ABSTRACT
Solid-liquid flows in geomorphological processes transport grains which are often supported by a combination of mutual collisions and turbulent suspension. The dominating support mechanism depends primarily on the size of transported sediment grains and on their local position within the internal structure of the sediment carrying flow. We focus on steady-state uniform turbulent open-channel flow in which some grains are suspended by turbulent eddies of carrying liquid and the rest is transported as (collisional) bed load. A modelling approach is discussed which employs the flow internal structure (distributions of velocity and solids concentration) and quantifies a distribution of turbulent characteristics across the flow depth. This enables to evaluate at the local level the ability of the carrying liquid to support transported sediment grains by the diffusive action of carrier turbulent eddies. The modelling results, obtained for flow conditions observed experimentally for lightweight sediment in our laboratory tilting flume, indicate that the widely used criterion for incipient turbulent suspension (the ratio of bed shear velocity and grain terminal settling velocity greater than 1.25) is appropriate. The chance for the local turbulent support (as additional mechanism to contact support) of grains appears to be more likely in the upper part of the transport layer developed in the flow above the mobile bed then in the lower part where intergranular contacts dominate. The part with turbulent support of grains increases with the bed Shields parameter.
