Graduation of Wouter Zijlstra

Hydraulic response of the Rhine-Meuse delta to Delta21

• Professor of graduation: Prof. dr. ir. M. Kok

• Supervisors: Dr. ing. M. Voorendt (TU Delft), Dr. Ir. S. van Vuren (TU Delft), Dr. ir. M. Rutten (TU Delft), Ir. P.T.G. van Tol (Witteveen+Bos)

The Dutch Rhine-Meuse delta is expected to require many dike reinforcements on both a short and long term, in particular due to accelerated sea level rise. Average yearly damages in unembanked areas will increase too, and a costly replacement or reinforcement of the Maeslantkering is expected after 2070. Delta21 is a an engineering project that aims to address these challenges related to flood protection, while also providing hydraulic energy storage and opportunities for valuable nature development. The design consists primarily of a large (salt)water storage lake attached to the Tweede Maasvlakte, and 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 an hydraulic battery, using the pumps and turbines to store or generate electricity as needed. However, during a storm event, the barrier closes and a spillway into the lake is opened. This allows the continuation of discharging river water, preventing water levels to rise in the upstream estuary.

Extensive one-dimensional statistical modelling has been performed to assess how large the effect of this enhanced storage is on water level exceedance frequencies reductions, and how they are distributed spatially. Furthermore, a sensitivity analysis is performed to describe how design variations affect the magnitude of the reductions at normative frequencies. In the current climate, Delta21 is capable of reducing the height of water levels at normative frequencies by as much as 0.4 - 0.5 m in the middle and southwestern area’s of the delta, growing by an additional 0.3 m in 2100 for the high emission IPCC SSP5 scenario. The influence of Delta21 is most noticeable during very extreme conditions, and its maximum capacity is rarely reached. Illustrative hydraulic loading around norms are however often reached for less extreme conditions. The operational control, and in particular the minimum criteria for using Delta21 as emergency storage, are particularly dominant on the magnitude of the reductions.

The effects in the Northern branches of the delta are negligibly small, and the lifetime of the Maeslantkering is not expected to be lengthened by Delta21. The expected yearly damages at the unembanked historical harbour and residential areas of Dordrecht exhibit significant reductions. By far the largest savings however, originate from saved costs due to obviated dike reinforcements. About 33% of the total length of primary river dikes in the Rhine-Meuse delta that need reinforcement according to the latest assessment rounds, no longer do when Delta21 is implemented. If no other drastic system-changing interventions take place before 2100, approximately 60% of dike kilometres don’t require reinforcement anymore by that year. 

Estimations indicate that these obviations would give an investment return of roughly €840 million, covering about one-fifth of the total Delta21 project costs. Alone, savings with regards to flood protection are therefore not sufficient to make the project viable. Future research is however recommended to split the total costs into a part that is already viable from the perspective of energy storage, and what additional costs are made to allow the additional flood protection functionality. Furthermore, additional savings can be expected after 2100, depending on what adaptive climate change direction the Netherlands decide to take. Further optimization of the operational control and closure criteria should yield a clearer picture of the true cost-benefit consideration.