Numerical Investigation on Downstream Increase in Peak Discharge of Hyperconcentrated Floods in the Lower Yellow River

Hyperconcentrated floods in the Yellow River usually accompanied with some peculiar phenomena that cannot be explained by general conceptions of ordinary sediment-laden flow (e.g., downstream increase in peak discharge, instability flow, ripping up the bottom). Up to date, the mechanisms for the abnormal phenomena are not well understood. The aim of this paper is to facilitate a new insight into the abnormal downstream increase in peak discharge of hyperconcentrated floods in the lower Yellow River. Numerical model experiments have been conducted on a typical flood occurred in August 1992 in the Lower Yellow River during which the peak discharge at Huayuankou station was 1690 m3/s larger than the value at Xiaolangdi station at upstream. It is found that a fully coupled model that incorporates the contribution of bed evolution to the mass conservation of the water-sediment mixture, can reasonably well capture the characteristics of peak discharge rise and severe bed scour, while separate numerical experiment using a decoupled model, which ignores the feedback effects of bed evolution, shows no rise in the peak discharge. This leads us to comment, if only briefly, that the entrainment of sediment due to bed erosion is the main reason for causing peak discharge increase along downstream course.

Hyperconcentrated flows as influenced by coupled wind-water processes

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 forma-tion and characteristics 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 combination 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 rela-tively coarse sediment is sufficient, but the supply of relatively coarse sediment is not. The com-bination 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 appears 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.

Sediment transport capacity of hyperconcentrated flow

As one of the most important components of river mechanics,sediment transport capacity of sediment-laden flows has attracted much attention from many researchers working on river mechanics and hydraulic engineering. Based on the time-averaged equation for a turbulent energy equilibrium in solid and liquid two-phase flow,an expression for the efficiency coefficient of suspended load movement was derived for the two-dimensional,steady,uniform,fully-developed turbulent flow. A new structural expression of sediment transport capacity was achieved. Using 115 runs of flume experimental data,which were obtained through two kinds of sediment transport experiments in the state of equilibrium,in combination with the basic rheological and sediment transporting characteristics of hyperconcentrated flow,the main parameters in the structural expression of sediment transport capacity were calibrated,and a new formula of sediment transport capacity for hyperconcentrated flow was developed. A large amount of field data from the Yellow River,Wuding River,and Yangtze River,etc. were adopted to verify the new formula and good agreement was obtained. These results above contribute to an improved theoretical system of river mechanics and a reliable tool for management of rivers carrying high concentration of sediments.

Motion Law of a Sphere in Bingham Fluid with Thixotropy

The thixotropy properties and the motion law of a sphere in the Bingham fluid have been studied. Through observation of the settling motion of a single sphere in the Bingham fluid on the X-ray screen, it has been discovered that the mud in estuaries and along sea bay, and the hyperconcentrated flow all behave as the Bingham fl fluid with thixotropy properties as the large sediment concentration. Through derivation, the theoretical relationship between the yield stress and non-settling maximum sphere supported by the stress for the Bingham fluid has been developed, the equations for calculating the increasing yield stress and the non-settling maximum sphere diameter with the duration at rest of the slurry have been obtained. In consideration of the effect of thixotropy on fluid motion, the Navier-Stokes equation group for the Bingham thixotropy fluid has been developed. Through further study of the flow boundary condition of settling motion of ii single sphere in the Bingham thixotropy fluid, and the solving of the Navier-Stokes equation group, under the small Reynolds number, the theoretical equation of the drag force of the Bingham thixotropy fluid flowing around a sphere has been deduced. The theoretical relationship between drag coefficient and Reynolds number has been derived. By use of the experimental data of rheological test of various slurries measured with viscometer and those of single sphere motion observed on the X-ray screeen, the above equations have been verified. The equations are in good agreement with the experimental data for various slurries.

TWO MODELS FOR ROLL WAVES IN A MUD LAYER

As has been extensively reported by hydraulic engineers in China, the flow of hyperconcentrated fluid mud in rivers is often pulsating and appears as roll waves. We report here two theories for such pulsating flows besed on two constitutive laws: the power law and Bingham plastic law. In order to account for the nonlinear bores convective inertia is kept in the approximate long wave equations. Kar man's momentum integral method with an assumed velocity profile is then employed. Results of the linearized instability analyses are discussed.Periodic bores are obtained either as stationary wave solutions or as the long- time limit of unstable disturbances which are initially infinitesimal. Implications on ho scour is examined.

FORMATION MECHANISM OF MEANDERS WITH HYPERCONCENTRATION

Ⅰ. INTRODUCTIONMeanders are a common river channel pattern. It is generally accepted that comparatively low sediment concentration, especially when bed material load is considered, is one of the fundamental conditions for the formation of meanders. With an increased sediment concentration, the river channel pattern will be transformed from a meandering one to a braided one. However, in our opinion, this holds true only for rivers whose sediment concentration is within the normal range. In studying alluvial rivers in wide valleys on the

ON THE STRUCTURE AND MOVEMENT MECHANISM OF FLOW WITH HYPERCONCENTRATION OF SEDIMENT

This paper, based on the information obtained from flume experiments and field observations, concerns with the analyses of the flow with hyperconcentration of sediment containing a certain amount of fine particles. Attention is focused on the classification of flow with hyperconcentration of sediment, the properties of the Bingham shear stress τB and rigidity coefficient η, the movement mechanism of fluid within flow-core and non-flow-core regions, the shear stress distribution and so on. Several formulae have been proposed to indicate vertical velocity distribution of 2-dimensional steady and uniform turbulent flow with hyperconcentration of fluid. The formulae can be applied either to the flow of the Bingham fluid or to that of the Newtonian fluid.