At TU Delft neighbour Deltares, the Ballast construction company is putting the finishing touches on the world’s largest wave channel. Last month, a group of science journalists were invited to take a peek.
No, there is no water in the 300-metre-long Delta channel, which is a pity for those hoping for a tsunami-like demonstration. Later on the empty tank turns out to be an advantage, as it allows us to get a much better idea of the size of the facility and the technology behind it. Visitors in the bottom of the channel make selfies in a site that will soon be covered by 9.5 metres of water. How cool is that?
‘Pick a pair of boots’. Substantive Project Manager Rob de Jong takes
10 visitors to the channel and ensures that they are outfitted according to
Ballast’s specifications. A short time later, the procession, kitted out in roomy boots, orange vests and yellow helmets, stomps its way through the halls of Deltares on the Rotterdamseweg – the site of the independent knowledge institution on water, subsoil and infrastructure. Once outside, the group follows De Jong to a white tent on the far left end of the concrete Delta channel. Having been instructed to proceed with care, they descend the steel ladder cautiously to just above the machine that will soon be causing the waves, which smells of oil and paint.
At one end of the footbridge are pressurised tanks with thick hoses connected to four horizontal cylinders.
Gleaming piston rods appear on the other side of the black cylinders, each mounted on the corner of a high bulkhead. From the bridge, visitors see a network of hoses, tubes and valves, which will soon control the cylinders. The four cylinders must be perfectly synchronised in order to hold the channel straight. This is no small job, as there will soon be around seven metres of water on the other side of the bulkhead. With a maximum speed of two metres per second, over a maximum length of seven metres, the bulkhead will set this mass of water in motion.
The static counter-pressure will be supplied by pressurised nitrogen; the dynamic pressure will come from a 1.9 megawatt hydraulic installation. ‘We will save energy by building up the pressure gradually’, explains De Jong, ‘and then using valves to transfer it to the cylinders. Because this will bring so much air into motion on this side, it will be impossible to close off the machine chamber. We will have to cover it with a sort of carport, so the air can escape. To absorb the blows, this part of the channel is outfitted with a 1.80-metre thick concrete floor, which will restrict the movement of the tank to one millimetre’.
Sealing is another area of concern: how can we keep it dry in here, with such a volume of raging water on the other side of the bulkhead? The exact solution is a trade secret of the American firm MTS, but it has something to do with inflated Teflon cushions gliding along the steel walls. The corners at the bottom are always the most difficult. They also cause the most leakage in the current channel in the Noordoostpolder. That is why in the new Delta channel they have a more rounded shape.
Longer and deeper
Even in this current age of enormous calculation capacity and advanced calculation models, practical tests are still needed, if only to validate the calculation models. This is one of the channel’s functions. The Deltares ecologist Minder de Vries mentions several others for testing at actual size: extreme conditions, the behaviour of clay, peat and sand mixtures (which cannot be calculated), the behaviour of natural materials (e.g. grass, willow and shellfish beds). These cannot be tested on a smaller scale.
Therefore, when it was time to replace the Delta channel at Marknesse (constructed in 1980), Deltares decided to expand the new channel and house it closer to the other laboratories. The Head of the Department of Hydraulic Engineering Structures, Dr Marcel van Gent, calculates that the new Delta channel will be able to generate 85% of the waves on the Dutch coast at actual size (this was 60% for the existing channel). This refers to a ‘significant wave height’ (crest-trough) of 2.2 metres, with a maximum wave height of 4.5 metres. Even higher waves, which are becoming increasingly rare, can reach the monstrous height of nine metres. These 15% outliers must thus be tested on a scale of 1:2. Although this is customary – water engineers know how to recalculate the experimental levels of mass, force, pressure and other factors to produce the actual values – it is undesirable, particularly for natural materials.
The size and the length of the channel have been scaled up in order to test soft sea walls. The popular principle of ‘building with nature’ requires testing of natural materials (e.g. willow and shellfish beds), accompanied slightly ascending gradients, which translate into greater length for the channel.
The decision to place the channel close by in the soft soil of Delft required 900 piles for the foundation, including tension piles to prevent the tank from drifting. In addition, 22 thousand cubic metres of concrete were poured, combined with 1.850 tons of reinforcement. The total costs of construction were around 25 million euro.
Large and expensive
Van Gent emphasises the relationships between the research facilities on the Deltares site. The channel is suitable
for research on two-dimensional sections.
The basins in the nearby hall are suitable for 3D research, albeit on a smaller scale. In the Delta basin (50m x 50m, 1m deep), waves crash back and forth through a scale model of the port of Costa Rica. The goal is to learn how best to protect the reclaimed land. In addition to the port, therefore, the ocean floor was also modelled.
In the nearby Pacific basin, a model of the new IJmuiden sea lock is being constructed. In this model, measurements of salt and fresh water are expected to identify the forces operating on the lock doors and on the seagoing vessels to be contained within the lock. They have also been modelled on a scale of 1:30. Through RWS, Deltares will deliver the results of the tests conducted here between November and January to the contractor combinations bidding on this project, which is valued at €850 million.
With a 40% share, the government remains the knowledge institution’s largest client. Projects also come from provincial and municipal governments, as well as from water boards and engineering firms, and Deltares receives 30% of its proceeds from foreign clients.
As an example of the types of experiments that will be conducted in the new channel, the ecologist De Vries mentions a test that he had previously conducted in the Delta channel at Marknesse. This involved sods being removed from a dike and placed in the wave channel in order to test its wave resistance. With eight weeks for construction and dismantling and one or two weeks of measurements, these were large, expensive experiments (costing €1–1.5 million). However, as De Vries cheerfully points out, it can make a big difference if the results indicate that the dike does not need to be reinforced or that it is resistant to overflow (water flowing over the dike).
On dry land
Project Manager De Jong guides the group over a concrete track to the other side of the channel. This track is strong enough to bear the weight of lorries carrying sand and stones, as those are required to create the set-up for the experiments. Under the track, 9.2 thousand cubic metres of drinking water is stored in a basin, to be pumped into the nearby channel for testing. Upon arriving at a scaffolding stairway at the testing building, De Jong says, ‘One or two people can go down to
the bottom’. A few moments later, everyone is down there. The measurements will take place in this shallow part. The part behind it, which ends in a wall of stacked stone blocks, is designed to allow the waves to play out. The visitors photograph themselves and each other. A nice thing to have for later, as soon everything will be under water.
View a test at the old Delta channel in Marknesse: youtu.be/t_p32yrLfcw