Analysis of near-bed sediment motion

Based on the experimental data obtained by using two-phase measuring technique,this paper analyzes the fluctuating features and mechanism of sediment movement near the bed in water-sediment flow.The special mechanism of sediment movement in the near-bed layer is found in that sediment particles have very intense non-turbulent fluctuation near the bed,resulting in fluctuating stress,which will greatly influence sediment movement.From this point of view,a systematical knowledge on the physical process of sediment particles being raised in water flow is presented.Some problems and insufficiencies existing in traditional theories are discussed,from which a new basic model of sediment entrainment is suggested.Using the two-phase flow theory,the mechanism of sediment entrainment are analyzed in detail.Finally,the profile of sediment concentration are investigated by the theory of momentum equilibrium of particle movements in vertical direction.It is demonstrated that,besides the gradient of sediment concentration,the gradient of fluctuating velocity intensity of particles is also a significant diffuse force,which further gives a reasonable explanation for the mechanism of two types of profiles of sediment concentration.