Tides and water levels

The tide is the daily rising and falling motion of the sea. This motion is primarily caused by the gravitational pull from the moon. There are approximately two high and two low tides per day in the North Sea. The period between high and low tide is called ebb and the period between low and high tide is called flood. The moment of high and low tide differs from place to place.

The tide is a complex phenomenon. The moon has the greatest influence on the tide. The gravitational force from the moon causes a high water level on the side of the earth where the moon is at that moment (see figure above). A high water level forms on the opposite side of the earth, however this comes from the movement of the moon and the earth around each other. Contrary to what most people think, the moon does not simply rotate around the earth. The moon and the earth rotate around a common point located somewhere near the center of the earth but not exactly in the middle. In this way, the water on earth is flung away.

Because the earth is also rotating at the same time, the raised water level moves around the earth. This is why one would expect to have high and low tide twice a day a specific place at the same time every day. However, because the earth and moon rotate around a common point, the moment of high and low tide shifts up around 50 minutes each day.

Ebb and flood

The period between high and low tide is called ebb. Seafarers often use the word ebb to designate a current. For example, they enter a sea channel with the ebb current. The word is also used to indicate low water. Sluices are usually open to discharge polder water into the sea during ebb.

Flood is the entire period between low and high tide. One can walk the flats when flood begins (just after low tide). A sailor sails with the flood because he has the current in his favor during the entire trip. However, the term flood is also often used to indicate high tide: 'Several meters of dunes were washed away during flood.'

Spring tide and neap tide

The sun also has a large influence on the tides. When the sun and the moon lie in one line with the earth, an extra strong pull is executed on the water. This creates extra high and extra low water levels and is called spring tide. When the sun and the moon counteract each other's force by standing at a right angle, the water levels are less high and low than normal. This is called neap tide. The graph below is spring and neap tide shown for the Zeeuws city Vlissingen.

Tides in the North Sea

The tides in the North Sea come from the Atlantic Ocean. When seen from the air, the tidal wave in the North Sea spins in a whirl around several central points in a counterclock-wise direction. These whirls are caused by the rotation of the earth (Coriolis effect). The center of such a whirl (called the amphidrome) barely moves, so that there is no tide difference here. The North Sea is influenced by three such whirls: one in the northeastern North Sea, one in the western central North Sea and one in the English Channel. The whirl in the central North Sea affects the tides in the Wadden Sea the most.

In the Netherlands, the tidal wave arrives first in Vlissingen, moves up north, and only arrives by Schiermonnikoog eight hours later (see graph below). The height of the tide is related to the distance from the center of the whirl.

Tides in the Wadden Sea

During flood, the tidal flats in the Wadden Sea are slowly inundated with water. Shallow sandbanks are exposed during ebb. The tidal movement in the Wadden Sea has drastically changed in the course of time. The two most important influences are the closure of the Zuiderzee in 1932 and the Lauwersmeer in 1969. In addition, the tidal region between the islands and the mainland has declined due to the construction of dikes. This has influence on the current velocity of the water and the sand transportation. Permanent changes have occurred in the composition of the tidal bottom, which again has influence on the animals and plants that can live in the area.

The times of high and low tide can be calculated beforehand. There are tide tables ready for a whole year, which have calculated the times for high and low tide and the expected water levels.

Water levels

The water level along the Dutch coast is partially determined by the tides but also partially by the wind. During a strong easterly wind, the water level in the Wadden Sea is lower than normal. A storm tide can occur when storm and high tide coincide. The water level in the rivers is determined by precipitation in thedrainage basin, melting snow and ice by the source and (in dammed rivers) the velocity of the riverwater discharge. Water levels in the polders is regulated with the help of pumps and sluices.

All water levels in the Netherlands are expressed in centimeters with respect to the Normal Amsterdam Water Level (NAP). In the 17th century, marble stones were immured in the walls of eight sluice in Amsterdam. These stones lie in one plain, and are eight feet and five thumbs higher than the Amsterdam Level, which concurs with the average high tide in the IJ in those days. In the course of centuries, this plain in the entire Netherlands was used as a basis for expressing the water level and ground heigth. In May 2004, the NAP was adjusted by two centimeters because it appeared to be lower than in the beginning of the last century. This is because the Netherlands is toppling.

Weblinks

More information explaining how tides work can be found at http://library.thinkquest.org/11776/tides.html.

Source: de Vleet, Ecomare