Automatic float-controlled flap weirs are purely mechanical water level control units and serve for volume activation at retention and storage basins. Through their use, water levels are kept constant, regardless of the amount of water. At the same time, the weir flap can also be used as backflow protection by closing the opening above the flap. A maximum storage volume utilization and better use of the storage volume in inflow channels, rain overflow basins, earth basins, dams and flood retention basins etc. is possible. The overflow area of the fixed weir threshold is additionally utilized.