Tide

The aim of this article is to present a broad overview of tides, for the generalist, with links to more specific articles for those who are interested.

Introduction

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.. This is the common definition for Tide, other definitions can be discussed in the article

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.. This is the common definition for Tide (more general), other definitions can be discussed in the article

On earth, our tides display much 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. Tides in many parts of the world (eg. English Channel) can be described as semi-diurnal (occurring twice a day), while some tides are purely diurnal (once a day) as evident in parts of the Mediterranean. Tidal resonance can produce strange behaviour, for example, on the south coast of the UK, Southampton Water famously exhibits two high waters, approximately 30mins apart.

The importance of tides in the coastal zone is often in the currents they generate, which can reach speeds of up to ?????m/s (Severn estuary). 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.

In contrast to the majority of coastal processes, the tides can be predicted with very good accuracy, for as much as 200 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), although 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

- 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.