by Dr Amanda Young
The twice daily rise and fall of the tide found around most of Europe must have been observed by the earliest visitors to the coast. The first written records of the phenomena are by peoples from the Mediterranean where tides are, in fact, almost negligible. Only when these peoples, Romans, Greeks and Phoenicians, sailed out into the Atlantic did they find, to their awe and fear, must how great tides could be. Indeed, when Caesar invaded Britain in 55BC the combination of strong winds and a 20ft spring tide left his boats high and dry on the shore.
The 'Ancients' were accurate observers of their natural surroundings
and Pliny the Elder (AD 23-79) explained how spring and neap tides are
the result of the combined effect of the sun and moon, and that diurnal
inequality is related to their positions north and south of the Equator.
We know that he was correct and that when the sun and moon are in line
at the new and full moon the gravitational forces which they exert
on the fluid which makes up the seas are largest and the tidal range greatest.
These are called spring tides. When the sun and moon are at right
angles to one another, at the quarter moons, the gravitational forces are
opposed and so decreased: the tidal range is then smaller and these are
called neap tides.
The Effect of the Tidal Wave
Globally the tides can be likened to a giant wave (or bulge of water)
that passes around the earth once every 12 hrs 25 mins. In reality,
the only place where the sea forms a continuous, uninterrupted belt around
the earth is the Southern Ocean.
However, because at these latitudes the great tidal wave (often called the primary wave) is only between 0.6 m and 0.9 m in height, the scattering of small island experience quite small tides. The generation of these tides is a dynamic process.
Whilst set in action by the sun and moon, it is the configuration of the ocean floor, the depth of the water, the effect of the Coriolis force and the friction felt by the water as it moves around, that results in any particular tide being a given height at a given location. Put simply the effect of the Coriolis force is to cause the water to spin around a number of points (called amphidromic points). At an amphidromic point there is no tide at all and as you move away from the point in a radiationg circle the tide becomes bigger and bigger. On top of this when the tidal wave reaches, first shallow water and then land, additional complications begin. Due to the enormous size of many of the land masses and their intricate contours the pattern of the tides can, and does, become extremely elaborate.
Take, for example, the tidal wave that sweeps across the North Atlantic Ocean. During its passage across the deep centre of the Atlantic its speed is relatively constant and around the Azores the height of the tide is about 1.5m. However, as it approaches the continental shelf the wave is slowed down due to friction. The shallower the water the slower the wave travels and the higher the tide becomes, so that around the Isles of Scilly the tide is about 5 metres and by Swansea 8 metres.
The tidal wave moves around the equator, virtually undetectable, at a speed of about 1,000 mph and in the open seas tidal currents are rarely more than a quarter of a knot. It is only when it reaches land are the resulting strong currents seen. Where ever large tides occur and vast amounts of sea water flow horizontally towards and away from the land strong currents form. It has been calculated that in the Bay of Fundy the twice daily tidal currents move in the order of 100,000 million tons of sea water. In places like this and in restricted passages and straits e.g. Discovery Passage and in Seymour Narrows, British Colombia, tidal currents are as strong as 10 knots. In Wales, Bardsey Island is called Ynys Enlli - Island of Currents - and between it and the mainland currents reach speeds of 8 Knots. Tides are also seen to great effect in estuaries. In these, the shape of the estuary is such that the water is squeezed together and eventually forms a tidal bore. This is one large wave, comprised of a turbulent mass of water in an almost vertical wall that is followed by a series of choppy waves. The bore rushes upstream at between 10 and 20 mph. In the River Severn the main bore is 1-2m high, whilst in China's Bay of Hangchow it can reach over 3m in height. Because of its large cone shape the River Severn experiences extra ordinary tides that, at Chepstow, can reach as much as 15m in height.
The largest tidal range in the world is that at the head of the Bay of Fundy, between Nova Scotia and New Brunswick. Here the water is funnelled along so that at the head of the Minas Basin the spring range reaches some 50feet in height.
Living in Britain most of us are used to two high and two low tides each day. This is not necessarily the case however, because tides everywhere are the result of oscillating waves and have both diurnal and semidiurnal periods. Around the European coastline it is the semidiurnal oscillations the predominate. In contrast, along the shores of the Mexican Gulf, where no tide exceeds much more than 0.6 metres, these semidiurnal oscillations are weak and the diurnal ones are displayed in the rhythm of one high water and one low water each lunar day of 24hrs 50mins.
Tides are a phenomenon regulated by the sun and moon and therefore predictable. Tide tables are published and in Britain they can be used a year or more in advance to find out what the tide will be at any given location, time and date, around the coast. However, tide tables are calculated for specific standardised meteorological conditions. In reality, the British weather, as at many other locations in the world, is infamously unpredictable. This is important because wind can hold back the tide or push it along. In addition, atmospheric pressure can cause substantial rises or falls in the predicted height and therefore expose or keep covered the foreshore to be visited.
On the Beach
No matter what the range, the effect of the tides on the shore is to
expose part of the seabed to air (or part of the land to the sea!)
Plants and animals normally living beneath the surface of the sea in a
fairly uniform environment have to be able to adapt to this exposure.
They may suffer desiccation from the sun and wind. They may experience
changes in salinity from freshwater streams and rain, or evaporation on
hot summer days . They may also have to endure extreme cold on frosty
winter nights, overpowering heat in midsummer or even predation from opportunistic
land animals. These and other factors have resulted in seashore
life has, in the intertidal zone, become well adapted to its unsettled,
constantly fluctuating environment.
ISBN 0-907151 817
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