Vízügyi Közlemények, 2004 (86. évfolyam)

3-4. füzet - Orlóci I.-Szesztay K.: Árvízi kockázat a Tisza vízrendszerében

Ar\'ízi kockázat a Tisza vízrendszerében 419 became to represent the highest risk of damage to the local societies when the growth of population and the economic development had reached a certain level (Tables II and III and Figures 2 and 3). Making efforts to enable the comparability of the highly varying interests one may express the flood risk "Z" with equation 1 in function of the values of the flood damage to human property and in that of the measures of probability of occurrence of a given flood in a given planning period. The risk of a flood that would cause the loss of a major part of the property of the larger part of the population can only be taken when the community or the state undertakes the responsibility of compensating the society against the losses of the flood concerned. The only problem is the lack of an appropriately accurate determination of a certain "design level of risk" that would correspond to the above basic principle. The task of giving values to a risk wears a high burden of the need of numerically expressing the factors, which affect that risk. This burden stems not only from the lack of data in our present time but also from the very high uncertainties in knowing the changes that might occur in the future within the planning period. The key task in developing a safety-strategy is the more reliable numerical determination (quantification) of the hydrological regime of the River Tisza, with special regard to floods. This quantification is hindered by the rise of the flood levels that have followed the building of the system of flood levees (Figures 4 and 5). The Tisza River is a very special river in terms of the vast floodplains and their topographic features and soil, and especially because of the high extremes of its hydrological regime and the innumerable variety of the types of floods. Some of its special features are still unknown and the confidence intervals of the flood statistics are such that would allow contradicting conclusions to be drawn. In this unique features the highly varying width of the floodplain (Figure 6) within the flood levees have a special role that would also result in special flood duration properties (Figure 7) and in varying frequencies of flood occurrences along the longitudinal profile (Figure 8). Owing to the extremely low slopes of the flat-land reaches of the River Tisza and its tributary streams and to the large size of the flood-flow channel (within the levees) the impounding effect of the floodplain and its longitudinal variation have decisive importance on the flood regime. In the passage of flood waves the floodplains of the Tisza valley act as a interconnected system of reservoirs where the water level would depend on the fullness of the river channel along the lower reaches. This means that the impounding (backwater) effect of the channels of the Danube and the tributary rivers (Maros and Körös river system) affect the water levels of upstream reaches to a large extent. For the estimation of this impounding effect one needs to separate the rising and falling limbs of the flow rating curves (Figures 9-11

) making also use of the relationship between the simultaneously measured water levels of different gauges (Figure 12). The most timely task of flood research is the quantitative numerical description of the dual-controlling simultaneous effect. To this come the flood localising tasks and the cutting of flood-way passages into the dense brushes and woods of the high-water channel as included in the Vásárhelyi Plan Update (the flood control development plan of Hungary). The dual-control of flood passage and the other features of the hydrodynamics of the Tisza have a common cause: the extremely low slopes of the water levels of the river along its flat-land reaches ( Table IV, Figure 13). This may explain the phenomenon of the reversed peaking of the flood hydrographs from downstream to upstream ( Table V) and the coincidence of the peakings of the rivers Maros and Tisza (Figure 14) as well as the reach-byreach variation of the passage of flood peaks (Figure 15). In creating the hydrological support for controlling flood-risk investigations aimed at the analysis of flood phenomena of longer river reaches and complex river systems, instead of those aimed at that of single cross-sections, should be improved (Figures 16-22). Multivariate statistical analysis (Table VI) and the probabilistic analysis of rarely occurring floods (Table VIII) as well as the continuing survey and inventory update of the wealth at risk in flood-basins (Table VII) have special significance in this procedure.