Wind congestion (hydrology)

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Elbe terraces in Otterndorf during storm surge (2007)

In hydrology , wind accumulation is defined as the increase in the water level under the influence of wind.

On the coasts of the sea, onshore and long-lasting winds cause water to be transported from one direction, which leads to a significantly higher water level, especially in offshore shallow water areas (e.g. Wadden Sea ). The wind congestion is the difference in height between the observed and the predicted or mean tidal high water level . This prevents the water from running off at low tide , so that even higher water levels can occur with the next high tide .

The size of the wind congestion depends on the wind conditions. In addition to speed, direction and duration, the decisive parameters are also the area of ​​the wind ( fetch ). The wind congestion is superimposed by the tides and can trigger storm surges , which can be even stronger , especially during spring tides . Spring tides are caused by certain constellations of moon and sun and produce significantly lower or higher tidal water levels.

On the German North Sea coast there is a large fetch length when wind is blowing from the northwest, so that the wind is very effective. Wind at gale strength from approx. 9 Beaufort - the first breakers form and the spray blows on the waves - creates a strong wind accumulation on the Lower Saxony and Schleswig-Holstein coasts. Especially in combination with a spring tide , there is a risk of a severe storm surge .

The wind build-up also pushes the water into river mouths when the wind blows against the direction of flow. Due to the direction of flow of the Lower Elbe against the main wind direction, there is always a risk of flooding in Hamburg when there is a storm surge. Well known is the devastating storm surge of 1962 , when dikes broke and 315 people lost their lives.

Wind congestion also occurs on inland waters. Especially large lakes and dams , if they are in the main wind direction, have a large fetch length, so that a higher water level occurs on the onshore banks. Wind congestion is also observed at the terminal points and locks in shipping canals with long, straight sections.

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