Vízügyi Közlemények, Kivonatok, 1965
Dégen Imre: Az 1965. évi dunai árvíz és árvízvédelmünk fejlődése
(26) over which it is constructed, and which may fail under the destructive influence of floods just as the levee itself. Effects of highwater on the levees are summarized in Chapter II /3. Detrimental seepage, saturation and leakage occurred in great numbers through both levees and subsoil. It was found necessary to defend the levees against 313 instances of leakage, over a length of 96 km against almost complete saturation and against 152 boils, or groups of boils. One of the interesting conclusions derived from observations on seepage phenomena was that hydraulic and soil mechanical aspects of seepage flow were closely related to the degree to whic h the subsoil was saturated prior to the flood. Taking into consideration that the travelling velocity of pressure waves in water is very high, it will become understandable that upward seepage and the formation of boils appeared in the case of rising stages considerably earlier than this could be calculated on the basis of seepage normal to the levee under the action of the increased head of water in the bed. During the flood period successive flood waves caused the pressure gradient under the levee to rise gradually, and thus the consequences of uplift pressure and seepage became apparent in much greater numbers than before. Chemical analyses and temperature readings demonstrated that some of the boils did not yield river water, but local groundwater influenced by the increased head in the riverbed. Besides seepage phenomena during the flood, the study of the sealing process formed an important part of the investigations. Forces developed by seepage through the levee body resulted frequently by the disturbance of equilibrium conditions on the landward slope. Groundwater which was placed under pressure and consequently seeped to the surface reached the landward toe of the levee before seepage water through the levee body. For this reason saturation of the landward slope of the levee occurred in many instances earlier from below than this was calculated by assuming saturation across the levee cross section (Fig. 11). An important experience was further, that seepage in the vicinity of the levee foundation, through the so-called base passages, was of far greater significance than previously believed. In Chapter II 1 4 measures taken to protect levees against the detrimental effects of seepage are described and discussed. Underseepage was adversely influenced by the proximity of borrow pits to the levee. One of the most important methods of increasing the safety of levees., consists of backfilling — within the critical distance — borrow pits and abandoned beds and of increasing the thickness of thin topcover sections by the hydraulic fill method. The fundamental task of protection was the regular supervision of the condition of the levee. On the basis of information gained thereby methods and forces were applied where the stability of the levee or of the subsoil was endangered, wich were capable of reinforcing the levee to a standard required by the stage and duration of the highwater. The active character
