UNSTEADY FLOW EFFECTS ON BED SHEAR STRESS AND SHEET FLOW SEDIMENT TRANSPORT

Recent measurements of velocity and concentration profiles in oscillatory boundary layers over mobile beds of cohesionless sediment have revealed important details of the physical processes involved. New estimates of the hydraulic roughness of mobile beds indicate that the equivalent Nikuradse roughness is much larger than the 2.5d(50) commonly assumed. The vertical gradient of the total shear stress is often large, making it unclear at what elevation we should evaluate the so-called "bed shear stress". Since this is the input for most simple sediment transport formulae, it is important to establish what is meant by this term.