Technical Papers TOWARDS A BETTER FORECAST FOR RESERVOIR SLOPE SLIDING Didier MAROT 1 , Alain ALEXIS 2 and Pierre THOMAS 3 ABSTRACT The siltation of reservoir causes many problems of management and environmental disturbances that are important to minimize. Solving these problems requires in particular a good knowledge of mud bank instabilities. In partnership with the Laboratoire National d'Hydraulique (LNH), a subsidiary of Electricité de France (E.D.F.), authors realize a study of soft soils sliding entailed by reservoir emptying. Using a geotechnical flat sliding mechanism, the theoretical formulation of the safety factor can be established when the sediment mass is submerged and emergent. The measurement campaign carried out makes it possible to obtain bathymetric and topographic surveys and a geotechnical characterization of the soil for the whole reservoir of Grangent dam, located on the Loire (France). The proposed model appears to agree satisfactorily with the instability of the zones emerged because of the emptying and with the stability of the profile after emptying. The use of this model within the framework of forecasts makes it possible to calculate significant minimal displaced heights. Key Words: Soft soils, Mud, Bank, Sliding, Dam reservoir, Emptying 1 INTRODUCTION Hydroelectric dams and their storage reservoirs allow for the production of electricity, the creation of water reserves for consumption or irrigation, and the regulation of river flow rate. The long-term future of these installations requires the dredging (often non-viable economically) or the emptying of storage reservoirs. This release of stored water is carried out with two objectives: - periodic inspection of the structure and the carrying out of maintenance work, - expulsion (flushing) of sediments accumulated as upstream slopes of the dam. The improvement of the sedimentary control and the development of operational solutions require increased knowledge of the impacts of emptying a storage reservoir on the slope stability of its silt deposition. Bouchard (1995), Paquier and Khodashenas (2002) presented numerical codes which simulate erosion and deposition in a storage reservoir at times of high waters. However, emptying is also accompanied by a considerable amount of slide of the upstream slopes. Zhou and Lin (1998) arbitrarily set a maximum slope gradient for the sediments, but the difficulty lies in the evaluation of this maximum slope as this is influenced by several factors: - the thickness of the sediment mass - the slope of the bedrock - the soils characteristics The objective of this study is thus to propose a criteria for upstream slope slide during emptying of storage reservoirs of dams, and to then validate it on a real site: the Grangent dam (Loire-France). An initial criteria is developed with a rectangular-shaped slice (Alexis et al., 2001). It supposes that the shear plane and the water-sediment interface are parallel. In order to deal with a case more typical of the natural configuration while freeing itself from this simplistic assumption, we develop a sliding criteria for a slice of sediment with a trapezoidal shape. Moreover to improve the validation of proposed criteria, the topographic profiles carried out before the construction of the dam are exploited. 1 Doctor, 2 Doctor-Engineer, 3 Professor, IUT de Saint Nazaire, Laboratoire Génie Civil Nantes-Saint Nazaire, BP 420, 44606 SAINT NAZAIRE cedex, FRANCE, Tel.: 00 33 2 40 17 81 50, Fax: 00 33 2 40 17 81 60, E-mail: marot@iutsn.univ-nantes.fr Note: The original manuscript of this paper was received in Dec. 2002. The revised version was received in Feb. 2003. Discussion open until Dec. 2004. - 278 - Internatinal Journal of Sediment Research, Vol. 18, No. 4, 2003, pp. 278-289