Like scribbling a note on the Mona Lisa. This is how Environment Agency chief executive Barbara Young described the environmental impact of the construction of a tidal power barrage across the river Seven estuary (NCE 8 November 2007).
Young's opinion that a £14bn structure between Cardiff and Weston-Super-Mare harnessing the tidal power that lies in the estuary is too environmentally expensive is one shared by most green groups.
The estuary’s 14m tidal range is the second highest in the world. This range is a potentially huge resource of tidal power.
But it also creates a natural phenomenon like the Severn bore – a wave which travels up the Severn during Spring tides – and has made a wetland habitat of mud flats and salt marshes, claimed by English Nature and the Royal Society for the Protection of Birds (RSPB) to be unique.
Home to a number of species of wading birds considered of international importance, the estuary is a designated Special Protection Area under the European Birds Directive.
The Department for the Environment Food and Rural Affairs (Defra) has also asked the European Commission to consider the Severn Estuary as a possible Special Area of Conservation under the European Habitats Directive.
The proposed 8.64GW Cardiff-Weston barrage is essentially a concrete dam stretching 10km across the estuary, with sluice gates allowing water to flow into the Severn’s tidal basin at high tide.
This water would be impounded until the tide on the coastal side of the barrage had reached its lowest point. Impounded water would then be released through turbines to generate electricity.
It is anticipated that construction could destroy up to 60% of the estuary’s inter-tidal habitat – 260ha according to Friends of the Earth (FoE) Wales – as it would be submerged in water held behind the barrage.
"At least three times the amount lost would need to be created in compensatory habitat in order to ensure the birds took to it and did not migrate elsewhere," says RSPB Wales environmental policy officer Peter Jones.
He was speaking at last week's ICE Wales Severn Barrage conference. Based on FoE Wales' figures, this would mean roughly 780ha of wetland habitat would have to be created.
"This could be the showstopper [for the Severn barrage]," says Sustainable Development Commission (SDC) commissioner for Wales Peter Davies, who also spoke at the ICE Wales conference.
Last year the SDC released a report concluding that a barrage was the best option for harnessing the tidal power in the Severn Estuary, pre-empting the Department for Business Enterprise and Regulatory Reform’s (BERR) two-year feasibility which began in February.
However, despite the SDC’s broad support for a barrage, Davies says the BERR study will need to answer three critical questions if it is to be able to justify the construction of a Cardiff-Weston barrage.
“Firstly, are there alternatives?" he asks.
"Secondly, is it in the overriding public interest? And thirdly is there compensation [for loss of habitat]?"
The first of these challenges looks as though it may be the easiest to answer: BERR has stated that it only wishes to look at renewable energy technologies that make use of the Severn’s tidal range.
This leaves the door open for only two established tidal power technologies: tidal lagoons and tidal barrages. A tidal lagoon is a circular concrete cofferdam which allows water in through sluice gates as the tide comes in. It then holds the water until low tide when it is released, generating electricity.
The SDC report concludes that tidal lagoons should be avoided as their effectiveness is unproven, but speaking at the ICE Wales conference, BERR Severn Tidal Power Feasibility Study deputy director Gary Shanahan said that no decision on the type of technology to be used had been made. But he later added that BERR wanted to avoid taking risks on emerging technologies.
"Additional risk will be factored into the cost, so you have to be careful how much innovation you put in," said Shanahan.
With the emphasis on proven technology, a barrage is looking increasingly likely. The Rance tidal barrage in Brittany, France has, after all been operating successfully since the 1960s.
Answering the question on alternatives therefore becomes a simple choice between the Cardiff-Weston barrage and the proposal for the smaller "Shoots" barrage further upstream.
This uses the same technology as the Cardiff-Weston proposal, but on a smaller scale. It would be located where the Severn is narrower and cost £1.65bn to build. Generation capacity would be 1.05GW.
Its impact on tidal levels downstream would be minimal, so environmental groups prefer it. But the Shoots proposal could be unstuck by another critical factor: is its construction in the overriding public interest?
In January, the European Union set Britain the target of generating 15% of its total energy demand – including transport and heating – from renewable energy sources by 2020, up from the current 2%.
To meet the target, it is anticipated that anything between 35% and 50% of the UK’s total electricity demand will have to come from renewables.
Given that the Cardiff-Weston barrage is projected to provide the equivalent of 5% of UK electricity demand, it looks likely it would meet the public interest criteria more comfortably than its smaller competitor.
But the issue then will be creating new habitats to compensate for those lost to the barrage. "It will be test three, on compensation, that will be difficult," says Davies.
It will be this question of compensation for lost habitats that will dominate the investigations of the consultant which carries out the Strategic Environmental Assessment contract – the first contract, shortly to be awarded, from BERR’s study.
Compensation on the scale being suggested is not without precedent: consultant Faber Maunsell was appointed in October to design a new 736ha wetland environment at Wallasea Island, Essex – the largest scheme of its type in Europe (NCE 9 October 2007). At £12M, the project, funded by the Royal Society for the Protection of Birds (RSPB), shows that a compensatory habitat for the Severn estuary would be cheap compared to the £14bn construction cost of the barrage.