Sediment transport modelling of a reservoir drawdown, example of tolla reservoir

Reservoir emptying generates sediment erosion and transport. Thus the sediment release downstream the dam may impact water quality. In order to mitigate water quality degradation during emptying operations, there are not so many parameters which could be modified. If possible, one can choose: (i) the upstream discharge, i.e. the emptying period, (ii) the rate of drawdown, (iii) the lowest attained water level (iv) dilution using tributaries (v) the use of settling tank. . . . This choice often relies on experience, however numerical modelling could be used to assess the sediment release and to define an optimal emptying scenario. This paper gives details on the use of a one-dimensional hydraulic and sediment transport numerical code, Courlis, to simulate several emptying scenarios. The model is constructed owing to sediment cores and bathymetric data. These measurements enable to estimate mechanical properties and the quantities of sediments, they also highlight the operation complexity there. The numerical model calculates the mass of eroded sediments and water quality modifications depending on emptying scenarios. Besides, a sensitivity analysis is performed to identify the main numerical and physical parameters and how they affect the results. This example could be used as a framework for reservoir emptying modelling.