The flood retention basin Odenkirchen on the river Niers in Germany was constructed around 60 years ago as consequence of a major, harmful flood event. The complete design and structures do not comply with up-to-date national regulations, the DIN 19700 [3], so that the owner, the Niers Association, decided to upgrade the basin. The upgrading works include different measures such as the adjustment of the reservoir operation management in consideration of the updated flood hydrographs and discharges as well as adjustments in respect to sewage disposal compensation. Latter purpose is an urgent consequence of the continuous urban development which led to a steady increase of the sewage discharges for which the required retention volumes cannot be realized locally. As part of the dam rehabilitation the reconstruction of the outlet works – a controlled weir with three bays and sluice gates – and construction of an overflow spillway as part of an embankment dam shall be realized. The complete project is dominated by the environmental restrictions comprising both environmental protection areas as well as requirements of fish passability and groundwater. The project area is located within a region with mining subsidence, groundwater management and high sediment loads.