Ille-et-Vilaine - the Marémotrice

It’s possible to drive over the Marémotrice without realising it’s there. The road across the estuary of the river Rance is apparently lacking in interest. It passes from Saint-Malo to Dinard without calling attention to the essential machinery hidden beneath it. But this is a proud and historic site for French engineering, because it was here in 1966 that France commissioned the first marémotrice in the world.

The north Breton coast sees the largest tides in Europe. Huge movements of water sweep in and out of the river Rance twice a day, and before the marémotrice barrage was built, the Rance had a “bore”, a wall of water forcing the incoming tide up river. The bore was powerful enough to be a considerable nuisance – moored boats were tossed about, fish and shell-fish traps were moved, jetties were damaged. Migrating birds avoided the river – it might be rich in fish but it was an uncomfortable place for a bird to be resting when the bore passed. It was also noisy! The name Rance has the same root as “raucous”.

Harnessing the power of the tide has been going on for millennia. The time-honoured method  is to enclose an area with a dam, allow the water to flood in until high tide, wait until low tide and then let the water out through the channel of a water-wheel. A short trip along the Rance will reveal a dozen sites where a mill building stood over the entrance to a tidal lagoon. One mill, the Moulin de Prat, has been restored for demonstrating the system to visitors. The mills were necessary for grinding flour, but they only produced power for less than an hour twice a day.

Over to the east of France there are great rivers with massive flows, there are mountain streams running continuously, and hydro-electric power was feasible. But in Brittany there are no mountains. In 1923 a dam was built on the river Blavet, forming the lac de Guerlédan, with turbines in the dam producing electricity from the exit flow. It worked, the power was useful, but it wasn't enough to power Brittany.

The eyes of the hydraulic engineers turned to the indisputable power of Breton tides. There were ideas for making a tidal barrage more efficient than the ancient tide-mills, but the war got in the way. When Charles de Gaulle returned to the presidency, he was concerned at the lack of power in France, a country which has neither coal nor oil nor gas. He commissioned nuclear power stations. Then he picked up the idea of a tidal barrage on the Rance and pushed the project into fruition. Work began in 1961, building the dam. Test runs of experimental equipment were carried out in 1964, the design was perfected and then 24 turbines were built. In 1966 they went into full production. They are still working. They provide 10% of the energy needs of Brittany, the highest amount from a renewable source.

The installation was described as a marémotrice, from the Latin words for tide and power. The innovation is that it runs almost continuously. The tides take just over six hours to complete a full cycle of coming in and going out. As the tide flows through the barrage from its lowest point onwards, the special “bulb” turbines are spun by it until the point of high water, when there is a short slack-tide period. Then as the flow reverses with the falling tide, so the “bulbs” reverse their movement, while still generating electricity. Handling this forward-and-reverse flow was a world first, embodying the genius of French engineering.

Theproven success of the marémotrice forward-and-reverse principle meant that other countries with powerful tides have now copied it, leaning heavily on French advisors – Annapolis in Canada in 1984, Sihwa Lake in South Korea in 2011 (now the largest marémotrice in the world) and Incheon Bay in South Korea in 2015. The UK government consented to a project in Swansea Bay in 2015 and a proposal for Cardiff Bay is mooted.

Power obtained from tidal movements is 30% cheaper than wind power, and tidal installations need less maintenance. Tidal movements are well-documented, so it’s obvious where to site a viable marémotrice and what energy can be expected from it, whereas windfarms have sometimes proved embarrassingly erratic at generating power.

The Rance Marémotrice has been relegated to second most powerful in the world after Sihwa, but no new projects are challenging its size. It is an astounding achievement.

It has had a series of knock-on effects as well. For a start, the bore has never been seen again, which makes life simpler and less expensive for all those working or dwelling close to the water. A well-behaved river is safer for novices to learn to sail and for wind-surfers and kite-surfers to enjoy themselves to the full. The bore’s disappearance has encouraged flocks of migrating birds to make a rest-stop on the river. The only drawback appears to be the face that the outgoing tide no longer sweeps all the mud in the river out to sea in the Bay of Saint-Malo - there has been some silting-up. But the overall picture is still an 80-year success story, with a long life of usefulness still to come.