Water is moved between the two

Published on 31 January 2008 (updated on 1 February 2008)
A surge tank: seen from Le Collet
A surge tank: seen from Le Collet
(Art Technique)

Water moves between the two via a gallery which splits into three penstocks that carry water through a height difference of 700 metres at a steep gradient.

A so-called “head race gallery” 7,100 metres long connects the two dams via a pressure circuit. This gallery was excavated by a powerful tunnelling machine. It is 7.70 metres wide and has a concrete lining 30 to 40 cm thick. Two valves at its upstream entrance at the level of the intake structure allow the flow of water to be stopped. A flood discharge has also been provided with a maximum rate of 50 m3 per second.

The gallery then splits into three penstocks. At the junction between them there is a chamber, which can be accessed by a service shaft, which contains valves that control the rate of water entering the three penstocks. The penstocks are 1,800 metres long and horizontal for their last 300 metres. Before this there is a drop of 700 metres at a gradient of 56%.

Last, a surge tank with a diameter of 10 metres also located at the junction, absorbs water hammer in the hydraulic circuit. This is reinforced throughout its length and opens 200 metres higher up at a locality known as Le Collet which overlooks Verney pondage.