Vízügyi Közlemények, 1989 (71. évfolyam)

3. füzet - Starosolszky Ödön: A vízlépcsők hatása a jégjárásra

384 Starosolszky Ödön The effect of river barrages on the ice-regime by Dr. Ö. STAROSOLSZKY, C. E., candidate of technical sciences River barrages affect the regime of ice in a river by way of an altered water regime and by different morphological conditions. Ice appears over the backwater reach earlier than previously and will stay for a longer time than before; ice-thickness will increase but in the lower reaches an adverse trend may take place. Here, less ice is produced but a continuous cover may appear earlier than under natural conditions. By operating the barrages properly, the development of ice sheet can be influenced. The effect of the barrage system of Gabcikovo-Nagymaros - built by Czechoslovakia and Hungary on a Danube stretch of common interest - exerted on ice development and behaviour had been studied first in 1952 ( VITUK1

1952). Followingly, several Slovak and Hungarian reports were published. Recently, the problem has been examined again in the framework of environmental investigations and the deployment of a monitoring system (VITUKI 1986, 1987, 1988). These investigations were extended to predict the expected ice-conditions in the reservoirs, to make use of experiences gained over the Austrian Danube-stretches, to learn more about the effect of heat-load and changes in the quality of water. The effect of ice-cover on the quality of water, the changes in ice-regime observed during the past 30 years were also studied. All these have led to establish principles for winter operation of the barrage system and to process available information to obtain reference data. Model experiments were carried out to support the design and operation of related structures as far as ice release and detention are concerned. The first observations on the Hungarian Danube stretch were carried out in 1818. Measurements in networks were started in 1851. Ice-related data were available in different shelves and were published in different reports (e.g. VITUKI 1974, 1982). Horváth's arrangement (1979) seemed to be most interesting for us. Ice-phenomena were investigated by different statistical analyses some of which are presented in Figs. I to 4. These analyses pointed to the fact that the appearance of ice is not scant and standing ice was available over the stretch between Bratislava and Mohács in 20 to 60 percent of the years. The longest period with standing ice may last for 100 days. Characteristic data for selected gauges are presented in Table I and in Figs. 5 to 7. Some important values are compiled in Tables II and III. A characteristic change observed during the last 3 decades - namely a decreasing frequency of ice-sheet formation at 5 gauges - is shown in Fig. 8. From these it is visible that statistical calculations are based on inhomogeneous series, because observations carry the impact of man-made interferences. Despite, the use of .Г-distribution was possible (Fig. 10), however, no convincing function could be fitted (evidently) to describe the distribution of days with standing ice sheet. Fair regression could be obtained for icy days for stations located upstream and downstream of Budapest (Fig. 11) but the trial was much weaker with days with standing ice sheet. Aerial photographs made in 1985 from a 9 km long stretch afforded the possibility to pursue detailed investigations (Starosolszky-Mayer 1988) the results of which are presented in Table IV and Figs. 14-15. The variability of ice-cover over the stretch is quite large but visual observation proved - surprisingly - to be realistic. Heat-load arriving from upstream sections - investigated by Déri - has been presented in Tables V and VI. To find interrelation between ice-cover and water quality, some investigations conducted in shallow lakes in Canada (Barica 1980, Mathias-Barica 1985), were examined. Ice experiences obtained at river barrages located in Austria were based on Dorfmeister's publication (1988). Accordingly, a stable ice cover upstream to a barrage is advantageous. To characterize the ice-conditions of reservoirs - primarily to analyze the influence of the length and thickness of ice sheets - observations taken from lakes Balaton and Fertő (Fig. 17), and from Tokaj - upstream to the river barrage of Tiszalök (Figs. 18 to 19) were used. Accordingly,