The Operating Principles of a Tidal Barrage
Once the site for the tidal barrier has been selected, an extensive Environmental Impact Statement must be prepared by the nominated contractor. This must take into account any effect on fish, waterfowl, and local flora and fauna. Once completed it is submitted to the relative government department and when approved, the massive civil works contract lasting up to five years can commence as noted below.
There are several proposed methods of construction, very briefly...
1. A foundation for the barrier is laid, usually from rocks from a local supply, then piles are driven in before concrete is poured to form the barrier. As it is being constructed, apertures are formed to accommodate the turbines and sluice gates along with a lock facility, if required, that will allow the passage of ships.
2. Foundations are laid then pontoons are constructed from concrete or steel incorporating the sluice gates, and shipping locks. The turbines are floated to the location and ballasted to sink onto the founds (var. "molds"). They are piled into the founds and joined to form the barrage and road/rail links are then laid atop.
The components of the barrage are as follows.
Sluice Gates – these allow the seawater to fill behind the barrage
Turbines – there are currently three types used to drive the power turbines.
Power Generators – these can be submerged or built above the turbines driven by a vertical or inclined shaft.
Lock Gate – some of the dissenters are against barrages because it stops the passage of ships up the river. A lock gate can facilitate this.
Basin – the area behind the barrier, this can prove a recreation facility for sailing, fishing and other water sports.
The incoming tide fills the basin, which is the area behind the barrage, through the sluice gates that are then closed at high tide. The sluice gates remain closed and once the ebb tide has receded below the barrier sluice gates, the turbine gates are opened. This allows the seawater to pass out through the turbines producing a large electrical output of power via the generators. This power generation can be extended by allowing the incoming tide to pass through the turbines as it fills up the basin on the flood tide, but this is both inefficient and uneconomical, and therefore most designs incorporate a one-way power generation system.
However, if the turbine can be reversed with the generator used as a drive motor, water can be pumped into a storage area using cheap electricity. This normally operates at night, and then used to generate power at high demand times charging premium rates.
The sketch below shows a section through the barrier turbine aperture, with a bulb turbine installed.