Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

and to check theoretical relationships against observation data. Formulae derived for bed-load transportation were readily accessible to verification by laboratory experiments and apparently successful efforts were made to obtain physically well founded relationships between stream-flow and the rate of bed-load transportation. However, in the case of bed-load comprising several size fractions, only a part of the particles is set into movement by a highwater wave of medium stages. This is indicated also by various bed-load formulae. The circumstance that only the smaller size fractions start to move results in a modification of the original granulometric composition. Moreover, if the comminution of particles is also taken into account, then it will become clear that a constant grain size distribution cannot be assumed even in the case of the highest critical flood, since the grain size composition depends on the streamflow regime during the period preceding the flood, which may extend to several years. The bed-load formulae failed also to allow for the sorting influence of spiral flow, as well as for the sorting of sediment moving in readily distinguishable bands. Further investigations became necessary which revealed eventually that in many rivers that were regarded to have a movable bed, the bottom material became cemented, the sediment material became arranged according to size and shape, and sorting occurred in the course of centuries in such a manner that larger gravel particles formed an armour layer over the more mobile sediments in the underlying layers. Bed-load arriving from upstream is conveyed through similar reaches without picking up additional bed-load corresponding to the transporting capacity of the stream. In this process the bed-load becomes abraded, comminuted and part of it increases the volume of suspended sediment. Problems concerning the abrasion of sediment, which assume increased importance on similar river sections, were investigated by K. Stelczer. A novel feature of these investigations was the tracing of representative bed-load particles, which were labelled by radioactive isotopes. In contrast to bed-load, no practical formulae are available for suspended sediment which would offer a positive and practicable relationship between the potential energy of the river and the rate of suspended sediment transportation. The sediment volumes carried in suspension are usually always smaller than estimated on the basic of the potential energy of the river. The volume of sediment available for suspension in the river bed is never sufficient to exhaust the transporting capacity of the stream and consequently the considerable volumes of silt suspended in the river originate in the catchment area and are transported into the watercourse by runoff. The rate of suspended sediment transportation is thus indirectly related to the rate of streamflow, the latter being controlled eventually by the runoff draining from the catchment and consequently increasing rates of runoff increase also the rate of erosion so that the rate of suspended sediment transportation is also increased. To derive a relationship between the rate of suspended sediment 36