Characteristics of nonuniform bedload motion under equilibrium sediment transport conditions

The influence of bedload characteristics and particle exposures on the nonuniform motion of the sediment was investigated. A formula for incipient motion of nonuniform sediment was developed according to the force balance relationship describing the rolling motion. The mathematical solution was obtained after analysis of the feature of the additional drag based on field measured data. The mathematical expression for the critical Shields number of nonuniform sediment was also derived by analysis of the kinematic equation of the nonuniform sediment. Then the difference of the incipient motion between the nonuniform sediment and uniform sediment was interpreted theoretically (uniform sediment is easier to move compared with nonuniform sediment with the equivalent grain size and vice versa). The kinematic feature of the nonuniform sediment in water flow was analyzed and it was found that the motion of the nonuniform sediment particles was directly linked with the bed conditions. Sediment has different motion pattern under different bed conditions. Finally, a sediment transport rate formula for nonuniform sediment under equilibrium sediment transport conditions was established, whose results agreed well with field measurement data. This formula is versatile because it can also be used to calculated the sediment transport rate of uniform sediment.