Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

Using Eq. (11) the determination of the critical slope by successive approximation can be avoided if the critical slope is computed with the help of the f

c value obtained with the help if Eq. (7). For a sediment of mixed grain size composition the relationship = 0,06 Ф\С

Х (rs — 7v)d (log 19d;/d)

2 where Ф is the shape coefficient of the particles, C

x the coefficient of frictional resistance, dj the particle diameter under consideration and d the weighted average diameter of the sediment material of mixed grain size composition. One of the objections to the determination of boundary conditions of sediment transportation is that the parameters of turbulent flow are not included among the hydraulic characteristics of the watercourse. It would be logical to assume that the extent, and especially intensity, of turbulence should be taken into consideration. The validity of this assumption is demonstrated by the experiments in which for the same sediment material of unchanged particle size and specific gravity the critical mean velocity was lower at higher intensities of turbulence. Unfortunately the relevant experiments have only been started recently so that no definite numerical expressions can be given as yet. The greatest problems are encountered during investigations into the rates of bed-load transportation. As mentioned earlier, there is practically no method available for estimating the rate of sediment transportation, which could be generalized. Quite obviously the rate of sediment transportation is not a simple function of the hydraulic properties of watercourses and the volumes which can be transported of a sediment c; given particle size and specific gravity, will depend on the relative roughness and slope. Investigations which have been started in this direction have already secured interesting results. It is perhaps surprising to note that little attention is paid to the forces, causing the individual sediment particles to move. This may be offered as an explanation for the fact that the uplift forces, which are necessarily present, are neglected by most investigators in favour of the frictional forces. The inclusion of these uplift forces would, of course, inevitably lead again to the extent or intensity of turbulence, the effect of which is — as mentioned before — not fully understood. It should be added that in the opinion of some investigators the formulae on rates of bed-load transportation, respectively the coefficients involved therein, can only be determined with the help of field observations. This conclusion is unquestionably correct, but before field measurements can be performed, the theoretical relationships must be cleared first by laboratory experiments. The developments in isotope techniques offered new opportunities for the application of radioactive isotopes to research into the boundary conditions of sediment transportation. Experiments using radioactive 2 17