Bubble screen separates salt and fresh water

Fresh water is precious, so contamination by salt water should be prevented as much as possible. Deltares, Royal Haskoning and Rijkswaterstaat have developed an innovative system that almost completely prevents the penetration of salt water through locks, allows ships to pass through the locks faster, and is also quite a lot cheaper to maintain and operate.

Dutch leisure vessels were introduced to the innovative fresh/salt barrier for the first time last summer in the Krammen locks in the province of Zeeland. It sometimes took some getting used to. The vertical currents generated by the bubble screen caused some turbulence and so sailing in and out of the locks demanded some steering skills. Even so, the inconvenience for individual sailors was more than made up for by the collective benefit of preventing salt water from entering the waters of the Volkerak through the lock.

Otto Weiler from Deltares explains: ‘If you do nothing and you open the lock gates on the salt side, a wave of salt water flows in at the bottom and the freshwater flows out along the top. Within about twenty or thirty minutes, almost all the fresh water in the lock chamber will have been replaced by salt water. When you open the gate on the other side, an enormous bubble of salt water flows out again. Slowly but surely, the salt content on the freshwater side starts to rise.’

LUCHTBELLENPLUIM STOPT ZOUT WATER

To stop that happening, Deltares teamed up with Royal HaskoningDHV and Rijkswaterstaat to design an improved bubble screen. A bubble screen consists of a curtain of air bubbles that generate a vertical current. Using screens like this to prevent salt intrusion is nothing new: they were already being used during the Delta Works, and they can still be found in the Terneuzen lock. The improvement is the design of the aeration heads, which have been positioned closer to one another over the full width. In addition, they produce exactly the right size of bubble (3 to 5 mm in diameter) to produce a vertically-flowing wall of water. This is fresh water, because lines and pumps are used to bring fresh water to the outer side of the line of aeration heads and produce a wide underwater fountain.

In addition to the bubble and water screen, fresh water is introduced into the lock chamber during the passage of ships by means of grooves in the lock gate, pushing back salt water. The fourth component in the innovative salt/fresh barrier is a movable threshold on the freshwater side to keep out the heavier salt water. In combination with an integrated control system that calculates the optimal lock-passage solution, this results in an almost impenetrable barrier.

Cost reduction

After testing in the Stevin locks in the Afsluitdijk Barrier Dam, the idea was to use the innovative barrier in the Krammen locks, which separate the salt waters of the Eastern Scheldt from the fresh water of the Volkerak and Zoommeer.  ‘In the meantime, those locks were urgently in need of major maintenance work’, recalls Otto Weiler. ‘Installing the new technology for the salt/fresh barrier gave us the opportunity to make considerable savings on that maintenance work.’

A pilot project was launched in one of the two smaller Krammen yacht locks in 2012. The introduction of the new system began on 25 April 2014. ‘We can now inform Rijkswaterstaat that the system is an effective salt/fresh barrier and that it can also be used in the larger locks for commercial navigation shipping. Furthermore, it can be used as an alternative for the major maintenance work on those locks.’

Suitable for worldwide application

The system can also be used elsewhere. The Netherlands has 23 locks that separate salt and fresh waters. The problem of saltwater intrusion is also an issue elsewhere in the world, where the system could be very useful indeed.