Instead of building a barrage across an estuary, it is theoretically possible to enclose an area of a tidal estuary or tidal region off the shoreline with an embankment or bund such as the St. Malo project discussed earlier so that it does not affect any part of the coast. The principle involved is the same, creating a reservoir that can be filled at high tide and then allowed to empty when the tide has fallen.
Although the St. Malo project never progressed beyond the design and planning stage, tidal lagoons of this type are being proposed again, together with what are proposed to be cost-effective ways of building them. Such designs have environmental advantages because they do not affect a tidal estuary or coastal land region itself. In addition, they can be built so that they do not obstruct waterways or shipping routes. Shallow tidal flats in areas of high tidal reach are judged to be the most economical sites for constructing such plants. There is some interest in such projects at the beginning of the second decade of the 21st century but no scheme of this type has so far been constructed.
TIDAL BARRAGE CONSTRUCTION TECHNIQUES
The construction of a tidal barrage represents the major cost of developing tidal power. As a result, much of the research work carried out into tidal power has focused on the most efficient way of building the barrage.
Construction of the French tidal power plant at La Rance was carried out behind temporary coffer dams, enabling the concrete structure to be built under dry conditions. While La Rance was completed successfully using this approach, the method is generally considered too expensive as a means of con- structing a tidal barrage today. There is also an environmental problem attached to completely sealing an estuary for the period of construction, which might easily stretch into years. For that reason, such an approach is unlikely to be adopted for the future.
A novel approach suggested for the construction of a barrage across the River Mersey in England borrows something from the construction of La Rance. The idea proposed here was to procure a pair of redundant bulk carriers (e.g., oil tankers) and sink them on the riverbed parallel to one another, sealing the ends and filling the enclosed space with sand to create an island. Concrete construction would be carried out on the island as if it were dry land. To create a watertight structure, diaphragm walls would be fabricated of reinforced concrete; the turbines and sluice gates required for the operation of the power station would subsequently be fitted to this concrete shell.
Once the first section of the barrage had been completed the bulk carriers would be refloated, moved along to the next section, and sunk again. This process would be repeated, until the barrage had been completed. The River Mer- sey barrage has not been built, so the efficacy of the method has yet to be tested.
Where an estuary is shallow, an embankment dam could be constructed instead of a concrete dam using sand and rock as its main components. Sand alone would not make a stable embankment; wave erosion would soon destroy it. Therefore, some form of rock reinforcement would be required on the sea- ward side. Concrete faces on both sides of the embankment could provide further protection. The sand needed for construction of such an embankment might be recovered from the estuary by dredging. Rock could also be removed from the riverbed by blasting or brought to the site from elsewhere. Rock is a more expensive construction material than sand so its use would have to be mini- mized to keep costs as low as possible.
While all these methods have their attractions, the construction method most likely to be used to build a large barrage today would involve prefabricated units called caissons. Made from steel or concrete, the caissons would be built in a shipyard and then towed to the barrage site where they would be sunk and fixed into position with rock anchors and ballast. Some caissons would be designed to hold turbines, others would be designed as sluice gates, and a third type would be blank. These would be placed between the other two types to complete the barrage.
Caisson construction was the favored approach in a study for construction of the Severn barrage in England completed in 1989 under the auspices of the Severn Barrage Development Project. A turbine caisson for this project would have weighed over 90,000 tons and would have had a draft of 22 m. The minimum height of the vertical faces would be 60 m. As a result of their size, special facilities would have been needed to construct them. Prefabrication of the caissons was expected to reduce construction time to a minimum. Even so, the Severn project was scheduled to take 10 years to complete.
For offshore lagoon construction, one company that is promoting such schemes proposes using a conventional rubble-mound breakwater. Barrage failure would have minimal safety consequences in an offshore lagoon because the project is self-contained, so this relatively cheap means of construction should be possible.