Hyperconcentrated flows as influenced by coupled wind-water processes

Abstract

Using data from more than 40 rivers in the middle Yellow River basin, a study has been made of the influence of coupled wind-water processes on hyperconcentrated flows. A simple “vehicle” model has been proposed to describe hyperconcentrated flows. The liquid phase of two-phase flows is a “vehicle”, in which coarse sediment particles are carried as solid-phase. The formation and characteristics of hyperconcentrated flows are closely related with the formation and char-acteristics of this liquid-phase and solid-phase. Surface materials and geomorphic agents of the middle Yellow River basin form some patterns of combination, which have deep influence on the formation and characteristics of liquid- and solid-phases of hyperconcentrated flows. The com-bination of high percentages of relatively coarse material with low percentages of fine material appears in the area predominated by the wind process, where the supply of relatively coarse sediment is sufficient, but the supply of relatively coarse sediment is not. The combination of low percentages of relatively coarse material with high percentages of fine material appears in the area predominated by the water process, where the supply of fine sediment is sufficient, but the supply of fine sediment is not. In the area predominated by coupled wind-water processes ap-pears the combination of medium percentages of coarse and fine materials, and thus both coarse and fine sediments are in relatively sufficient supply. The manner in which the mean annual sediment concentrations of liquid- and solid-phases vary with total suspended sediment concentration is different. With the increased total suspended sediment concentration, mean annual sediment concentration of liquid-phase increased to a limit and then remained constant; however, mean annual sediment concentrations of solid-phase in-creased continuously. Thus, the magnitude of total suspended sediment concentration depends on the supply conditions of relatively coarse sediment and the ability of the flow to carry these relatively coarse sediment particles. In the area predominated by wind process, both the liquid- and the solid-phases cannot develop well, and their concentrations are low. In the area pre-dominated by the water process, the mean annual sediment concentrations of liquid- and solid-phases are also low. Only in the area predominated by coupled wind-water processes, can the conditions most favor the development of both the liquid- and solid-phases, and then the peaks of mean annual sediment concentrations of liquid- and solid-phases appear. Low values of suspended sediment concentrations appear in the areas predominated by the wind process or by the water process, a fact indicating that the predominating wind process or water process does not favor the development of hyperconcentrated flows. Peak values appear in the area where the coupled wind-water processes are predominated, indicating that the cou-pled wind-water processes most favor the development of hyperconcentrated flows.