Vízügyi Közlemények, Kivonatok, 1965

Dégen Imre: Az 1965. évi dunai árvíz és árvízvédelmünk fejlődése

(24) crevasses reduced by about 20 to 25 cm the anticipated stages over the downstream Danube reaches. As far as the coincidence of flood wawes on the Danube and its tributaries is concerned it should be noted here, that the series of extreme floods in the year 1965 commenced with the flood wave which passed down the watercourses of Western-Transdaauvia in Hungary. One of the interesting and rare features of the 1965 flood on the Danube was that extremely high stages occurred at relatively low discharges and over the lower reach of the Hungarian Danube, stages surpassing previous maxima were due to the combination of overtaking flood waves, the free passage of which was obstructed by backwater resulting from highumter on the Dráva—Mura Rivers. This is reflected also by the following figures: stages of the sixth flood wave would have attained a probability figure of 0.1 0 0 corresponding to a recurrence frequency of thousand years had crevasses not takerplace. On the other hand the peak discharge had a probability of only 0.6 %, corresponding to 167 years frequency. The probability of actual stages was 0 4 %, corresponding to a "once in 200 years" flood. Flood wawes to be anticipated and the progression of flood wawes in genera], were forecast with a fair degree of accuracy by the Hydrographic Service. Dificiencies of forecasting were, at the same time, also exposed by the flood. It was found namely that existing forecasting aids are suitable for forecasting in the case of a single flood v/ave only, but are not capable of allowing for the contingency of flood waves which become superimposed upon each other. The period for which an advance forecast can be given and the reliability of the forecast are two problems of paramount importance for active flood control. The maximum error of forecasts issued at the time of peaking on the Engelhartszell gage with a time advance of 4 to 5 days for the Budapest gage and with a time advance of 6 to 7 days for the Mohács gage remained between 20 to 25 cm. It was demonstrated by the 1965 flood that discharges fluctuating between very wide limits may combine with widely different Danube discharges and the combined flows may show different rates and sequences of rising, or regression. Travelling velocities, stages and discharges of flood waves may be substantially influenced thereby. A^arying times and velocities of travel of flood waves are illustrated in Fig. 9. Whereas over the 780 km long reach between Passau and Mohács the average time of travel of flood waves is 135 hours, the third flood wave of the 1965 higwater took 245 hours to travel over the same reach. Crevasses resulted in a sudden change of discharges, as demonstrated by the graphical representation of the time of travel of the sixth flood wave. Without breaches the time of travel, as indicated by the extension of the graph for the reach above Dunaremete, would have been 350 hours. In preparing forecasts allowance should be made for the operation schedule of barrages and reservoirs on the upper reaches of the Danube and on the tributaries in Czechoslovakia.