Environmental Engineering Reference
In-Depth Information
reversible turbines. There have also been proposals for double barrages, with segmented
basins allowing for pumping, using the power generated from the other basin, although that
would of course add to the cost (Elliott 2004).
Whatever the actual mode of operation, since the barrage would in effect block off an
entire estuary to natural tidal flows, it would have significant environmental impacts. For
example, in the case of the Severn, the decreased tidal range would reduce the area of
mudflats exposed each day, which would have major implications for wading birds. Some of
the impacts, however, might be positive: the reduced turbidity would mean that silt would fall
out of suspension and the clearer water would be more biologically productive, supporting
more, and possibly new, species. That could offset the loss of feeding areas due to the
reduced tidal range. However the impacts are complex and need detailed study, and so far
most local and national environmental groups have been strongly opposed the idea (FoE
2004).
A study in 2007 by the governments advisory Sustainable Development Commission
(SDC) nevertheless backed the Severn barrage concept, although with conditions, including
full compliance with European Directives on habitats and birds. It also felt that it should be
'publicly led' as a project and publicly owned as an asset, to ensure the long-term public
interest was protected. However it was left open as to who should pay- although a public-
private finance arrangement seems to be one possibility (SDC 2007).
Although, given their high capital costs and cyclic outputs, the economics of barrages
look problematic at present, they might be more attractive, as a low carbon energy source, if
there were significant energy storage facilities. For example, it has been suggested that the
electricity produced could be converted to hydrogen gas by electrolysis and this could be
stored for later use, either in a gas turbine or a fuel cell, for the generation of electricity when
needed.
There have been proposals for smaller barrages on the Severn and also on other estuaries
around the UK, including the Mersey. Being smaller, they would be easier to finance and,
depending on location, could have less environmental impact. However, while supporting
smaller barrages, some environmental groups also favour what they see as a less invasive
option than barrages- tidal lagoons
Tidal Lagoons
Whereas a barrage closes off a complete estuary, a bounded lagoon, built entirely out to
sea, would have much less impact on tidal flows. The US company Tidal Electric has
proposed lagoons at various sites around the world, including the UK. One proposed project
is a 60MW lagoon off Swansea Bay, in Wales. The company is also in discussion with the
Chinese government, which, it says, has expressed interest in a 300 MW offshore tidal lagoon
to be built near the mouth of the Yalu River.
Lagoons would be in relatively shallow water and would be constructed like causeways
with rock infill. As well as generating power, they might be seen as an offshore pumped
storage facility. Moreover, as with the double barrage idea, segmented lagoons could enable
phased operation and pumping between segments.
The disadvantage of lagoons is that the entire containment wall has to be constructed. By
contrast, with barrages, the estuary shore provides free containment for the bulk of the water
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