Movement characteristics and transport laws of the bimodal bed load

The non-uniform bed load transport is a complicated coherent physical process of the fluid and solid in river. The research into the motion characteristics, transport laws as well as the action mechanism of influencing factors can help to not only complement the fundamental theories in river dynamics, and but also solve the urgent requirements from the practical engineering sediment. A series of flume experiments was carried out using the techniques of image recognition and instant monitoring for bed load. By introducing the comprehensive flow intensity Ψ_b which reflects the thickness ratio of the bed η, properties of the viscous sublayer, the characteristic Froude number Fr_b, the transport characteristics of the non-uniform bed load with a bimodal distribution were investigated under several conditions such as different flow intensities and various compositions of bed material, with the effect of particle non-uniformity on transport rate of bed load being especially considered. By means of key factor identification and dimensional analysis, the transport mode of the non-uniform bimodal bed load was proposed. A formula for calculating the transport intensity of bed load was developed, which is expressed as a dimensionless function of the transport intensity Φ' and comprehensive flow intensity Ψ_b based on characteristic factors near the bed. An analysis of the transport mechanism of the bimodal bed load indicates that the ratio of coarse to fine particles in the composition properties of the non-uniform sediment is an important parameter affecting the value Φ'. It is found that the collapsing effect of fine particles on coarse particles has an important influence on the movement of coarse particles, and therefore, the transport rates of bimodal bed load present a hump curve with the variation in the degrees of particle non-uniformity, wit the corresponding η_c of about 3∶7 for the peak rate.