Two-dimensional numerical modelling of flushing processes in water reservoirs

Abstract

This study describes a two-dimensional numerical model simulating flushing of sediments from water reservoirs. The numerical model solves the depth-averaged Navier-Stokes equations on a two-dimensional grid. A zeroequation turbulence model is used. The resulting flow field is extrapolated to three dimensions, and the convectiondiffusion equation for the sediment concentration is solved. A formula for the bed concentration is used as boundary condition, 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 field is used to calculate the location of the water surface. The grid is adaptive in the vertical direction, and changes according to the calculated water and bed levels. A porosity model is used to simulate the process when the water surface drops under the bed level at some locations of the geometry. The results from the numerical model were compared with data from physical model studies. The main features of the erosion pattern were reproduced, and the deviation between the calculated and measured scour volume was small. This showed that most of the simplifications made in the numerical model were reasonable. There were deviations between measured and calculated bed levels in a bend. This was most likely due to secondary currents that were not possible to model using a two-dimensional approach.