Hidrológiai Közlöny 1968 (48. évfolyam)

8. szám - Varrók Endre: A nyomásvonal alakulása töltésezett folyók mellett

26'6 Hidrológiai Közlöny 1968. 7. sz. HIDRAULIKA Head-lines in the area adjacent to a damraed river E. VAREKÓ Flood control is a ceaseless activity, not just a task arising time and time again. The most important flood control device, the levee is a construction of limited life. Slow changes of the climate affect the hydrological parameters of the stream. Bed load transport affects the river bed, bringing about as a resultant of a long succession of changes either a filling up or a scour. Observing the slow changes, establishing their trends, predicting their consequences and the suitable modification of the techniques and facilities of flood control are the most important items on the list of flood control operation to be performed continuously. The present paper diseusses somé results concerning certain details of research intopossible improvements of flood control techniques. As a result of observations and surveys carried out during and after the big floods of the last deeades the views concerning flood control have undergone a substantial change. In earlier times, the main task of flood control planning was the safe dimensioning of levees: the main task during the flood was the conservation of levees, possibly including their heightening also. More recently, it is not only the levee proper but also its environment in the horizontal as well as in the verticai sense (in depth) that occupy the field of the interest. Fig. 1. Schematic profilé of stream válley 1,2 — flood erest levels before und after the construction of levees, b — width of floodplain, B —• width of valley, m — thickness of impermeable toplayer, M — thickness of aquifer 1. ábra. Folyóvölgy vázlatos metszete 1 és 2 : árvízszintek tftltésezés előtt és után. B a völgy, b a hullámtér szélessége, m a fedőréteg, M a vízvezető réteg vastagsága Let us state the problem referring to Fig.l. The figure shows a stream valley of breadth B, fiiled to a depth M with high-permeability sediment. Directly below the ground surface there is a poorly permeable sediment of thickness m. Water level 1 is the flood stage before the construction of levees, water level 2 is the flood stage after that. The new floodplain of the river is of width b. This profilé can be regarded as characteristic of the entire length of the stream, considering that the dimensions of the profilé and the nature of the sediment undergo certain quite well-defined changes along the river. * * The cireumstance that the morphological eharacteristics of the river can be repeated along the river should also be taken into consideration. Disregarding local irregularities, one can establish the following essential changes along the river: The width B of the valley increases downstream, attaining several tens of kilometres in the aggraded floodplains of big rivers; the thickness M of the permeable layer exhibits no constant trend of variation, usually being a few tens of metres* *; the thickness m of the overlying impermeable layer increases downstream with a concurrent decrease of permeability. Ali in all, the ratio MjB decreases, the ratio m\M increases downstream, the latter varying from zero to unity. Considering these changes from the side of the ground water we may state that the probability of lying the piezometric surface in the top layer increases downstream. The actual ground water table may also lie in this layer. The construction of levees will affect also the movement of ground water. Damming increases the flood stages and also the rate of level changes, i. e. the fluctuation of water level turns into a more extent and more violent one. These level changes are transmitted — largely as pressure changes — from the river bed into the aquifer, and spread in it. This change of pressures spreading in the aquifer makes it necessary to take into consideration also the thickness of the permeable layer and top layer besides the actual body of the levee. In Hungary there are areas adjacent mostly to the Danube river in which the failure of top layer may occur in consequence of the insufficient thickness of the top layer and the high permeability of the aquifer. The danger of such failures may be further increased by the circumstance that the entrance point of the pressure fluctuation is quite often in the immediate proximity of the levee, either because the river Ised itself runs there or because the borrow pits of the levees, sited close by, have weakened or indeed broken through the impermeable layer. In view of the above circumstances it was indicated to determine, with due attention to the unsteady nature of ground water movement near the rivers, the factors affecting ground water pressure acting from below upon the impermeable top layer. The available theoretical solutions The differential equation of unsteady ground water movement, not limited by impermeable ** The sedimentary deposit is of course much thicker usually; here the uppermost permeable layer is had in mind.