# User Contributed Dictionary

### Pronunciation

### Noun

tides- Plural of tide
- "Yet the tides of war do not wait, and General Lee had come to the capital to try and shape their future course." - http://books.google.com/books?vid=ISBN0618485384&id=nSnw3YPGN-0C&pg=PA1&lpg=PA1&dq=%22tides+of+war%22&sig=hL4OXMUcIxma5nAXSP7YuJaEQsQ Stephen W Sears, 2004, Gettysburg

# Extensive Definition

Tides are the rising and falling of Earth's ocean surface caused by the
tidal
forces of the Moon and the Sun acting on the
oceans. Tidal phenomena can occur in any object that is subjected
to a gravitational field that varies in time and space, such as the
Earth's land masses.
(see Other
tides).

Tides noticeably affect the depth of marine and
estuarine water bodies
and produce oscillating currents known as tidal streams, making
prediction of tides very important for coastal navigation (see
Tides
and navigation). The strip of seashore that is submerged at
high water (also called high tide) and exposed at low water (or low
tide), the intertidal
zone or foreshore,
is an important ecological product of ocean tides (see Intertidal
ecology).

The changing tide produced at a given location is
the result of the changing positions of the Moon and Sun relative
to the Earth coupled with the effects of
Earth rotation and the local shape of the sea floor. Sea
level measured by coastal tide gauges
may also be strongly affected by wind.

## Introduction and tidal terminology

A tide is a repeated cycle of sea level changes in the following stages:- Over several hours the water rises or advances up a beach in the flood
- The water reaches its highest level and stops at high water. Because tidal currents cease this is also called slack water or slack tide. The tide reverses direction and is said to be turning.
- The sea level recedes or falls over several hours during the ebb tide.
- The level stops falling at low water. This point is also described as slack or turning.

Tides may be semidiurnal (two high waters and two
low waters each day), or diurnal (one tidal cycle per day). In most
locations, tides are semidiurnal. Because of the diurnal
contribution, there is a difference in height (the daily
inequality) between the two high waters on a given day; these are
differentiated as the higher high water and the lower high water in
tide
tables. Similarly, the two low waters each day are referred to
as the higher low water and the lower low water. The daily
inequality changes with time and is generally small when the Moon
is over the equator.

The various frequencies of orbital
forcing which contribute to tidal variations are called
constituents. In most locations, the largest is the "principal
lunar semidiurnal" constituent, also known as the M2 (or M2) tidal
constituent. Its period is about 12 hours and 25.2 minutes, exactly
half a tidal lunar day, the average time separating one lunar
zenith from the next, and
thus the time required for the Earth to rotate once relative to the
Moon. This is the constituent tracked by simple tide
clocks.

Tides vary on timescales ranging from hours to
years, so to make accurate records tide gauges
measure the water level over time at fixed stations which are
screened from variations caused by waves shorter than minutes in
period. These data are compared to the reference (or datum) level
usually called mean sea
level.

Constituents other than M2 arise from factors
such as the gravitational influence of the Sun, the tilt of the
Earth's rotation axis, the inclination of the lunar orbit and the
ellipticity of the orbits of the Moon about the Earth and the Earth
about the Sun. Variations with periods of less than half a day are
called harmonic constituents. Long period constituents have periods
of days, months, or years.

### Tidal range variation: springs and neaps

The semidiurnal tidal range (the difference in height between high and low waters over about a half day) varies in a two-week or fortnightly cycle. Around new and full moon when the Sun, Moon and Earth form a line (a condition known as syzygy), the tidal forces due to the Sun reinforce those of the Moon. The tide's range is then maximum: this is called the spring tide, or just springs and is derived not from the season of spring but rather from the verb meaning "to jump" or "to leap up". When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the earth, and the forces due to the Sun partially cancel those of the Moon. At these points in the lunar cycle, the tide's range is minimum: this is called the neap tide, or neaps. Spring tides result in high waters that are higher than average, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven day interval between springs and neaps.The changing distance of the Moon from the Earth
also affects tide heights. When the Moon is at perigee the range is increased
and when it is at apogee
the range is reduced. Every 7½ lunations, perigee coincides
with either a new or full moon causing perigean tides with the
largest tidal range. If a storm happens to be moving onshore at
this time, the consequences (in the form of property damage, etc.)
can be especially severe.

### Tidal phase and amplitude

Because the M2 tidal constituent dominates in most locations, the stage or phase of a tide, denoted by the time in hours after high water, is a useful concept. It is also measured in degrees, with 360° per tidal cycle. Lines of constant tidal phase are called cotidal lines. High water is reached simultaneously along the cotidal lines extending from the coast out into the ocean, and cotidal lines (and hence tidal phases) advance along the coast. If one thinks of the ocean as a circular basin enclosed by a coastline, the cotidal lines point radially inward and must eventually meet at a common point, the amphidromic point. An amphidromic point is at once cotidal with high and low waters, which is satisfied by zero tidal motion. (The rare exception occurs when the tide circles around an island, as it does around New Zealand.) Indeed tidal motion generally lessens moving away from the continental coasts, so that crossing the cotidal lines are contours of constant amplitude (half of the distance between high and low water) which decrease to zero at the amphidromic point. For a 12 hour semidiurnal tide the amphidromic point behaves roughly like a clock face, with the hour hand pointing in the direction of the high water cotidal line, which is directly opposite the low water cotidal line. High water rotates about once every 12 hours in the direction of rising cotidal lines, and away from ebbing cotidal lines. The difference of cotidal phase from the phase of a reference tide is the epoch.The shape of the shoreline and the ocean floor
change the way that tides propagate, so there is no simple, general
rule for predicting the time of high water from the position of the
Moon in the sky. Coastal characteristics such as underwater
topography and coastline shape mean that individual location
characteristics need to be taken into consideration when
forecasting tides; high water time may differ from that suggested
by a model such as the one above due to the effects of coastal
morphology on tidal flow.

## Tidal physics

see also Tidal force Isaac Newton laid the foundations for the mathematical explanation of tides in the Philosophiae Naturalis Principia Mathematica (1687). In 1740, the Académie Royale des Sciences in Paris offered a prize for the best theoretical essay on tides. Daniel Bernoulli, Antoine Cavalleri, Leonhard Euler, and Colin Maclaurin shared the prize. Maclaurin used Newton’s theory to show that a smooth sphere covered by a sufficiently deep ocean under the tidal force of a single deforming body is a prolate spheroid with major axis directed toward the deforming body. Maclaurin was also the first to write about the Earth's rotational effects on motion. Euler realized that the horizontal component of the tidal force (more than the vertical) drives the tide. In 1744 Jean le Rond d'Alembert studied tidal equations for the atmosphere which did not include rotation. The first major theoretical formulation for water tides was made by Pierre-Simon Laplace, who formulated a system of partial differential equations relating the horizontal flow to the surface height of the ocean. The Laplace tidal equations are still in use today. William Thomson rewrote Laplace's equations in terms of vorticity which allowed for solutions describing tidally driven coastally trapped waves, which are known as Kelvin waves.### Tidal forces

The tidal force produced by a massive object (Moon, hereafter) on a small particle located on or in an extensive body (Earth, hereafter) is the vector difference between the gravitational force exerted by the Moon on the particle, and the gravitational force that would be exerted on the particle if it were located at the center of mass of the Earth. Thus, the tidal force depends not on the strength of the gravitational field of the Moon, but on its gradient. The gravitational force exerted on the Earth by the Sun is on average 179 times stronger than that exerted on the Earth by the Moon, but because the Sun is on average 389 times farther from the Earth, the gradient of its field is weaker. The tidal force produced by the Sun is therefore only 46% as large as that produced by the Moon.Tidal forces can also be analyzed from the point
of view of a reference frame that translates with the center of
mass of the Earth. Consider the tide due to the Moon (the Sun is
similar). First observe that the Earth and Moon rotate around a
common orbital center
of mass, as determined by their relative masses. The orbital
center of mass is 3/4 of the way from the Earth's center to its
surface. The second observation is that the Earth's centripetal motion is the
averaged response of the entire Earth to the Moon's gravity and is
exactly the correct motion to balance the Moon's gravity only at
the center of the Earth; but every part of the Earth moves along
with the center of mass and all parts have the same centripetal
motion, since the Earth is rigid. On the other hand each point of
the Earth experiences the Moon's radially decreasing gravity
differently; the near parts of the Earth are more strongly
attracted than is compensated by inertia and experience a net tidal
force toward the Moon; the far parts have more inertia than is
necessary for the reduced attraction, and thus feel a net force
away from the Moon. Finally only the horizontal components of the
tidal forces actually contribute tidal acceleration to the water
particles since there is small resistance. The actual tidal force
on a particle is only about a ten millionth of the force caused by
the Earth's gravity.

The ocean's surface is closely approximated by an
equipotential surface, (ignoring ocean currents) which is commonly
referred to as the geoid.
Since the gravitational force is equal to the gradient of the potential,
there are no tangential forces on such a surface, and the ocean
surface is thus in gravitational equilibrium. Now consider the
effect of external, massive bodies such as the Moon and Sun. These
bodies have strong gravitational fields that diminish with distance
in space and which act to alter the shape of an equipotential
surface on the Earth. Gravitational forces follow an inverse-square
law (force is inversely proportional to the square
of the distance), but tidal forces are inversely proportional to
the cube of
the distance. The ocean surface moves to adjust to changing tidal
equipotential, tending to rise when the tidal potential is high,
the part of the Earth nearest the Moon, and the farthest part. When
the tidal equipotential changes, the ocean surface is no longer
aligned with it, so that the apparent direction of the vertical
shifts. The surface then experiences a down slope, in the direction
that the equipotential has risen.

### Laplace tidal equation

The depth of the oceans is much smaller than their horizontal extent; thus, the response to tidal forcing can be modelled using the Laplace tidal equations which incorporate the following features: (1) the vertical (or radial) velocity is negligible, and there is no vertical shear—this is a sheet flow. (2) The forcing is only horizontal (tangential). (3) the Coriolis effect appears as a fictitious lateral forcing proportional to velocity. (4) the rate of change of the surface height is proportional to the negative divergence of velocity multiplied by the depth. The last means that as the horizontal velocity stretches or compresses the ocean as a sheet, the volume thins or thickens, respectively. The boundary conditions dictate no flow across the coastline, and free slip at the bottom. The Coriolis effect steers waves to the right in the northern hemisphere and to the left in the southern allowing coastally trapped waves. Finally, a dissipation term can be added which is an analog to viscosity.### Tidal amplitude and cycle time

The theoretical amplitude of oceanic tides due to the Moon is about 54 cm at the highest point, which corresponds to the amplitude that would be reached if the ocean possessed a uniform depth, there were no landmasses, and the Earth were not rotating. The Sun similarly causes tides, of which the theoretical amplitude is about 25 cm (46% of that of the Moon) with a cycle time of 12 hours. At spring tide the two effects add to each other to a theoretical level of 79 cm, while at neap tide the theoretical level is reduced to 29 cm. Since the orbits of the Earth about the Sun, and the Moon about the Earth, are elliptical, the amplitudes of the tides change somewhat as a result of the varying Earth-Sun and Earth-Moon distances. This causes a variation in the tidal force and theoretical amplitude of about ±18% for the Moon and ±5% for the Sun. If both the Sun and Moon were at their closest positions and aligned at new moon, the theoretical amplitude would reach 93 cm.Real amplitudes differ considerably, not only
because of variations in ocean depth, and the obstacles to flow
caused by the continents, but also because the natural period of
wave propagation is of the same order of magnitude as the rotation
period: about 30 hours. If there were no land masses, it would take
about 30 hours for a long wavelength ocean surface wave to
propagate along the equator halfway around the Earth (by
comparison, the natural period of the Earth's lithosphere is about
57 minutes).

### Tidal dissipation

see also Tidal accelerationThe tidal oscillations of the Earth introduce
dissipation, at an average rate of about 3.75
terawatt.
About 98% of this dissipation is by the tidal movement in the
seas and oceans. The dissipation arises as the basin-scale tidal
flow drives smaller-scale flows which experience turbulent
dissipation. This tidal drag gives rise to a torque on the Moon
that results in the gradual transfer of angular momentum to its
orbit, and a gradual increase in the Earth-Moon separation. As a
result of the equal and opposite torque on the Earth, the
rotational velocity of the Earth is correspondingly slowed. Thus,
over geologic time, the Moon recedes from the Earth, at about 3.8
cm/year, and the length of the terrestrial day increases, meaning
that there is about 1 fewer day per 100 million years. See tidal
acceleration for further details.

## Tidal observation and prediction

From ancient times, tides have been observed and discussed with increasing sophistication, first noting the daily recurrence, then its relationship to the Sun and Moon. Pytheas travelled to the British Isles and seems to be the first to have related spring tides to the phase of the moon. The Naturalis Historia of Pliny the Elder collates many observations of detail: the spring tides being a few days after (or before) new and full moon, and that the spring tides around the time of the equinoxes were the highest, though there were also many relationships now regarded as fanciful. In his Geography, Strabo described tides in the Persian Gulf having their greatest range when the moon was furthest from the plane of the equator. All this despite the relatively feeble tides in the Mediterranean basin, though there are strong currents through the Strait of Messina and between Greece and the island of Euboea through the Euripus that puzzled Aristotle. In Europe around 730 A.D. the Venerable Bede described how the rise of tide on one coast of the British Isles coincided with the fall on the other and described the progression in times of the same high water along the Northumbrian coast. Eventually the first tide table in China was recorded in 1056 A.D, primarily for the benefit of visitors wishing to see the famous tidal bore in the Qiantang River. The first known tide table is thought to be that of John, Abbott of Wallingford (d. 1213), based on high water occurring 48 minutes later each day, and three hours later upriver at London than at the mouth of the Thames.
William Thomson led the first systematic harmonic
analysis of tidal records starting in 1867. The main result was
the building of a tide-predicting machine using a system of pulleys
to add together six harmonic functions of time. It was "programmed"
by resetting gears and chains to adjust phasing and amplitudes.
Similar machines were used until the 1960s.

The first known sea-level record of an entire
spring–neap cycle was made in 1831 on the Navy Dock in the Thames
Estuary, and many large ports had automatic tide gage stations by
1850.

William Whewell first mapped co-tidal lines
ending with a nearly global chart in 1836. In order to make these
maps consistent, he hypothesized the existence of amphidromes where
co-tidal lines meet in the mid-ocean. These points of no tide were
confirmed by measurement in 1840 by Captain Hewett, RN, from
careful soundings in the North Sea.

The exact time and height of the tide at a
particular coastal point
is also greatly influenced by the local bathymetry. There are some
extreme cases: the Bay of
Fundy, on the east coast of Canada, features the
largest well-documented tidal ranges in the world, 16 meters (53
ft), because of the shape of the bay. Ungava Bay in
Northern Quebec, north
eastern Canada, is believed by some experts to have higher tidal
ranges than the Bay of
Fundy (about 17 meters or 56 ft), but it is free of pack ice for
only about four months every year, whereas the Bay of Fundy rarely
freezes.

Southampton in
the United
Kingdom has a double high water caused by the interaction
between the different tidal harmonics within the region. This is
contrary to the popular belief that the flow of water around the
Isle
of Wight creates two high waters. The Isle of Wight is
important, however, as it is responsible for the 'Young Flood
Stand', which describes the pause of the incoming tide about three
hours after low water.

Because the oscillation modes of the Mediterranean
Sea and the Baltic Sea do
not coincide with any significant astronomical forcing period the
largest tides are close to their narrow connections with the
Atlantic Ocean. Extremely small tides also occur for the same
reason in the Gulf of
Mexico and Sea of
Japan. On the southern coast of Australia,
because the coast is extremely straight (partly due to the tiny
quantities of runoff
flowing from rivers), tidal ranges are equally small.

### Tidal analysis

It was the universal theory of gravitation due to Isaac Newton that first enabled an explanation of why there were two tides a day, not one, and, via calculation of the forces, offered hope of detailed understanding. Although it may seem that tides could be predicted via a sufficiently detailed knowledge of the astronomical forcing terms, the actual tide at a given location is determined by the response of the oceans to the astronomical forces accumulated over a period of many days. To calculate this response requires a detailed knowledge of the shape of all the ocean basins — their bathymetry and coastline shape.Instead of a direct calculation, the procedure
for analysing tides is pragmatic: At each place of interest, the
tide heights are measured for at least a lunar cycle. The tide
heights are compared to the known frequencies of the astronomical
tide-raising forces. The behaviour of the tide heights is expected
to follow the behaviour of the tide force, with the amplitude and
delays of those responses remaining constant. Because astronomical
frequencies and phases can be calculated with certainty, the tide
height at other times can be predicted once the response to the
astronomical states has been found.

The main patterns in the tides are

- the twice-daily combined lunar and solar tide, due to the rotation of the earth,
- the difference between the first and second tide of a day, due to the moon or sun being north or south of the equator,
- the spring-neap cycle in amplitude (due to the relative positions of the moon and sun), and
- the adjustment of spring tide heights due to the varying distances to the moon and sun.

When confronted by a periodically varying
function, the standard approach is to employ Fourier
series, a form of orthogonal analysis that uses sinusoidal functions as a basis
set, having frequencies that are zero, one, two, three, etc. times
the frequency of a particular fundamental cycle. These multiples
are called harmonics of the fundamental frequency, and the process
is termed harmonic
analysis. If the basis set of sinusoidal functions are
well-suited to the behaviour being modelled, relatively few
harmonic terms need to be carried in the analysis. Fortunately
orbital paths are very nearly circular, sinusoidal variations are
very suitable for tides.

For the analysis of tide heights, the Fourier
series approach is best made more elaborate. While the theorem
remains true and the tidal height could be analysed in terms of a
single frequency and its harmonics, a large number of significant
terms would be required. A much more compact decomposition for the
tides involves a combination of sinusoids having more than one
fundamental frequency. Specifically, the incommensurable periods of
one revolution of the earth (equivalently, of the sun around the
earth), and one orbit of the moon about the earth are used (for
simplicity in phrasing, this discussion is entirely geocentric, but
is informed by the heliocentric model).

To represent both the lunar and solar influences
using one frequency would require many harmonic terms, but allowing
two incommensurable frequencies requires only a few terms. That is,
the sum of two sinusoids, one at the sun's frequency and the second
at the moon's frequency, requires those two terms only, but their
representation as a Fourier series having only one fundamental
frequency and its (integer) multiples would require many terms. For
tides then, although the process is still termed harmonic analysis,
it is not limited to harmonics of a single frequency. To
demonstrate this offers a tidal height pattern converted into an
.mp3 sound file, and the rich sound is quite different from a pure
tone. In other words, the harmonies are multiples of many
fundamental frequencies, not just of the one fundamental frequency
of the simpler Fourier series approach.

The study of tide height by harmonic analysis was
begun by Laplace, Lord
Kelvin, and George
Darwin. Their work was extended by A.T.
Doodson who introduced the Doodson Number notation to organise
the hundreds of terms that result. This approach has been the
international standard ever since, and the complications arise as
follows: the tide-raising force is notionally given by sums of
several terms. Each term is of the form

- A·cos(w·t + p)

- A(t) = A·(1 + Aa·cos(wa·t + pa)) ,

- A·[1 + Aa·cos(wa + pa)]·cos(w·t + p)

Now, given that for any x and y

- cos(x)·cos(y) = ½·cos( x + y ) + ½·cos( x – y ) ,

Remember always that the astronomical tides do
not include the effect of weather, and, changes to local conditions
(sandbank movement, dredging harbour mouths, etc.) away from those
prevailing at the time of measurement can affect the timing and
magnitude of the actual tide. Organisations quoting a "highest
astronomical tide" for some location can exaggerate the figure as a
safety factor against uncertainties of analysis, extrapolation from
the nearest point of measurement, changes since the time of
observation, possible ground subsidence, etc., to protect the
organisation against blame should an engineering work be
overtopped. If the size of a "weather surge" is assessed by
subtracting the astronomical tide from the observed tide at the
time, care is needed.

Careful Fourier
data
analysis over a nineteen-year period (the National Tidal Datum
Epoch in the US) uses frequencies called the tidal harmonic
constituents. Nineteen years is preferred because the relative
positions of the earth, moon and sun repeat almost exactly in the
Metonic
cycle of 18.6 years. This analysis can be done using
only the knowledge of the period of forcing, but without detailed
understanding of the mathematical derivation, which means that
useful tidal tables have been constructed for centuries. The
resulting amplitudes and phases can then be used to predict the
expected tides. These are usually dominated by the constituents
near 12 hours (the semidiurnal constituents), but there are major
constituents near 24 hours (diurnal) as well. Longer term
constituents are 14 day or fortnightly, monthly, and
semiannual. Most of the coastline is dominated by semidiurnal
tides, but some areas such as the South China
Sea and the Gulf of
Mexico are primarily diurnal. In the semidiurnal areas, the
primary constituents M2 (lunar) and S2 (solar)
periods differ slightly, so that the relative phases, and thus the
amplitude of the combined tide, change fortnightly (14 day
period).

In the M2 plot above each cotidal line differs by
one hour from its neighbors, and the thicker lines show tides in
phase with equilibrium at Greenwich. The lines rotate around the
amphidromic
points counterclockwise in the northern hemisphere so that from
Baja California to Alaska and from France to Ireland the M2 tide
propagates northward. In the southern hemisphere this direction is
clockwise. On the other hand M2 tide propagates counterclockwise
around New Zealand, but this because the islands act as a dam and
permit the tides to have different heights on opposite sides of the
islands. (But the tides do propagate northward on the east side and
southward on the west coast, as predicted by theory.)

The exception is the Cook Strait
where the tidal currents periodically link high to low water. This
is because cotidal lines 180° around the amphidromes are in
opposite phase, for example high water across from low water. Each
tidal constituent has a different pattern of amplitudes, phases,
and amphidromic points, so the M2 patterns cannot be used for other
tide components.

### Tidal current

The flow pattern due to tidal influence is much more difficult to analyse, and also, data is much more difficult to collect. A tidal height is a simple number, and applies to a wide region simultaneously – often as far as the eye can see. A flow has both a magnitude and a direction, and can vary substantially over just a short distance due to local bathymetry, and also vary with depth below the water surface. Also, although the centre of a channel is the most useful measuring site, mariners will not accept a current measuring installation obstructing navigation, so a flexible approach is required. A flow proceeding up a curved channel is the same flow, even though its direction varies continuously along the channel. But contrary even to the obvious expectation, flood and ebb flows are often not in opposite directions. The direction of a flow is determined by the shape of the channel it is coming from, not the shape where it will shortly be. Likewise, eddies can form in one direction but not the other.Nevertheless, analysis of currents proceeds on
the same basis as tides: At a given location in the simple case,
the great majority of the flood flow will be in one direction, and
the ebb flow in another (not necessarily opposite) direction. The
velocities measured along the flood direction are taken as
positive, and along the ebb direction as negative, and analysis
proceeds as if these were tide height figures.

In more complex situations, the flow will not be
dominated by the main ebb and flow directions, with the flow
direction and magnitude tracing out an ellipse over a tidal cycle
(on a polar plot) instead of along the two lines of ebb and flow
direction. In this case, analysis might proceed along two pairs of
directions, the primary flow directions and the secondary
directions at right angles. Alternatively, the tidal flows can be
treated as complex numbers, as each value has both a magnitude and
a direction.

As with tide height predictions, tide flow
predictions based only on astronomical factors do not take account
of weather conditions, which can completely change the
situation.

The tidal flow through Cook Strait between the
two main islands of New Zealand is particularly interesting, as on
each side of the strait the tide is almost exactly out of phase so
that high water on one side meets low water on the other. Strong
currents result, with almost zero tidal height change in the centre
of the strait. Yet, although the tidal surge should flow in one
direction for six hours and then the reverse direction for six
hours, a particular surge might last eight or ten hours with the
reverse surge enfeebled. In especially boisterous weather
conditions, the reverse surge might be entirely overcome so that
the flow remains in the same direction through three surge periods
and longer.

A further complication for Cook Strait's pattern
of current flow is that the tides at the north end have the
ordinary two cycles of spring-neap tides in a month (as found along
the west side of the country), but the south end's tidal pattern
has only one cycle of spring-neap tides a month, as found on the
east side of the country. Tidal currents are much more complex than
tidal heights!

### Tidal power generation

Power can be extracted by two means: inserting a water turbine into a tidal current, or, building impoundment ponds so as to release or admit water through a turbine. In the first case, the generation is entirely determined by the timing and magnitude of the tidal currents, and the best currents may be unavailable because the turbines would obstruct navigation. In the second, the impoundment dams are expensive to construct, the natural water cycles are completely disrupted, as is navigation, but with multiple impoundment ponds power can be generated at chosen times. So far, there are few systems for tidal power generation (most famously, La Rance by Saint Malo, France) and many difficulties. Aside from environmental issues, simply withstanding sea-water corrosion and fouling by biological growths poses engineering challenges.Proponents of tidal power systems point out that,
unlike wind power systems, the generation pattern can be predicted
years ahead. However, weather effects are still problematic.
Another assertion is that some generation is possible for most of
the tidal cycle. This may be true in principle since the time of
still water is short, but in practice turbines lose efficiency at
partial operating powers. Since the power available from a flow is
proportional to the cube of the flow speed, the times during which
high power generation is possible turn out to be rather brief. An
obvious fallback then is to have a number of tidal power generation
stations, at locations where the tide phase is different enough so
that low power from one station is filled in by high power from
another. Again, New Zealand has particularly interesting
opportunities. Because the tidal pattern is such that a state of
high water orbits the country once per cycle, there is always
somewhere around the coast where the tide is at its peak, and
somewhere else where it is at its lowest, etc. so that via the
electricity transmission network, there could always be supply from
tidal generation somewhere. The most convenient situation is
presented with Auckland city,
which is between Manukau
harbour and Waitemata
harbour so that both power stations would be close to the
load.

But, because the power available varies with the
cube of the flow, even with the optimum phase difference of three
hours between two stations, there are still significant amounts of
time when neither tidal flow is rapid enough for significant
generation, and worse, during the time of neap tides, the flow is
weak all of the day, and there is no getting around this via
multiple stations, because the neap tides apply to the whole earth
at once. The most feeble neap tides would be when the sun's
influence is maximum whilst the moon's is weakest, and as far as
the sun is concerned, it is closest to the earth during the time of
the southern hemisphere's summer, which is when electricity demand
is the least there, a small bonus.

As a result, interest must fall on the Kaipara
harbour which not only is large, but also is two-lobed in
shape, and thus almost pre-designed for a tidal impoundment scheme
where one lobe could be filled by high water and the other emptied
by a low water, and then via a canal from one to the other
generation would be possible at a time of choice.

There is scant likelihood of any such scheme
proceeding, due to the disruption to natural conditions.

## Tides and navigation

Tidal flows are of profound importance in navigation and very significant errors in position will occur if they are not taken into account. Tidal heights are also very important; for example many rivers and harbours have a shallow "bar" at the entrance which will prevent boats with significant draft from entering at certain states of the tide.The timings and velocities of tidal flow can be
found by looking at a tidal chart or tidal
stream atlas for the area of interest. Tidal charts come in
sets, with each diagram of the set covering a single hour between
one high water and another (they ignore the extra 24 minutes) and
give the average tidal flow for that one hour. An arrow on the
tidal chart indicates the direction and the average flow speed
(usually in knots) for
spring and neap tides. If a tidal chart is not available, most
nautical charts have "tidal
diamonds" which relate specific points on the chart to a table
of data giving direction and speed of tidal flow.

Standard procedure to counteract the effects of
tides on navigation is to (1) calculate a "dead
reckoning" position (or DR) from distance and direction of
travel, (2) mark this on the chart (with a vertical cross like a
plus sign) and (3) draw a line from the DR in the direction of the
tide. The distance the tide will have moved the boat along this
line is computed by the tidal speed, and this gives an "estimated
position" or EP (traditionally marked with a dot in a triangle).
Nautical
charts display the "charted depth" of the water at specific
locations with "soundings" and the use of
bathymetric contour
lines to depict the shape of the submerged surface. These
depths are relative to a "chart datum",
which is typically the level of water at the lowest possible
astronomical tide (tides may be lower or higher for meteorological
reasons) and are therefore the minimum water depth possible during
the tidal cycle. "Drying heights" may also be shown on the chart,
which are the heights of the exposed seabed at the lowest astronomical
tide.

Heights and times of low and high water on each
day are published in tide tables.
The actual depth of water at the given points at high or low water
can easily be calculated by adding the charted depth to the
published height of the tide. The water depth for times other than
high or low water can be derived from tidal curves
published for major ports. If an accurate curve is not available,
the rule of
twelfths can be used. This approximation works on the basis
that the increase in depth in the six hours between low and high
water will follow this simple rule: first hour - 1/12, second -
2/12, third - 3/12, fourth - 3/12, fifth - 2/12, sixth -
1/12.

## Biological aspects

### Intertidal ecology

Intertidal ecology is the study of intertidal ecosystems, where organisms live between the low and high water lines. At low water, the intertidal is exposed (or ‘emersed’) whereas at high water, the intertidal is underwater (or ‘immersed’). Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as between different species of intertidal organisms within a particular intertidal community. The most important environmental and species interactions may vary based on the type of intertidal community being studied, the broadest of classifications being based on substrates - rocky shore and soft bottom communities.Organisms living in this zone have a highly
variable and often hostile environment, and have evolved various
adaptations to cope
with and even exploit these conditions. One easily visible feature
of intertidal communities is vertical
zonation, where the community is divided into distinct vertical
bands of specific species going up the shore. Species ability to
cope with desiccation determines their
upper limits, while competition
with other species sets their lower limits.

Intertidal regions are
utilized by humans for food and recreation, but anthropogenic
actions also have major impacts, with overexploitation, invasive
species and climate
change being among the problems faced by intertidal
communities. In some places Marine
Protected Areas have been established to protect these areas
and aid in scientific
research.

### Biological rhythms and the tides

Intertidal organisms are greatly affected by the
approximately fortnightly cycle of the tides, and hence their
biological
rhythms tend to occur in rough multiples of this period. This
is seen not only in the intertidal organisms however, but also in
many other terrestrial animals, such as the vertebrates. Examples include
gestation and the
hatching of eggs. In humans, for example, the menstrual
cycle lasts roughly a month, an even multiple of the period of
the tidal cycle. This may be evidence of the common
descent of all animals from a marine ancestor.

## Other tides

In addition to oceanic tides, there are atmospheric tides as well as earth tides. All of these are continuum mechanical phenomena, the first two being fluids and the third solid (with various modifications).Atmospheric tides are negligible from ground
level and aviation altitudes, drowned by the much more important
effects of weather.
Atmospheric tides are both gravitational and thermal in origin, and
are the dominant dynamics from about 80 km to 120 km where the
molecular density becomes too small to behave as a fluid.

Earth tides or terrestrial tides affect the
entire rocky mass of the Earth. The Earth's crust shifts (up/down,
east/west, north/south) in response to the Moon's and Sun's
gravitation, ocean tides, and atmospheric loading. While negligible
for most human activities, the semidiurnal amplitude of terrestrial
tides can reach about 55 cm at the equator (15 cm is due to the
Sun) which is important in GPS calibration and
VLBI
measurements. Also to make precise astronomical angular
measurements requires knowledge of the earth's rate of rotation and
nutation, both of which
are influenced by earth tides. The semi-diurnal M2 Earth tides are
nearly in phase with the Moon with tidal lag of about two hours.
Terrestrial tides also need to be taken in account in the case of
some particle
physics experiments. For instance, at the CERN or SLAC, the very large
particle
accelerators were designed while taking terrestrial tides into
account for proper operation. Among the effects that need to be
taken into account are circumference deformation for circular
accelerators and particle beam energy. Since tidal forces generate
currents of conducting fluids within the interior of the Earth,
they affect in turn the Earth's
magnetic field itself.

When oscillating tidal currents in the stratified
ocean flow over uneven bottom topography, they generate internal
waves with tidal frequencies. Such waves are called internal
tides.

The galactic
tide is the tidal force exerted by galaxies on stars within
them and satellite
galaxies orbiting them. The effects of the galactic tide on the
Solar
System's Oort cloud are
believed to be the cause of 90 percent of all observed long-period
comets.

## Misapplications

Tsunamis, the large waves that occur after earthquakes, are sometimes called tidal waves, but this name is due to their resemblance to the tide, rather than any actual link to the tide itself. Other phenomena unrelated to tides but using the word tide are rip tide, storm tide, hurricane tide, and black or red tides.## See also

- Aquaculture
- Coastal erosion
- Hough function
- Lunar phase
- Lunar Laser Ranging Experiment
- Orbit of the Moon
- Primitive equations
- Storm tide
- Tidal bore
- Tidal island
- Tidal locking
- Tidal resonance
- Rip current
- Tide pool
- Slack water
- Tidal power
- Red Tide
- Tidal range
- Tideline
- Head of tide

## External links

- Oceanography: tides by J. Floor Anthoni (2000).
- Myths about Gravity and Tides by Mikolaj Sawicki (2005).
- Tidal Misconceptions by Donald E. Simanek.
- Our Restless Tides: NOAA's practical & short introduction to tides.
- Tides and centrifugal force: Why the centrifugal force does not explain the tide's opposite lobe (with nice animations).

### Tide predictions

- National Oceanic and Atmospheric Administration (NOAA)
- WWW Tide and Current Predictor
- [http://www.mobilegeographics.com:81/ XTide Tide Prediction Server]
- Tides: Why They Happen -- Beaufort County Library
- Australian Tide Times
- Department of Oceanography, Texas A&M University
- UK, South Atlantic, British Overseas Territories and Gibraltar tide times from the UK National Tidal and Sea Level Facility
- UK Admiralty Easytide
- History of tide prediction

## References and notes

tides in Arabic: المد والجزر

tides in Bengali: জোয়ার-ভাটা

tides in Min Nan: Lâu-chúi

tides in Bosnian: Plima i oseka

tides in Breton: Tre ha lanv

tides in Bulgarian: Приливи и отливи

tides in Catalan: Marea

tides in Czech: Slapové jevy

tides in Welsh: Llanw

tides in Danish: Tidevand

tides in German: Gezeiten

tides in Estonian: Looded

tides in Spanish: Marea

tides in Esperanto: Tajdo

tides in Persian: کشند

tides in French: Marée

tides in Korean: 조석

tides in Indonesian: Pasang surut

tides in Icelandic: Sjávarföll

tides in Italian: Marea

tides in Hebrew: גאות ושפל

tides in Hungarian: Árapály

tides in Dutch: Getijde (astronomie)

tides in Japanese: 潮汐

tides in Norwegian: Tidevann

tides in Norwegian Nynorsk: Tidvatn

tides in Low German: Tiden

tides in Polish: Pływy morskie

tides in Portuguese: Maré

tides in Romanian: Maree

tides in Russian: Прилив

tides in Simple English: Tide

tides in Slovenian: Bibavica

tides in Finnish: Vuorovesi

tides in Swedish: Tidvatten

tides in Vietnamese: Thủy triều

tides in Turkish: Gelgit

tides in Ukrainian: Приплив

tides in Chinese: 潮汐