A caisson closure involves sealing the gap with a caisson, essentially a large concrete box. This method was first applied in the Netherlands for mending dike breaches resulting from Allied assaults on Walcheren in 1944. The following year, at Rammekens, surplus caissons (Phoenix caissons) sourced from England, originally used for constructing the Mulberry harbours post-Normandy landings by Allied troops, were repurposed for dike repairs.
In the aftermath of the 1953 storm disaster, the closure of numerous breaches with caissons was contemplated. Given the uncertainty surrounding the final sizes of the gaps and the time-consuming nature of caisson construction, a decision was made shortly after February 1, 1953, to pre-fabricate a considerable quantity of relatively small caissons. These were strategically employed across various sites, and later, within the Delta Works.
A limited supply of larger Phoenix caissons from the Mulberry harbours was also utilized for sealing a few extensive dike breaches, notably at Ouwerkerk and Schelphoek.