For more than a year, in this peaceful area north of Nantes, the landscape has radically changed: over the seasons, obviously, but above all due to the progress of an impressive construction site. In recent days, the low water level of Lake Vioreau seems frozen by ice while the temperature struggles to become positive. Around, the excavators and other machines which have been activated in recent months will begin to withdraw: the largest reserve of fresh water in Loire-Atlantique, essential to the supply and proper functioning of the famous canal from Nantes to Brestis preparing to begin the last phase of its renovation, dictated by that of its imposing 19th century dam.
After being completely dry, during the work, the 180 hectare hydraulic reservoir must really begin to fill, depending on the rains to come. In the Loire-Atlantique department, where nearly 17 million euros have been invested for this “extraordinary” operation, we are crossing our fingers that this precipitation will be regular, but also neither too weak nor too abundant. It would take, in theory, between one and six months for the level to rise completely.
One meter per week max to test the tightness of the dam
Because if we are talking about an artificial lake, the 7.5 million m3 of water that it should theoretically contain are indeed completely natural. However, there is no question of watching the storms pass by while twiddling our thumbs, at the risk of missing everything. “The authorized rise speed of the reservoir is a maximum of one meter per week, and must not exceed the following levels: 4.7 meters until the end of February then 6.7 meters until the end of March,” explains- we say at the departmental council.
Very strict regulatory constraints necessary in particular to ensure that the imposing 200-year-old stone dam, which had weaknesses, was truly waterproof and capable of coping with possible floods. “This work was essential in the name of hydraulic safety,” recalls Anne-Cécile Olivier, head of the infrastructure and waterways department in the Loire-Atlantique department. If the water passes above, around 8,000 inhabitants are affected, and many protected species are threatened on this Natura 2000 classified site.”
Navigation will resume this summer between Nantes and Brest
Already, the latest rains, which were too heavy and which therefore had to be partly evacuated, have thwarted the plans of the workers, who numbered around fifty at the peak of activity. But the teams are confident that navigation will be able to resume properly this summer on the Nantes-Brest canal, before optimal use in 2025. The nautical and leisure clubs in the Vioreau sector are also preparing to reopen their doors to the public, while fishermen, many of whom frequent this very famous spot, began to reintroduce the tens of tons of fish that they had caught before the lake dried up.
The dam operator has retired and will not be replaced although the new structure is now fully automated. “With our supervision tool, there will no longer be any need to come manually to open the valves, when it is necessary to regulate the water level”, informs Benjamin Gauthier, engineer in charge of modernizing waterways, at the department . Highly anticipated, the official inauguration of the Vioreau dam, now partly covered with a white coating, is scheduled for the end of May.