Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

As demonstrated by recent investigations the differences between results and observations performed under different conditions are due to the fact that for a given sediment material of a particular grain size and specific gravity, the critical tractive force depends on the hydraulic characteristics of the watercourse and consequently the critical velocity will also vary as a function of these latter. The least difficulty was encountered in demonstrating that being related to waterdepth the critical mean velocity of a watercourse cannot be constant for a sediment of given specific gravity and particle size. Investigators resorted then to the bottom velocity, the determination of which, either theoretically or by observations is extremely difficult, yet which appeared fully satisfactory for describing the critical condition. Unfortunately, as demonstrated by recent investigations, neither the critical bottom velocity is a constant value. Omitting details it should only be noted that according to latest information the magnitude of the critical bottom velocity depends also on the width of the channel in which it is observed. As long as the particle diameter remained smaller than 1—2 mm the channel width was of no influence, but beyond this size limit the critical bottom velocity was found approximately constant only in flumes wider than 1 m. The determination of changes in the critical tractive force presented substantially greater difficulties. The first attempt in this direction was made in 1936 by Shields, who was followed in 1937—38 by investigators in Iowa. It has been demonstrated by these experiments 30 years ago that the critical tractive force r

c is not a constant value even for given combinations of d and y\. Since the phenomenon was not sufficiently understood then to permit direct interpretation of these results, they were mostly forgotten during the intervening period. As a result of recent investigations the variable laws defining the critical condition are now available for the case of sediments consisting of identical particles, so that both the tractive force and the critical mean velocity can be estimated by them. For example with the help of these relationships the dimensionless r

c resistance coefficient f

c = TT introduced by Shields is obtained from {71—7) d the expressions í d .Л) |y/3g_l/3 ' d) (7) S ) <

8> Consequently, as indicated by Eq. (7) the critical tractive force pertaining to a sediment material of constant particle size d and specific gravity у 1 is a function of the relative roughness Did in such a manner 15