The Sand Motor is a mega-nourishment characterized by a very large sand volume of around 20 million m3 placed along the Dutch coast. The Sand Motor is a pilot project to evaluate the performance of an alternative nourishment strategy with respect to different functions of the coastal system. Within the scope of the coastal functions, the hook-shaped design of the initial morphology of the Sand Motor involves a sheltered (man-made) lagoon. The morphology of the Sand Motor and particularly the area around the lagoon entrance has changed over time since the construction in 2011. As a result of the changing inlet and lagoon morphology, the tidal conditions inside the basin have changed. We have set up a detailed numerical model to simulate the hydrodynamic conditions inside the Sand Motor’s tidal lagoon as a function of the water level at sea and the measured local morphology of the Sand Motor. It is found that the tidal response inside the lagoon changes significantly as a function of the changing morphology. The analysis reveals a prediction of the temporal development of the tidal range and mean water level inside the lagoon, which can be used to predict the closure of the Sand Motor lagoon.