Retention (water management)
Retention (lat. Retinere = to hold back) is a term used in water management . It dampens flood waves and occurs in natural lakes and dams . On a smaller scale, retention can also occur in longer rivers. The word also describes the filtering of rainwater caused by layers of soil before it gets into the groundwater.
The retention works with overflowing dams as well as with natural lakes as follows: At the beginning of the flood the lake or reservoir has a certain low level. If there is an increase in the inflow, the water level in the lake rises and only then can an increased The amount of water drains via the overflow / drain. The amount of water stored in the elevated water level forms the uncontrollable flood protection area . After the flood the water level sinks again; during this period the lake gives off more water than flows into it.
In addition to significantly dampening the flood wave, the retention also ensures a delay in the discharge peak. Due to the retention, dams can still strongly dampen a flood even when the flood relief system (overflow) is in operation .
Basically the following applies to lake retention:
- The larger the surface of the lake and the smaller the catchment area, the greater the retention effect.
- The narrower and higher the drain / overflow, the greater the retention effect.
- In the case of small lakes with a large catchment area, retention can also decrease to zero.
Retention on rivers
Since with increased water levels in rivers the increased water volume also offers a retention potential, a flood triggered in the upper course is weakened the further it gets into the lower course after hours or days. This effect is stronger, the longer the flow section is and the wider a possibly existing flood area is available to the river. The retention capacity of many rivers has been reduced as a result of the increasing embankment of rivers and river straightening in the past centuries.
A distinction must be made between controlled and uncontrolled retention. With controlled retention, a retention area (polder) is flooded in such a way that the apex of the flood wave is cut off. The uncontrolled retention describes a free flooding of the retention areas. The latter is ecologically more valuable, but it can happen that the retention area is already full at the time of the flood peak. The level-reducing effect is therefore less than with controlled retention
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