Vízügyi Közlemények, 1992 (74. évfolyam)
3. füzet - Illés L.-Jeczkó J.: A Tisza felső szakaszán kialakult szélsőséges kisvízállások vizsgálata
278 Illés L. és Jeczkó J. IRODALOM Pálfai /.: Az 1990. évi aszály Magyarországon. Vízügyi Közlemények, LXXIII. évfolyam, 2. füzet, 1991. * * * Investigation of the extreme low water levels of the Upper Tisza River by Lajos ILLÉS and János JEC.ZKO, civil engineers There have been very low water levels observed several times during the past years in the Upper Tisza River, and especially in the vicinity of the Hungarian/Ukrainian border (Tiszabecs), (Table I.). The authors searched for the factors that created such situation. They analyzed the low periods. The time series of the annual lowest daily water level (// s a) and that of the annual mean of the monthly lowest water levels (77 s mo) were analyzed. The time series of the annual lowest water levels (ff s a, Fig. I.) exhibited smaller decreasing trend (AA)before 1971 than after 1971. Similar results were obtained by the analysis of the annual means of monthly lowest water levels TJ sm n (Fig. 2.). Searching the cause-and-effect relationships it was found that - the trends of annual total precipitation (Fig. 3.) are basically the same as those of the annual lowest water levels; - evaluating channel changes (Fig. 4.) it was found that at Tiszabecs there was a channel subsidence in the period of 1974-85, followed by a channel rise between 1985 and 1990. Nevertheless the quantification of these data (Fig. 5.) indicated that the wetted cross-section area, corresponding to a given water stage was, increaed first by 32-37% and was then decreased by the same amount. - the cross-sectional mean velocity (K A m) has been steadily and significantly increasing (Fig. 7.). There was a trend of increasing velocities also in the nearby Tivadar river station (Fig. 8.) but not even nearly so marked and large than in Tiszabecs. - longitudinal channel and slope changes (Fig. 9.) indicate channel deepening unambigously. The authors claim that this was caused by excessive commercial gravel dredging on one hand, and by the lack of high floods since 1970 that resulted in the lack supplementary sediment depositions. - The surface and subsurface runoff conditions of the drainage basin (conditions of forest management and landuse structure) experienced no notable changes. The final results of the investigations are summarized in results the following recommendations were made: - more careful permitting of commercial gravel dredging; - redetermination of low flow water levels; - annual channel surveys to detect channel changes; - biweekly discharge measurements at Tiszabecs, the border station, and finally - informing the local public on the causes of the decline of water levels. * * *
