Difference between revisions of "Tide"
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Revision as of 19:49, 2 November 2008
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Contents
Introduction
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.
Definition of Tide:
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.[1].
This is the common definition for Tide, other definitions can be discussed in the article
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However, we should never lose sight of the astronomical perspective.
Definition of Tide (more general):
A tide is a distortion in the shape of one body induced by the gravitational pull of another nearby object.[2].
This is the common definition for Tide (more general), other definitions can be discussed in the article
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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[3] (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 c = \sqrt{g d} 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 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.
- Spring neap cycle
- Nodal? cycle
semi-diurnal
diurnal
The solar tide has a semi-diurnal period of 12 hours.
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.
The Earth and the Moon revolve around each other (around a common centre of mass) with a period of 27.3 days. 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. Sun and Earth is 365.25 day cycle
18.61 year nodal cycle due to the revolution of the moon's nodes
1600 to 1800 year cycle due to astronomical alignments
Tidal terminology
Admiralty charts provide useful information on the bathymetry of a coastal area, as well as tidal range and frequency. ODN / Ordnance Datum Newlyn Admiralty Chart CD / Chart Datum HAT LAT MHWS MHWN MLWS MLWN
Tidal measurement
Tide gauge Data sets
Tidal prediction
Since we can predict the movement of the planets to extremely good accuracy, we can use this to predict the tides.
-harmonic analysis Tidal constituents
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.
Tidal currents
Tidally generated bedforms tide dominated coasts
Tides in estuaries
estuary evolution estuary hydrodynamics stratified estuaries partially-mixed estuaries well-mixed estuaries halocline salt-wedge estuaries coriolis force flood-dominant estuaries ebb-dominant estuaries bar-built estuaries drowned river valleys rias fjiords Tidally generated bedforms intertidal
Related topics
Tidal power Internal tides Navigation Intertidal and biology tidal bore
The equilibrium theory of tides
Tide producing forces vary directly with the mass of the attracting body, but are inversely propoertional to the cube of its distance from Earth.
The dynamical theory of tides
See also
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.
References
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