CNR model

New French-Swiss management of the sediments of the Rhône

Ensuring the transit of sediments: a necessity

The Verbois reservoir in Switzerland has been flushed regularly by its operator the Services Industriels de Génève since it was commissioned in 1942. Why? Well, to prevent several districts of Geneva from being flooded due to the accumulation of sediments (clay, silt, sand, etc.) transported by the River Arve, a tributary of the Rhone, just downstream of the city.

32 - Vue aérienne EtournelThese operations are carried out every 3 to 4 years. The reservoir is lowered partially, causing the discharge of the sediments along the entire river and obliging CNR to assist these operations and monitor its own hydropower installations from the Swiss border to Lyon. The aim is to reduce the impacts and guarantee hydraulic safety, water quality and more generally the environment.

The Swiss flushing operations of 2012 (….) stood out due to exceptional conditions: the quantity of material removed from the Swiss reservoirs was three times greater than the previous operations and fish mortaility was observed upstream of Génissiat. CNR conformed to its commitments to ensure the transit of the sediment in the perimeter of its concession and did not generate any impact on the environment.

Following this new scenarios were studied and assessed to reduce risks and optimise sediment management on the Upper Rhone.

New joint and concerted management over 10 years

Centrale hydroelectrique Franco-Suisse sur le Rhone, Chancy, SuisseThe 3 industrial operators (SIG and SFMCP which operate the dam of Chancy-Pougny, and CNR) and the French Swiss authorities which issue the authorisations, decided on a new mode of sediment management according to a ten year plan. This plan provides for wide-ranging consultation between the stakeholders (elected representatives, environmental associations, the neighbouring population and industrial companies) during public meetings, and was officialised in May 2015 (….).

This new mode is composed of 3 sections and has been established for the period 2016-2026

  • The partial lowering of the Verbois reservoir (10 to 12 m instead of 18 m) and Chancy-Pougny every 3 years assisted by specific management by CNR of its 6 hydropower installations on the Upper Rhone.
  • Additional dredging performed on the reservoirs to prevent their filling and to ensure better management of the volume of material to be removed.
  • Support from the SIG regarding the flooding of the Arve to favour the transport of sediment as far downstream as possible, aided by the increased flow rate of the river.

A new and important aspect for risk management is that the Services Industriels de Génève have undertaken to control the rate of suspended matter at the Swiss border according to the standards already applied by CNR since 1981 at Pont de Seyssel. This will prevent the massive arrival of material in the Génissiat reservoir, the first of the French hydropower cascade on the Rhone.

At the request of the Services Industriels de Génève, the first operations to partially lower the water level of the Verbois dam are scheduled to take place from 19 to 31 May 2016.
See the communication of the Prefecture of Ain (…).

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