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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 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.

  • Influence moon and earth

    The tide is a cosmic phenomenon. The moon has the greatest influence on the tides. 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). Although less strongly, the moon pulls on the earth itself and even less forcefully on the water on the opposite side of the earth. In that way, a high water level, or rather a low earth level, is created on this side of the planet.

    Because the earth is also rotating at the same time, the raised water level moves around the earth. The earth turns under the tidal wave. At a specific spot along the coast, you can experience high tide and low tide twice a day, but not at the same time every day. Due to the rotation of the earth and moon, the moment of high and low tide shifts forward around 50 minutes each day.

  • Ebb and flood

    The period between high and low tide is called ebb. Ebb begins with high tide. Seafarers often use the word ebb to designate a tidal current. For example, they enter the tidal inlet between Vlieland and Terschelling with the ebb current. The word is also used to indicate low tide. 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 the flood begins (just after low tide). A sailor sails with the flood from Scheveningen to Den Helder because he has the current in his favor. 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, which is with new and full moon, an even stronger 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.

  • Tides in the North Sea
    Tidal wave in the North Sea, Ecomare
    Source RIKZ 1997

    The tidal wave in the North Sea originates in the Atlantic Ocean. It begins in Scotland. This wave reaches the Dutch coast the following day. 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 tidal difference here. The North Sea is influenced by three such whirls: one in the northeast, one in the middle and one in the south. 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

    The tidal movement in the Wadden Sea as we see it today exists since the closure of the Zuiderzee in 1932 and the Lauwerszee in 1969. The dams and dikes influence the current velocity of the water and sand transportation. In that way, permanent changes have occurred in the position and composition of the mud flats. In turn, this has influence on the plants and animals.

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

  • Water levels

    The water level along the Dutch coast is determined by the tides as well as by the wind. During a strong easterly wind, the water level in the Wadden Sea is lower than normal. During a northwesterly, the water level is much higher.

    The water level in the rivers is also determined by precipitation in the drainage basin, melting snow and ice and discharge from dammed lakes. Water levels in the polders are 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 sluices in Amsterdam. These stones determine the NAP. In May 2004, the NAP was adjusted by two centimeters. It is lower now. This is because the Netherlands is toppling.