Debate about the Severn Barrage, the vast energy generating project, has surfaced in the Green Party in the South West and South Wales. The majority are against it, because it is Big, will involve deomlition of a few mountains, and will threaten an internationally important Ramsar wildlife site.
There is a reasonably good summary of the issues on Wikipedia.
The whole issue it a complex system of advantages and disadvantages. Starting at the relavively trivial, I have ridden the Severn Bore in an inflatable (very interesting). The Barrage will diminish the Bore severely; but it will clarify the water of the Severn Estuary (nice) but will therefore tend to silt up (need siltation pans and organic farming exclusively in Severn Estuary); it will reduce the mudflats used by migrating birds, but they will go anyway with sea level rise, and we could protect them; there are, in short, lots of pluses and minuses, but in the end the Big Plus is that it will keep the flood waters from rising sea levels out of Bristol, Cardiff, Newport, Chepstow, Lydney and Gloucester would be protected by the Barrage, not to mention the rich agricultural lands bordering the Severn. It is clearly more economic to protect the area with one Barrage, rather than trying to trace out the whole coastline with banks dykes and bunds.
In protecting the land east of the barrage, it will worsen flooding west of the barrage.
Unfortunately, when the barrage is functioning as a sea wall, it will no longer act as an energy generator - apart from the wind trubines, current turbines and wave machines strung along its length.
As for the rest of the South West, they will benefit because if Bristol goes under a lot of Bristolians will come and want to live in there. Which will mean lots of new house building on green field sites, which is a Bad Thing.
On http://flood.firetree.net/ you can see the effects of sea level rise on our area. Serious changes occur at 4-5m, affecting mainly the low lying agricultural land.
Small is beautiful, but this does not mean necessarily that Big is always Ugly. We are confronting a Big Thing in Global Warming, and sometimes big responses are appropriate for big threats.