The Dutch Rhine-Meuse delta is expected to require many dike reinforcements on a short and long term, due to (accelerated) sea level rise. Average damage in unembanked areas will increase too, and a costly replacement of the Maeslantkering is expected after 2070. Delta21 is a project that aims to address these challenges related to flood protection, while also providing hydraulic energy storage and opportunities for valuable nature development. Delta21 consists primarily of a large (salt)water storage lake attached to the Tweede Maasvlakte. It is connected to the North Sea with a pump-turbine station. On the southern side, a closable storm surge barrier spans the remaining gap to the island of Goeree Overflakkee. Under normal circumstances the lake functions as a hydraulic battery, using the pumps and turbines to store or generate electricity as needed. However, when water levels threaten to exceed NAP + 2.5 m at Dordrecht, the barrier closes and an upstream spillway into the lake is opened. To determine the viability of the project in the context of flood protection, the question arises: “To what extent is Delta21 capable of providing a cost-effective alternative to the current flood protection policy in the Rhine-Meuse delta?” To answer this question, a one-dimensional hydraulic model is constructed with a detailed and accurate schemati-zation of Delta21. Results are processed into exceedance frequencies for a system with and without Delta21, to obtain reductions in water levels at normative frequencies. Furthermore, a sensitivity analysis is performed to de-scribe how various designs and configurations affect the magnitude of these reductions. Other than Delta21, no system-changing interventions are included in the scope of study. Three strategies are formulated in which Delta21 potentially reduces costs, and evaluated using the model’s results: 1 Extending the lifetime of the Europoortkering before it needs to be replaced. 2 Obviate dike reinforcements. 3 Reduce the average yearly damage of unembanked areas. Approach 1 is shown to be ineffective. Delta21 does not achieve changes in the Europoortkering’s closure frequency or failure probability per closure. Neither does Delta21 effectively mitigate a failure in the region where water level exceedance frequencies are dominated by the Europoortkering’s failure probability. Approach 2 is far more successful, accounting for 96% of the total cost reductions. Reductions in water levels at normative frequencies are translated to reduced failure probabilities of dike segments using fragility curves, which potentially yields a positive reassessment. Delta21 achieves this and obviates dike reinforcement for 41 km by 2050, and 150 km more by 2100. These lengths comprise respectively 33% and 60% of the total considered dike lengths that would need reinforcement. The net present value of obviated reinforcement costs is €752 million, with a 90% confidence interval of [€220 million, €2,821 million]. This large interval is due to large uncertainties in reinforcement cost estimations. Delta21 most effectively obviates reinforcements in trajectories with stricter norms and closer proximity to the storage dominated region. Water levels at normative frequencies are very sensitive to the opera-tional control (i.e. closure criterion) of Delta21. An additional nominal cost reduction of ca. €278 million can be achieved when a criterion of NAP + 2.5 m at Dordrecht is maintained in the future. The sensitivity is far weaker to varying dimensions or capacities of Delta21, which are generally unnecessarily large during illustrative conditions. Approach 3 provides the additional 4% contribution to total cost reductions. The only considered area is Dordrecht, due to its unique high economic value in low lying unembanked locations. Damage profiles are integrated with changes exceedance frequency curves to obtain the average yearly damage. This is reduced by approximately €53,000 per year now, and grows to €1.36 million per year in the future climate scenario (2100). The decrease in relation to the current system is about 42%, but the absolute value rises greatly with sea level rise. Implementation of lower closure criteria variations can yield an additional +20% now, and +10% in the future scenario. The total net present value of cost-reductions by Delta21 is €783 million, with a 90% confidence interval of [€251 million, €2,852 million]. This covers about 20% of the total construction costs of €3.7 billion, and is insufficient to make Delta21 viable alone. However, not all costs are related to flood protection exclusively. If the components required for energy storage are viable on their own, merely the additional costs of the spillway and storm surge barrier have to be included in the cost-benefit analysis. Operational costs, which heavily depend on the operational control, must also be added. Smaller design dimensions or pump capacity of Delta21 has been shown to be just as effective, and would further cut costs. Additional savings beyond 2100 are plausible, but outside of this research’s scope. Furthermore, less people displacement or flooding of unembanked areas may achieve additional societal value, but is difficult to quantify. More research will have to indicate whether attributing only specific costs, including additional value sources, and estimating reinforcement cost more accurately ultimately lead to Delta21 being fea-sible in the context of flood protection.