By closing off a series of broad, long, interconnected inlets that penetrate the area, runoff from the Rhine and its distributaries was channeled so that the estuaries were converted into three basins: fresh tidal water in the north, salt water tidal in the Oosterschelde, and salt water non-tidal in the remaining two arms. Four smaller basins, totaling 2,585 acres and holding 120,000-acre-ft, were built as reserves of drinking water, supplying 132 billion gallons annually. The project also shortened the coastline by 430 miles, rendering 440 miles of dikes obsolete, and providing a greater degree of security to the residents of the affected islands, as well as preventing existing farmland from being contaminated by salt water.

The Volkerak Works, consisting of a 2.8-mi-long dam, a 2.5-mi lock complex and a 0.75-mile bridge, closed off an inland waterway that connected two deep sea inlets. The core of the Volkerak Dam was made with floating concrete caissons, 149 ft long x 49 ft wide x 43.5 ft high, cast in a nearby dewatered building pit. The dam reduced tidal current velocities in four surrounding estuaries to allow subsequent construction of the primary dams. By modifying the unpredictable tidal currents, it also improved shipping. Begun in 1957, it was completed in 1969, with an additional set of locks added by 1977. One of the busiest waterways in Europe, its locks were handling 150,000 ships per year in 1969.

The 3-mi-long northernmost structure, Haringvliet Dam, features a series of 17 sluices and a shipping lock. The sluices let in salt water to prevent freezing of the Meuse and Rhine Rivers, and also can drain the rivers in case of flooding. Work on the Haringvliet Dam began in 1956, with large willow fascine mattresses placed in the 75-ft-deep channel. An artificial work island was constructed using caissons where the sluice gates were built. Each gate has two sluices including a seaward-facing one designed to break the force of the waves. Next a cableway was erected, and it was used to ferry 93,000 huge concrete blocks and dump them into the channel to form the core of the dam. Finally the remainder of the dam was built with sand and stone, and completed in 1970.

Next came the 3.7-mi-long Brouwershavensche Dam between Schouwen and Goeree Islands, across a 100-ft-deep channel. It was also built using mattresses. The dam’s sand core was placed hydraulically and in tidal regions placed between quarrystone quays. To protect the embankment toes from erosion, concrete sheet piles were jetted into position and augmented with stones caged in gabions. On the seaward side a 20-ft-wide layer of bituminous concrete stabilized the dam against wave action. The dam profile was designed with a low crest by adding an 82-ft-wide, almost horizontal outer berm to break storm-driven waves. A cableway was used to dump 260,000 2.5-ton concrete cubes to close off the southern section. The northern section was closed off with 14 concrete caissons, each measuring 223 x 59 x 53 ft and weighing 8,000 tons. Construction lasted from 1962 to 1971.

The last and most difficult of the Delta Works components was the Oosterscheldekering, a 5.6-mile-long series of dams and storm surge barriers across three channels up to 130 ft deep. A preliminary stage involved shallower portions of the estuary being sealed off by creating three work islands and a 2.3-mi, 120-ft-high sand dike.

Building Barriers

Originally conceived as a dam, environmentalists feared that closing the estuary would produce a brackish cesspool, killing its oyster and mussel beds and wildlife habitats and creating adverse tidal action at nearby beaches. After four years of study and debate it was decided that the centerpiece would be a 3.5-mi-long surge barrier. The barriers, split between the three channels, feature a total of 66 massive concrete piers, each weighing 18,000 tons and spaced 148 ft apart, supporting 63 steel gates. Dosbouw, a consortium of 11 Dutch firms, was the contractor.

Construction of the barriers kicked off in 1976, with dredges carving a 265-ft-wide trench varying from one to 105 ft deep. Then a specially-designed “deep densification pontoon” vessel, the Mytilus, moved into position—a 220 x 112 ft barge carrying four 44-ton compaction needles suspended from a 150-ft scaffold, with a 100-ft-long horizontal steel truss lowered to the bottom for support. The 138-ft-long steel pipe needles, each tipped with a finned bit, were driven into the sandy bottom with 200-hp vibrators, compacting the sand 50 ft deep. The compaction work took four years.

Two mats were placed under each pier to protect the bottom and keep the piers level. The first mat, 15-in.-thick, contains three layers—sand, fine gravel and coarse gravel. The top mat, also 15-in.-thick, contains three levels of coarse gravel. The mats were fabricated using 12-ft-wide rolls of wire-reinforced fabric stitched together and fed into chain conveyors, aggregate placed on them with wire baskets at intervals to prevent shifting, and steel pins installed to seal the layers together. A special barge, the Cardium, was built to place the 5,500-ton mats. Its 65-ft-dia reel suffered a series of equipment teething problems, leading to months of delays, but succeeded in unrolling the 165-ft wide, 660-ft-long mat sections into place. Additional nylon mats extend almost 2,000 ft on each side of the barrier, held in place by blocks of concrete. Foundation work wrapped up in 1984.

The monolithic piers were prefabricated in a diked work pit in the estuary, and after each one was completed the pit was flooded and an armada of vessels towed it into place over a 72-hour journey. Setting the piers into position required two special vessels working in tandem. The Oostrea floated over a sunken monolith, picked it up using grips hung from two gantry cranes and took it to the site. Then the Macoma, anchored in place at low tide, dredged away sand from the mat. Then the two vessels locked together and slowly lowered the pier into position within very precise parameters. Layers of progressively larger stones were then piled around each structure, and the piers were connected with concrete box beams at the top and bottom of the gate openings. Fabrication and installation of the piers took almost four years. The world’s largest movable flood barrier was finished in 1986, and has been closed 28 times since then.

The entire Delta Works program, including the raising of dikes along rivers and estuaries, totaled $7 billion, of which the Oosterscheldekering accounted for $2.4 billion.

The Delta Works scheme always intended for the New Waterway, the ship canal forming the northernmost arm of the delta, to remain undammed, as it handles ship traffic to Rotterdam, the world’s largest port by tonnage from 1962 to 2004. So to protect Rotterdam from storm surges, Rijkswaterstaat conceived a giant floating floodgate. Its two floating radial box girder structures lodge within dry docks in the canal banks when not in use, and constitute one of the largest floating structures in the world.

Each hollow steel gate is 690 ft wide, 50 ft tall and 72 ft deep. Shaped like curved ship hulls, each gate weighs 15,000 metric tons when empty. They are held by 750-ft-long steel trusses, each having three tubular longitudinal chords up to 6-ft-dia, connected to ball-and-socket joints embedded in 100,000-ton concrete and sand anchorage. When dangerous tides loom, the gates float out of their docks, closing off the 1,200-ft-wide channel. They are then flooded and sink 55 ft, resting on concrete sills in the canal bed. Construction started in 1991 and was completed in 1997. The gates cost $450 million. It has been closed twice, once in 2007 and again in 2023.