The Severn Barrage is a big project planned for the Severn estuary between England and Wales. It is a massive project, and an overview of the history and plans are given here.
It is my personal opinion that with such a large resource it would be foolish not to utilise it to generate renewable electricity. The main disadvantage against a Severn Barrage would be the environmental impact on the tidal area. I feel that it is likely that without this scheme there will be much more damage to the environment from global warming. However, it is important that the changes to the tidal mudflats are minimised as much as possible. For this reason I feel that the best option would be for a barrage from near Brean Down to Lavernock Point in south Wales. However, it would be my own personal idea that the environmental impacts of a barrage could be minimilised by allowing the barrage to spend periods where it doesn't interfere with the natural flows of the river/tides. For example, it is likely that there will be times when the barrage would be able to generate maximum output at times in the middle of the night etc when there is minimum demand. As such at these times I would propose that no generation is allowed, and the river is allowed to flow naturally.
In addition, if the barrage is given the go-ahead I would like compensatory schemes elsewhere for birdlife. This could involve the use of land elsewhere which was previously mudflats, such as the Norfolk Broads, being allowed to revert back to their natural state.