In the oceans, a ridge or swell formed by wind or other causes. The power of a wave is determined by the strength of the wind and the distance of open water over which the wind blows (the fetch). Waves are the main agents of coastal erosion and deposition: sweeping away or building up beaches, creating spits and berms, and wearing down cliffs by their hydraulic action and by the corrosion of the sand and shingle that they carry. A tsunami (misleadingly called a ‘tidal wave’) is formed after a submarine earthquake.
As a wave approaches the shore it is forced to break as a result of friction with the seabed. When it breaks on a beach, water and sediment are carried up the beach as swash; the water then drains back as backwash.
A constructive wave causes a net deposition of material on the shore because its swash is stronger than its backwash. Such waves tend be low and have crests that spill over gradually as they break. The backwash of a destructive wave is stronger than its swash, and therefore causes a net removal of material from the shore. Destructive waves are usually tall and have peaked crests that plunge downwards as they break, trapping air as they do so.
If waves strike a beach at an angle the beach material is gradually moved along the shore (longshore drift), causing deposition of material in some areas and erosion in others.
Atmospheric instability caused by the greenhouse effect and global warming appears to be increasing the severity of Atlantic storms and the heights of the ocean waves. Waves in the South Atlantic are shrinking - they are on average half a metre smaller than in the mid-1980s - and those in the northeast Atlantic have doubled in size over the last 40 years. As the height of waves affects the supply of marine food, this could affect fish stocks, and there are also implications for shipping and oil and gas rigs in the North Atlantic, which will need to be strengthened if they are to avoid damage.
Freak or ‘episodic’ waves form under particular weather conditions at certain times of the year, travelling long distances across the Atlantic, Indian, and Pacific oceans. They are considered responsible for the sudden unexplained disappearance of many ships.
Freak waves become extremely dangerous when they reach the shallow waters of the continental shelves at 100 fathoms (180 m/600 ft), especially when they meet currents: for example, the Agulhas Current to the east of South Africa, and the Gulf Stream in the North Atlantic. A wave height of 34 m/112 ft has been recorded.
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