Work on the Nantes-Brest Canal in Loire-Atlantique

In Loire-Atlantique, the Nantes-Brest Canal includes sections of natural waterways such as the Erdre and Isac rivers. It begins its first artificial section south of Nort-sur-Edre. Unlike a natural river, the gradient is not constant and the flow is not natural. The water is therefore conveyed from the large Vioreau reservoir (an artificial water reservoir) to the diversion reach via a 21.3 km-long feeder channel.

In 2023, major works will be carried out to secure the Vioreau dam, the main source of water for the Nantes-Brest canal in Loire-Atlantique.

The consequences? The water reservoir will dry up by the end of 2022, so it will no longer be able to supply the canal with water. The canal will not be supplied with water between the locks at Melneuf (Guenrouët) and Quiheix (Nort-sur-Erdre) for around a year. Cyclists will still be able to navigate along a canal with water but no boats. It should be noted that navigation will still be possible between Guenrouët and Saint-Nicolas de Redon, with a connection to the Vilaine.

 But why a canal in Brittany at all ❓

The idea of opening an inland waterway in Brittany dates back to the 16th century. It was only at the beginning of the 18th century, following the blockade of Brest by the British, that the project was launched under Napoleon I with the strategic interest of unblocking Brest via the hinterland and thus linking Nantes to the two major Breton war ports: Brest and Lorient.

After long and tedious construction work, the Canal de Nantes à Brest was opened to traffic in 1842, covering 364 kilometres and 238 locks in 5 départements. This waterway opened up Brittany and enabled its economic development. Its construction led to economic development in inland Brittany by facilitating the transport of goods, raw materials and products.

With the development of the railways and the improvement of road networks at the end of the century, transport activities were reduced on the Nantes-Brest Canal. In 1930, the construction of the Guerlédan hydroelectric power station and its dam interrupted navigation, thus removing the Canal de Nantes à Brest from the list of navigable or floatable waterways in 1957.

The tourist route has given it new impetus and economic dynamism: cyclists, walkers, horse riders, anglers, kayakers and pleasure boaters have repopulated the waterfront.

And how does a canal work?

A canal is a man-made hydraulic machine whose designer must ensure a permanent supply of water sufficient for its function. The Nantes-Brest Canal required the construction of 8 rivers, 3 diversion bays between the Loire and the Vilaine, the Vilaine and the Blavet, the Blavet and the Aulne, and 238 locks.

Here's how it works:

*Bief de partage: Division reach 

Rivière: River

Relief: Relief

To ensure the passage of boats, the locks are manual and are operated by lockkeepers.

Amazing works

The most important works were between Carhaix-Plouguer and Guerlédan, with the construction of :

• the "Grande Tranchée" at Glomel, where the highest point of the canal was reached at 184 metres above sea level. This trench was 3 kilometres long, 23 metres deep and 10 metres wide, making it possible to cross the highest point of the canal. It took the 4,000 convicts more than 9 years to move the 3 million cubic metres of earth.
• the double lock at Coat Natous in Mellionnec: two consecutive locks with a common gate, built because of the steepness of the gradient at this point.

On the Pontiy > Josselin stage, near Gueltas, 23 locks over a distance of 4.5 km were needed to overcome a difference in height of around 53 metres. This is known as a "lock ladder".