Difference between revisions of "Tide"
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Revision as of 11:43, 2 November 2008
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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.
The aim of this article is to present a broad overview of tides, with links to more specific articles for those who are interested.
Contents
Introduction
It is worth considering two definitions of tides. Within coastal science, tides are often taken to mean:
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, Southampton Water famously exhibits two high waters, approximately thirty minutes apart.
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), and 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).
Introduction to the equilibrium theory
The Earth and the Moon revolve around each other (around a common centre of mass) with a period of 27.3 days.
The tidal range is largest just after a full or new moon.
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 solar tide has a semi-diurnal period of 12 hours.
Introduction to the dynamical theory of tides
Kelvin wave Cotidal and corange lines. Figure Amphidromic point
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
The frequency of tides
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.
- Spring neap cycle - Nodal? cycle semi-diurnal diurnal
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
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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