Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

The above theoretical approach to the process of silting, if checked against the results of field observations may be expected to throw light on the problem. Whereas theoretical aspects appear to have been neglected, the problem of reservoir silting has been studied extensively by methods based partly on theory, partly on experience. For determining semiempirical relationships of this kind, the observations made on the silting of mountain reservoirs are especially valuable. However, neither these semi-empirical methods can dispense with the determination of the sediment transporting capacity of the watercourse. In this case, however, the sediment volumes can be checked against the observed extent of silting. In the course of latest investigations it has been concluded that the quantity 97 expressed by Eq. (13) in the preceding section, can be used to describe actual sediment transportation, provided it is written into a dimensionless form. Accordingly The rate of sediment transportation (kp/m

3) is seen to depend on the fall velocity со, the water depth H, the acceleration by gravity g, the velocity of flow v and on an empirical factor К (kp/m

3), which is representative for the watercourse. It has been suggested recently that the drawback of most formulae recommended for sediment transportation is that neither the bottom layer nor the granulometry of the suspended sediment are taken into consideration. Studies into this problem, based on the results of field observations have greatly contributed to the better understanding of this phenomenon. When using a semi-empirical approach it has been found advantageous to estimate the rate of bed-load transportation from the sedimentation of the reservoir and to refine with the help of data thus gained the available sediment transportation formulae. Considering sediment transportation of the Danube and its tributaries, the magnitude of the parameter involved in Eq. (17) has been determined for two extreme cases, namely for the settling proper from moving water and for the critical condition of scouring by the flowing water. It has been verified by field observations that depending on whether aggradation or degradation occurred over individual river stretches, they represented the corresponding extreme case, while actual sediment transportation could be described by the corresponding parameter value lying between the two extremes. Reservoir silting is commonly estimated with the help of relationships derived tín the basis of empirical data. This method is extremely valuable especially in the case of small reservoirs, as it offers tentative information also on the expected useful life of the reservoir. Several relationships have been published in which the annual rate of silting and К V со H g v

3 (17) 22