Low tide

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Low water of the Elbe in Dresden (water level at 1 meter, June 2005)
Ebbe , watercolor on paper, Sir William Fettes Douglas, 1882

The lower water level of bodies of water is called low water . A distinction is made between low water in the tidal area and low water in waters that are not influenced by the tide:

  • Tide-independent waters are waters on which the high tide and ebb have little or no effect.
    • As low water is then referred to the water level of water that is well below a defined normal state. This low water is basically due to the weather or the season
  • Tidal waters:
    • Low water occurs periodically every 12 to 12½ hours as normal. In Europe, this applies above all to waters that depend on the Atlantic Ocean , including the North Sea and the rivers that flow into it up to the tidal lock.

Low water in the tidal area

In the tidal cycle of the oceans, low water is the lowest water level at the transition from ebb tide to high tide . Colloquially, low tide is often confused with ebb.
The arithmetic mean of the tide rise (TS) and tide fall (TF) is called the tide range (TH).

Low tide in inland waters

In the case of inland waterways , low water is primarily the result of meteorological precipitation or water shortage. The regular seasonal fluctuations are measured by the mean annual low water (MJNQ), below which there is extreme low water. When the water level is lowest depends on the local climate ( runoff regime ), it can be in summer, but also - for example in high mountains - due to the binding of precipitation as snow in winter. In addition to this definition, low water is also defined on the basis of threshold values ​​that are set differently depending on the point of view ( ecology , water management , irrigation , shipping , etc.).

Extreme low water in Bonn (October 2018)

For inland navigation , low water can result in considerable restrictions due to a reduction in the unloading depth and the risk of accidents. Furthermore, the production of companies that rely on the withdrawal of cooling or process water can be restricted. If the discharge of pollutants remains constant, the lack of dilution water leads to increased pollutant concentrations. Since slow flow movements and high temperatures often occurring at the same time can lead to an insufficient oxygen content in the water, there is an increased risk of fish death . Bank protection and structures on wooden piles rot if they are exposed to the influence of oxygen due to the low water level. Low water can be exacerbated by human intervention (water withdrawal) as well as attenuating ( low water increase ).

Water withdrawal, for example for agricultural irrigation, which is legally bound to a minimum water level, can come to a standstill.

For water management and ecology, the problem of the amount of residual water at extraction systems, such as power plants, is also important.

Whitewater stretches can become impassable for paddlers at low tide due to strong blocks . Boats and water sports enthusiasts can be damaged by wrecks, scrap and stones that are then less deep under the water surface, but dangerous foreign objects can often be seen from above and removed more easily.

On the other hand, fords can be crossed with vehicles or on foot, for example with mountain boots, fishermen can climb further into a river bed with boots when the water level and current speed decrease. Construction work, for example on the foundation of a bank construction, is made easier if workers can stand in shallow water with only low flow speeds in waders . Mechanical hydraulic engineering measures in the river bed are also made easier because an excavator can be driven further into a shallow river bed at low tide and the position of built-in stone blocks, thresholds, groynes or a berm can be better assessed.

Web links

Commons : Low Tide  - Collection of pictures, videos and audio files
Wiktionary: low water  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Konrad Simmer : Foundation. Volume 2: Construction pits and foundations. 17th revised and expanded edition. BG Teubner, Stuttgart 1992, ISBN 3-519-25232-5 , pp. 269-279: Chapter wooden posts.