Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

computed from the relationships s It. <L 10"

1 and s > r while the full observation period T was taken as 100 s, meeting the condition T/t, I> 10

2 The natural gravel fractions from 1,47 to 2,27 and 2,27 to 4,47 mm diameter used in the experiments by L. Sárosi and L. Kaposi so far, cannot be regarded as bed materials of uniform size composition, so that the reliable observation of their critical condition is very difficult and the results may easily scatter between wide limits. Once the first observations were made with these wide gravel fractions, it would have been interesting to read some remarks on the variations of incipient conditions of movement as the intensity of turbulence increased (the duration of the vibration phase immediately before displacement, the number of particles starting to move simulateneously etc.). A variety of hydraulic parameters can be used for describing the beginning of bed-load movement, of which critical mean velocity is most readily determined, although its magnitude depends on waterdepth. These problems are dealt with in two papers. In his paper [4], Z. Hankó neglected the influence of bottom slope and combined the shape and friction coefficients into a single experimental constant. Assuming uniform gravel composition the critical condition is described with the help of seven variables, namely gravitational acceleration, waterdepth, kinematic viscosity, the specific gravities of water and sediment, as well as particle diameter. In his earlier papers the author developed the semi-empirical relationship of J. L. Bogárdi, who established a linear relationship between the tractive force per unit area and the diameter of bed-load particles. The validity of this relationship was extended in these papers to sediment and fluids of arbitrary specific gravity. In the present paper these generalized equations expressing the linear relationship are combined with the Chézy formula and solved for the critical mean velocity. However, in this manner the coefficient of frictional resistance of flow was introduced into the equations, which the author computed from the Colebrook

—White equation assuming that the roughness of the bed is identical to the particle diameter of bed-load. The correctness of the computation method is illustrated by numerical examples, which show a fair agreement with the results of laboratory experiments performed using sand- and powdered coal of uniform gradation as bed material. However, owing to the fact that the method is valid for uniform bed-load materials only, its field of application — especially on natural watercourses — is considerably limited. In his paper [1] J. L. Bogárdi uses the method of potential dynamic velocities introduced in several earlier publications, in an analytical approach to the determination of the resistance coefficient of the bed and the variations thereof. The potential dynamic velocities are no actual velocities, but quantities having the dimension of velocity and formed of 50