Natural forces

The Wadden Sea is one of the last large tidal regions where natural forces have free reign without a dominating influence from human activities. The interaction between natural forces, such as wind, current, tides and waves and the activities of plants and animals is the reason why the Wadden Sea is constantly changing.

Überfluten und Trockenfallen

Strömung und Sandtransport

The North Sea water flows into the Wadden Sea twice a day, flooding the mud flats with seawater. The water also flows out of the Wadden Sea twice a day, exposing the flats to the air. The tides are the force behind this process. Tidal currents can be found in all oceans throughout the world. They are caused by the gravitational forces between the earth, moon and sun. Water is pliable. When the moon pulls on the Earth, the water mass rises in its direction, causing high tide along the coasts. At other places, the water is pulled away, causing low tide. Ebb is when the Wadden Sea empties after high tide; flood is when it fills up again. This takes place twice a day. Because the difference in time between high and low tide is a slightly more than 6 hours, these tidal moments shift an average of an hour every day.

During high and low tide, the sea does not rise or fall to the same height every time. This is related to the position of the moon and the sun. Around new and full moon, the height of the tides take on extreme differences. In these cases, the gravitational pull of the sun plays a role. During these moon stages, the sun, earth and moon lie in one line. When the sun and the moon form a triangle with the Earth, there is less difference between high and low tide. The direction of the wind also influences the tides.

With strong easterly winds, the water is literally blown away causing a lower water level in the Wadden Sea. Given a strong westerly wind, more water remains in the Wadden Sea. These are all temporary situations related to all tidal regions. Should an extreme rise occur during a storm, then the results can be irreversible. In such situations in the past, islands have broken in half and large polders became permanently flooded. The inhabited Wadden Islands are presently more resistant to such extreme natural forces since dikes have been constructed along vulnerable parts.

The strong current along the North Sea coast enters through the openings between the islands. Until it is given more space, the rapidly flowing water cuts along the edges of the narrow inlets. This causes the banks to crumble and the sand and mud to be carried away. As soon as the water has more room to spread out, the current declines. The heavier particles start to fall to the bottom, creating sandy flats. At certain points, the current is practically motionless, as when two currents meet head on. Where there is so little movement in the water, the fine mud particles will sink to the bottom, forming muddy flats. The point where two major currents collide is called slack water. Because the water is so motionless at this point, the very finest particles sink to the bottom, forming large muddy surfaces. Each Wadden Island has its own point of slack water, which is easy to recognize by the huge mud flat extending from the island coast. When the water retracts during ebb, the current will also carry sediment but this time removing it from the Wadden Sea.

By some of the islands, the current takes sand from the North Sea coast and deposits it on the Wadden Sea side. In this way, the islands 'wander' slowly but surely towards the mainland. They seem to roll over; that is, when possible. Where dikes have been built along some of the islands, this process has been obstructed. All these forces are responsible for a constantly changing Wadden Sea. Some areas are expanding while other areas are eroding. Without the continual supply of sand, the Wadden Sea would deepen and the mud flats would no longer surface during low tide. This process of sand transportation helps the Wadden Sea to keep pace with sea-level rise.