Protecting the flood-prone areas outside the dikes of Dordrecht | Cost-Benefit Analysis of possible technically suitable, legally feasible, economically affordable and socially accepted solutions

Abstract

The Rhine Estuary-Drechtsteden region comprises both urban and rural areas, protected by flood defenses. However, within this region, numerous flood-prone areas exist outside the dikes (UFPAs), where many people live and work. With rising sea levels and increased river discharges due to climate change, these vulnerable UFPAs are frequently affected by flooding, leading to substantial damage. This anticipated damage is closely connected to the closure policy of the Maeslant barrier, underscoring the significance of UFPAs in terms of the barrier's functioning and potential replacement. Consequently, reducing damage in the UFPAs holds relevance in discussions concerning the Maeslant barrier’s lifespan.

This thesis centers on the UFPAs of Dordrecht, which stand out due to their high economic and historical value. No specific research has previously investigated or calculated into a local measure to protect the UFPAs by implementing a new primary flood defense. Associated costs, benefits and local effects of possible implementations have also never been investigated before. Therefore, this research aims to develop a technically suitable, legally feasible, economically affordable and socially accepted solution for the UFPAs of Dordrecht to reduce the impacts of flooding, summarized in a cost-benefit analysis (CBA).

This CBA adheres to standard guidelines while incorporating a range of supplementary methodologies to address technical aspects. The reference scenario assumes the absence of any new flood barrier. Six alternatives were formulated and compared to the reference situation, evaluating their effects, costs, and benefits. Six effects were identified to assess benefits: recreational boating, shipping, house prices, water storage capacity, retail, and (avoided) flood damage (both material and immaterial). While the first five effects were qualitatively described, the last one was quantified and monetized. Various required investments and related operation and maintenance costs were assessed. All absolute costs and benefits were discounted per alternative to determine their present value. Subsequently, net present values were computed to establish the CBA balances for each alternative, determining whether the benefits would outweigh the costs. Unanimously, alternative 3A exhibited the highest net benefits from the CBA and various sensitivity analyses, thus being the preferred alternative for future flood protection in Dordrecht. This alternative entails float barriers (rising quay walls), no storm surge barrier, supplemented by several lift barriers in the canals. Conducting similar follow-up research in other vulnerable UFPAs in the Rhine Estuary-Drechtsteden region or elsewhere would be highly relevant and interesting, providing insights into whether local small-scale solutions might outperform large-scale adaptations.