Sediment management of hydropower cascade: example of CNR run-of-river developments, French Rhone River, France

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1 Sediment management of hydropower cascade: example of CNR run-of-river developments, French 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 and hydropower context The Rhone River is one of the major rivers of Europe. Flowing from the Swiss Alps up to the Mediterranean Sea, the river winds through the French territory for 2/3 of its course. 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. Relatively steep slopes for such large river (Figure 1) characterizes the Rhone River. As many of its major tributaries come also from the Alps, it is currently concerned by significant sediment fluxes (Table 1). Note that these fluxes have been dramatically reduced since the end of the 19 th century. Many factors explain this evolution: sediment trapping into specific hydraulic structures built in the late 19 th to improve the navigation (so called Girardon spur-dikes), extraction works for civil construction, sediment capture into large reservoirs built on tributaries, soil restoration works done by the forestry Administration Thus, at the outlet of the basin, bedload transport has been divided by 10 whereas suspended load has been divided by 3 since the end of the 19 th century (SOGREAH, 2000). Disruption of solid transportation is a serious concern on the Rhone River since the lack of solid materials at the Rhone River delta limits the possibility of sediment supply to the Mediterranean shoreline. Figure 1: Longitudinal profile of the French Rhone River.

2 Table 1: Mean annual fluxes at different locations both for bedload and for suspended load on the Rhone River. Station Bedload (m 3 /y) Suspended load (Mt/y) Lyon 5,000 to 10, Valence 10,000 to 25, Avignon 25,000 to 50, Since 1933, Compagnie Nationale du Rhone (CNR) has been granted the concession to operate the Rhone from the Swiss border to the Mediterranean Sea. In that framework, CNR carries out three interdependent missions: hydropower production, navigation and irrigation and other agricultural uses. It has built 19 hydropower plants on the river, opened up a wide-gauge waterway of 330 km long between Lyon and the sea and permits the irrigation of crops (Figure 2). All hydropower developments operated by CNR are run-of-river developments, except Genissiat, which is a 70 meter high dam. CNR is France s second largest electricity producer and 100% of its electricity production comes from renewable sources of energy (hydropower, wind power and solar energy). The company s invests also for the benefit of the population of the Rhone Valley by the way of its plans of Missions in the General Interest (MGI). Figure 2: Overview of CNR's hydraulic developments regarding electricity production sources and navigation (left) and key figures concerning hydropower developments on the French Rhone (right).

3 Run-of-River features and management Typically, run-of-river scheme operated by CNR for hydroelectricity production consists of a barrage built across the mainstream that controls the flow of the river and diverts the major part of it into a headrace canal (Figure 3). This canal conducts the water to the power plant to drive the turbines. The remaining part of the discharge is released in the natural course of the river to preserve the ecological balance of the fluvial environment. The major point of run-of-river concept is that the storage capacity of reservoir is negligible compared to river flow volumes, especially regarding flood volumes. There is neither possibility of inter annual regulation nor seasonal regulation. Thus, the output flow sent downstream of the dam is equal to input flow entering upstream the reservoir. The turbine discharge has to be adjusted in real time to fit Rhone River flow fluctuations. Some modulation of turbinate discharge remains possible to produce high value energy at peak hour but the water level variation is in general lower than 50 cm. The consistency in the operation of the power stations in cascade is obtained by synchronizing the flow. Figure 3: Typical CNR run-of-river development scheme. In normal conditions, i.e. out of flood periods, the level of the reservoir upstream each dam is close to the horizontal (Figure 4). The main part of the flow is driven through the powerhouse turbines. The spillway and bottom gates are closed until the input flow reaches the installed flow capacity of the powerhouse. During flood conditions, the slope of the backwater curves increases upstream of each dam. Keeping the same regulated water level upstream dam would lead to increase the flood hazards. To avoid this unacceptable situation, water level is decreased and slope is tilted by a partial

4 opening of the spillway gates. This lowering is performed slowly in order to avoid an excess of flow downstream and a destabilization of the banks in the reservoir. For the design flood, which corresponds to a discharge of 1000 year return period (Q1000) for hydraulic developments operated by CNR, all spillway gates are completely opened. Water level and slope in the reservoir are both similar to values observed in natural conditions. Thus, extra inundation hazard is totally prevented (Figure 4). Figure 4: run-of-river cascade management for normal hydrological conditions and for design discharge. The tilt of reservoir slope during high flow conditions has also the advantage of increasing significantly velocities and facilitates the mobilization of sediments. As natural velocities are recovered in the reservoir, most of the sediments can pass the barrage during flood situations. Morphologically speaking, run-of-river developments are nearly transparent (Fruchart, 2012), especially if their design include spillway gates that can be completely opened during flood conditions (Figure 5).

5 Figure 5: Example of spillway gate that can be completely opened during flood conditions (Sauveterre dam operated by CNR on Rhone River, France). Sediment management of the cascade Despite a design that globally minimizes the possible impacts on sediment fluxes, the Rhone River channel experiences locally some sediment deposits. As expected, these accumulations are located in the sections of very low velocities. It mainly concerns navigation lock garages and channel tributaries immediately upstream of their confluence with the Rhone River. This knowledge results from a steady monitoring of the river conducted with a fleet of hydrographic boats of different sizes (Figure 6). This work is done at the minimum every 5 year or after each significant flood (above a discharge of 10 year return period). These observations mobilize a staff of approximately 50 people from CNR to cover the whole concession perimeter. In the framework of the concession contract, CNR has to ensure maintenance works on the riverbed. The objectives are to keep adequate conditions of navigation and operation. Another key point is also to preserve the original hydraulic capacity of the channel for the design discharge (Q1000). It means that the same water elevation than in natural conditions has to be respected for these hydrological conditions. Maintenance works concern both vegetation and sediments. Different types of intervention can be conducted: log jam evacuation, vegetation maintenance, gravel bar plowing, deposits dredging,... Many factors have to be taken into account to define the required procedure and equipment: areas to be treated, timing for a limited impact, access conditions, sediment size... In order to provide a long-term view of the situation and then to facilitate the works triggering, a 10 year management plan is established under French administration supervising (CNR, 2010).

6 Figure 6: CNR hydrographic boat used for Rhône River channel monitoring. On the Upper Rhone, flushing operations can be regularly planned by Swiss operators and CNR thanks to the artificial flood discharge possibly released from Geneva Lake (see communication of Peteuil in the same Feature Session). As flushing operations have appeared much more difficult to plan on the Lower Rhone, mechanical means have been more favoured for sediment deposits management. Many reasons justify this situation. First, water level in the reservoirs has to be lowered slowly in order to avoid a destabilization of the banks. As such operation takes tens of hours, hydrological conditions can change meantime in an unfavourable way. Second, precise hydrological forecasts on most tributaries are generally unreliable above 24 hours. Even on the largest ones, flood flows are too short to maintain efficient discharge conditions for flushing. Third, stopping fluvial navigation on the Rhone River requires a one year delay to perform the administrative procedure. A so long duration is clearly inconsistent with the opportunistic management often required to perform a flushing operation. Thus, accumulations of sediment moved artificially by CNR are distributed according to the following objectives: 46% for keeping adequate conditions of navigation, 37% for preserving the original hydraulic capacity of the channel, 9% for keeping proper operating conditions on the hydraulic structures and 8% for oxbows restoration (CNR, 2010). This latter point is out of the concession contract and is voluntarily both funded and carried out by CNR in the framework of its MGI plan. The mean annual volume of sediment accumulations treated by CNR has been around 660,000 m 3 for the last decade with a yearly average cost close to 6 M. The grain size distributions of the sediments show the following proportions: 75% are composed of silt and

7 sand while gravels represent 25% (CNR, 2010). For a given development, the volume of deposits corresponds on average to 3% of the suspended sediment fluxes passing through the river reach. The main section affected is currently located between Valence and Avignon. Sediment are put into suspension or artificially moved further downstream in order to minimize the potential disruption of sediment supply to the delta. Lessons learned The long term monitoring carried out by CNR shows that the riverbed elevation of the Rhone River is globally stable (Doutriaux, 2006). The specific design and operation of CNR run-ofriver developments are certainly key points to explain that long-standing situation. Including spillway gates as high as the barrage, these hydraulic developments conduct to very limited impacts regarding sediment transportation. Following frequent flood events, sediments are used to be temporally stored in reservoirs whereas a massive mobilization is observed after intense floods. For instance, 7 Mm 3 of sediments have been removed during the 2003 flood only. This figure has to be compared with the 660,000 m 3 or 1.3 Mt of sediments annually dredged by CNR for maintenance operations. To limit the potential impact on sediment supply to the river and its delta, these sediments are either put into suspension or artificially moved further downstream. For these maintenance works, a strict framework has been defined on the French Rhone River. For practical and administrative reasons, it includes in particular a 10 year management plan and the establishment of Environmental Impact Assessment studies. As pollution by heavy metals or PCB is a very sensitive issue, physical and chemical analysis of deposits are systematically performed before removal. Nevertheless, CNR surveys show that the pollution observed on the Rhone River is not significantly worse than the pollution experienced by other French Rivers. References Cited CNR, 2010, Entretien du lit du Rhone : plan de gestion des dragages d entretien Doutriaux E., 2006, Aménagements hydrauliques dans le cours du Rhône français. Bilan sédimentaire, Archives des Sciences. Fruchart F, 2012, Run-of-River: Revisiting the Definition and Interpretations, Workshop Towards the sustainability of hydropower in the Mekong Region: Options for improved project design and technologies. Bangkok SOGREAH, 2000, Etude globale pour une stratégie de réduction des risques dus aux crues du Rhône Etude du transport solide, Rapport de synthèse pour l Institution Interdépartementale des bassins Rhône-Saône.