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| − | {{revision}} | + | {{Definition|title=Tide |
| | + | |definition= The periodic rise and fall in the level of the water in oceans and seas as a result of gravitational attraction of the sun and moon and the rotation of the earth.<ref> CIRIA (1996). ''Beach management manual'', CIRIA Report 153.</ref>.}} |
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| − | == Introduction ==
| + | A more general definition from a geophysical viewpoint is |
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| − | 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.
| + | {{Definition|title=Tide (more general) |
| | + | |definition= A tide is a distortion in the shape of a body induced by the gravitational pull of another nearby object.<ref> Morrison & Owen (1996). "The Planetary System", Addison-Wesley. </ref>.}} |
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| − | The aim of this article is to present a broad overview of tides, with links to more specific articles for those who are interested. | + | The term 'tide' in the Coastal Wiki refers to the first definition. |
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| − | It is worth considering two definitions of tides. Within coastal science and engineering, the following definition is often given.
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| + | ==Notes== |
| − | Definition|title=Tide
| + | The tide (more precisely, the '''astronomical tide''') is the large-scale water motion generated by the rotation of the earth in combination with the varying gravitational influence on the ocean of celestial bodies, especially the moon and the sun. These phenomena cause predictable and regular oscillations in the water level, which are referred to as the tide. The astronomical tide at a specific location can be predicted and is published in Tidal Tables. |
| − | |definition= 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.<ref> CIRIA (1996). ''Beach management manual''. CIRIA Report 153.</ref>.}}
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| − | However, we should never lose sight of the astronomical perspective.
| + | The term 'tide' is sometimes used for the combined effect of astronomical tide and wind-driven set-up or set-down of the sea level (including [[#Storm surge|storm surges]]). |
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| − | {{
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| − | Definition|title=Tide (more general)
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| − | |definition= A tide is a distortion in the shape of one body induced by the gravitational pull of another nearby object.<ref> Morrison & Owen (1996). "The Planetary System".</ref>.}}
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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.
| + | ==Related articles== |
| | + | :[[Ocean and shelf tides]] |
| | + | :[[Tidal motion in shelf seas]] |
| | + | :[[Coriolis acceleration]] |
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| − | The importance of tides around our coasts is often in the currents they generate, which can reach speeds of up to 5 m/s<ref>Brown et al. (1989). Waves, tides and shallow-water processes. Ed. G. Bearman. Open University. Pergamon Press.</ref> (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.
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| − | [[Tidal resonance]] can produce strange behaviour, for example, on the south coast of the UK, Southampton Water famously exhibits two high waters, approximately thirty minutes apart.
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| − | 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.
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| − | 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]].
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| − | == Theory of tides ==
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| − | 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 surge]]s).
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| − | 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).
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| − | 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.
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| − | 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???>
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| − | 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).
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| − | This is explained more fully in article on the [[equilibrium theory of tides]].
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| − | 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 <math>c = \sqrt{g d}</math> 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.
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| − | The dynamical theory describes how the tide is changed by these added complications. The consequence is that the real tidal wave rotates around [[amphidromic point]]s (points where the tidal range is zero).
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| − | World-wide maps can be computed of the resulting amphidromic systems and typically show
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| − | * co-tidal lines, which radiate out from an amphidromic point and link locations where the tide occurs at the same time
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| − | *co-range lines, which lie around the amphidromic point and link locations of equal tidal range.
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| − | ==The frequency of tides==
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| − | 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.
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| − | - Spring neap cycle
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| − | - Nodal? cycle
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| − | semi-diurnal
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| − | diurnal
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| − | The solar tide has a semi-diurnal period of 12 hours.
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| − | ==Tidal terminology==
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| − | Admiralty charts provide useful information on the bathymetry of a coastal area, as well as tidal range and frequency.
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| − | ODN / Ordnance Datum Newlyn
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| − | Admiralty Chart
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| − | CD / Chart Datum
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| − | HAT
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| − | LAT
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| − | MHWS
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| − | MHWN
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| − | MLWS
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| − | MLWN
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| − | ==Tidal measurement==
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| − | Tide gauge
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| − | Data sets
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| − | ==Tidal prediction==
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| − | Since we can predict the movement of the planets to extremely good accuracy, we can use this to predict the tides.
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| − | -harmonic analysis
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| − | Tidal constituents
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| − | ==Tidal currents==
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| − | Tidally generated bedforms
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| − | tide dominated coasts
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| − | ==Tides in estuaries==
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| − | estuary evolution
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| − | estuary hydrodynamics
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| − | stratified estuaries
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| − | partially-mixed estuaries
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| − | well-mixed estuaries
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| − | halocline
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| − | salt-wedge estuaries
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| − | coriolis force
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| − | flood-dominant estuaries
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| − | ebb-dominant estuaries
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| − | bar-built estuaries
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| − | drowned river valleys
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| − | rias
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| − | fjiords
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| − | Tidally generated bedforms
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| − | intertidal
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| − | ==Related topics==
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| − | Tidal power
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| − | Internal tides
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| − | Navigation
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| − | Intertidal and biology
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| − | tidal bore
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| − | The equilibrium theory of tides
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| − | Tide producing forces vary directly with the mass of the attracting body, but are inversely propoertional to the cube of its distance from Earth.
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| − | The dynamical theory of tides
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| − | ==See also==
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| − | 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).
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| − | For more definitions of coastal terms and a sketch, see [[Definitions of coastal terms]].
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| − | An article related to tides is: [[waves]].
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| | ==References== | | ==References== |
| | <references/> | | <references/> |
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| − | Article by
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| − | |AuthorID=18394
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| − | |AuthorFullName=Somerville, Tracy
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| − | |AuthorName=Tracy}}
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| − | [[Category:Help]]
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