Tidal lagoons

Tidal lagoons are an adaptation of the barrage system. Similar to standard barrage models, tidal lagoons retain a head pond and generate power via conventional hydro-turbines. The difference is that orthodox barrage designs exploit the natural coast line to minimise barrage length. However, this entails blocking the estuary irrespective of depth – escalating costs considerably. Indeed, holding all estuarine water, even though the majority cannot be utilised for power generation, effectively ensures such schemes are prohibitively expensive 17 18. A lagoon, however, can essentially be developed anywhere within a high tidal range at low cost 18. Appearing like a normal sea wall at low tide and below the high water tide mark, a lagoon would have little visual impact 18.

The lagoon is proposed to be constructed from loose aggregates sourced from regional, national and international quarries or demolished structures. Rubble would be ‘dumped’ until an impound wall was complete. As any aggregate can be used, it is possible to restrict construction costs by implementing the cheapest materials available 18. As well as being cost effective, relatively uncommon resources, such as high quality sands, do not need to be used.

The company promoting tidal lagoons, Tidal Electric 18, argues that because it can be built without being constrained by geomorphological features, greater consideration can be given to the environment. Migrating fish can swim around unimpeded and without the danger of sluices or negotiating turbines. While the exact affect a lagoon would have on estuarine flows and tides remain unknown quantities, they will undoubtedly be less than that of a barrage. Additionally, a lagoon would not reduce the number of habitats available but actually add a calm water lake. No [known] studies have identified any potential shortfalls to marine biota, however, top ocean predators, sharks and cetaceans, would not be able to hunt in the lake. Therefore, smaller fish are likely to flourish; potentially providing significant feeding grounds for birds. On the perimeter of the lagoon, the 95 miles of piled aggregate 19 would create a vast artificial reef. This would further enhance the local habitat potential, increasing the opportunity for improved marine life.

Tidal Electric estimate that damage to the lagoon is most likely from erosion or earthquake. In such events, the damage can be repaired quickly by adding more aggregate to the impound structure. Although economic, and power, loss would be applicable until the wall is rebuilt, the timescale and cost would be dramatically lower than any other scheme 18.

Aggregates can be protected from erosion by installing artificial reefs, such as those provided by Artificial Reefs Inc 20. Their design is claimed to effectively control water flow speeds, as well as providing additional habitat for crustaceans and fish.

Tidal Electric does not indicate any potential damage to the turbines. However, valuable equipment can be substantially protected in concrete housing built to earthquake specifications; at no greater risk than a barrage.

Power would be derived from turbines based on reliable existing technology familiar in fresh water dams. Adaptation of turbines to work correctly in salt water is feasible provided that correct materials are used 21. This technology has been available for more than 120 years already 18.

The lagoon system, while using tried and tested technology, is a new concept that is without precedent. Prior to constructing a large scheme in the Severn, Tidal Electric is planning to test the concept in Swansea Bay with a relatively small 30MW output plant. AEA Technology, engineering advisors to the government, are confident of their methodology and that the technology will work, environmentally and financially 21.