High-water levels in the Rhine-Meuse delta are going to rise in the upcoming decades due to climate change with the accompanied sea level rise and the increase of the frequency of extreme Rhine discharges. The flood prone areas not protected by flood defenses at the Island of Dordrecht are directly influenced by the increase of these high-water levels, leading to an increase in the future flood risk. To lower this risk, the Delta21 project has been proposed. By pumping water from the Haringvliet, the high-water levels in the delta are lowered. It is unclear if an optimal operation of Delta21 for the flood prone areas of Dordrecht exists and if the reliability of the project impacts this high-water level reduction.

The main objective of this report is to determine if the inclusion of Delta21 to the flood protection system of the Rhine-Meuse delta can provide a significant reduction of the flood risk of the flood prone areas at Dordrecht. Furthermore, it should be determined if an optimal operational scheme is possible in which the flood protection system with Delta21 can comply with all the flood requirements of the flood prone areas at Dordrecht and limitations to the Europoort barrier and Delta21 project while considering the reliability of the Delta21 project and the climate scenarios of the year 2100.

It was determined that the present flood risk at the flood prone areas at the Island of Dordrecht is equal to €110,000 per year. For the minimum, medium and maximum scenario for the year 2100 this risk increases to €390,000, €1,300,000 and €8,100,000 per year respectively. The implementation of the optimal operation of Delta21 reduces this risk with 23, 15 and 64 %. To obtain a flood risk reduction of 64 % for the maximum scenario, the closure frequency of the Europoort barrier, which may not be larger than three times per year, is equal to ten times per year. Furthermore, the flood risk as a percentage of the average annual income per household at the flood prone areas may not exceed 1 %, but for the medium onward this limit is exceeded at the historical harbor (city center). The maximum allowable probability of failure per pump of the pumping station and per siphon of the spillway of Delta21 is about 0.5 if the correlation between the components of both these systems is smaller than 0.9.

The inclusion of Delta21 in the current flood protection system with the present Europoort closure level can provide a significant reduction of the future flood risk of the flood prone areas at Dordrecht. However, it is not possible to create an optimal operational scheme for all scenarios of the year 2100 in which the flood protection system with Delta21 complies with the flood requirements of the flood prone areas and the limitations of the Europoort barrier. Finally, the reliability of the new Delta21 components is non-decisive for the flood risk assessments that have been made as long as the components are not fully dependent.

Decision-makers in low-lying coastal zones are confronted with uncertain developments around flood risk.These drivers are surrounded by large uncertainties, which requires delta management to be adaptive. This research offers insights about adaptive delta management for the Rhine-Meuse estuary from three perspectives: a physical, a socio-political and a integrated perspective. In the physical perspective, the effect of pump capacity on the water system of the Rhine-Meuse estuary is quantified; one of the adaptation options against sea level rise. To assess the adaptation potential of pumps, the case study Delta21 is used; a plan to construct an artificial lake with an area of 35 km2 next to the Maasvlakte 2 in combination with a pump capacity of 10 000 m3/s. The Rhine-Meuse estuary is divided into four sub-areas depending on the
dominant hydrodynamic process: storm surge dominant area, flood storage dominant area, discharge dominant area and transition area. For each sub-area, the effects of Delta21 are computed on 1) water flows, 2) hydraulic loads and 3) failure probabilities. Delta21 succeeds in lowering the hydraulic loads and corresponding failure probabilities. At the same time, reductions are disproportionately over the Rhine-Meuse estuary leading to low reductions in some sub-areas. In the socio-political perspective, the focus is on decision-making and learning processes in management in delta management. The Rhine-Meuse estuary is characterized by a network structure, which means that knowledge and decisions do not belong to one single actor, but those decisions come about during interactions between various groups of actors. Not only the decisions need to be adaptive to cope with uncertain circumstances, but the learning process itself must also become adaptive. To achieve this aim, a conceptual mode is developed. Two case studies are used; the Delta Program and Knowledge Program Sea Level Rise. A longitudinal analysis of the Delta Program resulted in different narratives over time and various learning types. Subsequently, the Knowledge Program Sea Level Rise is analyzed with semi-structured interviews, process-tracing and the conceptual model. Observations and challenges are translated into a roadmap of learning activities. Moreover, the DEALTa learning handbook is developed, to support actors in designing learning activities. The physical and socio-political perspective are united in the integrated perspective. On the level of technical studies, insights are shared about the relation between both aspects and how the integration can be improved in the future.