On the crest of a wave

STUDIES are currently underway for a project which, if brought to fruition, will result in the production of tidal power in northwestern parts of the UK. Described as a deep water channel linking a series of lakes, starting in the Solway Firth in the west of the country, passing through Morecombe Bay and leaving England via the South Tyne Valley at Newcastle, the Western Water Highway (WWH) will allow shipping to reach European ports and markets by cutting across Britain instead of sailing around the coast.

The attraction for the hydropower industry is that climatic changes, and the special characteristics of the Irish Sea, will necessitate the construction of a coastal defence barrier across the Solway Firth to protect the entrance to the water highway. It is the construction of this structure which could lead to the production of tidal energy.

Plans for the WWH also incorporate a motorway-standard road, which will be built into the barrier, and the cost of which would be offset by the extraction of tidal energy. Ultimately, the WWH could lead to a substantial reduction in road freight traffic, consequently lowering air pollution.

Through the creation of lakes, the WWH will also help to maintain water levels and provide a considerable water resource for the UK. The waterway sits in the South Tyne and Irthing valleys with the Kielder reservoir to the north. The area has spasmodic but high rainfall and the catchment area is vast, but 90 per cent of this vital resource is being lost to the Irish and North Seas.

The locks on the WWH, coupled with the lakes, will conserve and store the water which would be transferred to the Lake District via a pipeline. From the Lake District, existing aqueducts and the re-opening of ancient canals could process this vital resource around England; a proposal which has already been put forward by the British Waterways and could revitalize the UK’s river network.

Whereas the construction of roads and rail, with freight depots, destroy wildlife habitats, the lakes will not only provide an important transport link and the provision of fresh water but offer the opportunity to create extensive wildlife centres. The Dutch, Belgian, French and German water highway builders have already demonstrated that environmental enhancement can be achieved.

Plans to utilize the abundant tidal energy potential along the northwest coastline of England have been around for some years, and the Solway Firth is not the only location in the northwest with tidal power potential (see table). With a population greater than Scotland, Denmark or Ireland (6.63 million), the northwest is the second largest region in the UK, and the energy produced from the WWH will meet the energy needs in this area for the next 50 years. Furthermore, if the Solway Firth barrage was constructed for tidal power usage, it would produce three times the amount of energy which comes from the Sellafield nuclear plant in the UK.

So far, $430 000 has been spent on preliminary studies for the WWH and over 15 different companies and individuals are currently involved in trying to put the scheme into action. The Western Water Highway Association believes that the combination of tidal power, a roadway and a coastal defence barrier will promote a ‘greener’ future for the northwestern parts of England.

Dr E.M. Wilson, consulting engineer from UK-based Wilson Energy Associates, examines the proposed tidal power scheme on the Western Water Highway:

The Western Water Highway is an imaginative concept which would require at its western end a means of entry for shipping through the wide, shallow inlet of the Irish Sea bordering the Scottish counties of Kirkcudbrightshire and Dumfriesshire and the English county of Cumbria. The inlet is known as the Solway Firth and is illustrated in figure 1.

To make the Firth navigable for ocean-going ships the construction of an estuary barrage is required. This will need ship locks to provide navigable water depths at all times in the inner part of the inlet, which would lead to the cross-country canal.

Once it is accepted that an estuary barrage is needed, then because of the tidal range in this region of the Irish Sea, it seems to offer the possibility of a location that may derive the additional benefit of accommodating a tidal energy generation station. Such an idea is not new and has been investigated in the past.1

The mean tidal range in the eastern part of the Irish Sea, off Cumbria, is about 4.3m, rising to 6.1m at Southerness Point as the converging sides of the Firth help to concentrate and increase the range. Maximum and minium ranges are 9.1m and 3.1m, respectively.

A barrage built across an estuary like the Solway Firth can result in modifying the tidal range as it changes the resonant length. Usually this means a reduction in the amplitude of the tidal wave and this aspect was examined in 1965 by the Institute of Oceanographic Sciences, when it was concluded that for zero flow at a barrage, as depicted in figure 1, the reduction might be 15 per cent, while for operation as a tidal barrage, it could be five per cent or less.

Allowing for these effects and using the results of many more recent studies of tidal energy around the world, it is now possible to determine, with reasonable accuracy, the size, capacity and annual energy output that might be envisaged for a Solway barrage.

The actual embankment would be 9km long, with a cross-section similar to that shown in figure 2, built on boulder clay sand. Its crest level would be at +7.5m OD. The power station section would be in the deep water channel and consist of 66 precast concrete caissons, 50m long by 18m wide and 25m high, each housing an 8m diameter runner Straflo or bulb turbine generator of 16MW capacity.

There would be a similar number of Venturi-type sluice gates, each 12m wide by 6m deep, and two ship locks, one probably 20m wide by 180m long and one perhaps 15m wide by 100m long. These would all be precast caisson-type structures founded on prepared fine rock rubble bases, dredged level.

The annual average electrical output would be approximately 3200GWh for an ebb generation system of operation, and perhaps ten per cent more if pumping into the reservoir was used.

In earlier studies, it was proposed that the energy output should be introduced into the electrical system by integration with the Loth Sloy pumped storage scheme, but such has been the growth of the country’s electrical capacity in the last 35 years that its output could now be absorbed directly into the national grid.

Environmental effects

The construction of the Solway tidal power barrage would alter the esturial regime on the landward side. Whereas now the difference in level of extreme high and low tides behind the line of the barrage can be as great as 9.8m, after construction the difference would be reduced to 4m, with the new range lying in the upper half of the existing range. This would mean that large areas of sandbanks and mudflats that are now exposed at low water would remain covered. Ultimately, this would have an impact on the numbers and species of wildfowl which use the area for stop-overs and feeding.

Fish are unlikely to be affected. Large relatively slow-rotating turbines with large inter-blade apertures offer a low risk for fish passage and experience at La Rance tidal power station in France (with 5.25m diameter turbines) confirms this.

All in all, although it could prove problematic if the Solway barrage was to be considered an economic proposition purely as an energy producer, there is no doubt that as part of a larger enterprise it could offer annual monetary benefits to the order of £150M at 1998 prices.

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