Throughout time, storm surges have regularly struck the low-lying areas by the Wadden Sea. Modern dykes have prevented fatalities for more than one hundred years, but climate change could lead to new challenges for the dykes by the Wadden Sea.

For residents living on the coast, the sea has always been a double edged sword. On the one hand they have benefited from the rich soil of the salt marshes and from the seaways connecting to the rest of the world. On the other hand, they have had to live with storm surges that regularly flooded their lands, destroyed their property and even cost lives. Technological advances have made it possible to build bigger and more efficient dykes. Even though storm surges in the Wadden Sea have become more frequent over the past few decades, it has been more than a century since there were any fatalities.

Wind, tidal waters and atmospheric pressure

The word ‘storm surge’ refers to water levels that are much higher than normal. Three factors can contribute to the occurrence of a storm surge. The first is wind stresses. This is when wind, possibly from a storm, pushes the sea towards land, which causes water to amass and to rise along the coast. The second factor is atmospheric pressure. Low atmospheric pressure in itself can cause the sea to rise by up to 40 cm. The third factor is tidal waters. There is a 1.5-metre difference between low tide and high tide in the Wadden Sea. Therefore, the severity of the storm surge depends on whether the storm coincides with high tide. The hurricane in December 1999 hit the Wadden Sea at low tide. So, even though it was probably the worst storm to hit the area in hundreds of years, the storm surge did not flood the dykes by Ribemarsken.

The Great Drowning

The Wadden Sea has been hit by many violent and destructive storm surges over time.

However, the years 1362 and 1634 still stand out in the history of the Wadden Sea. In January 1362, a storm hit the Wadden Sea and changed the coastline with such violent force that thousands of people died. The major harbour town of Rungholt was destroyed and abandoned.  The death toll was so high that the storm surge was called ‘The Great Drowning’. Survivors constructed dykes to protect themselves from the winter storm surges, whereas earlier dykes had merely been constructed to protect summer crops from salt water. The second Great Drowning hit in October 1634. It destroyed the island of Strand, and the fishing village Sønderside had to be abandoned. However, the island of Skallingen was created in the wake of the catastrophe. The storm surge was documented by terrified eyewitness accounts and is marked on many storm surge pillars.

Modern dykes

Sea walls were built to stop flooding from storm surges. The first dykes were constructed in Tøndermarsken back in the Middle Ages, but most of the Danish dykes have been constructed within the last 150 years. Modern dykes are built on a broad base and with a flat sloping surface that slowly weakens the intensity of the storm-surge waves. A broad base also means that dykes are more resilient to the pressure of rising waters. Today, dykes stretch along almost the entire 500-kilometre length of the Wadden Sea from Denmark in the north to the Netherlands in the south. The only places without dykes are where the hill islands meet the Wadden Sea, and around the bay at Ho Bugt.

Thousands evacuated

In recent times, three storm surges have raised questions regarding the strength of the dykes.

During the storms of 1976, 20,000 people were evacuated from Tøndermarsken and Ribe. The main dyke at Højer was built following the storms and the dyke by Ribe was reinforced. The storm in 1981 hit Mandø worst. The island was partially flooded when the dyke was breached in seven places.

Increase in storm surges

The number of storm surges with sea-level rises between two to three metres has tripled over the last four decades. During the same period, the Wadden Sea area has experienced the biggest storm surges since 1634. The Intergovernmental Panel on Climate Change predicts that water levels in the oceans will rise over the next hundred years, and storms will increase. The dykes will therefore be subjected to even more pressure and towns along the Wadden Sea will face new challenges from storm surges.

You can experience the history of storm surges here.

Visit the storm surge pillars at:

• Janderup Ladeplads
• Ribe Skibbro
• Mandø Forstrand
• Mandø Ebbevej
• Sønderho
• Nordby
• Marskmandshuset in Ballum Enge
• Rømø Harbour
• Vidå Slusen
• Højer Gamle Sluse

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