Uniform Bed-load Sediment Threshold Probability And Threshold Conditions Based On Total Threshold Probability

The threshold probability and transport law of bed load sediment is an important subject of river dynamics,and it has important significance in river dynamics theory and practical engineering.Based on previous studies,this paper studies the distribution law of three-dimensional sediment exposure angle,sediment threshold condition,threshold standards,total threshold probability and bed load sediment transport rate.The main research contents and conclusions are as follows:(1)By constructing a three-dimensional theoretical model of the sediment exposure degree,and used uniform glass spheres carry out the experiment experimental research for the exposure angle distribution law.The result show that probability density function approximated the normal distribution.(2)The instantaneous acting velocity and effective acting height were determined based on the relative position of sediment particles on the river bed.The relationship between the incipient conditions of sediment,and the sediment exposure on the bed surface was established,and the difficulty of bed sediment incipient mode is determined.Incipient motion by rolling and sliding is depended on the exposure angle of sediment particles.When the exposure angle?is less than 42.8°,a sediment particle is easier to rolling;when the exposure angle?is greater than 42.8°,the sediment is more likely to sliding.On this basis,the bed load total threshold probability formula considering two incipient modes was deduced,and the bed load transport formula was established.The validation of the classical measured data of bed load transport rate show that the formula has good applicability for high,medium and low sediment transport rates.(3)Based on the relative positional relationship of the three-dimensional bed surface sediment particles in a special accumulation form,taking into account the characteristics of water flow pulsation,the relationship between the critical shear stress with the boundary Reynolds number and the threshold probability formula are theoretically established in the three starting modes.It is concluded that the critical shear stress required for rolling mode is the lowest,and the corresponding relationship between the critical shear stress and the threshold probability is determined.It is further analyzed that the rolling critical shear stress in this paper basically coincides with the Shields curve at large boundary Reynolds numbers.Sediment incipient motion is affected by both flow and bed sand.When the relative exposure is averaged,the value of the drag force coefficient has no effect on the critical shear stress of the lifting mode,but the C_D value on rolling mode and sliding mode has great influence;the value of the lifting force coefficient has the greatest influence on the critical shear stress of the lifting mode.When the hydraulic force is rough,it has little effect on the critical shear stress in the rolling mode and slightly greater than the critical shear stress in the sliding mode.(4)Based on the distribution of the exposure angle in the general accumulation form of three-dimensional sediment particles,taking into account the randomness of the water flow and the randomness of the position of the sediment particles,and the total threshold probability formula of single sediment is derived by means of rolling mode.When the critical shear stress is 0.054,P=0.6887,which is in accordance with the results of By Sung-Uk Choi and Seungjoo Kwak(2001).Further based on the surface probability of a single sediment on the unit bed surface,the relationship between the threshold probability of a single sediment on the unit bed surface and the percentage of particles in motion is established;using the threshold probability of 0.2679 as the threshold standard,the threshold sediment bottom is deduced Speed formula and vertical average threshold flow rate formula.