Author(s): David Raus, Frederic Moulin, Olivier Eiff
Linked Author(s): David Raus
Keywords: Sediment transport, turbulent boundary layer, river hydraulics, rough bed, shear-stress partitioning
Abstract: Transport of sediment over a non-erodible bed of hemispheres was experimentally studied in two complementary flume experiments. Firstly, a morphological study at the flume scale showed the importance of bedforms on sediment transport and on the sediment elevation between the hemispheres: the protrusion of the hemispheres, P (defined as P=k/R where k is the protrusion height and R the hemisphere radius) was never greater than 30% in the lee of dunes, and reached high levels of around 40% only after the erosion front has crossed the measurement region. In a second experiment, the local hydrodynamics study of the turbulent flow over patches of spherical caps and glued sediment confirms that an analysis of only the total shear stress or the turbulence intensity near the bed is misleading for the question of sediment mobility and transport. Quadrant analysis of the instantaneous flow at z=d50 between the eroded hemispheres gives far more relevant information, showing in particular an enhancement of sweep and ejection events when hemispheres begin to protrude (at P=20%). This is followed by a gradual disappearance of these events as the level of protrusion increases. From P=60%, the sediment bed is dominated by intense but small turbulent eddies which have limited capacity to transport the sediment
Year: 2017