Retention of water for flood mitigation


The retention of water masses also occupies a central position in technical flood protection, in a similar way to natural retention potential. 

Below, you can find out more about the different options for water retention and their potential to reduce flooding.


Dams are suited to holding large quantities of water in morphologically very distinct terrain (or uplands), acting as a barrage sealing the entire transverse profile of a valley through which a water course flows. They are frequently located in the upper reaches of rivers, as the relief here offers ideal conditions with deep valleys. Nevertheless, construction of a dam on flat land is not impossible but it must be adapted to the local conditions with respect to the proportion of height & width of the arrangement.

Over time, the built up water forms a reservoir, which gradually floods broad areas of the land behind and thus reshapes previously terrestrial into aquatic ecosystems, representing a massive intervention in natural processes (a more detailed consideration of ecological consequences of flood protection measures will follow). The water can be released in a controlled way as required, holding the water level of the discharge at a constant level. The primary use of such a system frequently lies in drinking water and electricity production but it can also be used effectively for flood protection. As long as the capacity of the reservoir is sufficient, rainfall can be retained, which would otherwise have directly caused the level of the river to rise. Anticipatory control of the system is based on appropriate forecasting and can provide a very important service for flood protection.

One of the largest reservoirs in Germany is the Biggesee in Sauerland, North Rhine – Westphalia. The dam has a total height of 52 m and is 640 m wide. At 172 million m3, the volume of the reservoir is huge and it is therefore the 5th largest reservoir in Germany. For its part, the Bigge is a tributary of the Ruhr, which eventually cuts across one of the most densely populated areas in Europe and represents a significantly lower risk as a result of the control.



Retention basins

Flood retention basins play a special role in their function as a barrage. As long as a river flows along its bed at the normal water level, it crosses the structure with no problem. However, if the discharge increases above a certain limit during a flood event then the retention basin stops the excess water masses and allows only a certain basic discharge through. The lower reaches are thus protected from peak flows and receive only a calculable maximum discharge quantity.

Like dams, structures of this nature are frequently located in the upper reaches because of the topographical conditions and they serve exclusively to provide flood protection. Similar arrangements such as rainwater retention basins can be found primarily in urban sewer construction.

Special variants of retention basins are used in the Alpine area to protect against avalanches and mudslides. Structures here must withstand surges that occur suddenly and have a high solid content (trees, stones, soil material etc.), which presents particular structural demands.



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