Tidal reservations surface at the water margin

Figure 1. Proposed layout of the two principal Severn barrage schemes, Cardiff-Weston and Shoots, and the electricity transmission network (400 and 275 kV) around the estuary

Many schemes to exploit the Severn’s potential have been put forward in the 158 years since 1849 when Thomas Fulljames first suggested a barrage design (Figure 2). The building of such a barrage would be a huge engineering feat, comparable with some of the world's biggest construction projects, and it is the huge size and cost of most of the ideas over the years that have kept their plans firmly on the drawing board. Typically these schemes have been for transport links, flood protection, harbour creation, or tidal power generation. In recent decades it is the last that has come to be the primary focus for barrage ideas, and the others are now seen as useful side-effects. More recently the Severn Tidal Power Group, made up primarily of major construction companies, published a study, in 1989, which concluded that the electricity generated from the barrage (Figures 3, 4) would make the scheme economically viable if given certain ‘green’ advantages, and that the environmental impact was acceptable. Margaret Thatcher's government did not accept this, and shelved the plans.

Credit: www.bridgemanart.com

Figure 2. The 1859 Fulljames scheme

But interest in the Severn has sharpened recently with the publication of a report, Turning the tide – tidal power in the UK, by the Sustainable Development Commission (a UK?government funded think tank set up in 2006 to act as the government's watchdog and adviser on sustainable development) which considers the three possible approaches in terms of their prospects, what would be necessary to bring them to fruition, and the consequences of doing so. Of the three – tidal stream, tidal reservoir, and tidal barrage – the latter is considered the most immediately promising because its technology is the most mature, and there is a ready made example in France at La Rance that has been operating for some years. However it is extremely capital intensive and would require a strong lead from government if it is ever to be built. But the UK government now seems ready to give that lead. At the governing Labour party’s conference in Bournemouth in September business and enterprise minister John Hutton gave the strongest possible hint that the government is in favour of the scheme, and promised that a study to examine the feasibility of generating electricity from the Severn estuary will begin immediately.

SDC?Report

The SDC’s approach is markedly careful and considered, and has encouraged a climate in which those who object to any such scheme have been more vocal, and have had more attention paid to their objections, than might otherwise have been the case.

‘All tidal technologies’ says the report, ‘have a number of environmental, social and economic impacts that need to be considered. In particular the impact of the Severn barrage on internationally protected habitats and species is of great concern’. What that boils down to is concern for the 65 000 animal species that live in the estuary’s mix of semi-flooded environments, marshland and rocky islands, all at risk if the tidal pattern is changed as comprehensively as this scheme implies.

The river Severn’s great tidal range has created the regular miniature tidal wave known as the Severn bore, but it means also that its intertidal margin is enormously extended, giving rise to a wildlife habitat that supports thousands of species. These habitats and the bore itself will be destroyed or at the least severely damaged if the scheme goes ahead.

In fact there is not one scheme, but several, the Cardiff-Weston being what is usually referred to as the Severn barrage proposal. Such a barrage would cost £15 billion but would expect to be functional for over 120 years. An alternative version has a second basin on the seaward side to maximise the take up of the estuary resource and provide some protection against flooding of a low lying area known as the Somerset Levels. The Shoots or English Stones scheme generally regarded as the second serious proposal runs close to the existing river crossings and is designed to facilitate a rail link to replace the ageing Severn rail tunnel. This smaller-scale 1000 MW scheme would cost about £1800 million and produce 20% cheaper electricity. Engineers PB Power (of Parsons Brinkerhoff Ltd) consider that it is more practical than the larger barrage, at a tenth of the size.

Other schemes include the Dawson, which envisages another basin further down the estuary, enabling continuous power output, the Minehead-Aberthaw scheme, which has an even longer ‘outer’ barrage that would make maximum use of the estuary resource and is favoured by the Somerset local authority on flood protection grounds, the Severn Lake scheme which is an ambitious version of the Cardiff-Weston barrage incorporating a wave farm and four yachting marinas, and the Shaw two-basin scheme that incorporates deep set pumps for pumped storage capacity.

Figure 3. Construction of the barrage caissons as shown in the STPG proposal

Figure 4. Ebb tide turbine operation, as envisaged by the STPG proposal

Conditions

Developing such a barrage would result in significant climate change and energy security benefits says the SDC report, but it would also have a major impact on the local environment, with the loss of up to 75% of the existing intertidal habitat. There would also be a number of impacts on local communities and the regional economy, and a high risk that unsustainable ancillary development would take place alongside any barrage project. The SDC has therefore laid down a series of tough conditions which a Severn barrage would have to meet in order to be considered sustainable. These are the main ones.

• The barrage must be publicly led and publicly owned as an asset to avoid short-termist decisions and ensure the long-term public interest

• Full compliance with European directives on habitats and birds and a long-term commitment to creating compensatory habitats on an unprecedented scale

• Further investigation of the ‘environmental opportunity’ for combining climate change mitigation with adaptation, through a habitat creation package that actively responds to the impacts of climate change over the long term.

The report also points out that the lower rate of interest available to a government-led project is probably the only realistic way of financing a large-scale compensatory habitat package while also providing electricity to consumers at a competitive price.

But the Commission warns that the development of major tidal power resources should not be seen as a licence to ignore the need for dramatic reductions in our energy consumption, increased energy efficiency, and the wider decarbonisation of our energy supplies. ‘The Sustainable Development Commission is issuing a challenge to government to embrace a new way of managing this major project’ said its chairman Jonathon Porritt.

Output profile

Modelling of the barrage indicates that the method of operation with the highest electricity output would be ebb tide generation (Figure 4) with flood pumping, the pumping contributing about 3%. The load factor of such a scheme would not be high, at 22.5% for the larger project (compare with 27 % for onshore wind) but it would be predictable. A barrage operated in ebb tide mode would impound the water at high tide and then allow the receding tide to create a head before releasing the water through turbines.

The timing of tidal range devices is related mainly to two tidal patterns, semi-diurnal tides and the spring neap tide cycle. The optimum time for generation occurs between 2.5 and 4 hours after high tide and lasts for eight hours. Figures 5 to 7 show UK demand, the effect of the tides on barrage output, and the significant base load contributions to the national mix that can be made.

Transmission implications

The Severn estuary is well connected to the grid network and has significant capacity for new generation. But the availability of connection points is as important as capacity. Figure 1 shows the good connection points at 275 kV and at 400 kV on both sides of the estuary, quite close to the ends of the proposed construction.

For the larger Cardiff Weston scheme two connections at 400 kV would be required at each end of the barrage, mainly because the 275 and 132 kV networks are close to capacity in the area. The much smaller Shoots scheme would require only one connection, to the Hinkley Point-Melksham 400 kV double circuit. Both schemes would require some new infrastructure, but far more in the case of the Cardiff-Weston scheme.

Opposition to the barrage

Three environmental groups, WWF, the Royal Society for the Protection of Birds (RSPB) and Friends of the Earth Cymru, have voiced their opposition to the scheme, saying that it would cause irreversible damage to wildlife. There are, they say, other technologies which could be used in the Severn estuary with a much lower environmental impact. The estuary represents 7% of the UK's total estuary resource for wildlife.

Morgan Parry from WWF Cymru said the barrage was not the answer, adding: ‘The environmental damage caused by constructing a 10-mile concrete energy dinosaur will cause irreversible damage to Wales and England's most important estuaries.’

At the Welsh Renewable Energy Conference in Cardiff in November 2006, speakers challenged the record of La Rance, describing how its ecology is flourishing post-barrage, but also how blockage of the estuary during construction and a strongly reduced salinity wiped out the pre-barrage water life. The proper comparison estuary, they said, is not the tiny La Rance (20 times smaller in area and 30 times less powerful than the Severn), but the Bay of Fundy, Canada. There the existing small barrage has found problems in fish loss and shoreline erosion downstream, causing plans for a larger barrage to be dropped.