Incipient motion of non-uniform coarse grain of bedload considering the impact of two-way exposure

Abstract

The incipient motion of bedload is due to the interaction between the flow and sediment. It is stochastically correlated with the flow structure, the sediment gradation and the arrangement of grains on the bed surface. The random position of the sediment on bed can be represented by a hiding factor or an exposure degree. Based on the numerical simulation of the disturbed flow in the interstice of grains, the influence of the two-way exposure degree (the vertical exposure degree and the longitudinal exposure degree) on the coarse grain incipient motion was investigated in this work. Results show that the exposure degree varies with the position of the sediment on the bed, which influences the flow structure around the particle and the incipient motion. In this paper, the major research achievements on this phenomenon include: Firstly, a mathematical model is established for the rolling-pattern incipient motion of the coarse grain under a critical state of moment balance. The influence of the partial disturbed flow is considered. Secondly, the two-way relative-exposure-degree probability distribution functions are developed to reflect the influence of the disturbed flow and the random arrangement of sediments. Thirdly, a formula to calculate the incipient velocity is presented based on the above results, which considers the impact of the two-way exposure degree of sediment particles.