Numerical and physical modelling concerning the removal of sediment deposits from reservoirs

Removal of sediment deposits from reservoirs by means of flushing through lowlevel outlets is a widely used method to regain storage volume. However, the efficiency of flushing is very often highly overestimated. Flushing can only be effectivewhen it is combined with substantial water level drawdown. However, even at favorable conditions and unregarded the excavation level which might have been reached, the lateral extension of flushing is rather limited. Analytical treatment of flushing processes is difficult due to the three-dimensional flow pattern in the vicinity of the flushing outlets, and due to the high number of parameters involved in the phenomena. In order to estimate flushing efficiency, a simplified approach was set up which was merely based on continuity equation applied on a 3D-grid of streamlines and isotachs, and on some stability criteria for granular sediment deposits against flow-induced eroding forces.The simplified approach rendered promising results. It could be proved very clearly that the lateral extension of the excavation must necessarily be restricted to a limited zone close to the outlet unless the water level is lowered drastically. In a separate approach, a 3D numerical model based onFVMwas applied to evaluate the justification of the simplified approach.The simulation showed good agreement with the simplified approach as far as flushing at high water level was concerned, but rather poor agreement at water level drawdown. In addition to the two numerical approaches in a third step a physical model was applied. As expected, the agreement with the simplified approach and the 3D FVM simulation was satisfactory for high water level conditions. The comparison of the three approaches with respect to flushing efficiency at drawdown is discussed.