Construction of a barrage across a tidal river is bound to affect the conditions on both sides of the structure. Water movement patterns will be changed, sedimentation movement will be affected and the conditions at the margins of the estuary on both the landward and seaward side of the barrage will be altered. This could have a serious effect on marine and avian life.
The major effect of the barrage will be on water levels and water movement. Water levels will be altered on both sides of the barrage and the tidal reach may change behind the barrage, although the effect will be reduced as the distance from the barrage increases. Some areas which were regularly exposed at low tide will be continuously under water after the barrage is constructed. Though the volume of water flowing down the river should remain the same, patterns of movement will be changed.
Sedimentation will be affected in complex ways. The tidal waters of an estuary frequently bear a great deal of sediment. Some is brought in from the sea, some carried downstream by the river. Changes in current speeds and patterns caused by the interpolation of a barrage will affect the amount of sediment carried by the water and the pattern of its deposition. This will, in turn, affect the ecosystems that depend on the sediment.
Other areas of concern involve animal species. The effect on fish, particularly migratory species, is significant. Fish gates can be built to permit species to cross the barrage. Many can also pass through the sluice gates. However there is a danger that fish will pass through the turbines too, being injured in the process. Various methods have been explored to discourage fish from the vicinity of the turbines, with patchy success.
Many birds live on mud flats in estuaries. There is a possibility that such mud flats would disappear after a barrage had been built, and with them the birds whose habitat they formed. Salt marshes adjacent to estuaries are also likely to be affected. Studies have been conducted on potential UK barrage sites but much work remains to be done in this area.
Against these potentially adverse effects should be balanced with the absence of any emissions such as carbon dioxide, sulphur dioxide and the oxides of nitrogen. Unlike a traditional hydropower scheme, there is little possibility of generating methane within the reservoir of a tidal plant. A tidal plant is also a sustainable source of electricity.
Global experience with tidal power plants is limited. Nevertheless, and in spite of the caveats expressed above, the evidence available suggests that such projects need have no major detrimental effect on the environment. The evidence from La Rance, in particular, has provided no serious cause for alarm. Even so it would dangerous to make any assumptions. An extremely careful environmental impact assessment would form a vital part of any future tidal project.
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