Tides are the longest of oceanic waves. So long in fact, that often, we only observe them as a rise and fall of sea level over a period of several hours. However, a good understanding of tides and tide-generated currents is important for all areas of ICZM. Together with wind-generated waves, tides play an extremely important part in coastal processes, geomorphology, flood risk, species zonation and water quality. The aim of this article is to present a broad overview of tides, with links to more specific articles for those who are interested.
It is worth considering two definitions of tides. Within coastal science and engineering, the following definition is often given.
The periodic rise and fall in the level of the water in oceans and seas; the result of gravitational attraction of the sun and moon..
However, we should never lose sight of the astronomical perspective.
A tide is a distortion in the shape of one body induced by the gravitational pull of another nearby object..
The tides of our planet display extremely complex behaviour. For example, in the Mediterranean, the tidal range is very small (<1m), whereas in the Bay of Fundy, Canada, the shape of the bay augments the tidal range to over 15m. In Europe, the biggest tides can be found in the Severn estuary (12m) in the UK and near Mont Saint Michel, in France, where the tide goes out 9km and reputedly comes in faster than a person can run.
The importance of tides around our coasts is often in the currents they generate, which can reach speeds of up to 5 m/s (Bay of Fundy). The rising tide is usually referred to as the flood, whereas the falling tide is called the ebb. The tidal currents of the ebb and flood play a major part in shaping our coasts, transporting large volumes of sediment and moulding estuary environments.
Tidal resonance can produce strange behaviour, for example, on the south coast of the UK, in Southampton Water, the tide famously exhibits two high waters, approximately thirty minutes apart.
From the earliest times, it has been recognised that there is a connection between the tides and the moon. The most observable effect is that the tidal range is largest when the Moon is full or new.
Today, in contrast to the majority of coastal processes, the tides can be predicted with very good accuracy, for as many as two hundred years into the future. However, there is sometimes a difference between the observed and predicted tide, due to weather induced effects such as the storm surge.
Theory of tides
Sir Isaac Newton (1642-1727) was the first to suggest that the planets produced the tides on earth through their mutual gravitational pull. Astronomical tides are those tidal movements which are governed by the planets. The tide-generating forces can be precisely calculated through the motion of the planets (the equilibrium theory of tides), while the response of the oceans to these forces is modified by the effects of topography (the dynamical theory of tides) and by the transient effect of passing weather patterns (storm surges).
The Earth and the Moon revolve around each other (around a common centre of mass) with a period of 27.3 days. The centrifugal force within the Earth-Moon system exactly balances the gravitational force between the two bodies, so that the whole system is in equilibrium (otherwise we would either collide with or move away from the Moon). Together these two forces are called the tide-producing forces. Note that the centrifugal force of the Earth-Moon system should not be confused with the centrifugal force of the Earth's spin about it's own axis (the Coriolis force).
Hence the water on the Earth's surface is pulled towards the Moon by the gravitational pull, and directly away from the Moon, by the centrifugal force, creating a tidal bulge on either side of the Earth.
In the same way, the Sun also creates tide-producing forces. The magnitude of the Sun’s tide-producing force is about 0.46 that of the Moon, because, although enormously greater in mass than the Moon, the Sun is some 360 times further from the Earth. <ref???>
The relative positions of the Sun and Moon govern the lunar cycle and hence the tidal cycle. The complete cycle of new moon, first quarter, full moon, last quarter takes 29.5 days. We get spring tides (high tidal range) when the Sun and Moon are in alignment or sysygy (after full and new Moons) and we get neap tides (low tidal range) when the Moon is in quadrature (first and last quarters).
This is explained more fully in article on the equilibrium theory of tides.
Real tides, however, do not behave as equilibrium tides. The tide travels as a shallow water wave on the surface of the Earth (since it's wavelength is much greater than the depth) and it's speed is governed by the depth of the oceans (~4km). A time lag occurs due to the difference between the linear velocity of the Earth's surface with respect to the Moon and the speed of the wave in shallow water. Furthermore, the presence of land blocks the passage of the tidal bulges, so that the tides are forced to propagate within oceanic basins. Also, the movement of water is affected by the Coriolis force.
The dynamical theory of tides describes how the tide is changed by these added complications. The consequence is that the real tidal wave rotates around amphidromic points (points where the tidal range is zero).
World-wide maps of the resulting amphidromic systems can be computed, which typically depict co-tidal and co-range lines.
- co-tidal lines radiate out from an amphidromic point and link locations where the tide occurs at the same time, and
- co-range lines lie around the amphidromic point and link locations of equal tidal range.
The frequency of the tides
The most obvious timescale of tidal variation is that of the semi-diurnal (occurring twice a day) and diurnal tides (once a day). In addition, the equilibrium theory predicts the variation in tidal range of the spring-neap cycle, which has a period of 14.7 days. It should be noted that spring tides do not have anything to do with the season, the term is from the Old English word springan, meaning a rising or welling of water.
However, there is a seasonal (yearly) cycle, governed by the rotation of the Earth about the Sun with a period of 365.25 days. Even longer period cycles are the 18.61 year nodal cycle due to the revolution of the moon's nodes and the 1600 to 1800 year cycle due to astronomical alignments.
There are many accronyms associated with tidal levels.
A tide gauge is a device for measuring sea level. Data sets
UK tide gauge network
Tides can be predicted from analysis of long term (at least two years) tidal records using the harmonic method.
The observed tide can be regarded as the sum of a number of constituent sinusoidal waves, each possessing a defined amplitude and period. The period corresponds with the period of one of the relative astronomical motions between the Earth, Moon and Sun. The amplitude and phase of each particular constituent in unique to each location. The nature of the tide at a given location is governed by which of the constituents is dominant.
A table of principal harmonic constituents can be found in the article on tidal prediction.
Tides in many parts of the world (eg. English Channel) can be described as semi-diurnal (occurring twice a day), while some tides show purely diurnal behaviour (once a day) as evident in parts of the Mediterranean, where the amplitude of the semi-diurnal components is small.
If the tide behaves purely as a progressive wave, then maximum currents occur at high and low tides. However, if reflection results in a standing wave, then maximum tidal currents occur at mid-tide. In practice, however, the situation will be somewhere in between.
Extremely strong currents may be produced locally by a constriction, such as the narrow straits between two seas (eg Straits of Dover) or in the mouth of an estuary or river (eg. River Arun at Littlehampton).
In areas where the tidal current is strong enough, the frictional drag at the seabed produces turbulent mixing of the lower water layers. The boundaries (fronts) between such areas of mixed and of stratified waters are often sharply defined, and easily visible.
Strong tidal currents can play a dominant role in the movement and distribution of sediments and in tidally generated bedforms. Coasts where the tidal currents are large compared to the wave generated currents are termed tidally dominated coasts.
Strong tidal currents around headlands can result in steepening of wind waves and enhanced whitecapping.
Definitions related to tides are: tidal current, tidal flat, tidal wave, astronomical tide, highest astronomical tide (HAT), lowest astronomical tide (LAT), mean high water springs (MHWS), mean high water neaps (MHWN), mean high water (MHW), mean low water (MLW).
For more definitions of coastal terms and a sketch, see Definitions of coastal terms.
An article related to tides is: waves.
- ↑ CIRIA (1996). Beach management manual. CIRIA Report 153.
- ↑ Morrison & Owen (1996). "The Planetary System".
- ↑ Brown et al. (1989). Waves, tides and shallow-water processes. Ed. G. Bearman. Open University. Pergamon Press.
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