Bedload and Concentration Effects on Turbulent Suspension Properties in Heavy Particle Sheet Flows

A new set of open-channel flow experiments and turbulence resolved data are presented in heavy particle sheet flows (Shields number $0.35\le \theta \le 0.85$) for which the proportions of bedload and suspended load are both important (ratio of settling velocity and friction velocity in the range $0.8\le w_s/u_{*}\le 1.3$). The effects of sediments and particularly the bedload on the turbulent suspension have been addressed by gradually increasing the concentration from clear water to capacity conditions. Distinction between the bedload and the suspension layer is discussed on the basis of the linearity of turbulent mixing length profiles. It is shown that the bedload layer has important impact on the vertical structure of the particle-laden flow. An upward shift of the logarithmic velocity layer is seen to be accompanied by a strong reduction of turbulent momentum mixing. The modification of the mixing length affects the theoretical formulation of both velocity and concentration profiles in the suspension layer. A modified analytical solution is derived for the suspended sediment concentration profile taking into account the presence of the bedload layer for improved predictions compared with the classical Rouse equation. Based on the present experiments, as well as literature data, an alternative parametrization for the $\beta$-factor (ratio of sediment and momentum diffusivities) is proposed over an extended range of suspension number, $0<w_s/u_{*}<1.5$.