Research On Pattern Of Bimodal Sediment Incipient And Transport Motion

The regular pattern of sediment initiation and transport has always been one of the basic research directions of river dynamics.There are a lot of non-uniform sediment in natural river channels,and bimodal sediment,as a special form of non-uniform sand,has its special motion regular pattern compared with that of ordinary non-uniform sand,so it is necessary to conduct in-depth research on the law of bimodal sediment movement.In this paper,based on the randomness of bimodal sediment movement and the combination of probability theory and mechanics,the surface sediment position characteristics,probability of sediment initiation,bedload transport rate,bedload gradation and the prediction of bedload erosion gradation on bimodal sediment bed surface are investigated.The main research work and results of this paper are as follows:(1)Considering the randomness of the distribution of bimodal sediment particles,the sediment particles are simplified as a sphere model,and the glass balls with bimodal size distribution are used for flume tests.The distribution law of the bimodal sediment exposure angle is statistically analyzed.It is found that the bimodal sediment exposure angle is normally distributed,and the relationship between the bimodal sediment exposure angle and the median particle size,sediment fineness ratio,bimodal coefficient and other parameters is obtained,The formula for the average exposure angle of bimodal sediment was fitted.(2)Based on the non-uniformity of bimodal sediment,considering two starting modes of sediment particle rolling and suspension,the bimodal sediment hidden storm coefficient is introduced,and the lifting force coefficient and drag force coefficient of uniform sediment are corrected;Based on the randomness of near bottom flow,a bimodal sediment rolling probability model and a suspended probability model were established,and the sensitivity of parameters in the target model was analyzed.(3)Based on the derived bimodal sediment rolling probability formula,the sediment lifting probability is introduced,the equilibrium exchange between bed load,bed load and suspended load is considered,and the bimodal sediment equilibrium transport rate formula is established on the basis of Einstein’s modified bed load formula.Analyzed the calculation method of non-uniform sediment bed load grading,and used the method of graded sediment transport rate to calculate the bed load grading under the equilibrium sediment transport state of bimodal sediment.The accuracy of the formula is verified by actual measurement data.(4)Considering the influence of two starting modes of suspended and rolling sediment on bed sediment coarsening,a multi-step calculation model for predicting the hierarchical distribution of sediment coarsening in a bimodal pattern during clear water erosion was established,and a formula for calculating the hierarchical distribution of sediment coarsening in a bimodal pattern was obtained.