The Severn estuary has long attracted the attention of engineers and various governments, who see the ebb and flow of its tides – reckoned to be third highest tidal range in the world - as a potential major contributor to the UK’s use of renewable energy. By building a barrage across the estuary, engineers estimate that the seven to eight metre average tidal range could provide up to 2 gigawatts of energy, equivalent to some 17,000 gigawatt-hours of electrical energy per year. This would therefore make an extremely useful or indeed necessary component of the UK’s renewable energy mix, given concerns that greenhouse gas emissions from the burning of fossil fuels in power plants must be eliminated over the next 40 years.
Yet, since the Severn Barrage scheme, with an estimated cost of £15,000 million, was first proposed many decades back, it has triggered environmental concerns among a wide range of groups. A 2008 report, commissioned from Frontier Economics by the RSPB, WWF-UK, the Anglers’ Conservation Association, the National Trust, as well as fishery interests, such as the Wye Salmon Fishery Owners’ Group, concluded that the costs of the barrage could not be justified on economic grounds, let alone on the grounds of the environmental devastation that its construction and operation would cause.
In its final form, and after a massive undertaking, the Severn Barrage would not only have to be big enough to extract the potential energy from the tidal head of some 8 or more metres, but it would have to cope with powerful weather events, such as storm surges, and even sea level rise, now projected to be rising faster than estimated a few years ago. In essence electricity would be generated by a series of 40 megawatt underwater turbines during both extremes of the tide, thereby putting a considerable strain on the central grid which would somehow have to balance a single point surge in generation with demand twice over the course of 24 hours.
As pointed out in the Frontier Economics report, the barrage would lead inevitably to the loss of hundreds of square kilometres of mudflats and salt marsh - home to waders and other coastal birds, as well as to a host of migratory species. Furthermore, on account of the delay in the natural tidal rhythm from penning in the water and then from the surge of water over each of the turbines when the gates were opened, its construction and use would alter drastically the currents in the estuary, playing havoc with the deposition of silt and having a profound impact on estuarine life, including fisheries and salmon runs.
Cornish hydraulics engineer Rupert Armstrong Evans believes the Cardiff-Weston Barrage across the Severn Estuary (as currently planned) to be massively ill-conceived. Instead, he has proposed a substantially different concept that he claims would generate as much electricity, but far more steadily than the big barrage and which would simultaneously have a much reduced environmental impact, in particular in leaving most of the mudflats and salt marsh intact.
Having pioneered electronic control systems that revolutionised the use of mini-hydro in the 1970s, and having installed low-head hydraulic turbines which he designed for use in different parts of the world, from Cornwall, Wales, Scotland, to Nepal, India and South America, Armstrong Evans has come up with the innovative idea of a structure that has parallels with a tidal reef and so is designed to extract the energy from no more than two metres of tidal head.
In his tidal reef concept, Rupert has proposed a semi-floating set of caissons to stretch across the Severn Estuary, thereby avoiding the massive high head structure implied in the construction of the Severn Barrage. The fundamental difference between the barrage and reef is that, in the latter, the 1000 turbines of some 10 metres in diameter would be housed within the floating caissons, themselves designed to ride over a fixed base structure on the estuary floor. By using a moveable ‘crest gate’ to track the tide level and therefore to maintain a small head difference, irrespective of the stage of the tide, the turbines would operate for long periods, and for at least double the generation period compared to the proposed big barrage. In addition, the reef would be far less vulnerable under adverse conditions than the barrage, on account of its smaller size and lower operating ‘head’. In that respect, storm surges would easily top the structure rather than battering it, as would be the case for the barrage.
On account of the structure being more modest than the barrage, the saving on rock fill alone would amount to more than 10 million tonnes. At the same time, the passage of ships would be easier, as a single gate, similar in principal to the Thames Barrier, would allow the passage of even the largest ships with minimal disruption. This is only possible because of the small head difference across the structure. This system would also avoid the need to dredge a new deep-water shipping channel.
As Rupert points out, migratory fish should have no problems navigating the slow-moving turbines, and with the low head required any changes to the estuary flows will be significantly reduced, causing far less impact on the mud banks and salt marshes than would the barrage. Moreover, the time taken to construct the reef would be considerably less than for the barrage and Rupert refers to an excellent precedent in the construction of the Mulberry Harbour floating dock that was put together for the D-day Normandy landings. The various pieces of the dock were built in six months, before being successfully installed under enemy fire.
He has had the backing of WS Atkins, the international engineering consultant which, in 2008, declared, Rupert’s Reef Scheme, would not only generate more electricity but would cost considerably less – some £2,000 million less - while simultaneously avoiding the worst environmental aspects of the Severn Barrage.
In the face of mounting concerns over the ecological damage which would result from constructing the barrage across the Severn estuary, Rupert’s reef scheme has met with the approval of the Royal Society for the Protection of Birds (RSPB). Furthermore, the government has just announced that it believes the ‘Severn Tidal Power Reef’ project to have merit, and is to commit financial support towards its future development, thereby postponing, at least for a year, any decision as to which project it gives the go-ahead.
Peter Bunyard is the science editor of the Ecologist magazine, and a widely published freelance author and environmentalist. He has worked as consultant editor for the United Nations Environment Programme review on Industry and the Environment, and was secretary and editor of the Committee for the Study of Nuclear Economics. A fellow of the Linnean Society, he has conducted field work in the Colombian Amazon on the role of rainforests in global warming.
main photo: An overview of a small barrage built seaward off the Severn road crossing from the Severn Tidal Power Consultation website