Caractérisation des sédiments des retenues pour la prévision des risques écotoxicologiques liés aux vidanges

L'acumulation de sédiments dans les retenues de barrages hydro-électriques nécessite de procéder à des opérations régulières de désenvasement indispensables à leur bon fonctionnement.Ces opérations de vidange ont le plus souvent un effet destructeur sur la faune et la flore aquatique.Cet effet destructeur est principalement lié à une diminution de la concentration en oxygène dissous et une augmentation de la teneur en matières en suspension.La méthodologie présentée a pour but d'estimer préalablement à toute vidange les risques écotoxicologlques correspondant à une telle opération. Elle implique d'une part la description et l'analyse physico-chimique des sédiments en place, d'autre part la mesure en laboratoire de la consommation en oxygène dissous des matériaux remis en suspension au cours du temps.La fiabilité d'une telle procédure a été montrée en particulier à l'occasion de vidanges de retenues situées sur le cours de l'Isère.Le protocole est décrit précisément de façon à pouvoir guider les exploitants de retenues à l'occasion des opérations de vidange qu'ils dirigent.Sediment silting up in hydroelectric dams Implies regular draining operations in order to prevent disfunction of the dams.These operations often lead to drastic lethal affects on aquatic fauna and flora.A toxicological study has clearly shown two main factors responsible for acute toxicological effects : dissolved oxygen deficit and suspended solids increase.These effects have been quantified by experimental tests on Brown trout fry (Salmo trutta fario) the results of which are summarized.The second step consisted in the prediction of water quality evolution downstream during dam draining operation, regarding both factors suspected, in order to assess ecotoxicological hazard.The proposed methodology based on a sedimentological study made in Grangent dam, located on river Loire and immediately downstream of St. Etienne urban area, and in St. Hilaire dam situated on the river Isère downstream partIt consists in sampling cored sediments and in measuring dissolved oxygen kinetic by mixing sediments with water In a reactor.The sample conservation as carried out in jar glasses kept at 4 °C.Results are reproducible as long as residual dissolved oxygen concentration is higher than 3 mg/l. If this condition is respected, oxygen consumption is strictly dependent on suspended sediment concentration.An increase in test water temperature makes the oxygen consumption rise.In accordance with these results, recommendations for dissolved oxygen measures are about 5 g/l suspended sediment concentration, with an experimental temperature reaching the temperature observed during dam draining.The variability of the results for different samples depends on the dam studied. Surface tore samples collected in St. Hilaire dam consume dissolved oxygen faster than the same bottom tore samples.However, in Grangent dam, only one surface core sample collected near the dam construction has clearly shown reducing properties. The difference between two dam sediments is very important : Grangent dam sediment have, on average, dissolved oxygen consumption three more times higher than St. Hilaire in the same duration.Taking into account dissolved oxygen kinetics by mixed sediments, and a simple aeration model using water and sediment deposits from downstream dam, may expert both suspended sediment and oxygen consumption evolution.Hazardous mortality of trout fario fry in downstream dam are quickly graphically visualized.Dam draining operator may so assess, for different suspended sediment land the harmful exposition to fish at any distance from it.This knowledge allows him to plan more strictly floodgate working : pointly dissolved oxygen continuous measurement on downstream dam is sufficient to give a mark related to model hazard assessment.The model reliability has been demonstrated by the examination of data collected during St. Hilaire dam draining operation