Bed-Load Effects on Hydrodynamics of Rough-Bed Open-Channel Flows

Abstract

The extent to which turbulent structure is affected by bed-load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed-sized granular sediment with a d50 of 1.95 mm. Three different sizes of sediment ~d50= 0.77, 1.99, and 3.96 mm! were fed into the flow at two different rates ~0.003 and 0.006 kg /m/s !, and subsequently transported as bed load. Particle image velocimetry ~PIV! was used to determine the turbulence characteristics over the fixed bed during clear water and sediment feed cases. Mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases. The exception was with the transport of fine grains at the higher feed rate. In this case, longitudinal mean flow velocities increased compared to the clear water condition. The coarse grains tended to augment bed roughness, but fine grains saturated the troughs and interstices in the bed topography, effectively causing the influence of bed irregularities to be smoothed. The PIV technique permitted examination of both temporal and spatial fluctuations in flow variables: therefore many results are presented in terms of double-averaged quantities ~in temporal and spatial domains!. In particular, the form-induced stress, which arises from spatially averaging the Reynolds averaged Navier-Stokes equations and is analogous to the Reynolds turbulent stress, contributed between 15 and 35% of the total measured shear stress in the roughness layer. Flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production, suggesting that this process is highly intermittent near rough beds.