Retention areas are primarily intended for water storage in situations of excess water (water safety and flooding), but they can also fulfil a function for infiltration into the groundwater.
To prevent flooding of urban areas further downstream during heavy rainfall and high river water levels, retention areas can be created upstream. Often these retention areas also have nature functions or serve as pasture areas. Any construction in these areas should of course be water resistant. Another way to create additional retention areas is to excavate parts of the floodplain, giving the river more room at high water. After the excavation of the floodplain, the area will flood sooner than it does now. This does not mean that the area will be wet all year round. In the dry periods, between April and November, the area can be used on a regular basis.
Yet another way to realize more storage surface is to make rivers or waterways meander. Providing more space to watercourses and rivers generally also provides more space for biodiversity due to the greater dynamics.