Introduction to Tsunami                 Surf Life Saving Australia

Key points Coastal safety Role of Surf Life Saving Surf Life Saving response

Coastal conditions Coastal conditions and tsunami People at risk Quiz

Coastal Conditions - how can tsunami affect beach conditions?

Waves

Waves are affected in four ways:

• Mass of water in the waves.
• Speed of the waves.
• Type of waves.
• The set of waves.

Mass of water in the waves

Tsunami are usually generated by an undersea earthquake, never by the wind (for more information see What causes tsunami?).

Normal ocean and wind swell waves only cause motion in the water to a depth of 150 metres. These waves may cause motion without inundating normally dry land areas.

In contrast, the passage of tsunami involves the movement of water all the way from the sea surface to the seafloor. The energy moving through tsunami may result in them being able to travel long distances inland.

Depending on the size of the earthquake, and resulting tsunami, the increase in wave height can vary from five centimetres above the normal height, to a wall of water several metres in height. The consequences may vary from strong unusual currents that threaten swimmers and marine users, to large waves which may inundate beaches and low lying land, threatening buildings.

Speed of the waves

A tsunami wave can travel much faster than a wind swell wave, reaching speeds of up to 950 kilometres per hour in the open ocean. As they reach the coastline, their speed reduces and wave height can grow significantly - up to several metres. Due to the wavelength being so long, the front of the tsunami starts to slow down before the back part. As the back part of the tsunami catches up, the wall of water is created.

Type of waves

When tsunami reach the coastline, they may surge without breaking as in the case of very small tsunami. Larger tsunami may develop into a wall of water with a large mass of water behind it. The energy contained in the larger tsunami may cause them to push inland, resulting in serious inundation of coastal land.

On 17 July 2006, a group of campers at Steep Point, Western Australia, were lucky to escape when their site was inundated by a tsunami. This tsunami was caused by an earthquake south of Java (for more information see Tsunami in Australia).

Important:
Even a small increase in wave height may be enough to lift you off the ocean floor, and/or increase the force of the water moving back out to sea, causing difficulties for an inexperienced swimmer.

The set of waves

A tsunami is a series of waves travelling across the ocean due to a sudden displacement of a large body of water. Tsunami waves travel outwards from their source and have extremely long wavelengths - up to hundreds of kilometres between crests in the deep ocean. The first wave may not be the largest and the time between arrival of successive waves can be anywhere between ten minutes to two hours. In contrast, wind swell waves may have a wavelength of only a few metres and you can see the next one coming. It is important to wait until the 'all clear' has been issued before returning to the water's edge. This could take hours or even days.

Coastal conditions and tsunami

Currents

Any small increase in water moving towards the beach can affect rip and current activity. Even a relatively small tsunami may still be dangerous to swimmers and marine users.

Remember:
The larger the volume of water moving towards the beach, the larger the volume of water that needs to move back to sea.

The wind swell wave may increase the water level height by five centimetres, with water moving on the upper layer of ocean only. With tsunami, it is the water all the way to the ocean floor that is moving. It may increase in the strength of the rip without generating a noticeable wave. You may even find a large group of swimmers carried out to sea very quickly. This is what happened in Western Australia in 2004, as a result of the 2004 Indian Ocean Tsunami. Here, 20 people were swept off a sand bar and carried away from the shoreline because of a dramatic change in tides.

Tsunami may push water inland anywhere from 20cm up to many kilometres depending on its size. People camping in remote locations need to be aware of this.

As a beach manager it is important that you understand it will be difficult to estimate how far the wave will progress up the beach. It is highly recommended that all people be evacuated from the water's edge and surrounding beach (please refer to Surf Life Saving response section to find out exact procedures).

The wave swell caused by tsunami may not take on the regular shape of a wind generated swell. Wind swell waves generally travel through the ocean in a concentric manner, similar to dropping a pebble in a pond. However, tsunami waves travel in a finger-like projection from the source. This difference in wave patterns could also affect rip and currents along the coastline in ways that you may not expect.

Due to the different way that tsunami waves move through the ocean, the effects on the coastline may vary from one region to another. For example, if one region has water travelling 200m inland, it does not necessarily mean this will happen along the whole coastline.

Tsunami can affect normal wave patterns and tidal currents for days after the event. This may be due to tsunami refracting off the coastlines of other countries. If you are providing a lifesaving service to the community within this period, remember to be alert for unusual conditions.

Remember:
The first wave from a tsunami is usually not the largest. If there does not seem to be a noticeable effect at the predicted time of the arrival of tsunami, this does not mean that the worst has passed. The following waves in the set may cause the greatest damage/effect.