The Eastern Scheldt Barrier put to the test: what can a scale model teach us?

“When you mention at a birthday party that you’re working on the Eastern Scheldt storm surge barrier, you immediately have everyone’s attention,” says researcher Martijn de Jong of Deltares, visibly enthusiastic. “That just goes to show what an icon this is.” Jesse Simonse, project leader at Rijkswaterstaat, also feels that significance. “I’m from Zeeland,” he explains. “If you’d told me during my studies that I’d be working on a scale model of the Eastern Scheldt storm surge barrier, I wouldn’t have believed you.”

The Eastern Scheldt Storm Surge Barrier was built between 1976 and 1986 and designed for a lifespan of around two hundred years. After forty years of use, some components are nearing the end of their technical lifespan. For example, the mechanisms that open and close the enormous steel gates are being renewed. The side stops also need replacing. These are rubber buffers that absorb lateral movements of the gates when they close. The replacement or renovation of the 62 large gates is currently scheduled for after 2060.

Gaining a better understanding of the original design

At the same time, safety requirements have changed and rising sea levels are playing an increasingly significant role. This calls for detailed knowledge of the forces actually acting on the barrier. “Recreating the barrier is a form of reverse engineering,” says De Jong. “You work backwards: what exactly can it do, based on today’s insights?” Normally, a scale model is built before anything is constructed. Now the reverse happened: an existing structure was re-examined in detail. Not because there are doubts about the design, De Jong emphasises, but to understand it even better.

At Deltares in Delft, part of the barrier was replicated, including sluice gates, piers and traffic tunnels. In two large test basins, researchers were able to simulate various conditions: varying water levels, strong currents and waves from different directions. In doing so, they went beyond the storm for which the barrier was originally designed. In some tests, waves several metres high were simulated – the sort that statistically occur only once every 300,000 years.

Extreme conditions are particularly instructive. It is precisely such extreme scenarios that are of interest. They reveal the limits of the structure and which components are subjected to heavy loads first. “In the lab, you can say: I’ll run the storm for which the barrier was designed,” says De Jong. “And I’ll run another one that’s three times as severe.”

More than a hundred measurement signals recorded forces on the gates, cylinder rods and other components. The scale model provided new insights, particularly regarding waves approaching the barrier at an angle. In that situation, a lot happens simultaneously. Waves crash against the piers, bounce back and influence one another. This creates a complex pattern of currents and pressure on the sluice gates. “That longitudinal direction is even more complex than the perpendicular direction,” says De Jong. “Now we have a model in which all those interactions are represented very accurately.” Loads sometimes appear lower than expected, because these oblique waves are difficult to calculate, engineers in earlier computational models relied on conservative assumptions. The scale model tests now provide a clearer picture of how the forces are actually distributed. As a result, the loads on some components appear to be lower than previously assumed. This applies, for example, to the side stops and the cylinder rods that set the floodgates in motion. Such insights help to better determine where maintenance is genuinely required and how this can be carried out as efficiently as possible.

During the tests, the researchers also observed that, under extreme conditions, waves can strike the traffic tunnel – the part of the flood barrier over which the road runs. The original design took relatively little account of this, as waves can only reach the traffic tunnel under very extreme conditions. According to Simonse, this is not a cause for concern in the current situation, as the measured forces remain limited without sea-level rise and the structure is capable of withstanding them. “But it does provide additional information for future maintenance,” he says.

Original design is rock-solid

What is the main takeaway from the research? “It helps us to make the right investments,” says Simonse. “We were operating within a range of forces. Now we can assess this interplay of forces much more accurately.” The sluice gates remain the structurally critical component of the barrier and are currently scheduled for replacement or renovation after 2060. “With the results of the research, we can better estimate the remaining service life of the sluice gates. This follow-up research will be carried out in the coming years.” With the knowledge we have now, perhaps more attention would be paid to ease of maintenance, but the design still stands proud. Both Simonse and De Jong are full of praise and believe that the designers have delivered a ‘masterstroke’ in creating this. If the barrier had to be redesigned today, they would not do it radically differently. You always design for a certain level of safety.

You don’t design to infinity, because that’s impossible to build and unaffordable.

Martijn de Jong, researcher at Deltares.

Multiple generations involved

Engineers who were involved in the design in the early years came to watch the tests. “This may well be the last opportunity we had to actively involve that generation,” says de Jong. “That they could see what we are doing with their barrier. That felt special.” Simonse adds: “A structure like this might last two hundred years. That means several generations will work on it. What we are doing now is adding a new building block.”