Vízügyi Közlemények, 1970 (52. évfolyam)
4. füzet - Rövidebb közlemények és beszámolók
results of the theoretical considerations by Yalin, by which a uniform basis was created for the interpretation of sediment formulae, and some of the statements thereof are mentioned. The author concludes that according to the analysis of Yalin, from among the empirical formulae only the validity of the Meyer —Peter expression is fully corroborated. In this connection it should be noted that in this subsequent papers Yalin himself demonstrated with the help of dimensional analysis that bed-load transportation is controlled by Hour dimensionless groups of quantities (the Shields parameter r„ the shear velocity Reynolds number R», the relative roughness D/d and the ratio QSIq of sediment-water densities). Of these the first one is only taken into consideration in the Meyer —Peter formula, the validity of which is for this very reason mainly in the range of higher shear velocity Reynolds numbers, i. е., where the so-called Shields curve assumes a horizontal trend. The corresponding value of the Shields parameter is taken as 0,047 and this is considered critical for the commencement of movement. Hereafter we fully agree with the author in his conclusion that no bed-load transportation formula of general validity is as yet available and that existing ones fail to make adequate allowance for such important parameters and conditions as the grain size distribution and variations thereof in the course of transportation, wear and sorting and as a result of supply, the problem of partial and complete bed-load movement, bedload transportation under non-steady flow conditions, etc. In this context it should be added, however, that steps have been undertaken towards the solution of these complex and involved problems, during which some results have already been attained. Mention must be made here of the interesting theoretical investigations of Egiazaroff [22] concerning the movement of mixed-grain sediments, as well as of the radioactive and luminescent tracer studies conducted for several years now in Hungary, with the aim of determining the most important parameters mentioned by the author as not having been taken into consideration adequately so far. Bed-load discharge formulae familiar from the literature and regarded as classic to-day are critized also by Rossinskyi in his paper [8]. These are criticized for relating the specific bed-load transportation to the mean velocity of flow and other hydraulic parameters, while failing to make direct allowance for the influence of velocity pulsation. The relationship between the pulsation of velocity and the incipient stage of bed-load movement is analysed by a theoretical approach, starting from the case where individual particles are just displaced from their position by the vertical velocity components, but the gravels proceed by rolling, or sliding along the bottom. The total rate of bed-load transportation is interpreted as the sum of sediment discharges rolling and sliding on the bottom. It is assumed furthermore that the particles move in a single layer on the bottom surface by rolling, or sliding, and that the latter overtake the former ones, without, however, interfering with their movement. 48
