Vízügyi Közlemények, 1970 (52. évfolyam)
4. füzet - Rövidebb közlemények és beszámolók
The ratio of the particles moving and at rest on unit surface area of the bottom is described by two density coefficients. The effect of particle shape and sheltering by other particles is combined in the first, while the second represents the part of the area under consideration, where the instantaneous bottom velocities exceed the value required for displacing the particles, i. е., the magnitude of the vertical velocity components is great enough to overcome the weight of the particles. The probability of these velocities is described by a normal distribution. The instantaneous bottom velocity critical for scouring is expressed from the equilibrium equation of horizontal components, whereas the probability at which critical bottom velocities are exceeded are used to describe the average rolling velocity of particles. As revealed by the diagram introduced, the theoretical results are in fair agreement with those obtained by Dementieva in earlier laboratory experiments. The rate of bed-load transportation is computed using the shear velocity considered identical with the rolling- and sliding bottom velocities determined in the foregoing, by combining essentially the conventional impact force theory with the statistical description of velocity pulsations. Theoretical results are compared with results of laboratory experiments of various authors and with those obtained on some rivers. The same problem is dealt with from the aspect of the experimental investigator by L. Sárosi and L. Kaposi in their joint paper [9]. Velocity fluctuations were observed at a representative point in each of two crosssections of the glass flume and the results were recorded on tape over a compensograph. Mean velocities were determined from the tape using a polar planimeter. For describing velocity fluctuations areas both above and under the mean velocity were determined from the record. The movement of bed-load was found to commence at lower mean velocities when the area used as a measure of turbulence was greater, or, in other words, lower critical mean velocities pertain to increased turbulence. Similar laboratory investigations are contemplated by the authors using artificial bed-load material of uniform grading and of identical specific weight. The laboratory observations described in the paper are the first steps of an extended series of experiments, and aroused great interest and expectations. The use of hot-wire, or bubble photography techniques may greatly contribute to the improvement of observations. In fact, however small is the inertia of the current meter vane, the turbulent velocity pulsations acting on it are averaged to a certain degree. In order to study this effect, pulsation measurements have been carried out by Iwasa. and Imamoto in an open flume, using a vane of very small dimensions [25].From the statistical analysis of the results they arrived at the conclusion that if the ratio of the time over which averaging is performed and the time unit of integration by Euler, i. е., the quantity rIU is smaller than 10 _ 1, the inertia of the vane can be ignored. According to the authors, in free-surface flow the value of t, varies from 0,1 to 1 sec, while that of r was taken as 0.03 sec. The period s of sampling was 2 49
