Verbois Dam is located 12 km downstream of Geneva city on one of the major European Rivers, the Rhône River. The 11.4 Km long reservoir was built with a storage capacity of 13.8 million m3. The Dam is 34 m high, 340 m long and has 4 large outflow gates and 4 bottom gates along the left bank. On the right bank are 4 Kaplan turbines of 100 MW hydropower capacity.
The Verbois and Chancy-Pougny Dams are located on the Swiss Rhône River. They trap a large quantity of sediment that is largely contributed by the Arve River, a tributary of the Rhône River (see Figure 1). Owing to the location of Verbois reservoir, which is just downstream of Geneva City, sediment accumulation in the reservoir increases the risk of flooding. Empty flushing (also known as drawdown flushing) of Verbois Dam has been carried out since its completion in the 1940s to preserve the reservoir storage and reduce the risk of flooding the lower parts of Geneva City. While empty flushing had significant impacts on the downstream environment and reservoirs, an environmentally friendly approach to partially lower the reservoir in coordination with downstream French reservoirs was implemented from 2016 in a successful example of cross‐border coordination.
Hydrology and sediment
The Rhône River flows from the Swiss Alps through Switzerland and France to the Mediterranean Sea. At the location of Verbois reservoir, the Rhône River contains water from the Lake Geneva outlet (mean annual discharge 252 m3/s) and water of the Arve River (mean annual discharge 80 m3/s) which confluences with the Rhône River 2 km downstream of Lake Geneva and 8 km upstream of Verbois reservoir. Despite its much lower water contribution, the Arve River provides almost all the sediment load to the reservoir because of the intense erosion occurring in its alpine watershed that includes the Aiguilles Rouges and Mont Blanc mountains. The average annual sediment load is about 1 million tons (700,000 m3 per year) of which 50 percent settles in the reservoir.
The sediments deposits largely consist of fine sand and silt deposited in meanders and other locations of low flow velocity, with grain sizes decreasing from the upper to the lower reservoir basin. Gravel from the Arve River are deposited at the upstream end of the reservoir and may be transported along the main channel during flushing.
Approximately half of the annual average sediment flows (360,000 m3) are trapped in the reservoir, leading to bed aggradation and storage loss. Consequently, the reduction of the reservoir’s water storage capacity leads to a rise of the water level and increased flood risk in sensitive sites such as residential and industrial areas located upstream of the dam constituting part of Geneva City.
Whereas empty flushing has been carried out every three years since the dam was commissioned, flushing operations released high sediment loads resulting in major environmental impacts, mainly on fish populations, and increased sediment accumulation in the reservoirs downstream. Impacts were not restricted to a local scale but included the whole upper-Rhône River downstream towards Lyon. The growing discontent from civil society, nature conservancy organizations, fishermen associations and water management authorities led dam operators to consider alternative, less harmful sediment release strategies. Sediment flushing was stopped until 2012, until a new environmentally friendly management strategy was introduced from 2016 onwards.
History of sediment management at Verbois Dam
The Services Industriels de Genève (SIG) have regularly flushed out sediment from Verbois reservoir since its completion in 1942 to prevent flood risk to the lower parts of Geneva City. These flushing operations, carried out every 3 years, allowed the removal of large volumes of deposited sediment to be transported downstream with an average sediment removal efficiency of 91 percent. After 70 years of operation, only about 17 percent of the initial volume was filled with sediments. However, the flushing operations had significant negative impacts on the river ecology and downstream infrastructure. 40 km downstream of Verbois Dam, Génissiat Dam located in France regulated flow releases with limited sediment concentrations that the environment could bear. This strategy resulted in large-scale storage loss in Génissiat reservoir due to sedimentation.
In 2012 prior to discontinuing flushing from Verbois reservoir, empty flushing was carried out after a period of nine years, resulting in releases of exceptionally high sediment concentrations. Bathymetric surveys executed afterwards revealed that more than 200,000 m3 of sediment had deposited in the Chancy-Pougny reservoir. This prompted stronger transboundary collaboration between the French and Swiss authorities and three industrial operators (Services Industriels de Genève - SIG, the Société des Forces Motrices de Chancy-Pougny - SFMCP and Compagnie Nationale du Rhône - CNR) to manage sediment fluxes across the Upper Rhône River.
After that experience, development of an appropriate approach for sediment flushing considered various factors including environmental impacts, cost to operators, impacts on river users, risks to infrastructure and public health and technical feasibility to carry out the operation. A consensus was reached in 2014 around the ‘M’ scenario (see Fig. 4) which was the least sensitive across different factors.
New eco-friendly and transboundary flushing strategy: Controlled Sediment Flushing
The new approach to sediment management developed in 2014 by the Swiss and French authorities to manage sediment fluxes across the Upper Rhône River consists of a threefold procedure constituting:
1. A partial lowering of Verbois reservoir by 10 to 12 m instead of 18 m as previously done for a complete lowering, to control the suspended sediment concentration. The lowering of the Swiss Dams is carried out every three years, backed up by synchronized specific manoeuvres at the French reservoirs downstream.
2. Collaboratively opening the dam gates across Swiss and French Dams during the Arve River floods, to facilitate sediment transport as far downstream as possible; and
3. Localised complimentary dredging
The first partial lowering of Verbois reservoir under the new sediment management plan took place in May 2016. The operation lasted nine days and evacuated 1.3 Mm3 of fine sediment with a mean controlled suspended sediment concentration of 3.28 g per Litre (max = 11 g per Litre) released downstream. In comparison, the mean sediment concentrations during the 2012 empty flushing was 11 g per Litre with a max of 48 g per Litre. The total reservoir emptying of the 2012 flushing resulted in a 57 percent decrease in fish mass, with no recolonization process during the 16 following months, while the partial emptying of the 2016 flushing did not modify fish mass between the pre- and post-flushing periods. Controlled sediment flushing was therefore successful at minimising downstream environmental impacts on aquatic habitats (especially the fish community), since the concentrations of the released sediment were monitored and maintained at the reference measurement station of Pougny located at the French-Swiss border (see Figure 5). The same standards respected at Génissiat Dam during flushing operations since the 1980’s were adopted at Verbois Dam to maintain the threshold for suspended sediment concentrations at 5 g/L during the entire operation, complemented by further restrictions of 10 g/L and 15 g/L for maximum continuous periods of 6.5 hours. To achieve an efficient lowering operation, a real time monitoring system was implemented by Swiss and French operators 24 hours a day, enabling fine operational adjustments at Pougny and Seyssel to control suspended concentrations at reference measurement stations.
The new sediment management strategy combining partial emptying of the reservoir with opening dam gates during floods and localized dredging proved to minimize sediment accumulation in Verbois reservoir while concurrently preventing downstream environmental impacts. The collaboration to achieve sediment continuity along the upper-Rhône River between the Swiss and French operators along with both countries’ administrations is a good example of transboundary co-operation for sediment management across dams and reservoirs.
Loizeau, Jean-Luc, Justrich, Stéphanie, Wildi, Walter. Swiss examples of the impacts of Dams on natural environments and management strategies for sediment control. In: J. Dominik, D.V. Chapman, J.-L. Loizeau, A. Stanica and D.A.L. Vignati (Ed.). NEAR curriculum in natural environmental science. Genève : [s.n.], 2010. (Terre et Environnement; 88) https://archive-ouverte.unige.ch/unige:90834