Difference between revisions of "Tide"

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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

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

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.

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, for a number of reasons. 1. 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. 2. The presence of land masses blocks the tidal bulges. 3. The Earth rotates too quickly creating a time lag. 4. The movement of water is affected by the Coriolis force.

Kelvin wave Cotidal and corange lines. Figure Amphidromic point

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

The solar tide has a semi-diurnal period of 12 hours.

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

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

References

↑ 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.

Article by

The main author of this article is Somerville, Tracy
Please note that others may also have edited the contents of this article.

Citation: Somerville, Tracy (2008): Tide. Available from http://www.coastalwiki.org/wiki/Tide [accessed on 24-11-2024]

For other articles by this author see Category:Articles by Somerville, Tracy
For an overview of contributions by this author see Special:Contributions/Tracy

[1] CIRIA (1996). Beach management manual. CIRIA Report 153.

[2] Morrison & Owen (1996). "The Planetary System".

[3] Brown et al. (1989). Waves, tides and shallow-water processes. Ed. G. Bearman. Open University. Pergamon Press.

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