peer reviewed ; In the global framework of climate change, hydraulic structures rehabilitation is an up-to-date subject requiring appropriate design and impact studies. In this scope, upgrading the release facilities of dams is an important point since any badly controlled rise of water level in the reservoir leads usually to both up and downstream consequences, often irreversible. Suitable numerical models, coupled with contemporary computational possibilities, allow today engineers to forecast the complex situations induced on real structures by extreme events with increasing representativeness and accuracy. In this field, WOLF software, a process oriented free surface flows computation package completely set up by the HACH team, has proved its efficiency and reliability for years through numerous theoretical, experimental as well as on real structures applications. In this paper, the rehabilitation study of the Nisramont dam (Belgium) is presented. Located on the Ourthe River, a few kilometers upstream of the town of La Roche, this 21-meter high dam came into service in 1958 and was, in a first time, dedicated to be used as a cofferdam during the construction of a downstream larger reservoir of 250 hm³. But this second dam was never built and Nisramont became a definitive reservoir of 3 hm³ assuming the drinking water supply of a large part of the population of the south of Belgium and producing hydro-electricity. Due to its temporary primary tasks, the stilling basin downstream of the three 12.5 m large notches of the crest spillway has been designed for a short time use. Similarly, the 3 bottom outlets have never been equipped with valves and are thus unusable for the reservoir management. In order to secure the structure and to make it comply with its definitive working objectives, while taking into account climate change observations, the Ministère wallon de l’Equipement et des Transport (MET) - Direction des barrages de l’Est (D. 241) entrusted the HACH with evaluating up to date critical flood ...