Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

SEDIMENTATION OF WATERCOURSES, LAKES AND PONDS By DR. I. V. NAGY

1 The phenomenon of sedimentation is closely related to the sediment carrying capacity of water, be it that of watercourses, lakes, or ponds. In standing water, at higher concentrations, i. е., in the majority of cases of practical interest, the assumptions relating to the settling of the individual particle do not apply, much rather the group fall velocity of particle aggregates must be taken into consideration. This in turn is affected, among other factors, by concentration as well, so that the group fall velocity decreases as concentration is increased. In flowing water the effect of turbulent mixing must be added to the above factors. Viewed statistically, on the one hand the settling of particles is impeded thereby, on the other hand, the uplift force acting on the particles at the bottom, produce at certain flow velocities in the bottom layer a state of statistical equilibrium, where the number of particles settling to the bottom equals that of particles lifted therefrom. In this case the rate at which total solids are transported will equal the sediment carrying capacity of the watercourse, neither silting, nor scouring occurs, and the process is essentially of a steady character. Under conditions departing in any direction from the state of dynamic equilibrium, the problem of unsteady movements is encountered, which presents especially in the case of two-phase fluids considerable difficulties even as far as definition is concerned, not to speak of the complexities of solution. Foi this very reason at the present stage of knowledge no exact solutions are available even for the simplest problems of this nature; no theory can be applied to these cases unless grossly simplifying assumptions, semi-, or fully empirical relationships are introduced. It has been noted by M. A. Welikhanov [26] already that the dynamics of fluid flow and sediment movement differ on an essential point from each other. In the case of fluid dynamics three basis types of flow can be 1 Doctor of Technical Sciences, Professor of Hydraulic Engineering. Technical University, Budapest, Hungary