Aperçu sur le transport de sédiments en rivière. Application à l'Arc

Field: Environmental Science (Physical Sciences)

Management of sediment in rivers is necessary to avoid consequences such as floods and pollutions. Scientific basis and tools are described through the example of Arc River. Sediments are transported either as suspended load or as bed load. Bed load is strongly affected by the various structures across the river. In natural conditions, dynamic balance is obtained : local erosion and deposition processes do not change the global parameters of the bed (plane view, mean cross section, mean sediment characteristics). The engineer`s challenge is to define these averages and to estimate the evolution after man actions.Classical way to compute sediment balance is to use potential sediment transport capacity. Empirical relations such as Meyer Peter and Muller, 1948)`s relation (equation (1)) and (Engelund Hansen, 1967)`s relation (equation (2)) can provide estimates. They take into account a critical shear stress for bed load close to the beginning of motion and only a mean bottom shear stress in the other cases. The volume of transported sediments is mainly linked with the hydrological regime and more precisely with the more usual floods (return period of about one year). A lot of sediment transport models exist. They are mainly defined from the hydrodynamic model dimensions (1D, 2D or 3D) and the way sediment transport is considered (discharge, concentration, particle). According to the problem to be solved, 1-D model (that can be complicated by a distribution of sediments inside a cross section as in Fig. 1) can be enough considering the lack of calibration data.For the Arc valley, a 1-D model was used to simulate the large flood that occurs in 1993 during the construction of the motorway A 43. The river regime is influenced by the hydropower transfers (Fig. 2). To avoid expensive sediment removals from the bed, flood risk must be assessed for various scenarios. According to four periods, geometry of the river was built. The synthetic monofrequency discharge hydrographs were calculated along the river for various return periods. Homogeneous sectors for sediment transport were assessed (Fig. 3) and were used to define the stability of the bed for deposition of fine sediments (Fig. 4) using the fall velocity of various classes of sediments in front of the product of the mean river velocity multiplied by the energy slope. For the 100-year flood, a 1-D model with movable bed calculated the water levels for various scenarios of management and of floods in order to assess the flood risk. Various management strategies can be defined taking into account a monitoring system. / La gestion des sédiments en rivière est nécessaire pour éviter inondations et pollutions. On rappelle les approches classiques telles que la notion de capacité maximale de transport de sédiments illustrée par des relations empiriques. Les modèles numériques 1D, 2D et 3D répondent aux problèmes d`évolution des lits après modification d`origine anthropique. Pour l`Arc, un modèle 1D a été utilisé pour simuler la grande crue de 1993 pendant la construction de l`autoroute A43. Le régime de la rivière est influencée par les transferts pour l`hydroélectricité. Pour éviter des extractions de sédiments coûteuses, le risque d`inondation doit être estimé pour différents scénarios. Pour quatre périodes, la géométrie de la rivière a été construite. Des hydrogrammes synthétiques monofréquences ont été calculés le long de la rivière pour différentes périodes de retour. Des tronçons homogènes ont été estimés et ont servi à définir la stabilité du lit face aux dépôts de fines en comparant avec la vitesse de chute des particules multipliée par la pente d`énergie. Pour la crue centennale, un modèle 1D avec fond mobile a calculé les niveaux d`eau pour différents scénarios de gestion et de crues dans le but d`estimer les inondations. Différentes stratégies de gestion ont été élaborées prenant en compte le système de surveillance de la rivière.