the flow of the Vire has not yet returned to its usual level

Like many others in France, the Manche department has been very affected by drought
this summer. In the region of Saint-Lô and Condé-sur-Vire
mid August, we came close to the distribution of bottled water so low was the level of the Vire, which supplies drinking water.

Since then, of course, the rains have returned. And yet, in the department, rivers and streams have not yet returned to their usual level for the month of November.

Not back to normal yet

The flow of the Vire for example was 4 to 6 cubic meters per second in early November, compared to an average of 17 at this time of year. And if the rains of October made it possible to raise the level, the flow in recent days had “again tends to fall a little“, notes the vice president of Saint-Lô Agglo, in charge of the water cycle. The current situation is not alarming because “we are rather moving towards a period when the rains should be more frequent“, tempers Jean-Luc Lerouxel, “it still needs to rain” this winter !

These rains from November to February are particularly important for recharging groundwater, in particular the main one, located in the basement of the center of the Channel, in the marsh area, very busy in summer and whose level is low. Saint-Lois, for example, which has no underground water and whose drinking water comes from rivers, is partly supplied in summer by this water table and its boreholes in Sainteny and Saint-Germain sur sèves. In winter, conversely, the Saint-Lois returns water to these sectors to avoid pumping water from the aquifer during this period.

Work on the Semilly dam

This summer, at the beginning of August, at the height of this drought, a cofferdam has been temporarily installed on the Vire
to be able to maintain the pumping because the water intake ended up being too high compared to the level of the river. This region south of Saint-lô is particularly sensitive because the Fumichon drinking water production plant, which serves 11 municipalities and 34,000 inhabitantsis fed by the Vire and the Semilly dam.

However, the quality of stagnant water in this dam deteriorates in summer: the ammonium level is too high and it is no longer possible to exploit it. The agglomeration of Saint-Lô has therefore just carried out work to remedy this:

We have done aeration work, to be able to oxygenate the water of the dam so that it is treatable. This work was completed in mid-September. With feedback, we realize that it’s a good system, it works. This will make it possible to use the water from the Semilly dam in the summer and it gives a little security, from 15 days to 3 weeks, for the supply of drinking water, explains Jean-Luc Lerouxel

Interconnections and mutual aid

The other way to secure the supply of drinking water is to interconnect the different networks and territories. In Saint-Lois, work is planned for the start of 2023 to connect the networks between the Fumichon plant and the Couvains plant in the Elle sector which had less concern about water flow this summer.

An interconnection will also be created between Le Dézert and Graignes, which is one of the last communes in Saint-Lois, “without safety net“that is to say without an emergency solution if there was a supply problem. This work is estimated at 2.7 million euros.