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Treauray Dam : a spillway upgrade in a record time!

The Treauray dam (Brittany, France) was built in 1959 to provide a raw water storage of 770 000m3 for the drinking water supply of Morbihan, a French department of Brittany.

The project owner, Eau du Morbihan, undertook a revision of the regional maximum flood according to the “guidelines for dam spillway design” established by the French Committee on Dams and Reservoirs (Cfbr). The study concluded that :
- Design flood: peak inflow of 135m3/s for a return period of 300 years
- Extreme flood: peak inflow of 237m3/s for a return period of 10 000 years

The original gated spillway capacity was limited to 116 m3/s and proved inadequate to fulfill its safety requirements. Preliminary studies conducted by Safège Engineering focused on two alternatives for the additional auxiliary spillway: Hydroplus Fusegates and Piano Key Weir. The Piano Key Weir discharge rate proved to be insufficient to evacuate the design flood without overspilling above the dam’s crest. In comparison, the Hydroplus system concentrated all the outflow through the auxiliary sill while restraining the water level below the dam’s crest up to the extreme flood.

Fusegates were selected by Eau du Morbihan to upgrade the spillway capacity. Ten precast Fusegates, 1.50m high by 2.40m wide, were installed on a new sill. The 24.0m wide auxiliary spillway was opened within the right side of the dam buttressed wall. The new concrete sill platform was poured on top of a combination of precast beams and slabs. The extensive use of precast elements allowed compressing the overall civil works on site to 3 months.