Wpływ budowy geologicznej na zróżnicowanie procesów korytowych rzeki uregulowanej na przykładzie odcinka Wisły warszawskiej
Geologic structure influence on channel processes of regulated river in example of Vistula River reach in Warsaw

Summary/Abstract: The phenomenon of constant decrease of the Vistula River water level of the low, as well as mean state is observed from the mid-twentieth century (Wierzbicki et al. 1997). This process causes danger for the channel-connected infrastructures e.g. water intakes, regulation structures, bridges etc. It is genetically linked with channel bottom erosion induced by river engineering works and exploitation of alluvia (Skibiński 1963; Żelaziński et al. 2005). Currently, downstream and upstream of the Warsaw reach are also observed opposite processes such as aggradation. They are considered responsible for the increase of flooding danger. // The channel of the Warsaw Vistula River reach (so-called “Warsaw corset”) is narrowed by regulation structures from over 300 m to 220 m (mean flow) (Gutry-Korycka et al. 2006). The contemporary substratum of the alluvia consists of erosion-resistant deposits (e.g. boulder clay, clay, as well as coarse gravel and pebbles) building protrusions and cropping out in some places in the channel bottom (Falkowski E. 1990; Falkowski T. 2006). Between such elevations, the thickness of channel alluvia exceeds 15 m (below the mean water level). The examined 8- km long channel reach is representative for the phenomenon of simultaneous deposition and downward erosion in adjacent zones of the Warsaw Vistula. It comprises the aggradation zone of the Zawadowskie Islands and the erosional reach of the Siekierki region that are connected with the transitional reach. Bathymetric investigations (echosoundings) conducted in the channel zone have displayed that changes in the bottom level of the low and high water in the Siekierki region close to the river bank exceed 5 m and in the channel axis zone – about 1 m. In spite of this, depositional meso- and macroforms are completely lacking in the Siekierki reach. Changes in the channel bottom morphology are connected with the process of evorsion taking place close to the regulation facilities and with changes of the whirlpool dimensions. In the Zawadowskie Islands, changes of the bottom level exceed 3−4 m. This is a zone of constant changes of the position and morphology of meso- and macroforms. They constantly emerge and are washed away. Geological drillings conducted in the channel zone have displayed that geology is an important factor for the diversity of the studied reach dynamics. The shallow position of the substratum of the alluvia in the narrow, regulated channel reach near Siekierki is the reason for its transitional nature. The deep location of the substratum in the Zawadowskie Islands reach enables free thalweg migration, as well as the emersion, erosion and alteration of the depositional channel forms.

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