Three-dimensional Numerical Modelling of Reservoir Flushing to Obtain Long-term Sediment Equilibrium

Reservoir sedimentation is one of the major challenges in dam engineering today. A common approach for reducing sediment accumulation is reservoir flushing. Within this study the technical feasibility of a reservoir flushing is assessed in order to restore the storage capacity of a previously sedimented reservoir. For this purpose, a three-dimensional numerical model of the Schwarzenbach pumped storage reservoir is used to investigate the effectiveness of different flushing scenarios. The applied fully three-dimensional numerical program SSIIM 2 solves the Reynolds-averaged Navier-Stokes equations according to the finite volume method and the k-epsilon turbulence model for an adaptive, unstructured and non-orthogonal grid. The suspended sediment transport is calculated by solving the transient convection-diffusion equation.

In a first step, the model was set up based on bathymetric data, sediment characteristics, including the initial thickness of the sediment layer, and inflow discharge rates. In a second step, the relationship between water level changes during the flushing and sediment discharge (constant water in- and outflow rate) is evaluated on the basis of three specific scenarios, each with a duration of 350 hours. On the one hand, two pressure flushings are simulated (without lowering the water level and with lowering the water level to an intermediate water level) and on the other hand, the effectiveness of a drawdown flushing with a complete lowering of the water level is investigated. The simulated changes of the bed level as well as the calculated amount of flushed out sediments are presented in this study. The results show that drawdown flushing results in an about 80 times higher sediment outflow compared to pressure flushing without lowering the water level during flushing. The results reflect the dependency of the success of the flushing on lowering the water level. In the present case, pressure flushing without lowering the water level is not suitable for operational use of the Schwarzenbach dam while maintaining a permanent sediment equilibrium. However, flushing scenarios with a partial or a complete lowering of the water level guarantee a permanent solution for the use of the Schwarzenbach reservoir without expected future storage capacity losses.