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.
The complexity of inlet closure varies significantly with the size of the estuary involved. For smaller estuaries, which may naturally dry out at low tide, the process can be relatively straightforward. 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. The development of knowledge around such closures over time reflects a concerted effort to balance flood defence mechanisms with environmental stewardship, leading to the development of both traditional and technologically advanced solutions.
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. In the Netherlands, a number of inlets were closed by fully damming their entrances. Since such dams take many months or years to complete, water exchange between the sea and the inlet continues throughout the construction period. It is only during the final stages that the gap is sufficiently narrowed to limit this exchange, presenting unique construction challenges. 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.
Special techniques are required during this critical closure phase to prevent severe erosion of existing defences. Two primary methods are used: the abrupt or sudden closure method, which involves positioning prefabricated caissons during a brief period of slack water, and the gradual closure method, which involves progressively building up the last section of the dam, keeping the crest nearly horizontal to prevent strong currents and erosion along any specific section. Much engineering literature treats "closure works" as the construction phase that stops tidal exchange, distinct from the later conversion of the work into a permanent embankment or sea defence.
Purpose of a tidal inlet closure
The closure of tidal inlets serves various primary purposes:- Land reclamation
- Shortening sea defence length
- Creation of fresh water reservoirs
- Establishment of tidal energy basins
- Development of fixed-level harbour docks
- Construction of docks for marine activities
- Provision of road or rail connections
- Repair of breaches in dikes
- Creation of fish ponds.
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.
Side effects
Depending on circumstances, various hydrological, environmental, ecological, and economic side effects can be realised by the implementation of a tidal inlet closure, including:- change of tide at the seaward side of the dam
- change in bar and gully topography, outside the dam
- removal of tides on the inner side of the dam
- change in groundwater level in adjoining areas
- alteration of drainage capacity for adjoining areas
- loss of fish and vegetation species
- loss of breeding and feeding areas for water birds
- rotting processes during change in vegetation and fauna
- stratification of water quality in stagnant reservoir
- accumulation of sediments in the reservoir
- impact on facilities for shipping
- impact on recreation and leisure pursuits
- change in professional occupations
- social and cultural impacts.
Examples of closure works
Historical closures in the Netherlands
Several towns in the Netherlands bear names ending in "dam," indicating their origin at the site of a dam in a tidal river. Prominent examples include Amsterdam and Rotterdam. However, some locations, like Maasdam, have less clear origins. Maasdam, a village situated at the site of a dam on the Maas dating back to before 1300, was the site of the construction of the Grote Hollandse Waard, which was subsequently lost during the devastating St. Elizabeth's Flood of 1421. As a result of the flood, the Maas river is now located far from the village of Maasdam.One technique widely employed in historical closures was known as opzinken. This method involved sinking fascine mattresses, filling them with sand, and stabilising them with ballast stone. Successive sections were then sunk on top until the dam reached a height where no further mattresses could be placed. This process effectively reduced the flow, allowing the completion of the dam with sand and clay. For instance, the construction of the Sloedam in 1879, as part of the railway to Middelburg, utilised this technique.
Early observations revealed that during closures, the flow velocity within the closure gap increased, leading to soil erosion. Consequently, measures such as bottom protection around the closing gap were implemented, guided primarily by experiential knowledge rather than precise calculations. Until 1953, closing dike breaches in tidal areas posed challenges due to high current velocities. In such instances, new dikes were constructed further inland, albeit a lengthier process, to mitigate closure difficulties. An extreme example occurred after the devastating North Sea flood of 1953, necessitating the closure of breaches at Schelphoek, marking the last major closure in the Netherlands.
Modern dam construction in the Netherlands
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 and emptying of the basin. The speed of these currents is influenced by the tidal range, the tidal curve, the volume of the tidal basin, and the size of the flow profile at that location. The tidal range varies along the Dutch coast, being minimal near Den Helder and maximal off the coast of Zeeland, 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, 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.
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.
| Name | Length | Completion Year | Type of Dam | Closure Method |
| Zandkreek | 830 | 1960 | Secondary Dam | Two Closed Caissons |
| Veerse Gat | 2,800 | 1961 | Primary Dam | Sluice Caissons |
| Grevelingendam | 6,000 | 1965 | Compartmentalisation Dam | Cable Car |
| Volkerakdam | 5,000 | 1969 | Compartmentalisation Dam | Sluice Caisson |
| Brouwersdam | 6,000 | 1971 | Primary Dam | Sluice Caissons and Cable Car |
| Haringvlietdam | 5,000 | 1971 | Primary Dam and Discharge Sluice | Cable Car |
| Oesterdam | 10,500 | 1986 | Compartmentalisation Dam | Sand Closure |
| Philipsdam | 6,000 | 1987 | Compartmentalisation Dam | Sand Closure |