Vízügyi Közlemények, 1994 (76. évfolyam)

1. füzet - Hankó Z.-Alföldi L.-Starosolszky Ö.: A Felső-Duna és a Szigetközi hdirológiai jellemzőinek megváltozása a dunacsúni vízlépcső üzembehelyezése nyomán

Л Felső-Duna és a Szigetköz hidrológiai jellemzőinek megváltozása. 51 on appropriately frequent measurements. Discharge data of the Danube reach between Bratislava (Pozsony) and Budapest and the time series of data corresponding to discharge data of the same duration in the distribution curve of the individual years between 1401 and 1490 (Figure 2) indicate no appreciable tendency of a changing climate. Water level data of the same period, however indicate substantial changes due to anthropogenic intervention. One can state with great probability that the sediment budget of the Hungarian Upper Danube was positive until about the middle of the 20th century;-that is the quantity of sediment arriving from the headrace was larger than the quantity which left the reach towards downstream. This resulted in sedimentation, in rising channel bed, marked by the rising of water levels corresponding to the same discharge value. This tendency was reversed in the second half of this century, due mostly to the sediment retaining effect of German and Austrian river dams and partly to the effects of other human intervention along the reach of concern (Hood control, river training and mostly dredging). Recently, channel bed sinking due to the lack of sediment has exceeded 1.0 metre and this is associated with the similar rate of sinking of water levels which correspond to low flows. Water levels tend to drop on the inundated tloodplain also in the higher discharge domain, although in this domain deposition of the sediment is still continuing but at a reduced rate (Figures 1-7). Hie relationship lietween the hydrological regime of the main Danube channel and that of the side river arms of the flood-plain had to be also investigated along the river reach of Rajka-dönyű, so as to establish the baseline conditions to which the effects of the Cunovó Hiver Barrage can be compared. With the dale of putting the Cunovó River Barrage into operation (25th October, 1492) the hydrological regime of the river reach has been basically and substantially altered. At the present (end of 1443, beginning of 1444) the tíabcíkovó Hydroelectric Plant can release discharges up to 3600 m /sec (while the final design capacity is 5200 nvVsec) and the main channel of the River Danube receives 300 nr'/sec How across the ( unovó River Barrage (and via seepage flows). The largest side river arm, the Mosoni Danube, receives inflows of 10-20 nr'/sec. Due to the fact that the hydraulic structures are but partially completed this system can release the original design Hood How (the flow of the return period of 10,000 years) of 15,000 nr'/sec at head waters 1.5 m higher than the original backwater levels of 131.1-131.2 m M.S. (metres above Baltic Sea level), (Figure S). 'Ulis altered situation refers to the river reach between rkm 1851.75 (the Cunovó River Barrage) and the rkm 1811 (the point where tailrace canal rejoins the River Danube). Upstream of this reach in the Cunovó Reservoir (with water levels of 131.1 - 131.2 m H.S.) and along the river reach downstream of the confluence with the power canal the operation of the (labcíkovó Hydropower Plant also affects the hydrological regime (in addition to the flows arriving from Bratislava

/Pozsony), since there is a possibility for modest peaking operation mode. 'ITiese changes of the hydrological regime affect drastically not only the conditions of the main channel (between rkm 1852 and 1811), but also the natural water supply of the tloodplain, practically eliminating it (Figure '>). The sufficient supply of water to the floodplain, to the side river arms, is of outstanding importance also from the view point of assuring appropriate recharge of the groundwaters of the area. Nature conservation and water quality protection (ecological) interests are also associated with the maintaining of sufficient supply of water to the floodplain and to areas beyond the dikes. However, these interests can be met only by artificially providing additional supply of water. In the original design the DunakiUti River Barrage had an intake structure of 235 nr/sec capacity for releasing additional flows to the side river arm system. Ilié Cunovó River Barrage necessitates some work for the supply of artificial additional flows to the side arms by the utilization of the alxive intake works. For the characterization of subsurface waters affected by this situation one should consider an area bordered by the rivers Danube, Mosoni-Danube and Rába, by the Hanság Canal and by the western country border. The River Danube forming the northern border of this area flows on her own alluvial cone that had been formed in the Pleistocene and is of good permeability (Figure 10). The