach the shoreline, speed reduces and wave height can grow significantly - up to several metres. It is not so much this movement of water but the energy moving through it that makes tsunami so dangerous. This is when the potential for inundation of normally dry land occurs.
Dangerous rips and currents
A relatively small tsunami may still result in strong rips and currents that can be dangerous to swimmers and other marine users. This is the most likely impact we would experience in Australia. If dangerous rips and currents are likely to be produced by a tsunami, a marine threat warning will be issued by the Australian Tsunami Warning Centre.