Closure of tidal inlets
In coastal and environmental engineering, the closure of tidal inlets entails the deliberate prevention of the entry of seawater into inland areas through the use of fill material and the construction of barriers.
The aim of such closures is usually to safeguard inland regions from flooding, thereby protecting ecological integrity and reducing potential harm to human settlements and agricultural areas.
However, the management of larger estuaries demands a sophisticated blend of technical expertise, encapsulating hydrodynamics, sediment transport, as well as mitigation of the potential ecological consequences of such interventions.
In situations where rivers and inlets pose significant flood risk across large areas, providing protection along the entire length of both banks can be prohibitively expensive.
In London, this issue has been addressed by construction of the Thames Barrier, which is only closed during forecasts of extreme water levels in the southern North Sea.
As the gap diminishes, significant differences in water levels between the sea and the inlet create very strong currents, potentially reaching several metres per second, through the remaining narrow opening.
[5][6] Secondary purposes, such as tidal energy generation, harbour and construction docks, dams for transportation infrastructure, and fish farming, also emerged but had lesser environmental impact.
[7] In contemporary times, driven by a growing emphasis on quality of life, particularly in industrialised nations, inlet closure projects encompass a broader spectrum of objectives.
These may include creating freshwater storage facilities, mitigating water pollution in designated zones, providing recreational amenities, and combating saltwater intrusion or groundwater contamination.
Consequently, measures such as bottom protection around the closing gap were implemented, guided primarily by experiential knowledge rather than precise calculations.
[12] 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 procedure for closing a tidal channel can generally be segmented into four phases: 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.
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.
For instance, while there were plans to completely dam the Leybucht near Greetsiel, only a minor portion was ultimately closed—just enough to meet safety and water management requirements.
[21] In the 1960s, South Korea faced a significant shortage of agricultural land, prompting plans for large reclamation projects, including the construction of closure dams.
Over time, attitudes towards closure works in South Korea evolved, leading to considerable delays and modifications in the plans for the Hwaong and Saemangeum projects.
However, due to the country's low labour costs and high unemployment rates, methods employing extensive local manpower were preferred.
Notably, during the Afsluitdijk closure, boulder clay was utilised in a manner akin to stone, which circumvented the need for costly imports of armourstone.
In the Netherlands, such a technique was applied during the closure of the dike around De Biesbosch polder in 1926, where a temporary bridge facilitated the dumping of materials into the gap using tipping carts propelled by a steam locomotive.
Constructing an auxiliary bridge for larger and deeper closing gaps can be exceedingly cumbersome, leading to the preference for cable cars in the Delta Works closures.
The first application of a cable car was for the northern gap of the Grevelingendam, serving as a trial to gather insights for subsequent larger closures like the Brouwershavense Gat and the Oosterschelde.
However, the system's loading capacity proved insufficient, prompting a switch to 1 m3 (2500 kg) concrete blocks for subsequent closures (Haringvliet and Brouwersdam).
Although planned for the Oosterschelde closure, a policy shift led to the construction of a storm surge barrier instead, foregoing the use of the cable car for this purpose.
[32] The production capacity, which includes a sufficiently large extraction site for the sand, must surpass the maximum anticipated loss during the closure operation.
The horizontal axis in the diagram represents the closing gap's size, indicating that the depicted capacity is insufficient for a sand closure under these conditions.
Caissons were initially utilized as an emergency response for sealing dike breaches post the Allied Battle of Walcheren in 1944 and subsequently after the 1953 North Sea flood.
Subsequently, sand is sprayed in front of the dam, and the gates along with other movable mechanisms are removed, available for reuse in future closures.[25]: p.
[45] The formula for basin storage then simplifies to: This methodology facilitates a reliable estimation of current velocities within the tidal inlet, essential for its eventual closure.
Termed the storage area approach, this technique provides a straightforward means to gauge local hydraulic conditions essential for barrier construction.
