Vízügyi Közlemények, 1970 (52. évfolyam)
4. füzet - Rövidebb közlemények és beszámolók
of suspended sediment, bed load transportation, roughness conditions in movable channels of watercourses, the settling of sediment and thus the development of silting, together with several other phenomena may all be regarded as processes following or preceding the beginning of movement. In fact all the above phenomena are controlled fundamentally by the same physical quantities and it is indeed exceptional that additional variables must be introduced to solve particular problems. Consequently all detail phenomena related to sediment transportation are more or less closely connected with the beginning of sediment movement. In this respept a great number of theoretical but experimentally unsupported, and an equally great number of empirical but theoretically unjustified relationships have so far been published. Laborious and lengthy research work would be required to clear the reasons for the widely differing results and controversial interpretations. Although no conclusive solution has been attained as yet, it appears safe to state that the boundary conditions of sediment movement cannot be described uniquely and without consideration to the general hydraulic factors, even if the particle size and specific gravity of a particular sediment are known. For sediments of identical grain size and specific gravity the so-called variable laws defining the critical condition begin to emerge, but for sediments of mixed grain sizes the greatest difficulty is still encountered, although interesting new results are to be found in the very papers submitted to the Symposium. For describing the boundary conditions of incipient motion the jrictional drag theory and the impact theory have been introduced on the basis of theoretical considerations, which have lead to the theorems of the constant tractive forces, or constant critical velocities. Although the first attempts at the practical application of these relationships have already revealed appreciable inconsistencies, they are still used for average conditions especially in the absence of a sufficient number of actual observation data. With the help of one of the theoretical relationships for mean velocity, the relationship between critical tractive force and critical mean velocity can also be determined. Adopting the Chezy-formula, the critical mean velocity is obtained from the expression where f is the dimensionless resistance coefficient of Darcy — Weisbach, y\ and у are the specific gravities of sediment and water, respectively, v is the coefficient of kinematic viscosity, g the acceleration by gravity and d the particle diameter. The above relationship is, of course, founded on the law of constant tractive forces, but as mentioned before, its application is permissible for average conditions. (6) 14
