554 UNIDIRECTIONAL AQUEOUS FLOW Figure 11 Velocity profiles measured over a dune bedform, showing flow separation in the dune leeside and boundary layer recovery over the stoss side of the next downstream dune Flow left to right Reproduced from Nelson JM, McLean SR, and Wolfe SR (1993) Mean flow and turbulence fields over two dimensional bedforms Water Resources Research 29: 3935 3953, with permission from American Geophysical Union Other Factors Influencing Boundary Layer Structure In many unidirectional aqueous flows, the precise nature of the mean and turbulent flow is influenced by a range of variables that can significantly alter the flow structure, bed shear stress, patterns of sediment transport, and, hence, development of bed morphology Some of the most significant influences on the characteristics of unidirectional aqueous flows are described below The Nature of Bed Grain Roughness Particle roughness significantly increases the potential for turbulent mixing near the bed and often results in an increase in the gradient of the near-bed velocity profile, with a concomitant increase in the bed shear stresses derived from the velocity gradient, Reynolds stress, or turbulent kinetic energy budget Grain roughness may destroy the viscous sublayer and also increase the generation of turbulence near the bed, through either encouraging intensified bursting (quadrant events) from between the grains (and thus larger-scale return quadrant events) or generating regions of flow separation around individual grains or groups of particles, which may both create significant velocity gradients near the bed and generate large-scale coherent vortices, associated with flow separation both in front of and behind the particles, which can penetrate the entire flow depth (Figure 10) The Presence and Type of Bedforms Many bedforms such as ripples, dunes and largerscale bar forms, create their own flow field through topographic, convective accelerations and decelerations of fluid that may significantly change the Figure 12 Schematic diagram of the attenuation or enhance ment of turbulence due to the presence of sediment in a flow as a function of the volumetric sediment concentration, C, and the ratio of the particle response time to the turbulence time scale, tp/te (after Elghobashi S (1994) On predicting particle laden turbulent flows Applied Scientific Research 52: 309 329) nature of a unidirectional flow In addition, many bedforms are also associated with flow separation on their upstream stoss side or downstream lee side, which generates appreciable turbulence and a boundary layer that is recovering from flow separation downstream of the region in which the flow reattaches to the bed (Figure 11) The Type and Quantity of Suspended Sediment Many turbulent flows transport appreciable quantities of suspended sediment, with the suspended concentration in some flows reaching levels at which the flows become markedly non-Newtonian, such as in the Huanghe River in China, where concentrations of up to 1290 kg m have been recorded Suspension