Vízügyi Közlemények, 2004 (86. évfolyam)

3-4. füzet - Stelczer Károly: az 1954. évi dunai árvíz

370 Stelczer Károly However, there were also some substantial differences between these two floods at some reaches of the Danube. The causes of these differences were as follows: - Comparing the time series of the water stages of the two floods (Figure 4) it turns out that the flood of 1954 had a "preparatory flood hydropgraph" - In 1954 there were substantial rainfalls on the catchment basin of the rivers Isar and Vils (Figure 3) and the water stages of these rivers nearly approached the ever highest water levels of 1940. Therefore the flood levels donstream of the mouth of the river Isar were substantially highesr than those of the year 1899 (Figure J); - In 1954 the precipitation on the catchment basins of the rivers Traun and Enns (Figure 3) were much less than in 1899 and therefore these rivers carried smaller flows in 1954, and this was the case also for some other rivers of Lower Austria (Figure 5). - Nevertheless, none of the hydrometeorological processes of the Danube catchment corresponding to Vienna justified the fact that the flood peaks of 1954 had exceeded that of 1899 at all water gauges downstream of Vienna (rather downstream of Hainburg). This is especially interesting feature as neither the River Morava nor other rivers of the Carpathian basin carried considerable flows at the time of either of these floods. This situation can be explained as follows: The flood of 1899 arrived to a nearly empty channel in the Hungarian Danube reach, while in 1954 there was a smaller "preparatory" flood in early July and therefore the flood arrived to a full channel (Figure 4) and the two flood hydrographs were superimposed on each other at Pozsony/Bratislava. Another contribution to the observed phenomena was that in the period between these two large floods the flood-channel had been curtailed by the construction of flood levees both in the Bavarian Danube and downstream of Vienna. Consequently the flood of 1954 had a shorter time of travel both in the Austrian and Bavarian reaches, where the length of the channel was shortened. This provided less storage volumes and flood could flatten out less effectively. In course of the defence activities of the flood of 1954 large problems were created by the heightening of the levees and the controlling of boils was also problematic. Boils caused three out of the four dike-breeches of the main levee system. The vertical and transversal dimensions of the main levees of the Szigetköz were rather poor (Figure 6). The size of the levee was small for this flood, but also in comparison to those of the left bank in Csallóköz. The left-bank levees were by 20-40 cm higher and their crest width was 6 metres instead of that of the right bank (4 m). Strengthening of the levees of the Szigetköz started in 1951. Along the strengthened reach of 9 km length (Figure 6), between Cikolasziget and Kisbodak, no flood defence activities were needed during the flood of 1954. During the flood of 1954 there were large many "mini" boils in the Szigetköz (Figure 7). Counter-pressure basins (Figure 8) and gravel-banks built on the protected side proved to be the best defence facilities against mini-boils. During this flood there were three large or real boils (Figure 9) in the Szigetköz area, causing dike-breeches (at Upper Ásványráró, Kisbodak and Dunakiliti). The levee breech at Lower Ásványráró deserves a special attention as this was a never experienced breech in the history of flood defence in Hungary that far. The water flowing out the breech opening of Upper Ásványráró was conveyed by an old channel bank along the off-side of the flood levee (along the last row of houses of the village) and this flow washed away the levee gradually at Lower Ásványráró. The defence work at the circular dam (coffer dam) of Győr-Révfalu had some lessons. Large machinery and staff was sent to the site in order to provide protection of the Révfalu part of the City of Győr against the flood. The circular dike that had to be strengthened had been built to retain seepage waters only (Figure 13). The machines could hardly work in the narrow track that was available as the houses of the town were only 10-20 m away from the circular dike. The first lesson is that it would have been better to built an entirely new dike somewhat further on the plough land. Another problem was that the earth material used for strengthening the existing circular dike was