The challenge in sealing larger gaps with caissons lies in the diminishing flow area as more caissons are placed, resulting in significantly increased flow speeds (exceeding the aforementioned 2.5 m/s), complicating the final caisson's proper placement. This issue is addressed through the use of sluice caissons, essentially a box equipped with gates on one side. During installation, these gates are shut to maintain buoyancy, and the opposite side is sealed with wooden boards.
Once each caisson is positioned, the boards are removed, and the gates opened, allowing the tidal current to pass with minimal impedance. This approach ensures that the flow area doesn't drastically reduce, and flow velocities remain manageable, facilitating the placement of subsequent caissons. After all caissons are set, the gates are closed at slack water, completing the closure. Subsequently, sand is sprayed in front of the dam, and the gates along with other movable mechanisms are removed, available for reuse in future closures.
Sluice caissons were first employed in closing the Veerse Gat, and subsequently utilized at the Brouwersdam and the Volkerak. They were also deployed in the closure of the Lauwerszee.