Nature has the power to inflict terrible damage, and occasionally boats, buildings and even human lives are lost to the waves.

Tsunamis
A tsunami is a devastating tidal wave, usually caused by an underwater earthquake. Tsunamis are also known as tidal waves, but this is a misnomer as the wave has no relation to tidal effects. The earthquake that induces the wave occurs less than 50km (30 miles) beneath the sea floor, and has a magnitude greater than 6.5 on the Richter scale. They are also caused by volcanic eruptions or occasionally giant mudslides. The shock wave produced radiates outwards, travelling at speeds of up to 700km/h (435mph).

In deep water, the wave is unnoticeable, sometimes reaching a maximum of only 1m in height. But as it reaches shallow water, the great speed and power behind the wave means that it gathers in height very quickly, wreaking terrible damage.

Tsunamis are most common in the Pacific Ocean because of the volcanic activity around its rim. Thirty-six thousand people were killed in Indonesia in 1883 after the explosion of Krakatoa sent tsunamis halfway across the world. Some of the waves reached 35m high. More recently, in 1998, villages along the coast of Papua New Guinea were obliterated by waves 10m high caused by an earthquake. More than 3,000 inhabitants were killed. Perhaps the most destructive tsunami occurred in 1703 at Awa, Japan, killing more than 100,000 people.

The word ‘tsunami’ comes from the Japanese words for harbour wave.

Seiches
Seiches are rhythmic motions of water in lakes or land-locked bays and are caused by underwater earthquakes or surface winds. Water pushed by the wind or seismic activity can pile up on shore causing increases in water depth. When the wind or force reduces, the water mass continues to slosh back and forth like water in a bath. These oscillations may last for hours or even a couple of days. Seiches were first studied in Lake Geneva, Switzerland, in the 18th century. They can cause problems for ships by causing strong currents at the entrance to harbours, and occasionally drown people on the shore.

Whirlpools
Whirlpools are ocean currents that move in a rotating direction, produced by rising and falling tides. Some of these exhibit a downward pull, called a vortex.

One of the most famous whirlpools is the Maelstrom located near the Lofoten Islands, off the coast of Norway. The word comes from the Dutch for ‘whirling stream’. This is caused by a strong channel about 8km (5 miles) wide that travels through the islands of Moskensøya and Mosken. The currents can reach speeds of 11km/h (7mph) with the changing tides.

Others occur on the Strait of Corryvreckan (between Scarba and Jura, off the west coast of Scotland), along the coast of Calabria in southern Italy and off Messina, in the strait between Sicily and peninsular Italy. A vortex occurs in the Naruto Strait, which connects the Inland Sea (Japan) and the Pacific Ocean.

Waterspouts
A waterspout is formed when a tornado (rapidly rotating winds) moves over water, or with the build-up of atmospheric pressure. Waterspouts usually occur during periods of instability. Where cold front boundaries pass over the ocean, or over warm ocean currents, the instability and low pressure causes air and water to spiral rapidly inward and upward.

Some waterspouts are estimated to reach speeds of as fast as 64-80km/h. The water spout does not suck up water to great heights - the main visible cloud is formed by condensation of water vapour - although water does rotate around the lower end of the vortex.

Thousands of people witnessed one of the largest and most famous waterspouts in Massachusetts in August 1896. The spout lasted for about 35 minutes, although it reappeared three more times. It was estimated to reach a height of 1,095m, with a width of 256m at the crest, 43m at the centre and 73m at the base. It was thought to have been caused by thunderstorm-squall conditions.

Waterspouts can be a hazard to small boats, and people have been killed when the vortexes have moved inland. In 1879, two or three water spouts were reported to have caused the Tay Bridge rail disaster in Scotland.

Links

Watch how a seiche works with the University of Delaware’s Seiche Calculator
www.coastal.udel.edu/faculty/rad/seiche.html

Tornadoes and waterspouts in the UK
www.bbc.co.uk/weather/features/tornadoes_waterspouts.shtml

Videos and gallery of tornadoes and waterspouts
www.tahoetwisters.com

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