The coastal zone is becoming more vulnerable due to growing concentrations of human population, settlements, and socio-economic activities. The Rhine-Meuse Delta is low-lying land, which makes it vulnerable to flooding. The Maeslant barrier is a storm surge barrier which should prevent flooding in the low-lying land behind the barrier. Various climate models have predicted that sea level rise will occur. The predicted sea level rise is more than what was anticipated during the design stage of the Maeslant barrier. Due to the increasing water levels, the closing frequency of the barrier will increase. This has a negative effect on the reliability of the barrier, as the components are used more often. The Maeslant barrier also needs to discharge river water during a storm closure when the water level on the rear side of the barrier becomes too high due to river inflow. This discharging process is a complex process which contributes greatly to the failure probability of certain components.

Delta21 is a spatial plan to redevelop a part of the Dutch delta to mitigate the effects of climate change which are sea level rise and increased river discharges. Delta21 claims to improve the safety of the entire Rhine-Meuse delta till a sea level rise of 1.1 m. Delta21 pleads for a central approach to focus on improving pump capacity in the main water systems instead of raising and strengthening all the dikes. The main goal of Delta21 is to reduce the flood risk in the downstream area. Due to the large pump capacity available, Delta21 can replace the discharging function from the Maeslant barrier. This simplifies the closure operation of the Maeslant barrier to only retain seawater during storms on the North Sea. The effect of the simplified closure operation has consequences on the reliability of the Maeslant barrier.

Therefore, the main objective of this master thesis is to find out how the failure probability of closure of the Maeslant barrier with the simplified closure operation changes in the Delta21 configuration.

To fulfil the objective, the first step was to find out how the Maeslant barrier itself works and how the discharging procedure impacts the failure probability of closure. Then, a hydraulic system analysis was done to understand how Delta21 impacts the current configuration of the Rhine-Meuse delta. In the next step, the new situation with Delta21 was schematized into a simple hydraulic model which has been verified and validated.
By making the variables in this model probabilistic, a Monte Carlo simulation was done to find out what the probability of a negative head difference larger than 1.5 m was. The simulation was done for the situation without and with 0.6 m sea level rise.
The next step was the failure probability analysis. In the qualitative analysis of the failure probability analysis, different failure mechanisms and failure scenarios are identified for the Maeslant barrier in the current situation, and for the Maeslant barrier in the Delta21 configuration. In the quantitative analysis, the probabilities for the defined failure mechanisms and failure scenarios are calculated according to the upper bound approximation, for the current situation and with Delta21. The probability which was calculated with the Monte Carlo simulation is also used here. Ultimately, a fault tree can be composed on the basis of the qualitative and quantitative analysis for the Maeslant barrier in the current situation and the Delta21 configuration.

The failure probability of closure has been reduced by a maximum of approximately 10%. This is because many systems and operations were simplified because of the simplified closure operation.
Additionally, the failure probability analysis also provided insight in how the probability of flooding changed for Rotterdam with the Maeslant barrier in the Delta21 configuration. While the failure probability of closure was reduced by a maximum of 10% with Delta21, the probability of flooding for Rotterdam has increased by 53% when using the full pump capacity from Delta21. This increase is not significant since the probability of flooding remains very small. Due to the removal of the discharging function with Delta21, the probability of several flooding scenarios increased drastically which resulted in the increase in the probability of flooding for Rotterdam.
With a sea level rise of 0.6 m, the probability of flooding increased approximately 500% for the current situation (without Delta21). When comparing Delta21 with no sea level rise to Delta21 with 0.6 m sea level rise, the increase was also present with approximately 375%. This is less than 500%, which indicates that Delta21 is effective to mitigate the effects of sea level rise compared to the situation without Delta21.