Vízügyi Közlemények, 2003 (85. évfolyam)
4. füzet - Rövidebb tanulmányok, közlemények, beszámolók
628 Déri József VITUKI: Vízrajzi Atlasz, sorozat 11, Duna 3, Dunaföldvár-déli országhatár (1561-1433 fkm), Budapest, 1971. VITUKI-. Vízrajzi Atlasz, sorozat 29/2, Duna, Budapest-déli országhatár. Budapest, 1999. Changes in the sediment regime of the Danube by Dr József DÉRI, C.E. The regulation of the riverbed of the Danube started about 200 years ago and is nowadays still under way. The changes in its sediment regime is basically due to the excessive dredging of the riverbed and to the operation of water power plants. On the German and Austrian section of the Danube, altogether 31, on its tributaries 135 hydropower plants have been built. On the Austrian Danube stretch, 10 hydropwer plants have been put into operation during the last 50 years. It is particularly the latter plants which have a major impact onto the bedload and suspended load regime of the Hungarian Danube stretch. Before the creation of the barrages and the extensive industrial dredging, the annual suspended material discharge of the Danube, at its mouth into the Black Sea, used to be 15-70 million tons per year. This volume diminished by 1980 to 8-25 million tons per year. The missing sediment volume remained partly in the impoundments of the German and Austrian barrages and partly in the dredging pits of the riverbed. The diminishing of the bedload volume reaching Hungary is even more considerable. Before the creation of the barrage of Öúnovo (Slovakia), a volume of 200,000 m1 per year used to reach Hungary from the Austrian Danube stretch, while since the start of operation of that barrage, the bedload discharge downstrem the cross-section at 1810 r-km, is only in the range of 6-10,000 m\ These considerable changes in the sediment regime of the Danube resulted in measurable changes in the low-water riverbed conditions and in the history of fords, closely connected with the channel deepenings. Some former fords do not exist any more, others, disappeared for decades, revived, and also never-heard-of new fords came into life. Also the channel deepenings or dredging pits, resulting from excessive exploitation of riverbed material or industrial dredging, have a considerable retaining effect onto the bedload and suspended load transported by the river. An investigation was carried out on the Danube stretch between Nagymaros and Budapest, where during the period 1968-85 a volume of 18.2 million m 3 was exploited by industrial dredging. According to this investigation, the water stage corresponding to the flow discharge of 3000 m-Vs, is on this Danube stretch nowadays by 0,2 m lower than it used to be before dredging. Industrial dredging is no part of river training. In most cases, it is not serviceable but rather pernicious to water management. As a consequence of the changes in the sediment regime, also the system of quantitative and qualitative conditions of bankfiltered water production was changed. In the riverbank stripe exposed to selective bed erosion, the volume of produceable drinkwater volume may increase, but the filter effect is hampered and the risk of deteriorating water quality increases. The security of the cooling water supply to the Nuclear Power Plant of Paks required a coherent and complex investigation of the riverbed changes, the sediment-, runoff-, and ice regime of the Danube stretch in the surroundings of Paks as well as a prediction of the possible further changes. Table I shows the time series of bedload transport and bedload balance of the Danube stretch between Dunaújváros and Baja for the period 1955-1977. Table II contains the time series of suspended matter discharge and balances, Table III displays the long-range pattern of sediment balances, and finally Table IK the grain diameters and their changes during 27 years. Figs. 1 and 2 display the grain distribution curves for two selected cross-sections of the Danube.
