Turbulent momentum exchange over a natural gravel bed

Exchange processes occurring between a porous bed and the stream flow above play an important role in controlling the transport of contaminants and other substances in rivers and streams. This work examines the vertical momentum transfer occurring above the roughness layer of a uniform gravel bed for three shallow uniform turbulent flows. PIV measurements were used to calculate the boundary shear stress via the double-average method. The data was also used to define the different roles of the bed shape and of the flow conditions in controlling the momentum flux. Employing a cross-correlation technique it was observed that the bed shape strictly controls the spatial pattern of the momentum flux in the zone above the bed in all three cases, and that the flow conditions are less influential over the near-bed momentum flux pattern for the analysed flow conditions. These initial results may be generalised to other flows, therefore it is hoped that this study will stimulate further investigation on the subject.