The role of a movable sandy-bed in modelling open-channel flow

The sediment transport mechanisms in rivers have recently attracted the attention of civil engineers and decision-makers due to its importance for environmental issues and water resources management. The complexity of this phenomenon arises from several assumptions for its theoretical analysis, such as considering the water density as a constant with the value of 1000 m3 /kg. A movable sandy bed determines the variation of the fluid's density because of the sedimentation processes (e.g., lifting forces) that take place at the boundary between the channel bed and the fluid in motion. When the bed's material contains mainly coarse soils, such as sand or gravel, the bed constitutes a porous medium where the water is flowing as well. This paper describes some experimental investigations on how the water density varies with the depth, velocity and channel roughness under established hydrodynamic conditions. In addition, the velocity at the boundary between the bed and the water is estimated using a numerical model which calculates the flow through the sandy layer. The assumptions that WANG and WU [7] considered as the most commonly adopted in the models for sediment transport and free surface flow are discussed.