The Severn barrage

Nobody expects a Severn barrage to be built soon. But government opinion appears to be swinging in favour of the idea. The independent Sustainable Development Commission has just brought out a report that broadly supports a barrage. Though the environmental costs will be high, it says that mitigation measures will counterbalance some of the damage. We now also have a better feel for the economics of the scheme or, more correctly, for both of the two main options for blocking the Severn. The bigger scheme blocks the estuary between Cardiff and Weston-super-Mare. It will cost about £15bn and deliver just under 5% of the UK’s electricity. The smaller – just downstream of the Severn bridges – will cost a tenth as much, or £1.5bn, but will provide a sixth as much power as the bigger project.

£15bn to build a barrage that decarbonises less than 5% of the UK’s electricity supply is a high price to pay. Scaled up to the whole of the electricity business, this is about 20% of one year’s GNP to replace coal and gas power stations. Even over twenty years, this cost is similar to Stern’s estimate of the cost of reducing the UK’s emissions for the economy as a whole. The smaller barrier delivers much less electricity, but at a capital cost per kWh of little more than half its larger cousin.

The Sustainable Development Commission acknowledges that private financiers are unlikely to put up the cash for the bigger scheme. The report doesn’t really discuss the viability of the smaller barrage but it is much more financially attractive. In terms of total capital cost and expected yearly output, the upstream barrage is very similar to the huge wind farm development called the London Array. The Array will be constructed with private capital. I believe that if the current renewable electricity support scheme remains in place a barrage across the upper Severn can be built with risk capital.

The Sustainable Development Commission thinks that the bigger scheme should be built with public funds. I am not convinced by this. The offshore wind resources around the UK are orders of magnitude greater than the useful energy of Severn tides. If the larger Severn barrage has construction costs of nearly twice the typical figures for offshore wind, wouldn't it be better simply to speed up the licensing of wind farms?


The schemes The Sustainable Development Commission (SDC) looked at tidal energy around the UK. It assessed whether tidal energy could fill a substantial fraction of UK energy need. The report shows that the Severn contains a very large fraction of all 'tidal range' energy, and the Pentland Firth between north-east Scotland and the Orkneys had much of the usable 'tidal stream' power

How does a tidal range power plant work? A barrage that generates electricity from the tidal range will let the rising tide in without impediment. As the tide turns, the barrage will start to trap water. As the tide drops outside the barrage, and as the river water is held behind the barrage, a gradient develops. Some time after the tide has turned, the water is allowed to flow through turbines in the barrage. A tidal range barrage is essentially a hydro-electric power station working for about half the day.

Tidal energy is reliable. The output from a tidal barrage can be predicted with high accuracy for decades ahead. Nevertheless, tidal energy is intermittent. Before and just after a high tide the barrage will generate no electricity. It cannot completely replace a coal or gas power station for ‘baseload’ electricity.

Tides vary in size very substantially during the monthly lunar cycle. The monthly highest tides (‘spring’ or ‘flood’ tides) in the Severn have a range several times greater than the smallest ‘neap’ tides. The electricity that can be generated is a function of the range of the tide and so varies considerably from week to week as well as during the course of each tide. In addition, all tides are highest at around the spring and autumn equinoxes.

I didn’t know this before I read the SDC report, but spring tides occur at the same time of day at any given location. So we know that the larger barrage will always produce the most electricity at between 1 and 3 in the afternoon and 1 and 3 at night. The smaller barrage will peak an hour or so later. UK electricity use is at its highest in the early evening at around 6.30pm, meaning that a lower Severn barrage is only moderately useful as a source of replacement power. The upper Severn is substantially better.

The environmental issues

  1. Intertidal land: the huge tidal range of the Severn, which is as large as anywhere in the world outside Newfoundland, means that substantial areas of land are under water part of the time and exposed at others. These ‘intertidal’ areas are not particularly productive in terms of biological diversity but support a range of important bird species.
  2. Any tidal barrage will reduce the range of the daily tides. A large fraction of the land that is currently intertidal will either always be under water, or always dry. The ecology will therefore change. The SDC report talks of replacing some of the lost intertidal land with new land outside the barrage. The costings for this are incomplete and the likelihood of success is unknown. The intertidal range has substantial international protection but the Commission seems to believe that ways could be found to accommodate concerns.

    Much of the intertidal land is behind both barrages. The smaller scheme does not really avoid any of the environmental problems associated with the loss of tidal range.

  3. Fish: both barrages block rivers that contain large numbers of fish. Most fish going through a turbine at the barrage would be filleted. The SDC says that mortality will be extremely high, possibly resulting in a complete loss of certain species. (Friends of the Earth is more optimistic, seeing a 6% death rate.) The bigger barrage blocks the Usk and the Wye, the smaller just the Wye. The smaller barrage is likely to be less destructive than the bigger, but both are bad. The Severn is the principal source of the sea bass stocks of the Irish Sea and other species such as eels need the mixture of salt and fresh water for their breeding cycles.
  4. The river will be less muddy: sediment will be deposited rather than being stirred up by the fierce tides. By increasing the amount of light in the water, the growth of plant and plankton will be encouraged. This may or not be a good thing, but the decreased turbidity of the water will represent a major change.
  5. Wave energy will be decreased: there is little commentary of the effect of this in the SDC report.

Environmental organisations have reacted with horror to the SDC’s work. The Friends of the Earth criticised the SDC for its support for the larger barrage despite its environmental effects, particularly on birdlife. The vital Slimbridge bird reserve would be behind both the bigger and smaller barrages. FoE also notes the extremely high cost of the main barrage in terms of capital expenditure per unit of electricity delivered.

The alternative plan: lagoons Environmentalists pushed the SDC to give more support to the idea of tidal lagoons. These lagoons would be roughly circular in shape, just off the coast. They would trap water in the incoming tide and gradually let it out through turbines. Lagoons would not change the tidal range of the river and they could be constructed to avoid damaging most of the most sensitive intertidal areas. They would also leave the flow of the river largely untouched, cause fewer fish deaths and maintain wave heights. They would also probably leave the magnificent Severn tidal Bore largely unaffected.

Friends of the Earth argues that lagoons would create their electricity more cheaply than the larger barrage.

The SDC report is straightforwardly unimpressed by the arguments proposed in favour of lagoons. It points to the lack of detailed cost estimates and says that while barrage technology is well understood, lagoons are an almost completely new idea. The Commission is rather impatient in dismissing the claims of lagoon fans by pointing to the lack of any evidence that they can be built cost-effectively.

Like the Severn barrage, outline schemes for lagoons have been floating around for decades. A developer, Tidal Electric Ltd, is offering a plan to put a lagoon in Swansea Bay, westward from the proposed Severn barrages. The lowish cost estimates make the project seem interesting but I could find no external data to support the company's optimism.

One possible option for going forward would be to combine the smaller upstream barrage with one or more lagoons to the seaward side. Friends of the Earth thinks that this idea is well worth investigating further.

The barrage options: a brief financial appraisal The only weak point in the impressively thorough SDC report is the quality of its financial appraisal. Its preferred tool for evaluation and comparison is a slightly cumbersome one: the cost per kWh at various discount rates. As far as I can tell, this is the way the calculation is done.

  • The project has a capital cost, spread over about 6 years (or slightly less in the case of the smaller barrage).
  • It will produce an estimated quantity of electricity per year for over 100 years.
  • The amount of electricity generated in each year is discounted by a discount rate, much as in conventional financial analysis except that we are using kWh as the currency. With a discount rate, electricity made in year 10 is discounted slightly more than the electricity from year 9.
  • The total discounted total of kWh is divided into the capital cost, creating a cost per kWh.
  • I presume that yearly operating costs would then be added to provide an estimate of the cost of each kWh.

Pence per kWh of expected output

Discount rate 2% 3.5% 8% 10%
Lower Severn (Cardiff-Weston) 2.31 3.68 9.24 12.31
Upper Severn ('Shoots') 2.58 3.62 7.52 9.54

This is not a proper financial analysis. All these numbers enable us to do is compare the cost of generating electricity in Severn barrages with other technologies. Most alternatives will be in the 3 to 4p range. The current wholesale price of electricity is about 4.5p.

At first sight these numbers suggest that a Severn barrage, even the smaller one, would only be able to compete with existing technologies if it were able to raise money at 3.5% or below. Currently, of course, this would be impossible. Rather abruptly, the SDC therefore concludes that the barrage should be financed by public, not private money.

I think the argument is somewhat different. The economics of the small barrier are very similar to a large offshore wind farm, such as the London Array, a scheme for several hundred turbines off the north Kent coast. This proposal now has planning permission and appears to be fully financed. In my view, the smaller Severn barrage could attract financing on similar commercial terms. The financing of the barrage would require confidence that the payments under the Renewable Obligation would continue, but this is as much a risk for the London Array as it is for the upper Severn barrage.

The huge Severn barrage should be looked at differently. It only makes sense to build this structure if we are short of any other projects that could provide large amounts of renewable energy. The UK is not. We have offshore wind resources of at least 50 times the tidal energy present in the Severn. (Since the tidal power of the Severn is several times the level in any other estuary, this conclusion would not be changed if we included all the available tidal range power.)

In sum, therefore, there is no reason to support the large Severn barrage. It is an expensive distraction from the task of getting offshore wind farms all around the coast of the UK. The power of the lower Severn brings us very little we cannot get much more cheaply from the upper Severn and the grossly under-used power of offshore wind.