Vízügyi Közlemények, 1970 (52. évfolyam)
4. füzet - Rövidebb közlemények és beszámolók
is 8,5. Using these values in the computation 0.06 is obtained for r» c which is identical with the value of the Shields curve in the case of a high R, c value. For decreasing R», values a r and C D become greater and thus r, c shows a decreasing trend. The shape of the resulting curve is qualitatively the same as that of the Shields curve, but the two curves are not congruent. It should be noted in this context that rather than fitting a single curve among the points of observation, Shields himself used a fairly wide band to indicate the relationship between the plotted values of r„, and R, c. The critical shear velocity, or the mean velocity may assume different values even for the same particle size. This value may be influenced by the fact whether the bed is smooth, or dunes have already developed, further by the bottom slope, the granulometric composition of the sediment, etc. Detailed experiments have been performed into pressure and shear conditions on a bottom with ripples developed by Raudkivi [19]. Sediment movement was found to start at an average shear stress considerably lower than indicated as critical by Shields. From this it appears that the displacement of particles depends on the average tractive force prevailing on the bottom surface, as well as on turbulence. In other words, the particle is made mobile by turbulence, but it is transported by the shear stress. Concerning the causes and mechanism of the formation of different bed configurations the papers published in the literature are too copious to be even listed within the framework of the present report. Besides using a variety of analytical methods, the problem is approached by a group of investigators through the instability of the water-bottom interface, whereas others resort to dimensional analysis for this purpose. Latest experimental results have been usually plotted in terms of the Froude number, using the particle size, waterdepth, flow velocity, etc. as the second parameter. Bed configurations have been classified in different ways by different authors. The categories most commonly adopted are: 1) smooth bed, 2) ripples, 3) dunes, 4) transitions (wavy, or smooth) and 5) antidunes. In glass-flume experiments the smooth bed can be observed readily to become deformed in some places, these deformations increasing and spreading usually downstream. The first ripples are formed at random points on the bottom. Local accumulation of particles was observed to start immediately, as the velocity of flow exceeded the condition critical for incipient sediment movement. The factors responsible for this process are, among others, the non-uniformity of particle composition, the shielding effect of individual particles and the uplift forces due to turbulent eddies. Under the influence of these latter particles of a shape, or size may also be lifted from their original position, which the tractive force of the flow would be uncapable of conveying. It appears in general that the theoretical-empirical relationships published in connection with bed configurations yield fairly acceptable 46
