Vízügyi Közlemények, 1994 (76. évfolyam)
4. füzet - Stelczer K.: Negyven éve az árvíz uralta a Szigetközt
388 Stelczer Károly Forty years ago floods dominated the Szigetköz area by Dr. Károly STELCZER , C.E. Among the high floods of the 20th Century the flood of July, 1954 (Table I.) of the River Danube was extremely high upstream of Gönyű (Figure 2.). The author believes that one should remember the flood fighting activities of the Szigetköz area forty years ago for the following reasons: - Although there were 5 dike ruptures (Figure 2.), inundating 350 kin 2 area, but the danger of failure downstream of the Rajka sluice was eliminated, saving an even larger area; - It brought along a basic change of the principles of flood control development. Strengthening of the longest system of levees in Europe has started along with the updating the technical and organizational means of flood defence; - and finally because the flood defence capacity of the country seems to became weaker in the past few years along with certain signs of loosening the defence organization, due most likely to the successful fighting of the extreme floods of 1965 in the River Danube and of 1970 in the River Tisza and to the dry period of the last 25 years. Dike failures of the Szigetköz were caused by boils. Boils occurred, in every case, at the intersection of an old side channel and the flood levee. In the site of the ruptures the undersoil of the levee was deeply washed out, eroded (Figure 3.). After the flood of 1954 the crossings of old side channels (sedimcnted dead river arm) with the levees have been identified over the entire territory ot the country, using aerial photographic methods. At these sites 1-2 m thick and 50-100 m wide earth "blankets" were constructed behind the flood levee. The result is that there were no boil-induced dike ruptures in the country since 40 years. During the flood of 1954 many "mini boils" and so called underseepage flows occurred in the Szigetköz (Figure 5.). The usual method of counteracting such processes is to build a countcr reservoir over the boil (Figure 6.). Later counter pressure was provided by the construction of sand and gravel blankets behind the levee. Experiences indicated that counter reservoirs are very efficient in counteracting "mini boils", but they are rather inefficient in handling underseepage flow problems. Nowadays medium floods occur in the Szigetköz in nearly every decade causing 0.4-0.6 m deep inundations, which in turn cause vast damages without any dike rupture. After the diversion of the River Danube from its channel to the Bős/Gahóíkovo power station the occurrence of underseepage water was negligible and only smaller areas were concerned. In the Szigetköz area the major problem of flood defence was associated with the continuously rising peak water stages of the flood hydrographs (Figure 8.). This situation is well represented by Figure 9. showing the rise of peaking flood water levels of the Dunaremete station. The 2 cm/yr annual rate of rising did not decrease in spite of the fact that low water levels have been lowered by the very expensive river training measures (dredging). Eventually, the necessity of rising the height of the flood levees of the Szigetköz by 1.00 m in every 50-eth year has been eliminated by the diversion of the River Danube that also allows the conveyance of a substantial part of the flood flows into the by-pass canal of the Bos/Gabdkovo power station. The author finally draws the attention to the remarkable extreme flood of 1501 which has exceeded the next largest flood of 1954 by 1.39 m in Vienna. This has a serious relevance to the determination of design flood levels. Another important aspect of flood defence in the River Danube is the danger of ice-flood. Along the Hungarian Danube the safety of flood defence is the lowest, both in terms of ice- and non-ice floods, in the vicinity of Budapest (Table П.). The author illustrates the devastating effect of ice-floods of the River Danube in Figures 10-13, calling the attention to the necessity of the development of flood defence.
