Gestion écologique de l'étang du fret (Finistère)

The Frêt Pond at Crozon (Finistère) occupies the southwest part of Frêt's cove in Brest's harbour (fig. 1). This 18-hectare pond was isolated from the harbour by the natural accumulation of a mound of pebbles, which was reinforced by rocks so that it could support the D 155 road. At the free end of this mound, a tide mill had been built in the past, and it is through its release channel that water exchanges currently take place between the Frêt pond and Brest's harbour. The pond's catchment area occupies a surface area of 2.70 km2, of which 1.8 km2 are being farmed and 0.5 are forest area (fig. 1). The average flow over the year is estimated at 58 l/s based on a specific flow of 21.4 l/s/km2. It is therefore about 1.8 million cubic metres of fresh water that passes each year through the pond. In the catchment area, the number of inhabitants was estimated in 1980 at 200, to which were added nearly the same amount of summer holiday makers. Although without any major source of contaminants, there are some discharges, as well as in Frêt's cove near the pond's lock. The mathematical model of currents set up for the entire Brest harbour indicates that the bodies of water that occupy the southern half of Frêt's cove are almost stationary. The discharges that flow into it are therefore likely to enter on the flood tide into Frêt's pond. After the flour business stopped, the pond was abandoned. The water exchanges between the harbour and the pond were then reduced to leaks through the lock, which had more or less deteriorated, and percolation that occurred through the mound of pebbles supporting the road. Because the lock was no longer opened periodically, the mound of pebbles, through its natural slippage to the northwest, ended up obstructing the relief channel's outlet. In 1982, the Conservatoire du Littoral bought Frêt's pond. Then, at the same time that it seems most urgently needed (reestablishment of management of the exchanges between the harbour and the pond, reconstruction of the built-up parts in danger of falling into ruin, the Conservancy called upon CNEXO to carry out a study that would offer recommendations appropriate for ensuring a satisfactory and ecological management of the pond.