Water And Oceanography

Characteristics of Tides

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The rise and fall in the sea level is familiar to anyone who has taken a vacation in a coastal resort. The holidaymaker probably knows that they are caused by the orbit of the moon around the earth. An intelligent tourist probably realises that the inter-tidal zone is a unique habitat that may have been vital for the rise of life from the seas to the land. But what do we know of the details?

Scientists know a great deal about the tides. It is a story of astronomy, fluid dynamics, geography and meteorology.

First consider the basic reason for the tides. They arise through gravitational attraction. Newton’s law of gravitation states that gravitational bodies are attracted in inverse proportion to the distance between them.

The main feature of the tide arises because of the rotation of the Earth. In a time span a little longer than half a day each point on Earth moves between its closest and furthest distance from the Moon. This cycle generates half daily changes in the sea level that are known as the M2 tides.

The second most important feature of the tide arises because the orbit of the Moon around the earth is elliptical. At certain times of the month the Moon is closer to the Earth than at others. The Earth and Moon alternate between their closest and furthest points in the course of half a lunar month, in about 13 and a half days

The third most important tidal force arises through the gravitational interaction between the Sun and the Earth. As with the M 2 tide the solar tide varies due to the rotation of the Earth. Although the Sun is far away this is a significant force because of the Sun’s immense mass. The Sun is capable of producing a tide with about half the amplitude of that of the Lunar tide.

The basic observable characteristics of the tides arise from the three principal tidal generating forces. In most places the tide is dominated by the daily M2 tide in which high and low tides recur at 12 hour 25 minute intervals.  In some places there is only one tide a day – this is called the diurnal tide. It occurs in the Gulf of Mexico and on the West Coast of America.

The M2 tide is sometimes reinforced and sometimes offset by the other tide generating forces. The interactions are particularly noticeable at the so called spring and neap tides that occur in each lunar month. At the time of the spring tide the gravitation force of the sun is aligned with that of the moon and there is an exceptionally high tide. At the time of the neap tide the gravitational forces of the sun and the moon are in opposition and there is an exceptionally low high tide. The words spring tide incidentally have nothing to do with the season the word spring is used in the sense that the tide springs forward like a coiled spring. The spring tides are typically 20% higher than a normal tide.

Very high tides can occur when the spring tide coincides with the Earth’s closest approach to the Earth. Sometimes these high tides are harmless. If they combine with other influences such as a severe storm, hurricane or tsunami they can be devastating. Floods in southern and eastern United States in 1723, 1846, 1885, 1900 (Galveston), 1914, 1931 and 1978 are attributed to proxigean tides as floods in Europe in 1099 (Holland), 1606 (Bristol Channel, UK), and 1953 (North Sea coast of UK) and Holland

Tidal phenomena are very complicated. There are over 100 other astronomical influences called harmonics that contribute to the overall tide.

Having considered the gravitational influences the next task is to consider how the tide is dispersed. Each tidal component travels in a different way due to the effects of coastal geography. Due to the rotation of the Earth the tides generally propagate along the coast in anti- clockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. When oceanographers plot contour plots of which link equal times of high water they find that for each type of tide the tide times circulate around points in the deep ocean known as amphridromic points. The sea level at the amphridomic points is not influenced by the tide whatsoever.

When a tidal wave approaches the coast the coastline and the shape of the seabed becomes important. An estuary generally experiences a higher tide compared to the open sea. When a wave travels over a shallow sea bed the wave travels faster and the amplitude increases. The sides of the estuary funnel water which is forced to build in amplitude.

In Canada the Bay of Fundy experiences the greatest tidal range in the world. The tidal range is an astonishing 50 feet at the head of the Bay. The cause is not so much the funnel shape and bottom topography but geometry. The Bay is the correct shape to allow an M2 tidal wave to travel unimpeded.

A detailed knowledge of the tides has many uses. The most obvious use is in coastal navigation. Even today certain waterways are only open at high tide. A famous footpath between England and Scotland across the mudflats of the Solway Firth is clearly only open at low tide. Some scientists look at the viability of closing tidal estuaries with a barrage that generate tidal energy others. Others use their knowledge to provide predictive warnings for impending coastal floods and storm surge.  More detailed challenges involve using the knowledge of the tides to predict water quality in coastal waters. A knowledge of water quality allows tourist resorts to achieve the coveted clean beach awards.

Further reading

Physical Oceanography of Coastal Waters K F Bowden Ellis Horwood 1983

More about this author: Nick Ford

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