A storm tide is a natural phenomenon, and hence cannot be prevented. It happens when the normal water level rises high along the shore due to strong onshore winds and/or reduced atmospheric pressure (page 37) as in the North Sea. It is not an open ocean, being blocked from the ocean by the British Isles. As a result, the water dams up very quickly. The waves whipped up by the wind destroy or flood the dikes and beach dunes. Parts of the coast are also washed away. The Halligen Islands in Germany, for instance, have emerged as a result of storm tide action. These are small islands on the coast of Schleswig-Holstein, which were earlier connected with the mainland. They are regularly inundated by flood waters.