Environmental

Managing sediment behind an 80-year-old dam in Switzerland

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Managing sediment behind an 80-year-old dam in Switzerland

Verbois Dam is located 12 km downstream of the city of Geneva, Switzerland, 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 and 340 m long and has four large outflow gates and four bottom gates along the left bank. On the right bank are four Kaplan turbine-generator units with a total of 100 MW hydropower capacity. 

The Verbois and Chancy-Pougny dams on the Swiss Rhône River trap a large quantity of sediment that is mainly contributed by the Arve River, a tributary of the Rhône River (see Figure 1). Owing to the location of Verbois Reservoir downstream of Geneva, 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 reservoir storage and reduce the risk of flooding the lower parts of Geneva. 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 an example of cross‐border coordination.

Figure 1. Location of dams on the Upper Rhône River (extracted from Bretier et al 2019)

Editor’s note: This article was originally published on the International Hydropower Association’s Hydropower Sediment Management Knowledge Hub and is reprinted with permission.

Hydrology and sediment

The Rhône River flows from the Swiss Alps through Switzerland and France to the Mediterranean Sea. At the Verbois Reservoir, the river contains water from the Lake Geneva outlet (mean annual discharge 252 m3/s) and from 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% settles in the reservoir. 

The sediments 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 is deposited at the upstream end of the reservoir and may be transported along the main channel during flushing.

‍Sediment challenges

About 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 in 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.

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 toward 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. These flushing operations, carried out every three years, allowed the removal of large volumes of deposited sediment to be transported downstream with an average sediment removal efficiency of 91%. After 70 years of operation, only about 17% of the initial volume was filled with sediment. However, the flushing operations had significant negative impacts on the river ecology and downstream infrastructure. Located 40 km downstream of Verbois Dam, Génissiat Dam 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, before discontinuing flushing from Verbois Reservoir, empty flushing was carried out after a period of nine years, resulting in releases of exceptionally high sediment concentrations (see Figure 2). Bathymetric surveys executed afterward 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 (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.

Figure 2. Change in sediment volume in the Verbois Reservoir (bars) and flushing efficiency

‍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 Figure 3), which was the least sensitive across different factors.

Figure 3. Assessment results of transboundary sediment management scenarios

‍New eco-friendly and transboundary flushing strategy

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 of:

1. 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. Lowering of the Swiss dams is carried out every three years, backed up by synchronized specific manouvers 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. Localized 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 million m3 of fine sediment with a mean controlled suspended sediment concentration of 3.28 g/Litre (max 11 g per Litre) released downstream. In comparison, mean sediment concentrations during the 2012 empty flushing was 11 g/L with a max of 48 g/L. The total reservoir emptying of the 2012 flushing resulted in a 57% 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 minimizing downstream environmental impacts on aquatic habitats (especially the fish community) because the concentrations of the released sediment were monitored and maintained at the reference measurement station of Pougny at the French-Swiss border (see Figure 4). The same standards respected at Génissiat Dam during flushing operations since the 1980s 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.

Figure 4. Location of Upper Rhône River dams, hydropower plants and reference monitoring stations for suspended sediment concentration

‍Conclusion

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 cooperation for sediment management across dams and reservoirs.

‍Acknowledgement

Financial and technical support by the Energy Sector Management Assistance Program (ESMAP) is gratefully acknowledged. ESMAP is a partnership between the World Bank and 22 partners to help low- and middle-income countries reduce poverty and boost growth through sustainable energy solutions.

ESMAP’s analytical and advisory services are fully integrated within the World Bank’s country financing and policy dialogue in the energy sector. Through the World Bank Group (WBG), ESMAP works to accelerate the energy transition required to achieve Sustainable Development Goal 7 (SDG7) to ensure access to affordable, reliable, sustainable and modern energy for all. It helps to shape WBG strategies and programs to achieve the WBG Climate Change Action Plan targets.