Close to the city of Viersen (North-Rhine Westfalia) a natural retention area of around 50,000m³ was planned. Through this area, the river Niers flows during dry weather conditions. In the event of heavy rainfall, highly diluted mixed water is discharged into this area and temporarily stored there. The retention area is controlled by two weirs and surrounded by an earthdam structure. The river Niers will be given a natural, multi-branched watercourse. A floodplain will be created by lowering the terrain over a wide area, which will also provide additional flood retention space. The area presents a high presence of beavers that are used to dig holes inside the river dams and embankments. The designer needed to protect the dams from possible failure created by rodents.