tide & landscape
THE TIDE AND THE WADDEN SEA WHAT IS UP AND WHAT IS DOWN?
In the Danish part of the Wadden Sea, the daily difference in the sea level of up 2.5 meters. The tide and the Wadden Sea are inextricably linked since the changing sea level affects the landscape, the flora and the fauna plus man’s utilization possibilities. From Den Helder in Holland to Blåvand in Denmark, ca. 5,200 km2 of seafloor is exposed and flooded by 14 km3 sea water twice daily which, in the Danish part of the Wadden Sea, creates a daily difference in the sea level of up 2.5 meters. The tide occurs because the earth and the moon affect each other interrelationally. The sun’s contribution is not, however, without significance. So, one needs to look at things from a larger perspective while taking a good look at the solar system in order to understand why the sea level at the Wadden Sea rises and falls several meters approximately every six hours.
Astronomical waves
The moon’s orbit around the earth depends on two forces that each pull in a different direction while also affecting the oceans. The celestial bodies are pulled towards each other due to gravity, which is a universal, physical phenomenon that causes all bodies with a mass to attract each other. The pull of the moon can therefore be measured anywhere on the earth, but is strongest at the point nearest the moon and weakest on the side of the earth furthest from the moon. Why the moon does not crash towards the earth is
Experience the tide in ... SPRING
SUMMER
AUTUMN
WINTER
Tripping on the tide It takes ca. 13 hours for a tidal wave to move from Den Helder in the Netherlands to Blåvandshuk.
due to its orbit around the earth, which means that the gravitational pull is counterbalanced by an outwardly effective energy called the centrifugal force. This force is known from, among other things, carrousels and can be felt as the force that makes you hold on to the bar, so you are not hurled off. In the earth/moon system, the mutual rotational point is ca. ¼ inside the part of the earth nearest to the moon, so on the earth the centrifugal force is strongest at the point furthest from the moon. The tidal waves on earth occur because the ocean: • On one side of the earth is more attracted to the moon and therefore not hurled so far away • On the other side of the earth is hurled further away from the earth and therefore less attracted by the moon
Ulrik G. Lützen, Vadehavets Formidlerforum & Bent Jacobsen, NaturKulturVarde Translation: Nanna Mercer, Sirius Translation
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tide & landscape
MOONS GRAVITIONAL PULL CENTRIFUGAL FORCE RESULTING FORCE
EARTH
MOON
The tide occurs because the earth and the moon affect each other interrelationally. NEAP TIDE
SPRING FLOOD
HALF MOON
EARTH
SUN
SUN
EARTH
FULL MOON
NEW MOON
HALF MOON
The size of the tidal wave is also affected by how the celestial bodies stand in relation to each other
Monthly variations
The size of the tidal wave is not the same from day to day, but is also affected by how the celestial bodies stand in relation to each other. At the new and the full moon, the sun and the moon are in line in relation to the earth, and their respective forces work together. The difference between the high and the low tide is therefore especially great during this period, which is called spring flood and characterized by the lowest low tide as well as the highest high tide. The reverse phenomenon is called neap tide and occurs during the two monthly half moons when the sun and the moon are perpendicular in relation to the earth. The forces of the two celestial bodies are now opposing each other and the tidal wave is smallest during this period.
Wind and water
In the North Sea and in the Wadden Sea, the direction of the wind has great significance for the sea level by the coast, since the water is pushed in the direction of the wind.
During periods with easterly winds, the water is therefore pushed out of the Wadden Sea and larger areas than usual are left dry. When the winds are strong and westerly, water from the North Sea is pushed towards Jutland, thus giving an increase in the sea level along the west coast. In such situations, the high and the low tide are higher than normal, and when the sea level is extreme, it is call a storm surge. Before the dikes were built along the Wadden Sea coast, the huge storm surges caused loss of life and land.
The tide meets land
It is the local landscape that determines how large a tidal difference is experienced at the coasts. On the open oceans, a tidal wave can be ca. 0.5 meters high, while on a global scale, the sea level difference can be up to 16 meters and in the Wadden Sea, up to 3.5 meters. The reason for this is due to the phenomenon that waves running towards land are stopped by the seafloor that squeezes the waves together so they become narrower and higher. Therefore, the period when the water is rising (flood) is shorter than the
Ulrik G. L端tzen, Vadehavets Formidlerforum & Bent Jacobsen, NaturKulturVarde Translation: Nanna Mercer, Sirius Translation
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tide & landscape
Flocks of birds moving back and forth with the tide. period when the water is falling (ebb), since a huge amount of water is pushed into the Wadden Sea in a very short period of time, while the outflow takes place in a more moderate tempo and then only by the force of gravity.
The tide in the Wadden Sea
The Wadden Sea tidal wave starts in the North Atlantic and enters the North Sea through the English Channel and the waters between Scotland and Norway. Since water, just like wind, is pressed towards the right on the northern hemisphere, the tide flows down along the English east coast and joins with the tidal wave from the English Channel. The combined tidal wave now moves up along the Dutch and German coasts, and when it gets to the Danish border it is 2.5 meters tall, before it abates at 0.5 meters near Blåvands Huk.
The tide and its significance for the landscape
Within a short period of time, tidal waves push huge amounts of water into the Wadden Sea, thus bringing large amounts of sand and mud along to the rear of the islands. The water rushes through the deep tidal channels and is then led into the Wadden Sea though a network of channels that branch into smaller channels called tidal creeks, (in the marshland, called loer). Every tidal run can be viewed as an independent system, where the tidal wave runs in and out. Between
two runs, there will be an area where the two tidal waves meet and the water’s speed is close to zero. Here, an increased deposit will take place which has been used on Mandø, where Mandø Ebbevej is located right in the watershed between Knudedyb and Juvre Dyb.
Living with the tide
The Wadden Sea flora and fauna have adjusted to a life with changing sea levels. The soil animals in the wadden have developed different strategies that allow them to survive in the changing environment, while birds move back and forth with the tide. The marshland plants are extremely salt-tolerant and manage to cope with repeated and long-lasting flooding. Early on, man understood the tide and saw its many possibilities. Aside from digging for worms used for fishing, and collecting shellfish, the people living near the Wadden Sea invented special catch-methods utilizing the tide. For example, ”faghus-fiskeri” whereby a net is held across a tidal channel in order to catch the fish that follow in or out of the tide, while around Ho Bugt, people used ”skuller gårde”, specially designed fish traps called fyke traps. The tide has also left its mark on sailing in the area, where the traditional boats are flat bottomed for better to rest on the wadden at the low tide. The waddens have also been used for loading and unloading of larger ships, with the goods carried to and fro using carts and wagons.
Ulrik G. Lützen, Vadehavets Formidlerforum & Bent Jacobsen, NaturKulturVarde Translation: Nanna Mercer, Sirius Translation
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tide & landscape
Things to do ... The tide is clearly something to be aware of when one is tripping around on the wadden, for the dried seafloor can be flooded very quickly. Even very experienced Wadden Sea travelers can be caught by the heavy fog or the quickly rising tide. If you want to explore the wadden with its many wonderful things to do, you should bring a compass or a GPS, but not before having conferred with a local concerning the tide predictions. It is safer to start the trip on the ebb, the falling tide. A visit and an overnight stay on Mandø Trips with nature guides from Vadehavscentret or Tønnisgård, NaturKulturVarde, among others.
Learn about tides here ... NaturKulturVarde
Gl. Skovfogedbolig Roustvej 111 DK-6800 Varde T: +45 75 22 22 50 E: nkv@naturkulturvarde.dk W: www.naturkulturvarde.dk
Rav Museet
Vestergade 25 DK-6840 Oksbøl T: +45 75 27 07 03 E: vam@vardemuseum.dk W: www.vardemuseum.dk/dk.php/museer/ravmuseet
The Fisheries and Maritime Museum Tarphagevej 2-6 DK-6710 Esbjerg V. T: +45 76 12 20 00 E: fimus@fimus.dk W: www.fimus.dk
Vadehavscentret Okholmvej 5 Vester Vedsted DK-6760 Ribe
T: +45 75 44 61 61 E: info@vadehavscentret.dk W: www.vadehavscentret.dk
Naturcentret Tønnisgård Havnebyvej 30 DK-6792 Rømø T: +45 74 75 52 57 E: info@tonnisgaard.dk W: www.tonnisgaard.dk
Museum Sønderjylland - Højer Mølle Møllegade 13 DK-6280 Højer
T: +45 75 44 61 61 E: hoejer@museum-sonderjylland.dk W: www.museum-sonderjylland.dk/hojer-molle.html
tips for further reading
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The Wadden Sea landscape Storm surges on the Wadden Sea coast The salt meadows in the Wadden Sea Life on the waddens
About Vadehavets Formidlerforum... Vadehavets Formidlerforum is a partnership of visitor centers that mediate the Wadden Sea’s natural and cultural heritage. VFF’s main activity is to coordinate projects that highlight the nature and culture heritage of the Wadden Sea.. Learn more at www.vadehav.dk
Ulrik G. Lützen, Vadehavets Formidlerforum & Bent Jacobsen, NaturKulturVarde Translation: Nanna Mercer, Sirius Translation
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