The payback period of carbon costs induced by the production, transportation and construction of the materials to be used at the Severn Barrage would be less than six months, according to a recent study published in CIWEM's Water and Environment Journal.

The Severn Barrage project is the largest single source of renewable energy available to the UK, with the estuary providing one of the world's best opportunities to harness energy from tides. A barrage of 16 kilometres long would provide five percent of the UK's annual electricity demand, which is 25 percent of the UK target to cut emissions by 2020. This is similar to that of all other renewable energy projects operating currently in the UK.
 
A major concern for all forms of electricity generation is impact on carbon emissions. Renewable energy projects can be substantial in terms of the volume of equipment and land they require because they depend on low energy density sources, so the physical extent of this project is viewed by some as relatively demanding in materials and, by inference, in carbon content.
 
The barrage would be formed mainly of reinforced concrete caissons constructed at a number of shore based yards and towed to the site. It would require over 200 large water turbines and electrical generators and 166 sluice gates. The carbon content of producing these materials, transporting them to plant fabrication yards, manufacturing and constructing the units of equipment, and transporting the products to site for final assembly and installation have been quantified by the authors of the report, Charlie Woollcombe-Adams, Michael Watson and Tom Shaw.
 
In just over four and a half months the barrage can be expected to have paid back the carbon costs incurred from the sourcing of materials, manufacture of components and their transportation to the barrage site. When including the on-site works used to construct the barrage, this increases the payback period to five and a half months.
 
Constructing the Severn Barrage would take about nine years but power generation could begin before the barrage is completed as long as it is 'closed'. Hence the barrage could begin its carbon payback when still incomplete and in less than a year of operating at 85 percent annual output, it will have paid back the construction-related costs of the whole project.
 
Compared with its construction, the annual carbon cost of its operation is nominal. No energy intensive activities are involved as it should need little more than low-intensity maintenance during its expected 120-year lifespan.
 
The authors say: 'The technical simplicity and reliability of tidal power schemes is dwarfed by the complexity of their political realisation, not least when environmental legislation is being tightened at a time of growing international concern about the security of energy supplies and, paradoxically, their predicted serious environmental effects. In time honoured fashion, the decisions that have to be taken must be based on facts and rational debate. This paper adds an important dimension to that process.'
 
For those interested in finding out the implications of the Severn Barrage on water quality, Roger Falconer from Cardiff University will be giving a presentation at CIWEM's Annual Conference on the 29 April at the Olympia Conference Centre, London. For more information visit www.ciwem.org/events.