During a failure event on the 2nd of November 2023, non-closure of the spill gates in IJmuiden resulted in sea water inflow into the adjacent water system. This water system consists of the North Sea Canal, Amsterdam-Rhine Canal and multiple side rivers and basins. Considerable research is available on the ability of the complex IJmuiden in discharging the fresh water system and the connected polders. However, research on sea water flowing into the water system is limited. This research focusses on the behavior of a flood wave due to sea water retaining failure of the spill-lock complex in IJmuiden. Several scenarios are modeled in which sea water level, the failure mechanism and the duration of failure are varied. The model is created with the software of HEC-RAS. With the probabilistic analysis in combination with an estimation for the costs of damage, the research concludes with a flood risk estimation for 6 hours of spill failure.