Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

Another method for the theoretical determination of sediment transportation is founded on the principle of chemical streamflow measurement. The tracer material of known total weight К (kp) is introduced into the moving band of width b (m) in a preselected cross-section and the changes in concentration by weight of the tracer material mixed with bed-load is determined in terms of time by regular sampling as long as the entire tracer cloud has passed through the observing section. If the area under the curve characterizing the variation of concentration is C, then the specific rate of bed-load transportation is obtained from the expression = (15

) A third approach to observation consists of introducing the tracer material for an extended period at the uniform rate q, (kp/sec) into the channel. By regular sampling in the observing section the time is determined when the concentration of the passing tracer becomes steady. If this steady concentration is denoted by C

k, then the specific rate of bed-load transportation becomes «•=тк (1

6> In the first of the three above methods steady flow conditions are not a criterion for the success of application. For this reason in many cases this method should be given preference over the other two. In cases, however, where sediment transportation is very violent and the moving tracer is difficult to follow, the use of one of the other two methods may become advisable. Studies on sand-bar travel form an organic part of field sediment observation. The travel of sand-bars, the changes in their shape and dimensions, are obviously consequences of bed-load transportation. For this reason sand-bar travel is started as soon as the mean velocity of flow exceeds the critical value. As a result of extensive investigations the velocity of sand-bar travel can be described in terms of the Froude-number and one of the parameters of sediment mobility, such as the ratio of flow velocity to the settling (fall-) velocity of the particle fraction under consideration. Sand-bars may be looked upon as mezo-configurations of the channel, whereas the development of the so-called micro-configurations depends in general on physical conditions only. The occurrence of mezo-configurations on the other hand is intimately related to the geometry and dimensions, as well as to the hydraulic variability of the channel. It is interesting to study the influence of sediment conditions on bed development in the light of geographical aspects as well. According to one more recent approach the character of different river stretches is charac20