Speakers' Corner: Geoff Collard, Green campaigner

THE most recent Severn Barrage design, unlike previous versions, will in theory be bi-directional. That is, it will generate electricity on both the ebb and flow tides.

In terms of energy generation, this is obviously better than one-way generation and was always one reason why other technologies to harness the Severn's tidal energy, such as tidal lagoons, are better than a barrage.

But the essential point is that a Severn Barrage is still in effect a dam across the Severn.

This will inevitably create problems for fish and other estuarial wildlife such as wading and migrating birds.

As has so often been pointed out by the RSPB, Friends of the Earth and others.

The barrage will also have important economic effects on shipping and the Bristol Port, as the company has very recently pointed out, not to mention the likelihood of serious flooding of the coastal and estuarial towns such as Weston, Gloucester and Cardiff.

There will be only one lock for ships to pass through in Peter Hain's design. What will happen if more than one ship needs access to or from the Bristol Port at the same time?

Result: interminable and costly delays to shipping.

Even if more than one lock were built, the time needed for even one ship to navigate a lock will be long and costly, as any owner of a canal boat will know.

Mr Hain's barrage would not be a small affair – the intention is to build a road across the top of it, creating yet more traffic and pollution.

We already have two multi-lane motorway road bridges over the Severn and a rail tunnel under it.

We do not need another road or rail traffic highway.

It will be expensive, and even if there is no direct public subsidy of the cost of building his barrage, it is likely that in order to generate profit for shareholders, the cost to the national grid of the energy produced, and therefore the cost to consumers, will be considerably higher than if it were built with public money as a publicly owned asset.

Mr Hain's suggestion that his barrage could be a flood defence against rising tides as a consequence of global warming ignores reality.

You cannot mix energy generation with flood defence systems: the two things are completely different.

If you shut the barrage because of rising sea levels to prevent flooding, energy generation will inevitably stop. Pointless.

You can't mix the two to get flood defence on the cheap. We may well need better flood defences in the Severn because of global warming, but this will have to be done by building a dedicated flood barrier, like the one in the Thames estuary.

It will need to be quite separate and independent of tidal power energy generation and only ever shut in case of one-off storm surge, or more importantly, in time of increasingly rising tides and sea levels.

Yet paradoxically, a Severn barrage, because it is a dam, will cause man-made flooding behind it when flooding due to natural causes is not even present.

It will also cause constant turbulence to the water and estuary bed, consequently filling the estuary sea with particles, blocking out sunlight and hence preventing the growth of small marine plants on which small wildlife and fish feed, leading to a 'dead' estuary.

This in turn will lead to wildlife devastation higher up the food chain.

There is no evidence that a Severn barrage will generate more energy than other much more environmentally friendly energy generation technologies such as tidal lagoons and fences.

The amount of electricity generated by any technology will obviously depend on the number and disposition of turbines.

The advantage of tidal lagoons and fences is that they can be placed at various points in the Severn estuary, and indeed added to in the future if required.

They can be mixed together as well, without affecting energy generation, because there is no damming effect.

So, in my view, tidal lagoons and fences are very small in their environmental impact compared with a barrage, are probably cheaper to build, and offer future "upgradeability", unlike a fixed barrage.

Unlike a fixed barrage tidal lagoons and fences will not impede shipping or the movement of fish and wildlife.

Windmills could be built on top of them to add to energy capacity, unlike the road proposed on top of a barrage.

Tidal reefs fall somewhere between the two, but in my view have not dissimilar disadvantages to a barrage and I do not think they are the answer.

I and other Greens want clean green energy generated from the Severn's tidal power, but a barrage across it is not the technology to use to do it.