Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

tion of the microscope). By creating the possibility for obtaining new information theoretical research is also stimulated. While it is generally recognized that the movement of bed-load is one of the phenomena of Nature least accessible to observation, and consequently the level of bedload studies on the site remained far below that of theoretical investigations, great significance must be attributed to the tracer methods of bed-load studies introduced a scant two decades ago. The paper by K. Stelczer [10] contains a report on several research results which could not have been achieved without this method. Even the observation in the field of similar phenomena would have appeared impossible a few decades ago. The field studies of the author into bedload wear and movement have been started in 1964 on the Danube using three different gravel fractions labelled with different radioactive isotopes. Labelled particles were traced for several months with the help of radiation detectors and retrieved at the end of this period. Besides offering opportunity for checking various semi-empirical formulae on wear forth eir correctness, valuable information was gained also on the direction and rate of sediment movement. As far as bed-load wear is concerned, it is interesting to note that the formulae of Dull and Schoklitsch developed on the basis of revolvingdrum experiments yield much lower wear losses than the values actually observed. The considerable discrepancy is probably due to the substantial differences between the mechanisms of wearing under natural conditions and that realized artificially in the laboratory. The observation that no perceptible sorting according to particle size occurs even after extended periods of time, agrees well with the observations of the reporter, carried out using luminescent tracer material and for appreciably shorter periods. This seems to indicate that at low rates of bed-load transportation, when movement by rolling and sliding predominates, wearing is the main factor controlling the size reduction during downstream travel. Sorting of a major extent may probably occur at times of highwater and more intensive bed-load movement, when a significant proportion of the bed-load particles over the upper Danube section is carried — as a result of more violent turbulence — by saltation. The observation that some of the labelled particle fractions were moving already at tractive forces lower than the value indicated as critical by theory, may be due at least in part to mixed size composition. In fact, in laboratory experiments with luminescent tracer materials and sandy gravel mixtures it was observed that the mean velocity pertaining to initial displacement of the fine gravel fraction was lower by about 15% in the case of mixtures, than in that of uniform material [24]. We are in complete agreement with the closing remarks of the author, in which the continuation of extensive site observations is proposed, with the aim of deriving dimensionless groups involving all significant factors. Tracer techniques underwent considerable development during the two decades since they were introduced. A striking example thereof is 53