ABSTRACT
In sediment supply-limited river systems, fine sediments are often transported over a layer of immobile gravel. The level of protrusion of the coarse immobile gravel strongly influences the transport rate of fine sediments which can be sheltered or scoured. This laboratory study examines the effect on the local hydrodynamics of the increasing protrusion of immobile gravel as the fine sediment erodes. The fine sediment and coarse gravel are modelled by fine plastic granulate and two layers of spheres arranged in a staggered pattern, respectively. Simultaneous stereoscopic PIV and stereo-photogrammetric measurements show that the bed is characterized by high erosion rates for mean levels of protrusion P = k/R < 0.7, where is the height of the protruding sphere above the sediment bed and R is its spheres’ radius. This is consistent with the observation, that the spheres locally enhance the shear stress for low levels of protrusion. For P = 0.7, though, local sheltering regions appear in the lee of the spheres, which reduce the shear stress acting on the fine sediments. Consistently, the evolution of the bed is seen to slow down and effectively stops for P = 1, when the whole region in between the spheres is sheltered.
