Vízügyi Közlemények, 1980 (62. évfolyam)
1. füzet - Stelczer Károly: A görgetett hordalék mozgása. I. rész
26 Stelczer Károly Field measurements using radioactive tracers have indicated (Stelczer, 1979) that to a given particle size fraction a minimal bottom velocity Vfcmin belongs below which no movement occurs. The number of particles set into movement increases together with the bottom velocity until eventually a maximal bottom velocity vrcma is reached, at which all particles of the given size fraction start moving. The critical condition is consequently characterized by a velocity range, within which total bed load movement is induced. The displacement of individual particles, the megnitude of the bottom velocity, further the condition of total movement are typically random phenomena. For this reason it is logical to specify the velocity range corresponding to the critical condition with the help of some distribution. For determining the distribution function characterizing the critical condition, radioactive tracer experiments have been carried out in three cross-sections of the Danube. The cohesionless (alluvial) channel had a hydraulically rough bottom (Re x s= 170—300) and three bed load fractions of mixed size composition (Deo = 0,03138,0,02584 and 0,01417 m) were used. The measurements yielded the following data: the number of "stationary" and "moving" particles for successive 0,5 cm/sec increments of the bottom velocity, ordered into class intervals adopted arbitrarily, further the relative frequency p of the moving particles (Tabel I). The relative frequencies p have been plotted in Figs. 4,5 and G according to increasing values of the bottom velocities characterizing the class intervals adopted, for the individual particle sizes. It was observed consistently for each of the three particle size fractions that the relative frequency p of the moving particles assumed values increasing from 0 to 1 as the botom velocity increased. Thus it may be regarded as a relative frequency values pertaining to the bottom velocity intervals adopted arbitrarily. This, however, offers the possibility of regarding the bottom velocity as the random variable and of approximating the corresponding relative frequencies — as an empirical distribution — by one of the continuous distribution functions. For this purpose the one-dimensional normal (Gaussian) distribution function proved most convenient. The normal distribution functions determined for the critical condition of the three particle size fractions (in the three test sections) displayed practically identical standard deviations with <7 = 0,06 m/sec (Table II). This implies in turn, that within the range of field measurements (0,005 = Deo 0,05 m) the magnitude of the standard deviation a characterizing the normal distribution is unaffected by the factors representing the stream- and bed-load characteristics and may be assumed constant with the value ст = 0,06 m/sec. It follows further from the virtually identical standard deviations that the range of flow velocities (the values between 1 and 99%) may also be assumed to be constant with the value of 0,28 m/sec. * * * Geschiebebewegung Teil I.: Charakterisierung des kritischen Zustande« auï walirschcinlichkeitstheoretischer Grundlage von Dr.-lng. Károly Stelczer Die Bewegung des Geschiebes hat einen intermittierenden Charakter (Bild 1.). Die intermittierende Bewegung wird mittels der virtuellen Geschwindigkeit des Vorwärtskommens (Dhv) ausgedrückt, die wiederum von der Sohlengeschwindigkeit abhängt (Stelczer , 1971). Mit Abnahme der Sohlengeschwindigkeit verringert sich auch die virtuelle Geschwindigkeit des Vorwärtskommens. Offensichtlich gibt es einen kritischen Grenzwert der Sohlengeschwindigkeit, zu welchem der Übergang zwischen Stillstand und Bewegung des Geschiebes gehört. Die in der Natur durchgeführten isotopischen Messungen (Stelczer , 1979) ergaben, dass zu einer gegebenen Korngrösse eine minimale Sohlengeschwindigkeit («rc, mm)
