In 1989 the Dutch government decided to build a storm surge barrier in the New Waterway near Rotterdam being a good and cheap alternative for the necessary strengthening of dikes along the lower regions of the Dutch rivers Rhine and Meuse. To be feasible the barrier had to meet several goals. The most important ones being : * closing frequency less then once every 10 years now and once every 5 years after 50 years from now (including 25 cm sea-level rise) * prescribed reduction of design water-levels at two representative locations, being Rotterdam and Dordrecht. These and other parameters are calculated by means of a probabilistic calculation method. This method involves a mathematical open-channel network model of which the results are combined with the statistical properties of input parameters. A risk-analysis of the performance of the barrier is included. Finally the model is adapted to study operational aspects.