A tidal barrage is designed to extract energy from the rise and fall of the tides and is relatively simple in concept. Thus a tidal barrage has only four main components: turbines, located in water passages which are designed to convert the potential energy of the difference in water levels across the barrage into kinetic energy in the form of fast-moving water. This kinetic energy is then converted into rotational energy by the blades of the turbines and then into electricity by generators driven by the turbines. Openings fitted with control gates, called sluices, which are designed to pass large flows under modest differential heads. These have a dual role. During construction, they allow the tides to continue to flow into and out of the basin behind the barrage with relatively little obstruction and thus enable the last parts of the barrage to be built without undue difficulty. Once the barrage is in operation, they refill the basin (or empty it) ready for the next power generation period. Locks or similar apparatus, to enable ships or boats to pass safely across the barrage after it is complete, and to pass safely through a part complete barrage where the remaining openings would have relatively fast-flowing water and/or construction activities in progress. Embankments, or else simple concrete caissons, which fill the remaining gaps across the estuary. These have to be reasonably 'opaque' to water flow so that water and energy are not wasted. They also provide a route to the working parts of the barrage for operation and maintenance staff and equipment, and a safe route for power cables from the barrage to the shore.

'Caisson' is the name given to a prefabricated structure which is floated into position. Since the early days of civil engineering, caissons have been widely used for forming bridge piers, a concrete or steel box being lowered onto the river bed and the base sealed into the ground, generally by skilled labourers working inside in compressed air.

Only in the exceptional circumstances of a narrow, steep-sided site for a barrage would the 'working' components of a barrage, namely the turbines in their power house, the sluices and the ship lock, occupy the full width of the estuary. The remaining gaps would have to be closed by embankments or plain, i.e. non working, caissons. This chapter considers both options. Except where access from one bank of the estuary has to be provided early in the construction programme, the non-working parts are the least cost items and should be least sensitive to tidal currents during construction. Thus they would normally be built when the expensive turbine caissons and sluices had been placed. This aspect, loosely referred to as the 'closure' of the estuary, is discussed in Chapter 8.

Before the start of the 1978-81 studies of the Severn barrage, there was concern as to whether the completion of the final stages of construction of a Severn barrage would be feasible because of the currents to be expected in the final gap in a barrage across an estuary with a large tidal range. Consequently, the UK Department of Energy appointed the Netherlands firm of consulting engineers, NEDECO, to carry out a brief study of this aspect in the light of their experience in the Delta flood prevention works, where a number of tidal estuaries had been closed with barriers designed to prevent a repetition of the disastrous flooding which occurred as a result of the exceptionally high tide in January 1953 (Ref. 1978(1)).

A tidal power scheme will reduce the tidal range in the basin it encloses and will therefore reduce the volume of water entering and leaving the basin during each tide, the tidal 'cubature'. These fundamental changes will have quite profound effects on the environment in its broadest sense. Predicting the changes in tidal range is relatively straightforward. Predicting the consequential changes in water quality and then on through the ecosystem, and then assessing their importance, is much more difficult. Separately, assessing the effects of building a barrage and its subsequent operation on the human environment is largely a qualitative exercise. In this chapter, an attempt is made to describe the various changes and set them in perspective.

This chapter summarises the locations around the world where the combination of large tidal range and suitably indented coastline are of interest for tidal power development.