Vízügyi Közlemények, 1998 (80. évfolyam)

1. füzet - Rövidebb tanulmányok, közlemények, beszámolók

A magyarországi Felső-Duna és a talajvíz kapcsolata 151 been continuing ever since). This large basin was filled up during the Quaternary Period by the sediments of Danube and its tributaries. The river was then flowing along the crest of a huge alluvial fan. Sediment loads arriving from upstream exceeded the carrying capacity of the river and the river was forked into several side arms forming the Csallóköz bordered by the Kis-Duna (Little Danube) on the North and the Szigetköz on the South, bordered by the Mosoni Danube and the River Rábca (Figure /.). Floods of the river which was split into multiple branches had frequently overtopped the banks and contributed to further deposition of sediments. Thus the Holocene upper layer consisting of gravel and sand interwoven by layers of finer particles has been formed in the last ten thousand years. River regulation activities of the Hungarian Upper Danube proved to be not the most successful ones, since they resulted in the deepening of the low water channel and thus to the drop of the water surface profiles of the low- and medium water regimes (Figure 2.). Simultaneous upsilting of the floodplain (the flood water channel between the mean-flow channel and the flood levee) resulted in the gradual rising of the flood water levels (Figure 2.). Since these problems can not be handled by the methods of river training the canalisation of the river reach became necessary. The groundwater budget of the Kisalföld is affected partly by the hydrometeorological conditions (precipitation, evaporation and évapotranspiration) and partly by the open channel flow of the respective Danube reach. The latter has two kinds of effects; regional and local. The regional effect is due to the subsurface seepage flow between the Vienna Basin and the aquifers of the Gönyü Komárom Nővé Zámky (Érsekújvár) triangle. The local effects include the interaction between the open-channel flow of Danube and the neighbouring ground waters (Figure 3.). The direction of groundwater flow, inward or outward from Danube, depends on the relative position of the groundwater level, e.g whether it is higher or lower than the local water level of Danube in the channel. In the early 1950-ies groundwater recharge was the characteristic situation in the low-flow periods along the Danube reach between Dévény and Rajka. On the other hand ground water flow towards the Danube characterised the reach between Sap (Szap) and Komárom (Figure 4.). In the reach between Rajka and Sap (Szap) there was no significant exchange of water between the Danube and the groundwater. In the high-water periods of the fifties, however, Danube flow had recharged the groundwater along the whole reach between Dévény and Komárom (Figures 5. and 6. ). In the planning phase of the Gabcikovo (Bös)-Nagymaros hydropower scheme it became evident that the Dunakiliti river barrage is the key structure of the upstream sub-system of this scheme. This was the structure which could have regulated both the flow of the old Danube channel and the recharge of groundwater, outside the flood levee. After the construction of the Cunovo (Dunacsúny) river dam (on Slovak territory) the original mean flow of the Danube was cut into one-tenth and resulted in the respective decrease of the water surface profile along the 30 km long river each, until the confluence with the tailwater canal of the Gabcikovo (Bös)-power plant, where the main flow rejoins the river. Drop in the open channel water surface profile results in tum in the drop of the groundwater level, mainly in the floodplain (under the flood-channel, but also beyond this, outside the levee, over a few km wide band. Subsidence of the groundwater levels might be as high as 3 meters in the most affected sites, namely at the downstream part of the Dunakiliti river barrage. The construction of the Cunovo (Dunacsúny) dam disabled the operation of the originally planned water supplementation system and resulted in the lengthening of the zone along the river bank, where supplementary flows into the channel are required, with about 10 km, because Cunovo (Dunacsúny) is located 10 km further upstream (in Slovak territory) than Dunakiliti. Thus it became evident that the situation would have been much better if the Dunakiliti river barrage was put into operation in due time, instead of the river barrage of Cunovo (Dunacsúny).