Several Fundamental Aspects Of Bed-Load Transport Mechanics

Bed-load transport has been one of the basic topics in the field of river dynamics for more than a century. Due to its complexity, however, the topic still challenges the research community as only a small number of theories have been well established.The present thesis presents investigation of bed-load transport in friction coefficient, sediment entrainment, bed-load transport intensity, and the bed armoring process. Major results are summarized as follows.? A new physical concept, the angle of surface friction, was proposed based upon review of typical methods for determining the angle of repose. The difference between the angle of repose and the angle of surface friction was clarified via experiment. The relationship between the sorting coefficient and the angle of repose for non-uniform sediment was investigated, and the influence of the asymmetry and kurtosis coefficients was quantified. Recommendation was proposed on optimized design of non-uniform sediment gradation in some specific engineering projects.? The rolling model for describing sediment entrainment which is characterized by stochastic turbulent fluctuation and irregularity in particle configuration indicates that the Shields number falls in a range rather than at a single value. In the framework of the Einstein's(1950) sediment entrainment probability, the area ratio, quantity ratio and time ratio were reinterpreted to demonstrate the physical picture of sediment entrainment probability. A new model of sediment entrainment was developed, lending support to Einstein's(1950) probability model which was questioned by later investigators. Three so-called defects in Einstein's(1950) theory, the wrong distribution of lifting force, the improper integral region of the probability model, and the neglect of the drag force, were re-examined based on theoretical analysis and data verification.? Based on the modification of the Engelund's(1976) theory, a simple bed-load formula was proposed incorporating both the deterministic and the probabilistic approaches. A total of 382 sets of experiments were conducted for bed-load transport in equilibrium conditions with uniform particles in flat beds. The performances of some well-known bed-load formulas were examined against classical data and the present experimental data, and the results show that the present formula performs well in calculating both low and high bed-load transport rates while other bed-load formulas tend to perform well only in either end.? To deal with non-uniform bed-load transport, a series of armoring experiments with two kinds of sediments were carried out by recording real-time bed-load transport process, measuring the sediment size distribution of time-phased bed-load and the armor layer, and reconstructing the three dimensional terrain of the armoring bed. Data analysis indicates that the bed-load transport rate and the sediment size distribution exhibits strikingly different varying characteristics in both the first armoring process and the re-armoring process. Particularly, a “secondary armoring” phenomenon(i.e., bed-load transport rate and the quantity of coarse particles increase abruptly in the armoring process) occurs in the case where coarse particle dominates in the bed material, and the surface morphology of armor layer is regularly wavy under the scouring by intense flows. The surface morphology of armor layer turns to groove when fine and coarse sediments are almost equal in quantity. When the bed material and the maximum discharge in the stair constant flows remain unchanged, the total quantity of bed-load transport, the sediment size distribution of the armor layer, and the scouring depth are independent of scouring patterns(i.e., the different stair constant flows).