The Afsluitdijk:

In 2006, an inspection found that the Afsluitdijk was in need of an overhaul. Under the Water Act, the Afsluitdijk must be able to withstand a storm with a probability of occurrence of 1/10,000 per year, which, for the Afsluitdijk, would result in waves with a height exceeding 4 metres. The dyke and sluices and locks did not meet this requirement. The Afsluitdijk should also be able to discharge more water from the IJsselmeer into the Wadden Sea in the future. Until then, discharge was only achieved by free-flow discharge through the sluices, but there is insufficient discharge time available in the (near) future due to rising sea levels to maintain the target IJsselmeer water level in winter. In addition to water safety and discharge, the motorway on the dyke no longer complied with the latest guidelines, and therefore its safety had to be improved.

From 2012, Witteveen+Bos handled the planning study for Rijkswaterstaat, which focused on improving water safety and increasing water discharge capacity. The signing of the zoning integration plan in May 2015 was a milestone after years of efforts in design, impact study, engagement and stakeholder management and contract preparation.

The Afsluitdijk is the 20^th-century symbol of the Dutch fight against water. All the measures necessary for discharging water and safety must make the Afsluitdijk a renewed hydraulic engineering icon of the 21^st century. This presents more than just a technical challenge. Such was the vision laid down by central government and the region in the Afsluitdijk Fidelity Master Plan. Intensive collaboration between central government and the region would result in an innovative, safe and well-integrated Afsluitdijk by the end of 2023. 

To ensure that the Afsluitdijk could meet water discharge and water safety requirements by the end of 2023, the planning process was accelerated. To avoid delays, Witteveen+Bos delivered the draft plan development design file two months earlier. In April 2015, the draft RIP (Zoning Integration Plan) was signed by the Minister of Infrastructure and the Environment. This paved the way for the launch of the tender procedure. The invitation to tender for the DBFM (Design Build Finance Maintain) contract was issued in late 2016. 

Prior to the tendering phase for the further development and construction, the Levvel construction consortium (BAM, Van Oord and Rebel), as one of the participating parties, asked Witteveen+Bos to participate as a subcontractor. In 2017, Witteveen+Bos contributed to the tender by providing knowledge from the planning study and expertise in various specialisations, such as structural hydraulic engineering, life extension of existing structures, water safety and water drainage. In early 2018, the Levvel consortium was commissioned by Rijkswaterstaat to start construction. On behalf of Levvel, Witteveen+Bos was part of the integrated design teams.

In the design and construction phase, Witteveen+Bos was able to contribute a lot of knowledge on subjects including:

• modelling water discharge and determining the pump deployment strategy
• hydraulic loads on the structures (wave impact)
• mechanical engineering design of new operating mechanisms in the existing structure
• design of the sliding mechanism (probabilistic assessment of the reliability of opening and closing), plus tribological research into innovative material in this application (D-Glide)
• factoring ecological factors into the design, including growth predictions in relation to maintenance
• ice load scenario analysis.

Rijkswaterstaat encouraged innovation within the project. Among other things, this resulted in designs for new revetments for the dyke. To verify the stability of these innovative revetments, sections of the dyke were simulated to scale (1:20 and 1:3) and subjected to real waves in a wave flume.

The renovation of the sluice gates meant guiding the wave load away from the sliders and other components. In view of the complex geometry of the sluice gates, this was impossible using existing design methods. To this end, parts of the sluice gates were also recreated on scale to measure the wave load on these components. Witteveen+Bos prepared and supervised these model tests.

After completion of the initial designs in 2019, the construction works on site were started. Working outside on the Afsluitdijk means working in a very restricted space; it is precision work that requires careful planning and coordination. The available space for the work to be performed in relation to daily functional use, natural values and safety were the biggest challenges. Strict tolerances and coordination on the reuse of materials also made for a fun and challenging puzzle. This makes that as an on-site supervisor of such a large project, you play a pivotal role.

The contractor has a contractual maintenance obligation for 25 years after completion. Witteveen+Bos is also involved in this maintenance phase. It must be demonstrated annually that the minimum strength as built and specified in the design is actually compliant. To this end, annual surveys are conducted, which are used to test the actual strength of the dyke against the minimum promised strength necessary under the Water Act. This includes monitoring the location of 75,000 innovative concrete Levvel blocks.

Since the Afsluitdijk is a barrier for fish migration, various innovative measures are devised to restore migration. One of these measures is the creation of a fish migration river at Kornwerderzand. In early 2023, it was announced that a consortium of Strootman Landscape Architects and Witteveen+Bos will create three design variants to improve the transition between salt and fresh water at Den Oever.

In all three variants, there will be a gradual transition between salt and fresh water, to prevent fresh water fish from accidentally swimming into the Wadden Sea from the IJsselmeer. The brackish water warns fresh water fish that they are approaching salt water. The gradual transition also helps migrating fish, such as salmon and eels. These animals live in the sea as well as in rivers and fresh water lakes. A gradual rather than direct transition between salt water and fresh water helps migratory fish find their route to the Wadden Sea. The aim is to prevent salt intrusion in the IJsselmeer to the extent possible to ensure the ecological values are preserved.