Vízügyi Közlemények, Kivonatok, 1965

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

(25) Short-term flood forecasts are prepared with the help of aids constructed using graphical correlation. However, in view of the stage of development of computation techniques and the tremendous capacity or electronic computers, it should deem possible and necessary to allow for potential variations of factors influencing the passage of flood waves. In Chapter II 11 flood control in the Hungarian Danube Valley is described. The significance of flood control in Hungary is illustrated by the fact that out of the 93 thousand sq.kilometres which form the territory of Hungary, 25 % lies in the flood plains of rivers, where 50 % of the population is living and where 30 % of towns and villages and 50 % of the highway and railway network are situated. For protecting flood plains against inundation earth levees have been constructed in a length of round 4000 kilometres and flood walls for short sections. The levee system along the Danube River provides insufficient safety as regards both freeboard above design stages and cross sectional dimensions, in spite of the considerable efforts after the flood catastrophe in Szigetköz in 1954 and the ice-run flood in 1956. Of the 1.5 thousand million Forints expended on flood control and river training during the past 10 years, 490 million Forints have been diverted for strengthening the Danube levees by 9 million cu.m of earthwork. In Chapter II 2 a survey of the existing flood levee system is given. Levees along the Danube have in the past been constructed, regardless of type of soil used for building and subsoil conditions, to 4 m crest width and to 1 : 3 and 1 : 2 slopes on the riverside and landside, respectively The material originated in general from the silt layer covering the flood plain, but over some stretches coarse sand has been used for construction. The levees vary as regards structure

, both density and the embankment material are in many places unsatisfactory and wide differences exist in cross sectional dimensions and safety freeboard. Over the upper reach of the Danube a top cover of sandy silt to clayey silt and of a thickness varying from 0.5 to 6 metres is underlain by a highly pervious sandy gravel sublayer of great thickness. Proceeding down the river, the gravel forming the sublayer becomes gradually finer and alternates in some places with medium to coarse sand layers. South of Dunaföldvár gravel occurs at isolated spots only, its place being taken by pervious coarse sand. Over the lower reach the top cover disappears in places and the sand sublayer emerges to the surface. The bed of the Danube is everywhere cut into the pervious sublayer (Fig. 10). During the flood the levees were found to be extremely loose and it was again demonstrated that repeated and extended exposure to alternating seepage flow caused the levees and their subsoil to become loose, interwoven with passages, the levees "aged" and offered a diminished resistance to seepage flow. One of the most important lessons gained during flood protection was that the retention of highwaters is never accomplished by the levee alone, but wj,th the participation of the soil block of varying width and depth