Alluvionnement des retenues par courants de turbidité
Reservoir sedimentation by turbidity currents All lakes created on natural rivers are subjected to reservoir sedimentation. The construction of a dam significantly modifies the flow conditions of natural streams inside and downstream of an artificial lake. Considering the sediment concentration, often high during the flood season, the entering flow shows a greater density than the ambient fluid. Suspended load can therefore be entrained along the reservoir bottom all the way down to the dam in the form of turbidity currents. Research was undertaken to better understand the physical phenomena that contribute to reservoir sedimentation. The study of turbidity currents at the bottom of a lake is based on a review of the relevant literature, on measurements of water and sediment motion on site, on physical modelling of turbidity currents and on numerical flow simulation. The two-year on site investigation in the test site of Luzzone in southern Switzerland showed the relationship between precipitation, water and sediment flow and turbidity current in the reservoir even for minor events. A simple relationship describing the inflow hydrograph and the sediment transport for small floods in the incoming river was established as a set of non-dimensional equations. These relations were implemented in the numerical flow code as an upstream boundary condition. No significant occurrence of high water was observed. It was therefore impossible to establish the necessary elements for extrapolation to important flood events with their implication for the reservoir sedimentation process. The laboratory investigation confirmed the results from numerical simulation of turbidity currents. The progressing flow proceeded as predicted by theory and numerical simulation. The numerical model developed is based on the general Navier-Stokes solver code CFX-F3D from Computational Fluid Dynamics Services. User defined sediment deposition and erosion modules were added in order to take into account sedimentation and the interactions between the turbidity current and the bottom of the reservoir. The flow in the physical model as well as in the real reservoir geometry was simulated with exactly the same boundary conditions. Comparison between computer simulation, physical model and on site measurement results showed a satisfactory agreement. The code was then used to simulate turbidity currents in two dimensions in a simplified reservoir geometry with all available flow - sediment interaction models for three different particle sizes. The results were compared to data from an existing 2Dflow simulation programme and they agreed well. The study of a thousand-year flood in the Luzzone reservoir using the developed computer model revealed the potential of such a tool. In particular, the impact on the sediment deposits was analysed. A Rapid evaluation of the incidence of such an extreme turbidity flow was made. Through this study, considerable knowledge has been acquired. It can be used to formulate proposals in order to allow limited transit of sediments through the lake during floods to evacuate them directly downstream of the dam. This operation has to take place during flood periods, when the upstream and downstream reaches carry enough water. The bottom outlet therefore participates partially in the sediment evacuation at the moment of their major occurrence during floods. Another advantage of this procedure would be the reduced impact on wildlife and vegetation in the downstream river.