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Flood risk reduction through natural retention and physical protection: Provide adequate, but controlled retention

Flood water storage and floodable land along the rivers, so-called polders, are important means to reduce flood crests. While flood storage without any human intervention or infrastructure (such as artificial basins or inlets/outlets) would be the most natural approach, it is not the most efficient. Uncontrolled polders often flood before the peak of the flood has been reached, and are already ‘full’ when they could achieve their optimum impact by taking up water. Land along the rivers is also typically fertile, attractive and scarce. Natural polders would require too much space in order to achieve a meaningful peak flood reduction in large flood events. In contrast, ‘controlled’ polders offer greater benefits relative to costs. These are filled at the point when they can achieve greatest impact based on the anticipated flood crest. German water authorities have calculated that such polders may need, on average, only one quarter of the space that a natural polder would use to achieve the same effect. But polders and their impact on flood crests are still not perfectly understood. There is need for additional research on how polders work, and what can be done to alleviate floods during periods when waters crest. This could help increase acceptance of polders, as, again, only that which is widely understood will be accepted or gain support among the general population. Polders, especially ‘controlled’ polders, have been regarded as a controversial approach in some areas. Landowners upstream and communities downstream must both participate in discussions to find a fair and financially viable solution for all, so as to achieve the necessary protection level. It is hoped that a solution can be found without having to expropriate land. Retention space for flood water is limited, and any additional polder space that becomes available needs to be managed wisely to achieve the best possible flood control.

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