Vízügyi Közlemények, 1974 (56. évfolyam)

3. füzet - Ligeti László: Keszthely és a Balaton

424 Ligeti László These two regulation measures have created a new situation for the town, since the Zala River discharged into the closed bay formed before the town. The conditions within the bay have successively deteriorated since, marked by an increasing rate of silting. The natural silting process in Lake Balaton, the rate of which is controlled by the prevailing environmental factors, is most severe in the Bay of Keszthely, the average rate being estimated at 1.3 to 1.7 mm/year in contrast to the present 0.8 mm/year average in other parts of the lake. The factors giving rise to the currents causing silting, the relevant research and construction experiences as well as the potential methods of silting control are discussed in the paper. In Chapter 2 on the currents prevailing, the primary aim is to arrive at a better understanding on wind conditions (Fig. 2). The winds stronger than 3 deg. В strike the southern shore most frequently at an angle of round 70 degrees, so that the component parallel to the shore tends to drive the piled up water masses eastwards. This is promoted by the depression created along the northern shore. Consequently, strong currents are induced by wind along the northern shore, the direction of which is controlled by the prevailing with direction and which tends to pick up also the suspended sediment to transport it into the Bay of Keszthely. In Chapter 3 the investigations performed and the experiences gained by diverse research institutions are summarized. These appear to support the conclusions arrived at in Chapter 2 (Fig. 4). The strong current along the northern shore has been indicated by the sand drift studies made with luminiscent ground glass, by the studies on the quality pattern of organic substances, as well as by the drift towards Keszthely of the 2 — 20 mm sandy gravel spread before the Badacsony—SzigligetGyőrök camping site. It is concluded eventually that silting in the Bay of Keszthely is due not only to the sediment discharged by the Zala River, although it is recognized that the bulk of this sediment is trapped in the bay. The finest sediment fraction present in the entire lake migrates towards the Bay of Keszthely. Variations in the factors responsible for silting in the lake and influencing the lake sediment, as well as the potential remedial measures are considered in Chapter 4. Changes by orders of magnitude have occurred during the past 10 years in the composition according to origin of the sediment moving in the lake, in tliat the sediment of biological origin has assumed predominance over that resulting from shore erosion. This fact is explained by the appearance of reed stands on the southern, eroding lake shore. Beed areas were unknown on the southern shore in 1900 (Fig. 5), whereas the maps based on the surveys from 1958 and 1968 indicate extensive reed areas. The leaching of fertilizers and nutrient salts from agricultural areas and the phosphate-nitrate effect of communal sewage discharged into the lake are identified as the main causes thereof, which in combination with the small fluctuations of the regulated water level and with the wide shallow strip of beach along the souther shore promote the spreading of reed-covered areas. Filling on the shallow strip and the construction of shore protection works are suggested as effective remedial measures. As outlined in Chapter 5, river regulation principles should be applied to control the silting process in the Bay of Keszthely. The movement of sediment in the bay should be influenced by the energy of flowing water in a manner, to flush the suspended sediment from the bay by currents accelerated with the help of structures. For this purpose — the Zala Biver should be diverted into the Bay of Szigliget, whence the sediment would be carried eastwards by the prevailing north-western winds ( Figs. 1-4), — at the Cap of Győrök, at the entrance to the bay the lake bed should be contracted to accelerate the currents due to wind and surface oscillations ( Figs. 7, 8 and 9), — the shores of the bay should be stabilized, the foreshore cleared from reeds, to permit flushing the shallows by the entering currents, which would at their return entrain the suspended sediment and remove it from the bay, — to the south of Keszthely town, facing the contraction an impact wedge should be constructed which would under certain circumstances divide the entering currents. * * *