Severn barrage study unleashes tide of paper

The government has shortlisted five tidal barrage schemes for the Severn estuary, as it continues consulting on what would be Britain’s biggest, most expensive renewable energy project.

Its latest set of documents - running to nearly 3,000 pages - highlight the huge environmental impacts of barrage schemes and the uncertainties about compliance with EU nature conservation law.¹ Yet a scheme could greatly reduce UK greenhouse gas emissions.

The consultation shows a large investment in compensation and mitigation for damage to habitats and species would form a significant part of the total cost of any scheme.

However, the Department of Energy and Climate Change (DECC) has also kept the door open for novel types of tidal energy schemes in the estuary that should be less destructive. Energy and Climate Change Secretary Ed Miliband announced £500,000 to fund development work on tidal reefs and tidal fences.

Tidal barrages are essentially long, low hydroelectric dams that hold back huge volumes of water on the outgoing tide. They reduce the area of mudflats exposed at low tide, where tens of thousands of wading birds feed. For the biggest barrage on the shortlist, running from Cardiff to Weston-super-Mare, the government’s consultants estimate 200 square kilometres of mudflats would be lost.

Barrages also kill fish swimming past their turbines and interfere with shipping. By altering the flow of the several billion tonnes of water washing up and down the estuary twice a day, they affect sediment deposition, which would in turn affect water quality, marine life and the estuary’s shape.

A tidal fence is a long array of underwater turbines across most of the estuary. Tidal currents sweeping up and down the estuary turn the turbines, but there is no difference in water levels on either side of fence and ships can pass through gaps in it.

A tidal reef acts like a barrage but operates with a much lower difference in water level, of only two metres, on either side of the structure. As a result, there is far less change to the natural tidal range on the barrage’s landward side. Furthermore, the water velocities and turbine rotation speeds are much lower than in a more conventional barrage and so are less likely to kill fish.

The feasibility study considered a tidal reef and a tidal fence stretching between Minehead, Somerset and Aberthaw on the South Wales coast. This is longer and further to the west than the largest of the shortlisted barrage schemes.

The government has recognised that tidal reefs and fences need more research before they can be fairly assessed against the range of barrage schemes on the shortlist. Too little is known about their cost and engineering feasibility.

Three barrages running across the estuary are on the shortlist: the 10-mile-long Cardiff to Weston-super-Mare barrage, the 5.5-mile Shoots Barrage downstream of the newer Severn motorway crossing and the one-mile Beachley Barrage just above the River Wye’s mouth and next to the original Severn Estuary suspension bridge.

Estimated peak electricity outputs are 8,600 megawatts, 1,050MW and 625MW respectively. Construction cost estimates range from £21 billion for the largest, which could generate nearly 5% of UK electricity, to £2.3 billion for the smallest.

The other two shortlisted schemes are tidal lagoons, in which a long embankment runs out from the shore and comes back to land miles away, impounding huge areas of mudflat exposed at low tide. The rising water flows into the enclosed area through sluices, which shut when the tide falls and forces the water to exit through turbines. The two lagoons - one proposed for the Welsh side of the estuary, the other for the English bank - are each estimated to have a peak output of 1.36 gigawatts and construction costs of about £4 billion.

The shortlisted projects were chosen from a list of ten schemes drawn up following the government’s public call for proposals in May 2008. The notion of an electricity-generating Severn barrage across the Severn estuary, which has one of the world’s highest tidal ranges, is nothing new. The last time the government seriously studied the idea was more than 20 years ago. The main reason it was not built then was the high costs.

Among the schemes not shortlisted were two longer barrages, further west down the estuary than the Cardiff to Weston scheme, and offshore tidal lagoons, which are circular embankments not connected to the land. Costs per unit of electricity generated and scale of environmental effects were factors in choosing the shortlist.

Despite having drafted a shortlist, the government is still consulting on all ten schemes. Its new interim consultation document seeks views on which of the ten should be shortlisted for detailed assessment this year and on the shortlisting process.² With a view to avoiding future legal challenges, it is also consulting more widely on the issues that its Severn tidal power study is considering and how these are being approached. Ministers are also consulting on the scope of the project’s strategic environmental assessment (SEA) which it must carry out under EU law.

Within the next year the government aims to announce a final shortlist of schemes to go forward for further analysis, with the aim of learning enough about tidal fences and tidal reefs to decide whether they should go on this list.

DECC says a final decision on whether and on what terms government could support Severn tidal power generation will be taken in 2010 or 2011, following a second public consultation. That means it will come after a general election. As well as DECC, several government departments, the Welsh Assembly Government and the South West of England Regional Development Agency are involved in the study.

Spending on external consultants to date runs to some £3 million, with several million pounds more to be spent over the remainder of the study. Most of the money has been spent on an options analysis and SEA carried out by a consortium headed by two big, multinational firms with US-headquarters, Parsons Brinckerhoff and Black and Veatch.

The initial SEA document is a scoping report that identifies the environmental issues at stake, plus social and economic issues. It is accompanied by 16 topic papers covering areas such as birds, fish, terrestrial and marine ecology and shipping. There is even one on noise and vibration; at 62 pages it is one of the shortest. A second phase of the SEA will assess the environmental impacts of the shortlisted options in detail.

Even though there was a major government-funded study of Severn tidal power two decades ago, basic unknowns still have to be addressed, largely through computer modelling of how the structures will affect waves, currents, erosion and sediment deposition. There is, for example, a dispute over whether construction of a large barrage will lead to more or less wave erosion of the estuary’s shoreline on the landward side. Statutory nature conservation bodies say more, engineers say less.

Among the many consultation documents are two about the EU habitats Directive, which will have a critical bearing on any Severn tidal power schemes. The estuary is a Special Area of Conservation (SAC) under the Directive, as are three rivers draining into it that would be affected by any scheme: the Twyi, Usk and Wye. They have this designation because of migrating fish like the Atlantic salmon and twaite shad, or unusual marine estuarine habitats.

The Severn estuary is also designated as a Special Protection Area (SPA) under the EU birds Directive because of the 68,000 wildfowl and waders it supports. And it is also protected under the Ramsar Convention, an international wetlands conservation treaty. Further afield lie other sites designated under the birds and habitats Directives, such as the Somerset Levels and Moors SPA. An initial screening report, written to comply with the UK regulations implementing the Directives, has identified 34 designated areas that could be affected.

The next stage required by the Directives and Regulations is to assess the impacts on SPAs and SACs in detail. If these are significant - and for Severn tidal power schemes they certainly will be - the government has to consider alternative solutions. It can go ahead with a damaging project on the grounds of ‘imperative reasons of overriding public interest’, but it has to draw up measures to mitigate or compensate for the damage.

This is covered by a second document on the habitats Directive, which takes a first broad look at how a Severn tidal power scheme could comply with the legislation by way of mitigation and compensation.³ This report makes it clear problems exist with both. For instance, there is "currently very low confidence" of finding ways to avoid serious harm to migrating fish.

One way to compensate for the loss of mudflats and salt marshes caused by a scheme would be ‘managed realignment’ of coastal defences elsewhere, carefully letting the sea invade farmland. But, says the report, "the scale of managed realignment that might be needed is unprecedented" and would "pose significant challenges in terms of public acceptability and deliverability".

The consultants have tried to judge the compensation costs for the loss of habitats under the Directives. This "increases the capital costs by between 6% and 20% for larger barrage options and typically between 10% and 35% for the other options". The EU water framework Directive could also pose problems for any scheme.

DECC says electricity from a Severn scheme would be expensive compared with prices offered by today’s renewable power sources. Even so, a scheme has the potential to reduce the overall costs for the UK of meeting its renewable energy target set under the EU renewables Directive. By 2020, 15% of the UK’s total energy supply must come from renewable sources.

Any Severn scheme would need more support from electricity consumers and taxpayers than is currently on offer to renewable power generators. The consultation document says the private sector could finance the construction of one of the smaller schemes, given enough certainty about long-term electricity prices. But the government would have to help pay for the construction of a large scheme, although the private sector could then operate it.

The consultation closes on 23 April.