Case studies

Successful barrages

The Great Western Power Barrage will require substantial investment in time and money. Whilst modelling and outline designs have shown it to be viable, this is even more persuasive since the concept is strongly supported by two schemes which have been operating successfully for many years.

These two schemes both operate on the same principles. They are La Rance Tidal Power Plant (La Rance) near Dinard on the north side of Brittany, France and Lake Sihwa Tidal Barrage Power Plant (Sihwa) in South Korea.

The energy production of these schemes is around 240GWh/annum and 250GWh/annum respectively.

It is most instructive to consider the environmental impacts during construction and the subsequent recoveries for both of these schemes.

Both were conceived and built at a time when there was less emphasis on safeguarding the environment as well as the welfare of future generations.

La Rance Tidal Barrage

The main build period for this scheme affecting the environment was between the start of construction in January 1961 and July 1963 when there was final closure of the dam across the estuary.

The impact on the wildlife in the estuary was significant during this period largely because the upstream basin was drained of all water to allow the use of a cofferdam so that the concrete for the main parts of the barrage could be cast in situ.

A new ecological equilibrium took about 10 years to re-establish. The operator, EDF Energy, considered the estuary to be richly diversified again by around 1976. A survey in 1980 reported the basin to be home to 110 warm-blooded, 47 crustacean and 70 fish species.

The operation of this barrage has resulted in a 2.5m rise in the mean water level. In addition, there has been a reduction in the hydrodynamic regime, including reduced velocities in the water within the upstream estuary, where the slack period is also longer.

The general flora and fauna distribution became increasingly diverse post- construction. The patterns of distribution, their grouping into ecological units and the nature of their inter-relationships indicate some biological adjustment to the new environmental conditions.

However, the bird varieties visiting the estuary are the same as before the construction of the barrage – around 120 species. The operator reports well developed communities of fish-eating birds including gulls, guillemots and shags. However, some numbers may have altered because of the decrease in the intertidal area – from the 2.5m rise in mean water level. Birds can also find food in other bays (mudflats).

Seabass and cuttlefish have returned to the estuary and there are new fishery activities; scallops and belon oysters are now harvested.

There have also been some changes to the patterns of sedimentation. Even with the slackening of water currents, it appears that the distribution and composition of the estuary sediments is comparable with what is observed in neighbouring natural estuaries. However, one study suggested that the modification of the tidal stream in the estuary, in particular during ebb tides, has provoked more silt deposit in the lower intertidal zone.

Sihwa Tidal Barrage Power Plant

This scheme did not start with the proverbial clean sheet of paper for creating a tidal barrage; it was a retrofit to alleviate the effects of an earlier scheme that was causing pollution.

In 1994, South Korea created a 56.5km² freshwater lake at Sihwa by constructing a 12.7km dyke between Oido Island in Sikeung City and Daebudo Island in Ansan City. The purpose of this lake was to secure agricultural and irrigation water and to reclaim 173km² of land near the local metropolitan areas comprising the three cities around the lake.

However, within a few short years after the completion of the embankment, it became apparent that without seawater circulation, Lake Sihwa was experiencing an inordinate inflow of polluted wastewater from a nearby industrial complex severely contaminating the basin and making it unusable as a freshwater reservoir.

By 1997 and partly driven by the scandal of the pollution, officials had to reformulate their plans by retrofitting a tidal power barrage to circulate seawater for regular flushing as well as for generating power.

This tidal power barrage only generates as the tide in the Yellow Sea outside of the lake rises. During the ebb tide when the tide falls the lake is emptied by opening sluice gates. The reason for this unusual regime is that several buildings had been erected on the lake shores after the building of the original seawall and, as a consequence, the level of the water in the lake had to be reduced by 1.0m relative to the sea level outside. This meant that it was not possible to design turbines which could generate satisfactorily in both directions.

The tidal power plant has effectively dispersed the accumulating pollution in the lake. There is not much other information on what the coast was like before constructing the original dyke and the tidal power plant was an ‘add-on’ to deal with the unexpected pollution problems.

The most remarkable impact has been the recovery of water quality and ecosystems. Approximately 60 billion tonnes of sea water flows in and out of the turbines and the floodgates annually, which equates to a change of about half of the total water in Sihwa Lake every day. This continuous circulation of water between the lake and the outer sea during the power generation process has improved the water quality.

In 1988, the chemical oxygen level in Sihwa Lake was 17ppm, but has since been reduced to 2ppm, resulting in an improved habitat for all fish species.

This site attracts interest for learning about live ecosystems, with over 146 bird species including stork and mallard, and some 23 million birds living in and around the lake.

The Sihwa seawall is a popular spot for leisure activities and sports. Since the addition of a 75m tall observatory in 2014, this project and surrounding area now attracts some 1.5 million people annually.

The document below, Energy security through tried and tested technology, gives more detail of these sites.