Whichever, if any, tidal scheme is built on the Severn, it is sure to anger some environmentalists. Being a renewable source of electricity, tidal generators might be assumed to be popular with the green lobby. Yet there are serious reservations over the environmental costs of a barrage or lagoon in the estuary — and they have split the environmentalist movement.

On the one hand there is the appeal of doing something positive about climate change by turning to a renewable, rather than burnable, source of energy. Environmental activists have been urging governments, power companies and the public to embrace renewable energy because it is cleaner than fossil fuels and nuclear power.

On the other hand, thousands of hectares of shoreline will be destroyed as a feeding ground for birds — an internationally important feeding ground, no less.

There are also deep concerns about the impact on the fish and invertebrates in the Severn. Barrages and, to a lesser extent, lagoons form a physical barrier to species such as salmon and eels as they migrate.

The dilemma is balancing the potential damage to habitat against the gains made in combating climate change.

If measures such as the Cardiff-Weston barrage are not taken, how much of the river will be claimed anyway by sea-level rises from melting ice caps and how many creatures will be forced to find somewhere else to live because temperatures have become unbearable?

Some of the projects that missed the shortlist are regarded as having less of an impact on the environment but they are the most unproven schemes and, however attractive their merits, their effectiveness is questionable.

When coming to their decision on tidal schemes for the Severn — and perhaps one day the Mersey, the Wyre and the Thames — ministers will have plenty of factors to weigh up.

There will be the jobs created — the bigger the scheme the bigger the job creation prospects — and there will be the economic damage caused by limiting navigation of the Severn and access to upstream ports.

There will be the attraction of plumping for a huge barrage that will be a monument to their tenure in office, to be set against the affordability of constructing such an edifice.

But most of all they will have to judge whether the wider environment will best be served by sacrifice or preservation.

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