Vízügyi Közlemények, 1969 (51. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(15) structures built up to 1940 were renewed. For securing the navigation channel dredging operations were intensified along the fords from 1951. The filling process of the main bed could not be stopped, however, and no final balanced regime was attained. The process of bed-aggradation could not be interrupted by the regulations. Owing to the rising stages accompanying bed aggradation the meanwater regulation structures have turned relatively low and now they are called upon to withstand more severe efforts. Meanwhile the structures of low-water regulation became also filled up and became unable to function properly. As a consequence, in the middle 50-ies ruptures of parallel dykes and damages to bank protection structures occurred frequently (Figs. 1 to 4). Between 1946 and 1963 — the beginning of regulation work according to the new principles — Hungary and Czechoslovakia placed round 850 thousand cil. m of rock and from 1949 Lo 1966 they dredged jointly 6.4 million eu. m from the bed. Detailed figures are contained in Tabic I and Figs. 5 to 7. Nevertheless, it must be concluded that the efforts towards the stabilization of the main channel of the Upper Danube resulted only in temporary and uncomplete solutions. Earlier regulation efforts had therefore to be reviewed and new methods of regulation had to be devised. The main reasons are summarized as follows: a) Conventional methods of mean- and low-water regulation failed lo establish a balanced regime in the main channel; b) Costly dredging operalions at an increased rate also failed to produce a final solution. On the basis of investigation results obtained for Lhe Upper Danube new principles were established by the CTC in 1963 for further regulation work, the basic features of which may be given as follows: a

) In the first step the water losses from the main channel must be minimized by closing off the lateral branches and establishing a uniform bankline; b) In the second step by a sequence of branch closures the branch systems must be regulated; c) In the third step the meanwater channel must be narrowed further, so that the resulting increase in tractive force should at all stages ensure the conveyance of at least the bulk of the sediment arriving from above. The general plan for the regulation of the section from R. St. 1942 to 1916 km of the Upper Danube has been prepared according lo these principles. In this plan allowance has been made already for lhe river regulation aspects of hydroelectric power utilization contemplated on this section ( Figs. S and .9

). As mentioned already, navigation conditions over the Upper Danube deteriorated seriously by the middle of the 1950-ies. Especially frequent troubles were encountered at the Nagybajcs ford (R. St. 1801 km), which was in lhe early 60-ies the worst lord on the Upper Danube (Table 111). The problem was further complicated by the fact that at the time of planning no final agreement was reached on Lhe principles of further regulation work. It was therefore necessary to prepare plans in a manner to give the regulation measures contained therein an experimental character — to show what consequences are to be expected form severely narrowing the meanwater channel — and to permit their ready modification in the course of subsequent regulations. The basic objective of regulation was to prevent the further deposition of bed load which continued to accumulate over the years in the ford section and to promote thus the development of a stable low-water channel, i. е., of a safe navigation channel. Computations have shown that the esisting 300 m wide meanwater bed must be narrowed in the apex section by an additional 80 m, in order to ensure the conveyance of most of the bed load arriving on the reach. Since this contraction was appreciably more severe than those applied so far, the structures called for dimensions differring from those commonly used earlier (Figs. 11 to 15). The contemplated low-water regulation measures were checked also in a model, but the model results failed to support the ideas embodied by the plans (Figs. 16 and 17). Low-water regulation was carried out according to the original plans. From the results of observations performed during the period passed since the completion of