The Delta Barrier

Abstract

The world is heating up quickly. Since 1901, the temperature in the Netherlands has risen about twice as fast as the global average and the effects of climate change seem clearly noticeable. With sea levels rising and peak river discharges increasing an enormous pressure is mounting on the current Flood Protection Programme in the Netherlands and measures to prepare for an uncertain future are being developed. Although expensive, the most commonly used strategy as in the current Flood Protection Programme is to increase the retaining heights of the existing dikes. A new method to improve the flood defence system in the South West Delta of the Netherlands is the Delta21 concept. With discharging instead of raising the dikes in mind, Delta21 poses an alternative strategy to reinforcing dike sections for the downstream area of the Rhine and Meuse. The idea of Delta21 is a future-proof solution for the Southwestern Delta of the Netherlands. A solution for not only flood risk management, but energy transition and nature restoration as well.

Between the coastline of the Tweede Maasvlakte and that of the island of Goeree-Overflakkee, the Delta21 project aims to construct a flood defence in the form of a row of dunes, multiple pumping stations and a closable storm surge barrier. Considering protection against flooding, Delta21 strives to limit the water level at Dordrecht to a maximum of NAP + 2.5 m. The flood defence ensures that a lake of about 20 km2 is created in the sea, as it were. By closing off the newly created Tidal Lake by means of a storm surge barrier and opening up a spillway to the Energy Storage Lake, pumping stations are able to pump excess river water from the Energy Storage Lake into the sea. The dunes and the closable storm surge barrier together provide sufficient protection against high seawater levels.

This thesis aims to come up with a preliminary, integral design for the Delta21 storm surge barrier taking into account the new Delta21 landscape design as such that the required functionality of Delta21 is met. This design must fulfil the functional and structural requirements and must fit within the ideology of Delta21. To achieve this, the hydraulic engineering design method is used. The first step in this cycle is to analyse the system with the purpose to better understand the situation in order to formulate the requirements and boundary conditions more accurately. The first design phase is concluded with the basis of design where the processes and functions of the Delta21 system explored
in the system analysis are translated into the requirements and boundary conditions used for the design. The Basis of Design provides mostly "SMART" formulated functional- and structural requirements and boundary conditions. Boundary conditions at sea have been determined using Hydra-NL software.
Climate change plays a vital role in this, as mostly sea level rise influences the boundary conditions significantly. Wishes from stakeholders, which are also explored in the system analysis, are translated either in requirements or evaluation criteria.

In the second step a gate type variant study is conducted. From an inventory of multiple concepts, the vertical lift gate and the segment gate remained after verification. After evaluation of the two remaining concepts using the functional requirements and criteria from stakeholders, the vertical lift gate came out as most suitable for the Delta Barrier. Subsequently, in the second step, a spatial and functional design is presented which satisfies all functional requirements. A new Delta21 flood protection process is proposed in order to more effectively
protect the hinterland from flooding. The reliability of the closure operation has been researched after which is concluded that the closure procedure can be considered reliable until a sea level rise of ca. 1 m. It is verified with a hydrodynamic model that an effective flow area of 6000 m2, defined below NAP,
is to be preferred where mostly the impact on the ecosystem was governing. In order to enable passage of shipping, a location of a separate lock complex is proposed and bridge girders are incorporated into the design in order to enable the passage of road-traffic over the barrier. Furthermore, it is concluded that using tidal turbines to generate electricity with tidal flow behind the Delta Barrier is unfeasible due to the shallow water depth behind the barrier and the relatively low tidal flow velocities through the barrier. Hydraulic cylinders are chosen as driving mechanisms for the gates as it is expected that hydraulic cylinders are less expensive and have a lower failure probability than e.g. a rack and pinion system. Lastly, using a climate adaptive pathway approach, three general strategies are presented to the closure reliability problem of the Delta Barrier after ca. 1 m of sea level rise. Firstly, the barrier could be closed permanently as part of the "protected closed" strategy. Secondly, the dikes in the hinterland
could be strengthened, the bottom protection could be strengthened, the probability that the closure of gates fail could be decreased and the Tidal Lake water level could be increased during a non-closure event. Additionally, the Water Act allows for redistribution of failure probabilities over dike segments
and failure mechanisms. Changes in the closure regime could also prove beneficial. These solutions are part of the "protected open" strategy. Thirdly, inhabitants of the lower lying areas of the Netherlands could be stimulated to move to higher ground in the Netherlands as part of the "move along" strategy.
It should be noted that a combination of either three strategies is very much possible and perhaps to be preferred. The Delta21 project enables the adaptation of any climate adaptive pathway where especially the "protected closed" and "protected open" strategies seem in line with the ideology of Delta21. Whereas the solutions above are presented within the context of the Delta21 flood protection system and the Delta Barrier, the problem is not specific to the Delta Barrier alone and a solution placed within an integral flood protection strategy for the entire (South West) Netherlands might be preferred.

As the third step, a structural design is presented which satisfies all structural requirements. Firstly, it is determined that a prefab construction method suits the construction of the Delta Barrier best, where the piers are to be sunk on to a prepared bed and most of the assembly of the barrier is to take place from the water using specialised vessels and equipment. Secondly, the 40 m long and
10.4 m high steel gates (25 in total) are designed. Thirdly, all main elements of the concrete civil superstructure are designed consisting of the 25 top beams, 25 sill beams and the 26 piers. In general, all concrete elements are restrained from cracking and post-tensioned. Finally, the global stability of the Delta Barrier is verified, taking into account horizontal, vertical and rotational stability, uplift,
scour and internal erosion. A geometrically closed granular bottom protection is designed to tackle the latter two failure mechanisms. In a structural sense, the Delta Barrier is able to withstand a sea level rise of 3.81 m corresponding to the median projection of climate scenario SSP5-8.5 and has a design life of 200 years. The construction of the Delta Barrier will take ca. 5 years, is assumed to be
completed in 2050 and will cost between ca. 2.67 and 3.50 billion euro (price level 2022).

It is recommended to further investigate the ecological impact of the Delta Barrier. Furthermore, a detailed driving mechanisms design and as such a more profound estimate for the probability that the closure of the gates fail is to be recommended.