In recent times, the construction of larger dams in the Netherlands has been driven by both the necessity to protect the hinterlands and the ambition to create new agricultural lands.
The formation of currents at the mouth of an inlet arises from the tidal actions of filling (high tide) and emptying (ebb tide) of the basin. The speed of these currents is influenced by the tidal range, the tidal curve, the volume of the tidal basin (also known as the storage area), and the size of the flow profile at that location. The tidal range varies along the Dutch coast, being minimal near Den Helder (about 1.5 metres) and maximal off the coast of Zeeland (2 to 3 metres), with the range expanding to 4 to 5 metres in the areas behind the Oosterschelde and Westerschelde.
In tidal basins with loosely packed seabeds, current channels emerge and may shift due to the constantly changing directions and speeds of currents. The strongest flows cause scour in the deepest channels, such as in the Oosterschelde where depths can reach up to 45 metres, while sandbanks form between these channels, occasionally becoming exposed at low tide.
The channel systems that naturally develop in tidal areas are generally in a state of approximate equilibrium, balancing flow velocity and the total flow profile. Conversely, when dike breaches are sealed, this equilibrium is often not yet achieved at the time of closure. For instance, rapid intervention in closing numerous breaches following the 1953 storm surge helped limit erosion. For the construction of a dam at the mouth of an inlet, activities are undertaken to reduce the flow profile, potentially leading to increased flow velocities and subsequent scouring unless pre-emptive measures are taken, such as reinforcing the beds and sides of channels with bottom protection. An exception occurs when the surface area of the tidal basin is preliminarily reduced by compartmentalisation dams.
The procedure for closing a tidal channel can generally be segmented into four phases:
# A preparatory phase with a slight reduction in the flow profile (to 80 to 90% of its original size), during which dam sections are constructed in shallow areas and soil protection is placed in the channels.
# A sill is then erected, serving as a foundation for the closing dike. This sill can help distribute the dike's pressure on the subsoil and/or act as a filter between the bottom protection and the closing structure. The closure gap at this stage must be wide enough to allow the ebb and flow currents to pass without damaging the sill and the protective measures.
# The actual closure, where the final gap is sealed.
# The final phase involves constructing the dike over and around the temporary dam.
Under specific circumstances, alternative construction methods may be applied; for instance, during a sand closure, dumping capacity is utilised in such a manner that more material is added per tide than can be removed by the current, typically negating the need for soil protection.
When the Zuiderzee was enclosed in 1932, it was still possible to manage the current with boulder clay, as the tidal difference there was only about 1 metre, preventing excessively high flow velocities in the closure gap that would require alternative materials. Numerous closure methods have been implemented in the Delta area, on both small and large scales, highly dependent on a variety of preconditions. These include hydraulic and soil mechanical prerequisites, as well as available resources such as materials, equipment, labour, finances, and expertise. Post-World War II, the experiences gained from dike repairs in Walcheren in 1945, the closure of the Brielse Maas in 1950, the Braakman in 1952, and the repair of the breaches after the 1953 storm surge significantly influenced the choice of closure methods for the first Delta dams.
Up until the completion of the Brouwersdam in 1971, the choice of closure method was almost entirely based on technical factors. However, environmental and fisheries considerations became equally vital in the selection of closure methods for the Markiezaatskade near Bergen op Zoom, the Philipsdam, Oesterdam, and the storm surge barrier in the Oosterschelde, taking into account factors like the timing of tidal organism mortality and salinity control during closures, which are critical for determining the initial conditions of the newly formed basin.