The two ball-joints are designed to ensure that the gates can move in all directions, both horizontally (when the gates are floated out) and vertically (upon submersion). In addition the gates must be able to ride up and down with the waves upon opening and closing. The only type of hinge capable of accommodating all these movements is a ball-joint. The ball-joint, which may be compared to a hip or shoulder-joint, must also be capable of transmitting the enormous pressure of the water on the gates to the foundation. In extreme weather conditions the retaining walls float out into the Nieuwe Waterweg, sink to the bottom and hold back the seawater. The water level on the seaward side is higher than that on the river side; the difference in force is so great that a ship of comparable dimensions would immediately capsize. The shape is particularly important. The final design of the retaining walls was arrived at experimentally. Keyword at the design stage: stability. The trusses, each 240 metres in length, are a prominent visual part of the Storm Surge Barrier. Their function is to transmit directly to the ball-joint the loads exerted on the retaining walls in the closure process. During closure the truss will ride up and by approximately 40 centimetres into a camber, levelling out again when the load eases. The sill structure on the bottom of the New Waterway has three functions: •The formation of a flat base and foundation for the retaining walls, which are positioned on the sill- blocks with the aid of fenders. •Cutting down the flow of water once the barrier is closed. •Holding in place the subgrade on which the blocks are based.   Upon closure of the barrier, operating equipment drives the retaining walls out of the parking dock to close the Nieuwe Waterweg. This equipment is operated from control buildings on the north and south sides. The equipment consists of three elements: the operating equipment of the dock-gate, the locomobile and the ballastsystem in the retaining wall. The decision as to whether or not to close the Storm Surge Barrier is made by a computer programme known as the Decision and Support System (BOS). The decision is left to computer as the risk of error would be many times greater if the decision were to be taken by human beings: a computer follows procedures, is not prompted by intuition and is not affected by bad environmental factors. In the event of a closure the BOS issues commands to the New Waterway Storm Surge Barrier Control System (BES), which then carries them out.