Vízügyi Közlemények, 2002 (84. évfolyam)
3. füzet - Keve Gábor: A jégmegfigyelések korszerűsítési lehetőségei
376 Keve Gábor great part of Budapest but also the larger part of the Danube valley between Esztergom and Mohács. There were a number of other ice floods, worth mentioning, in that ccntury in the years 1839, 1850. 1876. 1878, 1883 and 1891. Ice floods of the 20 l h Century caused large damages in the first part of the century at Budapest ( 1920, 1923 ). in the upper Hungarian Danube ( 1926, 1929) and downstream of Budapest (1940 and 1941). The largest ice flood of the 20 , h century was that of 1956 (Figure /.), causing 58 levee-brceches, inundating a total 750 km 2 area and causing a total damage of 540 million HUF (which at the price level of those years was an enormous damage). Figure 2. shows the ice conditions of the Danube in the period 1901—2002 on the basis of the data of 102 winters of this period. In this period there was only 24 ice-free winters. Nearly half (II) of these ice-free years occurred in the past 30 years. Ice-free period occurred also earlier but not with this present frequency Figure 3. shows the trends of the winter water levels of the Baja station of the River Danube. In this context it should be mentioned that flow rates do not show such a significant trend, which means that presently lower water levels correspond to flows higher than before. Figure 4. shows the probability of occurrence of ice. The probability of ice occurrence was decreased to the half and their length is also decreasing. The changes of the ice regime in the reccnt years are apparent. Figure 4. also indicates that the most probable period of icc occurrence is January-February. The variation of temperature of these months is shown in Figure 5. It can be stated that no considerable changes took place in the variation of temperature. Comparing figures 5 and 2 I is found that monthly mean temperature lower than -5 °C is needed for the development of icc and —120 -140 °C negative temperature sum is needed for standing ice cover ( Table /.). In Hungary ice observation is going on for more than 100 years. The estimation of the degree of icc-coverage is a rather subjective procedure. There is usually a great difference' in ice coverage when observed from the river bank and when from the air (Figures 6-7). The employee of the Water Authority of the Lower Danube (ADUVÍZIG) have developed a technique in the winter of 1971/1972, which allow the avoidance of the large error of this estimation (Figure 9—10). This old idea coupled with recent technical tools allowed us to maintain instrumented ice-observation of the Baja segment of the River Danube (Figures 11—12). This instrument system includes a web-camera installed at a high position (Figure 8.) and a wire-less micro-wavc digital data transmitter. An interesting feature of this device is that the digital signs induced by the moving-image can be processed not only in our Centre but also at any place of the worl There is a research going on into the use of techniques of flow-velocity estimation on the basis of particle movement. Wc hope to be able to automatise the determination of both the degree of ice-coveragc and the travel velocity of drift-ice. These image-processing techniques have only a short past at global scale, but they inevitably face a great future. * * * Möglichkeiten zur Modernisierung der Eisbeobachtungen von Dipl.-Ing. Gábor KEVE Unter den Hochwässern der Donau sind immer die Eishochwässer die gefährlichsten gewesen. Die 24 Eishochwässer des XVIII. Jhs. haben mehrere dichtbesiedelte Gemeinden völlig zerstört, ein Teil deren heutzutage nur noch in den historischen Aufzeichnungen existiert. Von den 6 Hochwässern der ersten 30 Jahre des XIX. Jhs. wurde jedes vom Eisgang verursacht.
