Overview

Génissiat is a 70 m high dam located on the Rhône River. The Rhône originates in the Swiss Alps and flows through Switzerland and France to the Mediterranean Sea, with a cascade of 7 dams. The dam impounds a 23 km long reservoir with an initial storage of 56 million m3. Approximately 25percent of the reservoir's storage has been lost to sedimentation to-date. Génissiat is located in France, downstream of two dams on the Swiss Rhône river (Verbois and Chancy Pougny). The upstream dams trap a large quantity of sediment from the Arve River. The five dams in the French Rhône are jointly operated by Compagnie Nationale du Rhône (CNR). The high sediment loads from the tributary Arve River are safely managed through the Génissiat dam through eco-friendly flushing to maintain downstream river health and the reservoir storage.

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Hydrology and sediment

The Rhône River is one of the major rivers of Europe. At the outlet of the catchment, the river delivers a mean annual discharge of 1700 m³/s, for a basin area of 95,500 km². In its Swiss part, the Rhône is a mountainous river with steep slopes and high sediment transport capacities. After crossing Lake Geneva, the river is characterized by very low sediment fluxes due to the trapping effect of that natural reservoir. 2 Km downstream of Lake Geneva, the Rhône confluences with the tributary Arve River, which carries high sediment loads from the steep mountain range of Mont Blanc whose catchment is affected by intense erosion processes.

The total sediment supply of the Arve River has been estimated between 1.0 and 3.0 Mm³ per year (Sogréah, 2000), providing almost all the sediment load in the Upper Rhône River. The volumetric ratio between bed load and suspended load is around 1Percent. The catchment area after the confluence is 10,320 km², delivering a mean annual discharge of 335 m³/s.

Most of the hydropower plants operated by CNR are run-of-river, except for three, including the Génissiat Dam in the Upper Rhône. It is crucial to keep the fine suspended sediment concentrations downstream of the dams low enough for several river uses, such as supporting aquatic life, and the operation of intakes for cooling systems of nuclear power plants, well-fields for drinking water supply and bathing areas. Moreover, floodwater routing must be ensured through guaranteeing adequate hydraulic capacity of channels and by avoiding adverse obstructions of dam spillways, possibly resulting from sediment deposits. Good navigation conditions also have to be maintained through regular monitoring and maintenance of the riverbed. To deal with these requirements, CNR has adopted an overall sediment management program established under the supervision of the French authorities. Complementary to this masterplan, “environmentally friendly flushing” operations are also regularly conducted at the Génissiat dam as a result of the historical transboundary issues of the Rhône River.

14 m m3 of Génissiat’s reservoir storage volume has been lost due to sedimentation (25percent of initial storage). Gravel and coarse sand have been deposited in the upper part and a predominance of silt in the last 10 km of the reservoir.

Sediment challenges

Downstream of the confluence between the Avre and Rhône Rivers, two dams are operated in series on the Swiss territory. The Verbois dam (commissioned in 1942) is upstream of the Chancy-Pougny dam (commissioned in 1926). The Verbois reservoir contains 500,000 m³ of deposited sediment, which is of great concern to the lowest parts of Geneva City because the bed aggradation increases flood hazards significantly.

Since the Verbois dam’s construction, flushing procedures have been refined. In particular, it has been decided to use artificial flood discharge from Lake Geneva to mobilize the stored sediments.

The French section of the upper Rhône River is characterized by several hydropower developments carried out by CNR. The first two are the 70-meter-high Génissiat dam and the Seyssel run-of-river development, respectively commissioned in 1948 and 1951. Between 1980 and 1986, four more run-of-river developments, Chautagne, Belley, Bregnier-Cordon and Sault-Brenaz, have been constructed by CNR.

Sediment management

The main reason to conduct flushing operations on the upper Rhône River is to mitigate flood

hazards due to the bed aggradation caused by sedimentation in the Verbois reservoir in the lowest parts of Geneva City. These operations have been conducted for 40 years at the request of the Swiss authorities with the assistance of both Swiss hydropower operators and CNR. The releases from the Verbois dam increases the sediment load at the Génissiat dam, leading to sediment deposition in its upstream reservoir.

History, framework and regulations

Since 1981, CNR conducted environmentally friendly flushing to release suspended sediment concentrations tolerable by the fluvial environment downstream of the Génissiat dam. This followed the 1967 French-Swiss agreement (updated in 1981) to improve the coordination of the flushing process between the Verbois and Génissiat dams carried out every three years between May and June. Full drawdown flushing had been performed every three years by the Verbois dam, since its completion in 1942, to prevent increased flood hazards that may be caused by sedimentation in the reservoir. Full drawdown flushing led to very efficient release of sediments in large quantities, but the high concentrations (reaching 40 g/l) negatively affected aquatic life as large quantities of sediment were released from the Verbois dam during these operations. To counter these effects, the eco-friendly sediment releases from the Génissiat dam follow specific restrictions to maintain bearable life conditions for the aquatic fauna and prevent adverse effects on downstream ecosystems of the French Rhône River.

Sediment flows are routed and regulated by CNR across the Génissiat dam within the following limits:

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• not to exceed 5 g/l on average over the entire operation;
• 10 g/l on average over any 6 hours period; and
• 15 g/l over any 30 minutes period.

The dam is equipped with three outlets that make the operations possible: a bottom gate (LLO), an outlet at halfway up the dam (ILO), and a surface spillway (HLO).

The coordinated eco-friendly flushing operations have reduced deposition of sediment in the Génissiat reservoir by 80percent. While sediment deposits of 23 Mt could have been expected in the reservoir since commissioning, only 4.5 Mt has been deposited since. In addition, this has improved the safety of the Génissiat dam by avoiding blockage of the bottom gate, limiting the impact of the flushing operation on the fluvial environment, preserving the natural sections of the Rhône River, and limiting the impact of sediment releases on water use for human activities, such as the water intake for a nuclear plant and the wellfield for drinking water of Lyon City both located downstream of the dam.

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Flushing operations, including monitoring

Using the principle of environmentally friendly flushing, the sediment concentration from flows discharged from the Génissiat dam is monitored and controlled to levels favourable that for the downstream fluvial environment. This principle has been applied at the dam over 40 years in two steps:

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• Firstly, lowering the water level in the Génissiat reservoir to flush out sediment that had previously accumulated in the reservoir; and
• Secondly, routing sediment flows from upstream dams through the reservoir. Upstream Swiss dams are opened in a coordinated manner to discharge flows with high sediment concentration to the Génissiat reservoir.

Managing the water level in the reservoir is critical to ensure successful operation of eco-friendly flushing. For instance, having the water level too high in the reservoir during routing may result in high settlement of inflowing sediment within the reservoir, while very low water levels would result in the release of high sediment concentrations from the Génissiat dam. Thus, maintaining specified levels of clear water in the reservoir facilitates the discharge of controlled sediment concentrations from the dam.

Since 1997, the specific procedure that has been applied is:

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• First the Génissiat reservoir water level is lowered by about 20m during the first week of the flush, and then raised by 10m during the second week to accommodate the Verbois dam flushing.

• Then, an appropriate gate opening program and mixing of the sediment laden flows released by each of the three outlets (i.e. mixing water with high sediment concentrations from the bottom of the water column with enough “cleaner” water from higher in the water column) are performed, to stay within the required concentrations further downstream.

• Finally, the LLO discharges high sediment concentrations, the ILO releases flows with lower sediment concentrations, and the HLO discharges clear water (Figure 3).

The high sediment concentration is monitored and controlled along the reservoir with strict regulation to maintain specified sediment concentrations downstream of the dam. This is ensured through continuous monitoring of suspended sediment at the entrance of reservoir, at the outlets and immediately downstream of the dam during the entire operation, using real-time monitoring devices (Gamma Ray devices) and several gauging stations that are synchronized with the command centre. Other complementary field monitoring methods include Picnometre (water density measurement with temperature correction) and the Pan Cake method (filtering, quick drying and weighting). Concurrently, ecological surveys and monitoring of aquatic life is carried out in the field to identify ecological impacts during the flushing operations.

Specific actions may vary depending on the flushing conditions. For instance, when sediment concentrations exceed 2 g/l during a continuous period of 1 hour at Seyssel sediment gauging station (downstream of Génissiat), dams located downstream are closed to preserve the ecological integrity of the Old Rhône River. All hydropower facilities downstream of the Génissiat dam are low head run-of-the-river systems.

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One major setback of the flushing operation is its high operational cost in terms of energy losses and labour requirements. The operation is carried out over a period of 10 days and employs 400 people, 24 hours a day. For example, in previous years, the costs are estimated at 1.4 M€ (62 percent energy losses, 15 percent for staff costs and 23 percent for subcontracted services (impact surveys, specific monitoring, communication etc.). Nevertheless, given the high volume of sediments flushed during the operation (1.8 Mt in 2003), the current flushing operation provides a much more cost-effective alternative compared to other techniques like dredging in the reservoir.

Conclusion

Several key factors allow eco-friendly flushing at the Génissiat dam:

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• Adequate design and positioning of the three dam outlets at different levels
• Adhering to the concentration thresholds of sediment discharges
• Optimization of operation rules that allow for the progressive recovery of natural flow conditions. This procedure was developed based on long and practical experience and feedback from field observations like bio-parameter measurement and electrical fishing before and after flushing.
• Regular monitoring of the river channel and hydraulic structures, and finally adaptive management of the river depending on a comparison of target states and observed evolution.  

Consequently, the sedimentation rate of the reservoir has been reduced by half and only 15 percent of the inflowing sediment is trapped within the reservoir. In addition, the downstream sediment transfer throughout CNR run-of river facilities has been also increased by 50 percent compared to previous events. Additional gains are expected as the upstream Swiss operators comply with the same restrictions on suspended sediment concentrations released from the dams since 2016.

With the benefit of such design and operation patterns, the impact on sediment fluxes is significantly minimized. These changes have been observed during the continuous monitoring of the river and hydraulic structures performed by CNR with a fleet of hydrographic boats. Additionally, a comprehensive update of the bathymetric state of the river is carried out with a minimum frequency of five  years or after significant floods.

There is a management plan in place (updated every 10 years) for the maintenance of sediment deposits to keep adequate conditions for navigation, dam operation and hydraulic safety.

The sediment deposits are artificially resuspended by the flow and deposited on the riverbed further downstream, considering fine and coarse particles respectively. This requirement minimizes the potential disruption of sediment continuity down to the Rhône River Delta and contributes to prevent its degradation.

The management scheme defined and implemented by CNR has demonstrated that achieving sustainable management of sediment fluxes through hydropower cascades is possible.

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References

Guertault, L., Camenen, B., Peteuil, C. and Paquier, A., 2014. Long term evolution of a dam reservoir subjected to regular flushing events.

Peteuil, Christophe. 2012. Eco-friendly flushing downstream Génissiat dam, French upper Rhone River, France.

Peteuil, Christophe. 2018. Sustainable Management of Sediment Fluxes in the Rhône River Cascade.

Acknowledgement

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