UPDATE 19.10.2010: The government has now dropped the Severn hydroelectric plans entirely. Shame they still didn't look at the lagoons version.
Quiz question: Which is Great Britain's longest river? Handy hint, it's not the Thames. In fact it is the Severn, which also happens to end at the place which has one of the largest tidal movements on our planet. Therefore, the UK government is considering to meet a large chunk of its renewable energy target by building a barrage across the Severn estuary. Environmentalists and especially Friends of the Earth are critical of the proposal and have suggested tidal lagoons instad, where a certain volume of the estuary is walled in but the rest is left open to the tides.
I have to say I was skeptical of the FoE idea at first (as it would take a lot more concrete to build a circular wall around a given water volume, compared with building just the base of a triangle), but the simple diagram on their web page won me over -- with a walled-off circle one could exploit the tidal cycle much more drastically than with a barrage, hence produce at least as much, if not more, power per mile of wall built, so it would be no more expensive and definitely greener than the barrage. But try to explain such complicated things to a UK government.
Read my story in Current Biology, Volume 19, Issue 5, R180-R181, 10 March 2009:
Muddying the waters