Vízügyi Közlemények, 1999 (81. évfolyam)

3. füzet - Gauzer B.-Bartha P.: Árvízi szimulációs vizsgálatok a Felső-Tiszán

Árvízi szimulációs vizsgálatok a Felső-Tiszán 387 Comparative simulation analysis of the 1970 and 1998 floods of the Upper Tisza river by Dr. Balázs GAUZER C.E. and Péter BARTHA C.E. M.Sc There were two significant floods (1970, 1988) of the Upper Tisza river in the past 30 years (Figure /.). The river network is shown, schematically, in Figure 2., while the flood forecasting system is illustrated by Figure 3. The analysis of these flooding events was made for the purpose of supporting the developments aimed at increasing the safety against floods, by analysing the consequences of various scenarios of combinations of meteorological (precipitation) and hydrological conditions of realistic probability of occurrence. For the development of scenarios to be investigated the similarities and differences of the flood events of 1970 and 1978 were analysed and the causes of flooding was identifed. The most important similarity in these two floods was, that the occurrence of very intensive rainfall over only a partial area of the catchment of the Upper Tisza river (upstream of Tokaj) was suffcient to induce both runoff events (Table II.). The difference was that the 1970 flood occured mostly on the left-bank tributaries (on the rivers Visa, Iza, Szamos, Túr) in Kárpátalja (Ukraine) and ErdélylTransilvania (Romania), while 1988 one in the right-bank rivers (rivers Tarac, Talabor, Borzsa and Bodrog). Results of the investigations and the flow measurements of 1998 indicate, that in the case of the 1970 flood the peak discharges were significantly overestimated. This can be explained by the occurrence of water stages substantially exceeding the earlier Hra level and also by the catastrophic situation. It can be stated that the water volume, which passed the gauging station Tiszabecs during the 1998 flood, was much larger than the same parameter of the 1970 flood what, as calculated on the basis of the peaking water levels of Tiszabecs. The main reason is the deepening of the channel in the Tiszabecs section, although the levee failures in Kárpátalja (Ukraine) also contributed to this (Figure 4.). This substantial difference is the main reason of the occurrence of the very high water stages at Tivadar station during the 1998 flood and also of why in 1998 the water stages of Vásárosnamény have exceeded those in 1970, in spite of the relatively low flow contribution of the River Szamos in 1998. The time of concentration and the time of passage did not significantly differ in the two floods (Table VI). This allows the drawing of the following important conclusions: The effect of the levee failures in Kárpátalja (Ukraine) during the 1998 flood was considerable in the river reach at Tiszabecs (Figure 8), decreasing the peak water stages by some 0.15-0.20 m. During the 1970 flood the levee failures of the River Szamos at Szatmárnémeti (Romania) and Tunyogmatolcs (Hungary) have lowered the peaking of the River Szamos at Csenger only to a small extent, but decreased the peaks of the river reach betweeen Vásárosnamény and Tokaj of the River Tisza substantially, by about 0.50-0.90 m. In the case of the 1970 flood the peaking water stages of Vásárosnamény and Záhony would have been only slightly affected due to the delayed arrival of the Szamos flood wave and the rapid recession of the Upper Tisza flood (Figure 9.). Coinciding, simultaneous precipitation events (of those in 1998 and 1970) of the right- and left-bank side tributary catchments of the Upper Tisza River (Table VI.) could result, in the Tisza river reach between Tiszabecs and Tivadar, in 0.3-1.00 higher peaking flood water stages than the ever observed maximum and this would cause the overtopping of the flood levee crest levels over considerable length (Table VII.). If the above mentioned flooding event was associated with the Szamos river flood of 1970, then it would result in maximum water stages of the Tisza river reach of Vásárosnemény-Tokaj which are 1.20-2.20 m higher than the ever observed maximum. This would cause the overtopping of the levee crests over the bulk of the river system, resulting in a situation, which the flood defence could hardly cope with (Figure 10-11.)