Upstream, this tunnel is extended by a variable sectioned, 939m-long pipe, which forms a circuit around the base of the Eau de Bérard valley, located at an altitude of 1992m, capturing small streams as it passes by means of summary intakes.

The West Collector tunnel was created from an intermediate hole located at a place called Loria. An intermediate hole, opening through the passage under the Tré-les-Eaux stream, was used to evacuate the section from the upstream section of the tunnel and is now used as access to the valve chamber of the corresponding water intake. A structure for measuring the inflows from the West Collector is installed at the downstream end of the tunnel at a place called Le Débouché.