Vízügyi Közlemények, 1970 (52. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

Danube tends to restore the disturbed conditions over the dredged section as regards both bottom elevations and stages. Consequently, a permanent reduction of stages is impossible, unless the bed is dredged continuously, or major dredging operations are repeated periodically. It should be emphasised, however, that the results obtained by studies in a movable-bed model offer qualitative, rather than quantitative information on bed changes and stage reduction caused by dredging. As mentioned earlier, sediment and bed-load conditions in a particular watercourse are closely related to its hydraulic properties and morphological features. Consequently, when an obstruction is built across a river, the sediment conditions as well as the morphology of the bed will be appreciably modified over the backwater reach. Over such reaches waterdepths increase, the velocity of flow is reduced and some of the sediment is deposited, or its rate of transportation — the sediment discharge —

1 is decreased. On the other hand, downstream of the weir the movable bed of the river is usually scoured to greater depths. The extent and rate of sedimentation and scouring must be known for the solution of a number of problems in water management. For this purpose the general relationships existing between all significant parameters of channel building processes should be understood better. Since no similar relationship could be established so far (the formulae developed as yet yield results of acceptable accuracy only for the weir, or dam for which they were derived from observation data), and since the results obtained from formulae of local validity are usually of questionable reliability, it is common practice to observe the quantities characterizing sediment transportation and bed changes of the watercourse, in the attempt to derive conclusions therefrom on the probable extent of sedimentation and silting. These estimates are made more realistic, if there is already a weir in operation on the watercourse and the results of observations made in its vicinity can be utilized. This is the case at the second barrage under construction on the Tisza river, where the results of field observations were used by J. Csuka [2] for predicting the extent of bed changes above and below the structure. He succeeded furthermore in deriving a relationship between the hydraulic and sediment parameters. The annual volume of sediment deposited over the backwater reach was estimated, and on the downstream reach the points were predicted where scouring will necessitate the construction of regulation structures. Relationships between flow and sediment characteristics — determined on the basis of site observations — were used also by S. Bruk [9] for predicting the bed changes resulting from the weir under construction in the Iron Gate section of the Danube. The ingenious method of computation is based on the division of the backwater reach into several (in this particular case 71) sections, on conceiving the mathematical model of flow and sediment movement over the backwater reach after the construction of the weir, and on establishing 2 65