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Quelques mois après le début de la mise en eau du barrage de Petit Saut, la mise enservice normale de l'usine conduisait à une désoxygénation de l'eau du tronçon de rivièreaval, le rendant incompatible avec la vie aquatique. La solution retenue a été laconstruction d'un seuil, afin d'apporter de l'oxygène et d'éliminer les gaz réducteursproduits au fond de la retenue, notamment le méthane, consommateur potentiel d'oxygènedissous.Un seuil métallique à deux lames déversantes successives a été construit ; saconfiguration prend en compte les principaux critères physiques jouant un rôle significatifsur l'oxygénation de l'eau (hauteur de chute, épaisseur de la lame déversante, ledimensionnement du bassin de réception des chutes, la présence de dispositifs favorisantl'éclatement de la lame d'eau). Placé dans le canal de fuite de l'usine, à une centaine de mètres à l'aval du barrageprincipal, il est à l'abri des crues et ne crée pas d'obstacle supplémentaire enrivière.L'article chiffre l'effet d'aération de ce seuil pour les deux gaz O2et CH4 dans deux configurations : celles consécutives à l'abaissementpartiel de la chute amont réalisé en deux étapes. Après décembre 2001, pour le débit moyenturbiné (près de 200 m3 /s), l'efficacité d'aération du seuil a baisséde près de 10 % (gain de 80 % en oxygène dissous et élimination de 70 % et 75 % du méthanedissous). Après février 2003, pour un débit de 100 m3/s, 75 % du déficit amont en oxygènedissous est comblé et près de 70 % du méthane dissous éliminé.From the moment tropical reservoirs are impounded, climatic conditions cause rapid(within several weeks) and marked thermal stratification, especially during the dry season.This phenomenon is further exacerbated by the chemical and biochemical processes takingplace in the reservoir due to the decomposition of submerged organic matter. In densetropical forests, the overhead biomass is estimated at roughly 170 t(C)/ha, and the carboncontained in the soil is also not negligible since it is on the order of 100 t(C)/ha. Thedegree of biodegradability of the different compounds in the flooded biomass is variable,ranging from a few weeks for bacteria to several centuries for tree trunks.The studies carried out at Petit Saut (French Guiana) show that, immediately afterimpoundment, only the epilimnion (a few dozen centimetres thick) was oxygenated whereas thehypolimnion was characterized by complete anoxia and a very high methane content (about 15mg/L). Water quality in the river downstream from the reservoir was of course stronglylinked to variations in the water quality in the reservoir as well as to its operating mode.The waters passing through the turbines, coming from the bottom layers, were anoxic andloaded with fixed or volatile reducing compounds (e.g., CH4,H2 S), and were responsible for a high immediate or progressive oxygendemand. At Petit Saut, despite an inflow of good quality water, there has been a progressivedeoxygenation in the river downstream due to the high methane content (roughly 8 mg/L) ofthe turbined water. Thus, 40 km downstream from the dam, the oxygen content was less than 2mg/L and therefore incompatible with most aquatic life. To solve this problem, it wasnecessary to build an aerating weir capable of reoxygenating the turbined waters and, moreimportantly, eliminating reducing gases such as methane at the same time. The function of the overflow weir was to entrain air bubbles into the water and togive these bubbles a sufficiently long immersion time to ensure that they dissolve. At thetime of its installation, only three examples of oxygenating weirs existed in the entireworld, all located in the United States. The weir configuration was tested using a physicalmodel to qualitatively examine the form of the flow both across the weir and downstream fromit. The degree to which air bubbles were entrained in the water was also tested, but not thequestion of evaluating the flux of gaseous exchanges between the air and the water.The system that was finally designed by EDF, in October 1994, was a metallic weir withtwo consecutive falls, the configuration of which respected the main physical criteria thatplay a significant role in the oxygenation of water, i.e.:- the height of the falls (roughly 5.40 m, depending on the flow rate);- the thickness of the water stream, the function of which is to entrain air bubblesand keep them in the water for a sufficiently long period of time for the oxygen to dissolve(between 12 and 25 seconds, depending on the flow rate); - the dimensions of the receiving basin of the first waterfall where the air bubblesare held (5 hexagonal alveoli); and- systems to promote the fragmentation of the flow. This structure was placed in the tailrace channel of the plant, approximately 100 mdownstream from the main dam. This location protected it from floods and did not create anextra obstacle in the river. In addition, it allowed the water to be re-oxygenated as soonas it left the reservoir.The efficiency of the two waterfalls of the Petit Saut re-aerating weir was tested attwo different turbine flow rates: 80 m3 /s and 230m3 /s. In 1996, the results of the measurements showed that for a flowrate of 230 m3 /s, upstream of the weir the concentrations ofCH4 were around 5 mg/L and dissolved oxygen was 0.8 mg/L. Downstream fromthe weir CH4 concentrations were 1.3 mg/L and dissolved oxygenconcentrations were 6.8 mg/L. The dissolved methane elimination rate was approximately 75per cent. At a flow rate of 80 m3/s, upstream of the weir the concentration ofCH4 was 5.5 mg/L and the dissolved oxygen concentration was 0.7 mg/L.Downstream from the weir concentrations of CH4 and dissolved oxygen were1.0 mg/L and 7.1 mg/L, respectively. The dissolved methane elimination rate was around 80%.The efficiency of the re-oxygenation was always greater than 90%. These data prove that the efficiency of the Petit Saut weir installation was higherwhen the turbine flow rate was lower. This could be due to a greater waterfall height, thebetter entrainment of air bubbles per unit volume and/or a longer air bubble residence timein the downstream flow.Between December 2001 and February 2003, for a flow rate of 200m3 /s, the efficiency of the weir decreased by 10%, with the dissolvedmethane elimination rate at around 70-75%. The level of re-oxygenation was around 80%. SinceFebruary 2003, for a flow rate of 100 m3 /s, the efficiency of the weirhas decreased by 10%, the dissolved methane elimination rate was around 70% and the level ofre-oxygenation was around 75%.On a local scale, the effect on the quality of the river water has been very positive,as aquatic life has been maintained. Without the weir, the methane contained in the turbinedwater would have been progressively transformed, along the course of the river, into carbondioxide. In the absence of significant additions of good quality water and without the weir,a large part of the course of the river would have a dissolved oxygen content of less than 2mg/L, the critical threshold for the maintenance of aquatic life.At present time, the results of the current ecological survey are used to supportstudies on biogeochemical processes

revue des sciences de l'eau

Efficacité d'un seuil artificiel sur l'oxygénation de l'eau et l'élimination de CH4 contenu dans l'eau évacuée par la barrage hydroélectrique de Petit Saut (Guyane française)