Geographic context. Hydropower developments on the upper Rhone River

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1 Environmental friendly flushing downstream Genissiat dam, French upper Rhone River, France Christophe PETEUIL Compagnie Nationale du Rhone Engineering Department, River Systems and Climate Hazards Division 2, rue Andre Bonin Lyon Cedex 04 - France c.peteuil@cnr.tm.fr Geographic context The Rhone River is one of the major rivers of Europe. Flowing from the Swiss Alps down to the Mediterranean Sea, the river winds through the French territory for 2/3 of its course (Figure 1). 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 Rhone is still a mountainous river with steep slopes and high sediment transport capacities. But after crossing the Lake Geneva, the river is characterized by very low sediment fluxes due to the trapping effect of that natural reservoir. Figure 1: General location of the Rhône River and overview of the valley between the Swiss border and Lyon with Bourget Lake (France) in the background. Immediately downstream of the Lake Geneva, the Rhone River finds its confluence with the Arve River, which is one of its major Alpine tributaries. After the confluence area, the Rhone River catchment is around 10,320 km², delivering a mean annual discharge of 335 m³/s. The Arve River comes from the steep mountain range of the Mont Blanc and its catchment is affected by intense erosion processes. Thus, this tributary provides a significant amount of sediment to the Rhone River (Figure 2). The total sediment supply of the Arve River has been estimated between 1.0 and 3.0 Mm³ per year (Sogréah, 2000). The volumetric ratio between bed load and suspended load is around 1%. Hydropower developments on the upper Rhone River Downstream of the confluence, 2 successive dams are operated on the Swiss territory. The upper one is the Verbois dam. It was built in 1942 to replace the Chevre dam, an outdated development operated since The other one is the Chancy-Pougny dam, which has been in operation

2 since The observations available on the Verbois reservoir point out that the sediment deposits due to the dam impact usually affect an annual volume of 500,000 m³. This long standing situation is a great concern for the lowest parts of the Geneva City because the bed aggradation increases significantly the flood hazards. Figure 2: Confluence between the Rhône River (left) and the Arve River (right) showing a remarkable difference in sediment concentration between the two streams. Since the construction of the Chevre dam, the need to maintain an acceptable bed elevation for the safety of the Swiss riverine people has justified to conduct regular flushing operations. After Verbois dam construction, the flushing conditions have started to be better formalized. In particular, it has been decided to use the artificial flood discharge provided by the Lake Geneva for the mobilization of the stored sediments. In the meantime, the French section of the upper Rhone River experienced several hydropower developments carried out by the Compagnie Nationale du Rhone (CNR). The two first ones are the 70 meter high Genissiat dam (Figure 3) and the Seyssel run of the river development, respectively in operation since 1948 and Between 1980 and 1986, 4 more run of the river developments, the so called Chautagne, Belley, Bregnier-Cordon and Sault-Brenaz, have been also put into operation by CNR (see communication of Peteuil in the same Feature Session for the main characteristics of these developments). Framework of the sediment flushing operation In 1967, a French-Swiss agreement was signed to improve the coordination of the flushing process. This agreement includes in particular a definition of the frequency and of the time schedule for the operation. Updated in 1981, the current layouts are finally formalized as follows: Organization of the sediment flushing every 3 years, Timing between the end of May and the beginning of June. Several technical, environmental and administrative requirements are considered by CNR, after a large cooperation with the French authorities. For instance, at Genissiat dam and further downstream, CNR has to deal with the following challenges:

3 Ensure the safety of Genissiat dam by avoiding the obstruction of the bottom gate, Limit sediment deposits in the reservoirs managed by CNR, Pass the sediment fluxes coming from upstream, Regulate significantly the concentration of suspended sediments released from Swiss dams in order to limit the impact of the flushing operation on the fluvial environment. Preserve the natural sections of the Rhone River, so called Old Rhone, and in particular those recently restored for ecological purposes. These restoration works have been funded and carried out by CNR in the framework of its missions conducted for the general interest of the Valley. Limit the impact of sediment release on human activities closely connected with the river (water intake of Bugey nuclear power plant, wellfield for drinking water of Lyon City ). Principle of the environmental friendly flushing The principle of the environmental friendly flushing that CNR has carried out for 40 years is to send downstream of Genissiat dam only a concentration of suspended sediment that the fluvial environment can withstand (Fruchart, 2008). Three major points make this objective possible: (1) absolute respect of concentration thresholds for sediment released, (2) optimum design of the Genissiat dam and (3) procedure based on a long and practical experience. First of all, the suspended load released downstream of Genissiat should not exceed a maximum level of concentration. Different thresholds have been defined in cooperation with the French authorities and according to aquatic life considerations. The maximum limits admitted are as follow: Average concentration during the entire operation: below 5 g/l Average concentration during a continuous period of 6 hours: below 10 g/l Average concentration during a continuous period of 30 minutes: below 15 g/l Knowing that the Swiss operators have not to cope with such constrains and that sediment concentration up to 40 g/l is frequently released from the Swiss dams, the difficulties of CNR s challenge can be easily understood. The definition of these thresholds results from a 40 year feedback based on many observations like bio-parameters measurement, electrical fishing before and after the flushing Unfortunate operations, like in 1978, turned out to be very thoughtstimulating and have also led to significant improvements of the procedure. The design of the Genissiat dam includes 3 hydraulic outlets located at different levels (Figure 3): a bottom gate with a hydraulic capacity of 500 m³/s (intake level at m), a half depth gate with a hydraulic capacity of 1,500 m³/s (intake level at m) and a surface spillway with a hydraulic capacity of 1,200 m³/s (intake level at m). During the flushing operation, the reservoir level is lowered in order to increase the shear stress and ensure the sediment transportation. This procedure is also followed for obtaining a vertical gradient of concentration for fine materials. Then, the great concentration flowing through the bottom gate is diluted by the less turbid water provided by the half depth gate in order to regulate in real time the appropriate concentration downstream of the dam (Figure 4). If necessary, the surface spillway can also be used for providing even more "clear" water.

4 Figure 3: (a) Aerial view of the Génissiat dam during the 2012 flushing operation; (b) Detailed view on the outlets of the bottom gate (bottom left corner of the picture) and of the half depth gate (upper right corner of the picture). Figure 4: Principle of suspended load dilution at Génissiat dam.

5 The operation includes two successive steps. First, solid materials previously stored into Genissiat reservoir are progressively flushed. Then Swiss dams are opened, releasing high concentration of sediment in the river. But thanks to the sediment control allowed by the Genissiat dam operation, this unrestricted supply of sediment is significantly regulated downstream of the reservoir. Concentrations of suspended sediment are measured in real time by Gamma Ray devices and other complementary field methods: Picnometre (water density measurement with temperature correction) and Pan Cake method (filtering, quick drying and weighting). Several gaging stations for suspended sediment are located at different key points. These stations provide data to the Genissiat dam command centre throughout the duration of the operation. In the meantime, many ecological surveys are conducted by CNR in the field for a direct monitoring of the flushing impacts on the aquatic environment. Specific procedures can also be applied during the operation. For instance, if the sediment concentration exceeds 2 g/l during a continuous period of 1 hour at Seyssel sediment gauging station, dams located downstream are closed to preserve the ecological integrity of the Old Rhone (Figure 5). Figure 5: Closure of the Lavours dam during a flushing operation in order to preserve the Old Rhone.

6 Lessons learned The main reason to conduct flushing operations on the upper Rhone River is to prevent flood hazards due to the bed aggradation induced by the Verbois dam in the lowest parts of the 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. Thanks to an original procedure, the aquatic life and the other interests at stake downstream of Genissiat dam are weakly impacted by the flushing operation. Note that the major improvement is certainly to have taken into account of relevant sediment concentration limits for preserving the ecological balance of the fluvial environment. Given the sedimentation rate of the Genissiat reservoir, the environmental friendly flushing that CNR has carried out for 40 years appears to be an appropriate operation. While a sediment deposit of 23 Mt could have been expected in the reservoir since the dam completion, only 4.5 Mt have been stored inside the reservoir (Figure 6). Figure 6: Cumulated suspended load fluxes released downstream of Swiss dams and stored inside Génissiat reservoir (adapted from Thareau et al., 2006). For CNR, such an operation is nevertheless costly. It mobilizes instantaneously 120 people from the company during approximately 10 days. But as the operation lasts 24 hour a day, the overall staff needed is in fact around 400 people. Based on the figures of the 2003 flushing (Thareau et al., 2006), the cost evaluation is around 1.4 M distributed as follows: 62% for energy losses, 15% for staff costs and 23% for subcontracted services (impact surveys, specific monitoring, communication...). Given the volume 1.8 Mt of sediments flushed during the 2003 operation, this is much more cost effective than more expensive techniques like dredging.

7 By experience, organizing flushing is not always easy when different operators have to deal with their own constraints, and especially in case of cross-border rivers. In the case of new hydropower developments, the situation could be clearly facilitated by imposing in the concession contract to flush the reservoirs in cooperation with other operators, if any. References Cited Fruchart F., 2008, Why and How to flush a reservoir without environmental impacts, Regional Workshop on Discharge and Sediment Monitoring and Geomorphological Tools for the Lower- Mekong Basin Vientiane, Lao PDR, October 2008 at MRC Sogréah, 2000, Etude globale pour une stratégie de réduction des risques dus aux crues du Rhone Etude du transport solide, Rapport de synthèse pour l Institution Interdépartementale des bassins Rhone-Saône. Thareau L., Giuliani Y., Jimenez C., Doutriaux E., 2006, Gestion sédimentaire du Rhone suisse : implications pour la retenue de Genissiat, Congrès du Rhone «Du Léman à Fort l Ecluse, quelle gestion pour le futur?»