Two-dimensional numerical modelling of flushing processes in water reservoirs

This study describes a two-dimensional numerical model simulating flushing of sediments from water reservoirs. The numerical model solves the depth-av eraged Navier-Stokes equations on a two-dimensional grid. A zero-equation t urbulence model is used. The resulting flow field is extrapolated to three dimensions, and the convection-diffusion equation for the sediment concentr ation is solved A formula for the bed concentration is used as boundary con dition, resulting in a calculation of bed material load. Continuity for the cells closest to the bed is used to find the bed changes. The pressure fie ld is used to calculate the location of the water surface. The grid is adap tive in the vertical;direction, and changes according to the calculated wat er and bed levels. A porosity model is used to simulate the process when th e water surface drops under the bed level at some locations of the geometry . The results: from the numerical model were compared with data from physic al model studies. The main features of the erosion pattern were reproduced, and the deviation between the calculated and measured scour volume was sma ll. This showed that most of the simplifications made in the numerical mode l were reasonable. There were deviations between measured and calculated be d levels in a bend. This was most likely due to secondary currents that wer e not possible to model using a two-dimensional approach.