The Twin Dike project is an innovative concept to achieve the required flood safety while being able to preserve valuable areas. A lower, second dike is constructed behind the original existing dike. The original dike is only being strengthened to a minimal extent. The general idea behind this project is to more efficiently use the area near the dike without losing valuable nature or buildings during dike reinforcements. At the same time, this concept is expected to be cheaper, provides the required flood safety and the area between the dikes is used for experimental purposes. Currently, there are no pre-defined methods which can be used to easily assess the safety requirements of multiple lines of dikes. This research investigates if multiple lines can be an attractive alternative for traditional dike reinforcements regarding flood safety and costs, to provide the required protection against floods.

In order to answer the research question, it is analysed how flood defences are currently assessed in the Netherlands. Governing loads and failure mechanisms for the Twin Dike are determined. Using current and experimental methods the loads acting on both dikes are determined. Next, a sensitivity analysis is performed to analyse the sensitivity of various aspects on the results. Finally, a cost-benefit analysis is performed.

This research concludes that erosion of the outer slope and overtopping are governing failure mechanisms for the outer dike of the Twin Dike project. Overtopping volumes are, however, not large enough to cause considerable damage to both the outer and inner dike. Existing assessment methods are not good enough to determine the safety of the Twin Dike project with respect to erosion of the outer slope. A prototype model is used to better analyse the effects of erosion of the outer dike and the consequent loads on the inner dike. The model showed that breaches are formed in the outer dike at conditions which have a higher probability of occurrence than allowed by the safety norms. When a breach occurs the inner dike is not able to withstand the consequential loads. A total system failure immediately occurs.

A sensitivity analysis was performed on the overtopping volumes and erosion model results. Sea level rise was the dominant uncertainty for the overtopping volumes. If sea levels will rise extremely within the design lifetime, overtopping volumes can be three times as large compared to the case with mean sea level rise. However, the overtopping volumes are still low enough such that Twin Dike satisfies the required safety norms. For the erosion model, the average return period for which a breach occurs varies greatly with the uncertainty of various parameters. However, using a case with mean values for all variables, the probability of a breach is still larger than allowed by the safety norms.

Also, a cost-benefit analysis was performed to assess if the Twin Dike project is an attractive solution with regards to costs. In this case, from a cost perspective, multiple lines of dikes are not an optimal solution.