The construction
Dredging
After the disaster, the building of the barrier was sped up. Preparations started less than a year later, on January 18, 1954. The barrier would be built on the Hollandse IJssel estuary in the Nieuwe Maas (New Maas), between Krimpen aan de IJssel and Capelle aan de IJssel. At this point, the river is only 250 metres wide. A lot had to happen before they could actually start constructing the barrier. Firstly, the hindrance on shipping traffic had to be kept as small as possible. On average, 250 ships passed through each day. By dredging a part of the forelands, ships were able to sail without being hindered, and at the same time, work on the river could continue. Secondly, various clay layers had to be removed, because they were too weak to build a barrier upon. The clay was dug up and replaced with sand. The suction ship ‘Ahoy’ sucked up the sand from the Nieuwe Maas near Vlaardingen. The sand was transported via ships to Capelle where it was discharged by two floating transport cranes. The Nieuwe Waterweg needed to be dredged anyway, so by replacing the clay with sand, they reached two goals.
Dam walls
In July 1954, they finished the dredging operations and they started driving the steel dam walls. On the inside of the dam walls they were able to start pumping away the water. 100 kilometres of dam walls were used to build this excavation. A large majority of the work wasn't done above, but under the water. The dam works by lowering down 80 metre wide metal screens. However, without proper precautions, these screens would be too heavy to be supported by the riverbed.
Thresholds
Therefore, a kind of threshold was built to carry the dam walls. A dry dock of 80 metres wide was needed, because the threshold had to have the same length as the screens. This appeared to be impossible, because a large part of the Hollandse IJssel would have to be closed for shipping traffic. Therefore, two enormous thresholds were built in parts. Every threshold consisted of two parts with a width of 27.5 metres each. These parts were placed in the middle of the river. Afterwards, by linking the parts together with two extra parts of 12.5 metres, a connection with the banks was made.
The thresholds were built with a special shape, because of the risk that the surrounding ground could be washed away. But unfortunately, due to this special shape, the stability of the thresholds was in danger. As the screens are lowered, the waterflow is increased because the area the water has to flow through is decreased. Immediately before the screens are completely down, the flow is at its fastest point. It is this high water flow that could wash away the ground.
The Screen lock
In the second phase of the project, a lock was built at the side of Kapelle aan de IJssel. Right next to the storm surge barrier, they worked on a 24 metre wide and 120 metre long lock for shipping traffic. The doors of this lock would weigh 60 tons. The foundations for the lift towers and the bridgeheads (the part that connects the bridge to the bank) were also laid. However, the riverbed was again too weak, so they had to drive piles again. In total, 33 kilometres of piles were rammed into the ground. If many people had lived in the area, they would probably have complained about all the noise...
Screens
While the work on the thresholds, the screen lock and the bridgeheads continued, the screens were also being made. These screens were 81.2 metres long and 11.5 metres high. The screens were made in parts, just like the thresholds, and connected later on. Because the screens weighed over 635 tons, they could not be moved easily. For raising and letting down the screens, a so-called counterweight was needed. This is a weight that compensates the weight of the screens. The contra-weight weighed 460 tons. This reason this was less than the locks was to guarantee that the screens would fully fall to the bottom. The screens were moved by electromotors in the towers. By means of its own generator, the Hollandse IJssel would always be sure of energy, even in the case of power failure. The risk that the barrier would not close had to be excluded. For this reason, they only worked on one screen at a time. Before they started working on the second screen, the western screen was completed. The hinterland would still be protected in the case of spring tide. Eventually, the standby screen was placed between the two rear towers in 1977.
Towers
The towers carrying the screens were no less than 45 metres high. After all, ships had to be able to sail under the slides. The towers ‘grew’ about three metres every week, until the construction of the slides was started in the autumn of 1957. Initially the mechanism for the slides was installed, followed by the lock doors, the movable bridge, the regular bridge and eventually, a few days before Christmas, the first steel storm flood screen was put into place. From that moment on, the Hollandse IJssel dam would protect a large part of South-Holland against the rising water and provide a better connection with the islands of Zeeland.
