Sorting Mechanism Of Non-uniform Bed-load Under Clear-water Scouring

It is of great significance to study the movement law of pebbles in the channel of mountain area for further study of the resistance law of mountain rivers and thus control the channel of mountain areas.The mechanism of scouring of river bed is the key to finding the scour depth of river bed,observing lower water level and the changing hydro geometric relation.Under certain water flow conditions,the form of bed load movement on the pebble bed is dominated by rolling.Different sizes of particles result in different movement distance—coarse particle move further and the finer particles move nearer,resulting the separation of bed surface sediment.When the ultimate impact of scouring stability,coarse grains form a thick protective layer on the surface of the river bed which limits further scour.In this research,the critical starting velocity of particles is analyzed based on the rolling mechanical model,and the formula of particle incipient velocity is obtained,which is verified by the measured data.The mechanical model of pebble rolling motion is established using the numerical solution,with which the rolling track of the gravel particles of different conditions could be drew.The mathematical expressions and expectations of single-step parameters such as single-step distance and single-step period are obtained based on the former research.Then,the influence of relative exposure probability distribution on particle starting is discussed.According to the analysis the particle rolling process is divided into initial step and continued step,and the particle probability of continuous rolling is calculated.It indicates that continuous rolling probability is a function which only has relationship with the flow velocity and particle size.The statistical characteristics of particle motion and the relationship between the single-step and single-time movement are obtained.The formula for grading the uneven gravel bed after scouring is deduced,and the change of gradation and erosion gradation after scouring in different time and different channel section is calculated through a practical example.The theoretical results of this paper could be applied to calculate the rolling of bed load and it provide a theoretical calculation method for the study of sorting process of pebble river bed.This calculation method is simple and feasible which could obtain consistent results with other scholars.Finally,two experiment is done to observe the separation of coarsening in water washed.The theoretical formula and the experimental data are compared and the error is small enough to show its high reliability.