Viscous effects on bedload sediment transport rates

Viscous effects on bedload sediment transport rates have been seldom examined in previous studies. They may become significant in the low transport regime, as evidenced by experimental data. However, the problem cannot be resolved using classical bedload models that involve the shear excess, for example, Meyer-Peter–Müller formula. By applying the concept of the critical pickup probability for incipient sediment motion, this study derives a new bedload formula that takes the fluid viscosity into consideration. The results show that the relationship between the dimensionless bedload rate and the Shields number varies significantly with the viscosity-dependent parameter in the low transport regime, which partially accounts for the scattering of bedload data. In addition, the present study proposes a simple approach to the evaluation of the critical bedload transport rate that is associated with the Shields curve for incipient sediment motion.