Vízügyi Közlemények, 1956 (38. évfolyam)

2. füzet - VII. Kisebb közlemények

(90) 2. Warm days at the beginning of March inducing the breaking up and drifting of the upper stretch of the ice cover were followed by permanent frosty days from March 8 to 16. The ice froze up again, ice barriers were consolidated and this coincided with the arrival of the flood wave. Fig. 2. shows the daily variation of the upper boundary of the drifting and of the solid ice and the calculated advance of the crest of the floodwave caused by snowmelt. This shows that the floodwave had to be discharged in the riverbed jammed with ice or beyond it in the flood plain. This was only possible at a stage when overtopping of the levees was inevitable. The upstream end of the ice cover moved on March 2, and its last residues left on March 19, the south border of the country, downstream from which it found already an ice-free stretch on the Danube, wherefore it could drift down through Jugoslavia without any particular trouble. The daily variation of the backup caused by ice jams during the 16 days of ice passage is shown in Fig. 4. The ice jams, which barred the passage of the flood wave principally at. Dunaföldvár, between 1566 — 1546 km and at Fájsz — Dunaszekcső, that is, between 1510 — 1460 km, caused such backups that, on the reach downstream from the 1580 km point along a stretch of 85 — 90 km, all so far observed maxima were surpassed. Maximum stages occuring along the Danube were compared with the so far observed highest flood free of ice in 1954, and the result is shown in Fig. 5. The Fig. also shows what stages the flood would have reached, if there had not been any breaches of levees, and also indicates the highest icy flood stages previous to the 1956 icy flood. Flood protection levees have generally been designed 1 m higher than the so far observed highest flood stage, but actually they have not yet been built so high in several places along the Danube. Therefore on those stretches of the levees, where there was any deficiency in height, the levees were topped by additional embankment during the passage of the ice, in the course of flood protection operations. Since, however, the icy flood by far exceeded the so far observed highest icy flood stages especially on the lower section, in many cases the emergency embankment could not be protected. The water overtopped them, ice floes parting from the channel and drifting toward the levees sheared off the additional embankment and the levee itself. The levee was breached in many places. Annex I. shows the location of the levee breaches arising along the Danube reach below the 1520 km point, the flooded areas, the position of the emergency dams built for the localization of the floodwaters, and the levee cuts made to drain water. Tabic II. gives the serial number, name, location (defined by the river and levee km) date and width of the dam breaches. Fig. 6. was drawn on the longest emergency protection line and shows the daily advance of water in the flooded area. Dam breaches are attributed to three causes : 1. The great majority of the breaches were caused by overtopping of the levees, by the waslioff of the embankment and of the dam itself, though in several places the additional embankments gave good protection. At breaches between 1460 — 1465 km the overbank water first sheared off the additional embankment, afterwards the bank itself (Pict. 7. and 9.). 2. Breaches also occurred through internal erosion of the upper part of the levees perforated by vermin-holes, which occurred imperceptibly : previously to the flood the river side of the frozen dam thawed up, the water penetrated into the dam but could not flow out on the landside slope because of the frozen crust. However, when it broke out somewhere, the erosion was speeded up, and before the soaked part could be sealed off, the breach occurred within minutes. 3. Finally, because of the high water stages the subsoil of the levees was stressed by overpressure, to which the subsoil, made porous at places by repeated floods, could not resist, and breach due to soil failure originated. This is indicated by some breaches where scours of 10 — 12 m depth occured. After the levee breaks emergency protection lines were built including roads and railway embankments for localizing the extension of the flood ; of these the Hajós —Miske — Bátya — Fajsz line was 30 km wide (Fig. 6.). These localization lines were built in 1—3 days by mechanized work and successfully prevented the progress of the water.