Lac des Dix - Underwater monitoring

In order to manage the sand-silting of the Lac des Dix, the former EOS dam, which is nowadays submerged most of the time, serves as a sand catcher.

It stops sediment and prevents its accumulation around intakes and outlets. An open breach in the wall of this old dam permits the transfer of water downstream and also lowers the working value below the former dam level. The owner plans to raise this breach to compensate for its sand-silting. Before doing so, he will need to know the exact conditions of the site in order to organise and plan the work.

 

 

 

The Mission

HYDRO Exploitation specialists were mandated to control the sand-silting condition of the breach and to ensure that it was still functional. They also had the task of controlling the outlets of the new dam. To complete its mandate, HYDRO Exploitation employed an underwater robot.

This underwater monitoring operation was carried out during one week in September 2009.

 

Intervention

• Collection of information and archives.
• Installation of the dam site: a pontoon, two boats, a robot.
• Place the pontoon, the boats and the robot into the water with the aid of a crane.
• Monitoring phase :
  • taking of pictures (overall views – close-up) and films with the aid of sound camera to a depth of 90m for the breach and 200 metres for the monitoring of outlets;
  • measurement of the level of the sand silting in the breach and the outlets.
• Dismantling of and withdrawal from the building site.
• Analysis and interpretation of the pictures and films.
• Drawing up recommendations.
• Writing and remittance of the final report.

Conclusion

The underwater monitoring of the Lac des Dix enabled the precise state of sand-silting of the breach in the former dam to be evaluated and the feasibility of the planned works to be confirmed. As far as the new dam was concerned, the project verified that none of the outlets presented any particular visible anomalies.

The comparison between the recorded information and the evolution of sand-silting enabled the desired work deadlines to be defined.