Special waterway transport: enormous concrete threshold for storm surge barrier from Kallo to Nieuwpoort

Jan De Nul says that within the next few days, a special transport will leave Kallo, in Port of Antwerp. A pontoon carrying an enormous concrete threshold. Final destination: Nieuwpoort harbour channel. This is a major component of the storm surge barrier currently under construction by the Agency for Maritime Services and Coast (MSC). This storm surge barrier will protect Nieuwpoort against extreme storm surges. Minister for Mobility and Public Works Lydia Peeters came to have a look at how this enormous threshold is being completed.

23.5 metre wide, 42.1 metre long and five metre high, that is the size of this concrete threshold that was under construction at Kallo over the past few months. This threshold will soon be installed in the harbour channel at Nieuwpoort between both abutments of the storm surge barrier. Later, it will hold the steel barrier in normal, open position.

Water transport
Getting the threshold from Kallo to Nieuwpoort can definitely be qualified as special transport. The threshold is built onto a pontoon that can be submerged. Two tugboats first pull the pontoon through the Scheldt, and then over sea to the harbour in Ostend. A trip that will last about twelve hours.

For the transport over the Wester Scheldt and over sea, several links in the nautical chain work together closely. The Common Nautical Authority (CAN) gives approval for the river leg, and the Maritime Rescue and Coordination Centre (MRCC) issues the permit for the sea leg. For both the part of the trip on the Scheldt and over sea up to Ostend, a pilot will be on-board.

Permits have been issued for this special transport, stipulating certain safety conditions.

Upon arrival in Ostend, everything preventing the threshold from shifting or tipping over during transport (seafastening) is removed. Then, the threshold is hooked onto a crane on a second pontoon, the Matador III. The concrete threshold weighs more than 4,500 tonnes. Too heavy to lift for the Matador III. If you still remember Archimedes’ Law, you will know that a submerged object is lighter to lift. Therefore, the pontoon will be submerged so that the threshold is entirely covered by water. The load to be lifted then ‘only’ weighs 1,210 tonnes. This operation takes 16 to 20 hours. A number of checks will still need to be carried out. Only then will the threshold, suspended from the crane on the Matador III, be ready for the last leg of the trip to Nieuwpoort. This journey takes approximately six hours.

Once the threshold has arrived in the harbour channel at Nieuwpoort, it is lowered between two abutments. This takes approximately 1 day. Finally, the threshold still needs to be anchored. For this, the harbour channel will be closed off completely for about 10 days. During the whole process in Nieuwpoort, divers are present to check that the positioning happens correctly.

Work sequence
Installing the concrete threshold is a major milestone in the construction of the storm surge barrier. The MSC Agency launched the works in the spring of 2018. In the Nieuwpoort harbour channel, both concrete abutments and guide walls were built and the trench for the laying of the threshold was excavated.

This autumn, construction works will start for the bypass drains. These are lateral pipes in each of the abutments. They will ensure that the spring tide flow rate never exceeds three knots. Then, the steel barrier still needs to be installed, as well as the mechanical parts and the fenders, and the service building will be installed on the abutment on the right bank.

The project should be completed in 2025.

The threshold in numbers:
Weight: 4,570 tonnes
Dimensions: 23 metre wide, 42.1 metre long, 5 metre high
1,732m³ concrete
572,450 kg reinforcement steel