Tidal barrages

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 focussed 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 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 constructing 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 was to procure a pair of redundant bulk carriers, oil tankers for example, 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.

Figure 9.1 Cross section of a typical tidal barrage

This process would be repeated, until the barrage had been completed. The Mersey 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 using sand and rock as its main components. Sand alone would not make a stable embankment; wave erosion would soon destroy it. Hence, some form of rock reinforcement would be required on the seaward side. Concrete faces on both sides of the embankment could provide further protection.

The sand needed for construction of such an embankment can easily be recovered from the estuary by dredging. Rock, which must generally be blasted from the riverbed, is a more expensive material and its use needs to be minimised.

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 favoured 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 tonnes and would have 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. There remained some uncertainty about how easy it would be to place the caissons in position, uncertainty that could only be dispelled by actual construction.

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