Seven years after the start of the construction works at Vieux Emosson, the reservoir has reached full capacity for the first time to supply water for the generation of electricity in the future. While the reservoir was being filled – the process having begun in the spring of 2017 – surveillance of the dam was reinforced. In addition to daily telemetry readings and weekly monitoring measures (with pendulums, rock meters etc.), further steps were initiated. Additional research was carried out at each stage, involving geodesic measurements, a detailed site inspection, opening of the outlet gates, and inspections by experts and the Swiss Federal Office of Energy. The reservoir priming programme is continuing with the first draining, which is due to start in spring 2020. During this phase, Vieux Emosson will again be subject to extra monitoring.
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OKVieux Emosson dam height raised: reservoir now filled!
The Vieux Emosson reservoir at the beginning of October 2019
The reservoir was drained in 2012 to allow the height of the dam to be increased, with the aim of meeting the needs of the Nant de Drance pumped storage power plant. Between 2012 and 2014, the dam’s height was raised by 21.5 m, enabling a doubling of the storage capacity of the reservoir, and so adding to the flexibility of the future power station. What was originally an arch-gravity dam has been transformed into a double arch structure. Due to its design geometry, the upper part of the dam was initially reduced by 10 metres on the upstream side and by 20 metres on the downstream side. The total height the dam has increased from 55 metres to 76.5 metres. Further rock injections were carried out in 2015 and concrete injections in 2016.
Key figures
| Before | After | |
| Years under construction | 1952 - 1956 | 2012 - 2016 |
| Type | Arch-gravity dam | Double arch dam |
| Maximum height | 55 m | 76.5 m |
| Top water level | 2,205 m. above sea level | 2,225 m. above sea level |
| Crest length | 170 m | 205 m |
| Reservoir volume | 11.2 million m3 | 23.5 million m3 |
| Owner | Swiss Federal Railways | Nant de Drance SA |
