Guide High Tide: WHERE THE SEA MEETS THE SKY

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In the early morning hours of June 6, , the complex operation code-named Overlord began to unfold. The Allies had assembled an armada of 5, ships and landing craft carrying , soldiers across the English Channel to the Normandy beaches. Airborne operations carried a total of 24, troops using more than a thousand transports and gliders. Near sunrise, aerial and naval bombardment shook the German coastal strongpoints, and landing craft started the long runs in to the beaches.

Any invasion date in May or June would leave the entire summer for the Allied drive across France and toward the German homeland before bad weather in fall or winter could slow the advance.

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But the invasion planners later made clear that the selection of June 6th in particular was for astronomical reasons: moonlight and the effects of the lunar phase on the tides came into play. The Allies required a low tide near sunrise, and, on this part of the Normandy coast, such a tide occurs only near the times of either new Moon or full Moon. General Dwight D. Eisenhower, the Allied Supreme Commander, realized that preparations were not complete in May and postponed the assault until June. His book, Crusade in Europe , explained why moonlight and a low tide were important:.

Admiral Chester W. Nimitz, in his memoir The Great Sea War, likewise recalled the significance of the lunar phase and the tide times:. This publicity photograph from the film, The Longest Day , shows the early assault waves advancing on foot through mined beach obstacles, a mixture of stakes and hedgehogs. Prime Minister Winston Churchill, in his book Closing the Ring, stressed the astronomical and tidal factors:. Only on three days in each lunar month were all the desired conditions fulfilled. The first three-day period The Allies initially intended to invade on June 5th, but bad weather forced a postponement of one day.

Slanting moonlight illuminated the ground below for the troops of the 82 nd and the st Airborne as they started dropping from the sky between and a. Brigadier General James Gavin of the 82 nd Airborne provided an eyewitness account in a monograph titled Airborne Warfare. The German defenders had employed several types of mined obstacles on the beaches, as shown in the accompanying illustrations.

Left: This German plan of beach obstacles shows barbed wire connecting stakes, ramps, hedgehogs, Belgian Gates, and tetrahedrons, as the beach would appear near low tide. At low tide the beach defenses lay exposed … We would assault when a rising tide reached the obstacle line and give the engineers 30 minutes to clear it before the water became too deep. Successive assault waves would then ride the rising tide nearer the sea wall through gaps in the obstacle belt. Our Texas State group wrote a computer program to calculate the tide levels at Omaha Beach.

The calculated morning tide range was about 18 feet, with a rapid rise from low water at a. Russell Doescher. This rapid rise had a significant effect. The initial landings at Omaha Beach took place at a. Over the next 30 minutes, the water level rose 2. Even a small delay had serious consequences. These calculations help explain why demolition crews cleared only five of the planned 16 gaps among the obstacles before the advancing tide forced them to wade ashore.

While tidal and astronomical considerations meant that the date of the Normandy invasion had to fall near a full Moon, its gravitational effects were then responsible for the rapidly rising spring tide. First British glider lands at the Caen Canal bridge, bright Moon in southeastern sky a. Radio message sent: British captured the canal bridge and Orne River bridge intact a.

American paratroopers begin to land inland from Utah Beach a. Lunar transit, Moon at its highest point in the sky for the night a. Beginning of civil twilight a. Low water exposes the beach obstacles on Omaha Beach a. Naval bombardment of Omaha Beach begins a. Sunrise a. First assault wave lands on Omaha Beach, on a rising tide a. First assault wave lands on Sword Beach, on a rising tide a. High water covers Omaha Beach almost to the sea wall p. Approximate time when Lord Lovat leads British commandos inland from Sword Beach and they link up with the airborne forces at the canal bridge. The shape of the shoreline and the ocean floor changes the way that tides propagate, so there is no simple, general rule that predicts the time of high water from the Moon's position in the sky.

Coastal characteristics such as underwater bathymetry and coastline shape mean that individual location characteristics affect tide forecasting; actual high water time and height may differ from model predictions due to the coastal morphology's effects on tidal flow. However, for a given location the relationship between lunar altitude and the time of high or low tide the lunitidal interval is relatively constant and predictable, as is the time of high or low tide relative to other points on the same coast.

For example, the high tide at Norfolk, Virginia , U. Land masses and ocean basins act as barriers against water moving freely around the globe, and their varied shapes and sizes affect the size of tidal frequencies. As a result, tidal patterns vary. For example, in the U. From ancient times, tidal observation and discussion has increased in sophistication, first marking the daily recurrence, then tides' relationship to the Sun and moon.

Pytheas travelled to the British Isles about BC and seems to be the first to have related spring tides to the phase of the moon. In the 2nd century BC, the Babylonian astronomer , Seleucus of Seleucia , correctly described the phenomenon of tides in order to support his heliocentric theory.

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He noted that tides varied in time and strength in different parts of the world. According to Strabo 1. The Naturalis Historia of Pliny the Elder collates many tidal observations, e. 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.

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All this despite the relatively small amplitude of Mediterranean basin tides. Philostratus mentions the moon, but attributes tides to "spirits". In Europe around AD, the Venerable Bede described how the rising tide on one coast of the British Isles coincided with the fall on the other and described the time progression of high water along the Northumbrian coast. The first tide table in China was recorded in AD primarily for visitors wishing to see the famous tidal bore in the Qiantang River.

Albans in , based on high water occurring 48 minutes later each day, and three hours earlier at the Thames mouth than upriver at London. William Thomson Lord Kelvin led the first systematic harmonic analysis of tidal records starting in The main result was the building of a tide-predicting machine using a system of pulleys to add together six harmonic time functions.

It was "programmed" by resetting gears and chains to adjust phasing and amplitudes. Similar machines were used until the s. The first known sea-level record of an entire spring—neap cycle was made in on the Navy Dock in the Thames Estuary. Many large ports had automatic tide gauge stations by William Whewell first mapped co-tidal lines ending with a nearly global chart in 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 by Captain Hewett, RN, from careful soundings in the North Sea. The tidal forces due to the Moon and Sun generate very long waves which travel all around the ocean following the paths shown in co-tidal charts. The time when the crest of the wave reaches a port then gives the time of high water at the port. The time taken for the wave to travel around the ocean also means that there is a delay between the phases of the Moon and their effect on the tide. This is called the tide's age.

The ocean bathymetry greatly influences the tide's exact time and height at a particular coastal point. There are some extreme cases; the Bay of Fundy , on the east coast of Canada, is often stated to have the world's highest tides because of its shape, bathymetry, and its distance from the continental shelf edge. Southampton in the United Kingdom has a double high water caused by the interaction between the M 2 and M 4 tidal constituents.

The M 4 tide is found all along the south coast of the United Kingdom, but its effect is most noticeable between the Isle of Wight and Portland because the M 2 tide is lowest in this region. 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.

Elsewhere, as along the southern coast of Australia , low tides can be due to the presence of a nearby amphidrome. Isaac Newton 's theory of gravitation first enabled an explanation of why there were generally two tides a day, not one, and offered hope for a detailed understanding of tidal forces and behavior. Although it may seem that tides could be predicted via a sufficiently detailed knowledge of instantaneous astronomical forcings, the actual tide at a given location is determined by astronomical forces accumulated by the body of water over many days. In addition, accurate results would require detailed knowledge of the shape of all the ocean basins—their bathymetry , and coastline shape.

Current procedure for analysing tides follows the method of harmonic analysis introduced in the s by William Thomson. It is based on the principle that the astronomical theories of the motions of Sun and Moon determine a large number of component frequencies, and at each frequency there is a component of force tending to produce tidal motion, but that at each place of interest on the Earth, the tides respond at each frequency with an amplitude and phase peculiar to that locality.

At each place of interest, the tide heights are therefore measured for a period of time sufficiently long usually more than a year in the case of a new port not previously studied to enable the response at each significant tide-generating frequency to be distinguished by analysis, and to extract the tidal constants for a sufficient number of the strongest known components of the astronomical tidal forces to enable practical tide prediction.

The tide heights are expected to follow the tidal force, with a constant amplitude and phase delay for each component. Because astronomical frequencies and phases can be calculated with certainty, the tide height at other times can then be predicted once the response to the harmonic components of the astronomical tide-generating forces has been found. When confronted by a periodically varying function, the standard approach is to employ Fourier series , a form of analysis that uses sinusoidal functions as a basis set, having frequencies that are zero, one, two, three, etc.

These multiples are called harmonics of the fundamental frequency, and the process is termed harmonic analysis. If the basis set of sinusoidal functions suit the behaviour being modelled, relatively few harmonic terms need to be added. Orbital paths are very nearly circular, so sinusoidal variations are suitable for tides. For the analysis of tide heights, the Fourier series approach has in practice to be made more elaborate than the use of a single frequency and its harmonics.

The tidal patterns are decomposed into many sinusoids having many fundamental frequencies, corresponding as in the lunar theory to many different combinations of the motions of the Earth, the Moon, and the angles that define the shape and location of their orbits. For tides, then, harmonic analysis is not limited to harmonics of a single frequency. Their representation as a Fourier series having only one fundamental frequency and its integer multiples would require many terms, and would be severely limited in the time-range for which it would be valid.

Doodson extended their work, introducing the Doodson Number notation to organise the hundreds of resulting terms. 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. There is one term for the Moon and a second term for the Sun. The phase p of the first harmonic for the Moon term is called the lunitidal interval or high water interval. The next step is to accommodate the harmonic terms due to the elliptical shape of the orbits.

Accordingly, the value of A is not a constant but also varying with time, slightly, about some average figure.


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Replace it then by A t where A is another sinusoid, similar to the cycles and epicycles of Ptolemaic theory. Thus the simple term is now the product of two cosine factors:. Given that for any x and y. Consider further that the tidal force on a location depends also on whether the Moon or the Sun is above or below the plane of the Equator, and that these attributes have their own periods also incommensurable with a day and a month, and it is clear that many combinations result.

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With a careful choice of the basic astronomical frequencies, the Doodson Number annotates the particular additions and differences to form the frequency of each simple cosine term. Remember that astronomical tides do not include weather effects. Also, changes to local conditions sandbank movement, dredging harbour mouths, etc.

Organisations quoting a "highest astronomical tide" for some location may exaggerate the figure as a safety factor against analytical uncertainties, distance from the nearest measurement point, changes since the last observation time, ground subsidence, etc. Special care is needed when assessing the size of a "weather surge" by subtracting the astronomical tide from the observed tide.

This analysis can be done using only the knowledge of the forcing period , but without detailed understanding of the mathematical derivation, which means that useful tidal tables have been constructed for centuries. Semi-diurnal tides dominated coastline, but some areas such as the South China Sea and the Gulf of Mexico are primarily diurnal. In the M 2 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 Peninsula to Alaska and from France to Ireland the M 2 tide propagates northward.

In the southern hemisphere this direction is clockwise. On the other hand, M 2 tide propagates counterclockwise around New Zealand, but this is because the islands act as a dam and permit the tides to have different heights on the islands' opposite sides. The tides do propagate northward on the east side and southward on the west coast, as predicted by theory. The exception is at Cook Strait where the tidal currents periodically link high to low water.

Each tidal constituent has a different pattern of amplitudes, phases, and amphidromic points, so the M 2 patterns cannot be used for other tide components. Because the Moon is moving in its orbit around the Earth and in the same sense as the Earth's rotation, a point on the Earth must rotate slightly further to catch up so that the time between semidiurnal tides is not twelve but The two peaks are not equal. The two high tides a day alternate in maximum heights: lower high just under three feet , higher high just over three feet , and again lower high.

Likewise for the low tides. When the Earth, Moon, and Sun are in line Sun—Earth—Moon, or Sun—Moon—Earth the two main influences combine to produce spring tides; when the two forces are opposing each other as when the angle Moon—Earth—Sun is close to ninety degrees, neap tides result. As the Moon moves around its orbit it changes from north of the Equator to south of the Equator. The alternation in high tide heights becomes smaller, until they are the same at the lunar equinox, the Moon is above the Equator , then redevelop but with the other polarity, waxing to a maximum difference and then waning again.

The tides' influence on current flow is much more difficult to analyse, and data is much more difficult to collect. A tidal height is a simple number which applies to a wide region simultaneously. A flow has both a magnitude and a direction, both of which can vary substantially with depth and over short distances due to local bathymetry.

Also, although a water channel's center is the most useful measuring site, mariners object when current-measuring equipment obstructs waterways. A flow proceeding up a curved channel is the same flow, even though its direction varies continuously along the channel. Surprisingly, flood and ebb flows are often not in opposite directions. Flow direction is determined by the upstream channel's shape, not the downstream channel's shape.

Likewise, eddies may form in only one flow direction. Nevertheless, current analysis is similar to tidal analysis: in the simple case, at a given location the flood flow is in mostly one direction, and the ebb flow in another direction. Flood velocities are given positive sign, and ebb velocities negative sign. Analysis proceeds as though these are tide heights. In more complex situations, the main ebb and flood flows do not dominate.

Instead, the flow direction and magnitude trace an ellipse over a tidal cycle on a polar plot instead of along the ebb and flood lines. In this case, analysis might proceed along pairs of directions, with the primary and secondary directions at right angles. An alternative is to treat the tidal flows as complex numbers, as each value has both a magnitude and a direction. Tide flow information is most commonly seen on nautical charts , presented as a table of flow speeds and bearings at hourly intervals, with separate tables for spring and neap tides.

The timing is relative to high water at some harbour where the tidal behaviour is similar in pattern, though it may be far away. As with tide height predictions, tide flow predictions based only on astronomical factors do not incorporate weather conditions, which can completely change the outcome. The tidal flow through Cook Strait between the two main islands of New Zealand is particularly interesting, as the tides on each side of the strait are almost exactly out of phase, so that one side's high water is simultaneous with the other's low water.

Strong currents result, with almost zero tidal height change in the strait's center. Yet, although the tidal surge normally flows in one direction for six hours and in 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 continues in the same direction through three or more surge periods.

A further complication for Cook Strait's flow pattern is that the tide at the north side e. The graph of Cook Strait's tides shows separately the high water and low water height and time, through November ; these are not measured values but instead are calculated from tidal parameters derived from years-old measurements. Cook Strait's nautical chart offers tidal current information. Near Cape Terawhiti in the middle of Cook Strait the tidal height variation is almost nil while the tidal current reaches its maximum, especially near the notorious Karori Rip.

Aside from weather effects, the actual currents through Cook Strait are influenced by the tidal height differences between the two ends of the strait and as can be seen, only one of the two spring tides at the north end Nelson has a counterpart spring tide at the south end Wellington , so the resulting behaviour follows neither reference harbour. In the first case, the energy amount is entirely determined by the timing and tidal current magnitude. However, the best currents may be unavailable because the turbines would obstruct ships.

In the second, the impoundment dams are expensive to construct, natural water cycles are completely disrupted, ship navigation is disrupted. However, with multiple ponds, power can be generated at chosen times. So far, there are few installed systems for tidal power generation most famously, La Rance at Saint Malo , France which face many difficulties. Aside from environmental issues, simply withstanding corrosion and biological fouling pose engineering challenges.

Tidal power proponents point out that, unlike wind power systems, generation levels can be reliably predicted, save for weather effects. While some generation is possible for most of the tidal cycle, in practice turbines lose efficiency at lower operating rates.


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  • 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 are brief. Tidal flows are important for navigation, and significant errors in position occur if they are not accommodated. Tidal heights are also important; for example many rivers and harbours have a shallow "bar" at the entrance which prevents boats with significant draft from entering at low tide.

    Why are there two tides a day?

    Until the advent of automated navigation, competence in calculating tidal effects was important to naval officers. The certificate of examination for lieutenants in the Royal Navy once declared that the prospective officer was able to "shift his tides". Tidal flow timings and velocities appear in tide charts or a tidal stream atlas. Tide charts come in sets. Each chart covers a single hour between one high water and another they ignore the leftover 24 minutes and show the average tidal flow for that 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 tide chart is not available, most nautical charts have " tidal diamonds " which relate specific points on the chart to a table giving tidal flow direction and speed. The standard procedure to counteract tidal effects on navigation is to 1 calculate a " dead reckoning " position or DR from travel distance and direction, 2 mark the chart with a vertical cross like a plus sign and 3 draw a line from the DR in the tide's direction. The distance the tide moves 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 water's "charted depth" at specific locations with " soundings " and the use of bathymetric contour lines to depict the submerged surface's shape. These depths are relative to a " chart datum ", which is typically the water level at the lowest possible astronomical tide although other datums are commonly used, especially historically, and tides may be lower or higher for meteorological reasons and are therefore the minimum possible water depth during the tidal cycle.

    Tide tables list each day's high and low water heights and times. To calculate the actual water depth, add the charted depth to the published tide height. Depth for other times can be derived from tidal curves published for major ports. The rule of twelfths can suffice if an accurate curve is not available.

    Intertidal ecology is the study of ecosystems between the low- and high-water lines along a shore. At low water, the intertidal zone is exposed or emersed , whereas at high water, it is underwater or immersed. Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as among the different species. The most important interactions may vary according to the type of intertidal community.

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    The broadest classifications are based on substrates — rocky shore or soft bottom. Intertidal organisms experience a highly variable and often hostile environment, and have adapted to cope with and even exploit these conditions. One easily visible feature is vertical zonation , in which the community divides into distinct horizontal bands of specific species at each elevation above low water. A species' ability to cope with desiccation determines its upper limit, while competition with other species sets its lower limit.

    Humans use intertidal regions for food and recreation. Overexploitation can damage intertidals directly. Other anthropogenic actions such as introducing invasive species and climate change have large negative effects. Marine Protected Areas are one option communities can apply to protect these areas and aid scientific research.

    The approximately fortnightly tidal cycle has large effects on intertidal [61] and marine organisms. Many other animals such as the vertebrates , display similar rhythms. Examples include gestation and egg hatching. In humans, the menstrual cycle lasts roughly a lunar month , an even multiple of the tidal period. Such parallels at least hint at the common descent of all animals from a marine ancestor. 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.

    In addition to oceanic tides, large lakes can experience small tides and even planets can experience atmospheric tides and Earth tides. These are continuum mechanical phenomena. The first two take place in fluids.

    Where the sea meets the sky - Picture of Wells Next The Sea Beach, Wells-next-the-Sea

    The third affects the Earth's thin solid crust surrounding its semi-liquid interior with various modifications. Atmospheric tides are negligible at ground level and aviation altitudes, masked by weather 's much more important effects. Earth tides or terrestrial tides affect the entire Earth's mass, which acts similarly to a liquid gyroscope with a very thin crust.

    Precise astronomical angular measurements require knowledge of the Earth's rotation rate and polar motion , both of which are influenced by Earth tides. The semi-diurnal M 2 Earth tides are nearly in phase with the Moon with a lag of about two hours. Galactic tides are the tidal forces exerted by galaxies on stars within them and satellite galaxies orbiting them.

    The galactic tide's effects on the Solar System 's Oort cloud are believed to cause 90 percent of long-period comets. Tsunamis , the large waves that occur after earthquakes, are sometimes called tidal waves , but this name is given by their resemblance to the tide, rather than any actual link to the tide. Other phenomena unrelated to tides but using the word tide are rip tide , storm tide , hurricane tide , and black or red tides.

    Many of these usages are historic and refer to the earlier meaning of tide as "a portion of time, a season". From Wikipedia, the free encyclopedia. The periodic change of sea levels caused by the gravitational and inertial effects of the Moon, the Sun and the rotation of the Earth. For other uses, see Tide disambiguation , Tidal disambiguation , High tide disambiguation , and High Water disambiguation. Low tide at the same fishing port in Bay of Fundy , Play media.

    For other uses, see Ebb tide disambiguation. Main article: Tidal range. Spring tide: Sun and Moon at the same side cycle restarts. See also: Tidal force and Theory of tides. See also: Tidal acceleration. Further information: The article on A. Doodson has a fully worked example calculation for Bridgeport, Connecticut , U. Main article: Tidal power. Main article: Intertidal ecology. Further information: Intertidal zone.

    Main article: Earth tide. Nautical portal Water portal Moon portal Earth sciences portal Geophysics portal. Driftwood, floating seaweed, foam, and other floating debris may accumulate. Descriptive physical oceanography: State of the Art. Taylor and Francis.


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    McGraw-Hill Professional. In estuaries , seasonal river outflows influence tidal flow. Do not confuse with the astronomical lunar day on the Moon. A lunar zenith is the Moon's highest point in the sky. Introduction to physical oceanography. These are mean values in the sense that they derive from mean data.

    Washington Department of Ecology, State of Washington. Retrieved 5 April Land Information New Zealand. Retrieved 20 February Mathematical astronomy in Copernicus's De revolutionibus. Oxford English Dictionary 2nd ed. Oxford University Press. The Danish niptid is probably from the English.