Experience the tides along the Wadden Sea

The clay soil deposited by the sea is incredibly rich in nutrients. As a result, the marshland bordering the sea is very rich compared to the moorland plains further inland. Over time, the marshland has provided a livelihood for a relatively large population along the Wadden Sea coast. The problem, however, was that the low-lying marshland was prone to storm surges that have claimed thousands of lives over the years. The Wadden Sea people were forced to settle on the edges of the higher-lying geest areas. To better utilise the marshland, they began to create artificial elevations on which to build their farms. These are the so-called shipyards, which can be found in many places in the southern part of the Danish Wadden Sea. As early as the Middle Ages, people began building permanent dykes to keep the sea out. However, it is only in the last 150 years that effective sea defences have been built. These modern sea defences have been successful in protecting people from storm surges. Fortunately, storm surges have not caused any deaths in the Danish part of the Wadden Sea for more than 100 years.

The difference between high and low tide is usually around one and a half metres, but weather and current conditions can magnify this difference. In stormy weather, the difference can be up to 5 metres and in extreme cases lead to storm surges. During the 1999 hurricane, the water level in Ribe was measured at 5.12 metres - then the gauge broke down. That water level was even measured at low tide. If the hurricane had hit at high tide, the water level would have been 1-1.5 metres higher and would have come close to breaching the 7 metre high dikes at Ribe.

The largest of the Danish Wadden Sea islands, Rømø, is a testament to how the tides build the land. Rømø is named after a so-called rime, which originally referred to a low pile of sand off the coast. But as the water washes up on the rime, leaving sand behind, the rime grows. When the sand is thick enough, it blows together to form dunes, which eventually form an island after a long enough time. Behind the dunes, there is so much shelter that the water stops moving long enough to deposit mud and the marsh begins to form. 

Another example of the power of the tides is the Skallingen peninsula, located about 10 kilometres south of Oksbøl and one of Denmark's youngest landscapes. When the catastrophic storm surge of 1634 ravaged most of the Wadden Sea coast, the Langli peninsula was partially washed away and turned into an island. In the decades after the storm and up until the early 1700s, the sea transported huge amounts of sand from Blåvandshuk and Horns Rev further south, where Skallingen literally grew out of the sea in a short time. For the first 200 years, Skallingen was a barren upland with a few dunes along the coast. The development into the marshland we see today is largely due to the interaction between plant life and the tides. The extremely salt-tolerant plant, the queller, was the first to establish itself. When the tides bring clay, silt and organic material twice a day, the salt marsh plant captures and retains some of it, slowly increasing the height of the surface. Marsh formation is now underway and this allows other plants to establish themselves, trapping even more sediment. The marsh grows slowly but steadily and Skallingen is constantly changing.

In this way, the characteristic landscape types of the Wadden Sea area emerge as a product of the tides.

The rise and fall of the tides every six hours is mainly due to the moon's influence on the Earth's mass. The gravitational pull of the Moon tugs on the Earth's water masses, and the force varies depending on the distance to the Moon - it varies throughout the month it takes the Moon to rotate around the Earth. When the pull is weakest, we call it niptide, while conversely, when the Moon and Sun together have the strongest effect on tidal rise, we call it spring tide.