FR EN ES Search
  1. Home
  2. Past Events
  3. Sediment management on the Rhone a fine example of cross-border cooperation


All the news

Sediment management on the Rhone : a fine example of cross-border cooperation


Last May, on the occasion of the operation to manage sediments on the Upper Rhone, French Swiss cooperation acted in the field to perform a mission that brought together the authorities and industrial operators of both countries.



The French and the Swiss have learned to work together and benefit from feedback from experience acquired from operations aimed at emptying the reservoir dam of Verbois in Switzerland to avoid flood risks to the low-lying districts of Geneva. The sediments removed from this dam are flushed in diluted form downstream towards France. The manoeuvres regarding the operations of the dams are highly technical and difficult, since they must not exceed the maximum rates of suspended matter. They are completed by permanent monitoring of water quality and biodiversity reserves throughout the operations.

In 2012, the rates of suspended matter coming from Switzerland upstream were difficult to control and threatened fish life (read the box “Sediment management, a sensitive operation”). A technical committee was set up following these operations in view to studying t

he new procedures for maintaining the Verbois

reservoir and managing the sediments. The French and Swiss authorities and industrial groups met together for two years to determine a strategy. Since 2016, the sediment management mission, called APAVER (French acronym for the Partial Lowering of VERbois), has been managed jointly by the operators of both countries, according to the procedure established by the technical committee.


A unique management mode

“At present, we’ve moved on from suspicion to confidence,” said Laurent Tonini, Territorial Director for the Upper Rhone at CNR, the manager of the River Rhone from the Swiss border to the Mediterranean Sea. The procedure implemented, known as mixed sediment management, is based on three pillars (see box). It has now become a reference in the international scientific community.

“As early as 2012, Laos showed interest in our sediment management procedures,” said Daniel Jouve, Director of Engineering and Major Projects. We received a response from the Laotian Vice-Minister of Energy and Mining, since this country had 5 hydroelectricity projects on the Mekong upstream of Vientiane, the capital. The managers had been warned by NGOs of the stakes involved by sediment management. Following this visit and recommendations from CNR, the Laotian projects were revised to optimise their dimensioning and the installation of lower level and bottom gates, like at Génissiat, that allow the passage of sediments.”

Following this, CNR received a large number of delegations and forged links with world renowned scientists, leading in particular to the current partnership with the University of Stuttgart. Exchanges also took place with the Mekong Commission that groups Laos, Vietnam, Cambodia and Thailand. “We were able to share a great deal of experience and knowledge, especially regarding the measurement of solid flows.”


The importance of measurements

Measurement is pivotal in sediment management operations. Water quality and rates of suspended matter (SM) must be monitored to protect aquatic fauna. France and Switzerland coordinate their actions to lower the reservoir of Verbois very closely.

We constantly exchange information, whether data on discharges, heights or MS rates,” declared Laurent Tonini. “To do that, we have ensured that our information systems can communicate with each other. Lastly, several meetings are organised every day between the management stations, which are in contact with each other 24 hours a day.”

This procedure is far from being applied in all countries.


A potential cause for conflict  

Sediment removal is not integrated by all companies responsible for developing infrastructures,” explained Daniel Jouve. “Different countries have different cultures regarding this issue. For example, the United States does not intervene in the management of sediments stored in reservoirs. That’s one of the reasons why an increasing number of American dams are erased, that’s to say eliminated. There are certainly ecological objectives, and also certain American dams can no longer store water, since they are too clogged by sediments. Other countries apply what could be termed brutal practises: the gates are opened without any heed taken of the impacts occurring downstream.”

Such practises are not beneficial for relations between countries or for the health of ecosystems. “A river cannot be managed in small segments. It must be considered in its entirety,” observes Elisabeth Ayrault, CEO of CNR, who praises French-Swiss cooperation. Sediment flows have an impact in particular on the life of the river, from upstream to downstream: water quality, the preservation of biodiversity, deltas, coastlines, etc.  “In the same way as we’re seeing conflicts over the way the water of the same river is shared, sediment management gives rise to international tensions. The Mekong delta is sinking because the sediments no longer reach it. Afterwards, it’s difficult to know what’s held back by structures and what is taken out of the rivers,” analyses Daniel Jouve.


Sediment management is a sensitive affair

Trapped by dams, sediments accumulate in reservoirs. Over time, this inflow of matter leaves less and less place for water, thereby aggravating the risk of flooding for populations and property downstream. Therefore, it is necessary to periodically remove these sediments, on the one hand to prevent floods, on the other to allow these sediments to nourish the river and its ecosystem. This removal of sediments, sometimes called “flushing”, cannot be done without taking into account the repercussions. If too concentrated, the fauna and flora are asphyxiated. It is necessary to both ensure the removal of the sediments further downstream while limiting their impacts on the environment; thus, there is a delicate balance between protecting property and people and protecting the environment.

Since 2016, Swiss regulations have been aligned with French standards and impose an average maximum threshold of 5 grams of suspended matter per litre of water, i.e. the equivalent of a teaspoon. This sharing of the same standards has facilitated the implementation of “mixed sediment management” on the Rhone by the two countries.

It is based on three principles:

  • The partial reduction of the Verbois reservoir takes place every 3 or 4 years. It is assisted by CNR’s specific management of its 6 dams installed on the French Rhone.
  • The management of the floods of the Arve, a tributary of the Rhone, which transports nearly 700,000 m3 of sediments a year of which half is deposited in the Verbois reservoir. The aim is to favour the transport of the sediments carried by the Arve as far downstream as possible.
  • Lastly, dredging is carried out periodically in the reservoirs to extract part of the sediments lying on the bottom.


Programmed in 2020, the lowering of the Verbois reservoir had to be postponed until 2021 due to the health crisis. “This year, it is estimated that 1.2 million tonnes of sediments were released into the Rhone on the Swiss side”, declared Laurent Tonini. 300 CNR employees were called on to assist this operation on the French stretch of the Upper Rhone. This year, the operation was complicated by the COVID-19 crisis and worsened by extreme hydrometeorological conditions, with heavy rainfall and several floods occurring on the tributaries of the Upper Rhone at the beginning and during the operation.

It’s a unique and exemplary experience. There are so many countries that enter into dispute around water issues. Here, we demonstrated our ability to work together. It’s a real human adventure that federates our teams. I’m very proud of them and the work achieved”, concludes Laurent Tonini, a few days after the end of the operation.


More information about CNR engineering here

Mettez à jour votre navigateur pour consulter ce site