Vízügyi Közlemények, 2001 (83. évfolyam)
4. füzet - Nagy István-Schweitzer Ferenc-Alföldi László: A hullámtéri hordaléklerakódás (övzátony)
A hullámtéri hordalék-lerakódás (övzátony) 561 Sediment deposition on the flood plain by Dr. István NAGY C.E., Dr. Ferenc SCHWEITZER geographer and Dr. László ALFÖLDI geologist The first high flood (of 1888) that occurred after the completion of the regualtion of the River Tisza by flood-levees built on both sides arrived with a peak water level (H ma x) higher than that of the last high flood (of 1830). The maximum water levels had been gradually increasing along the river and reached 2.5 m in the Middle Tisza region (Szolnok). Until the first large flood (1970) that followed the water level rise of the first period the rise was only small (0.8 m). The two high floods of the more rainy period that followed the long, dry drought period resulted in peaks 1.7 m higher than those of the former ones (0.65 and 0.67 m), while the maximum discharges (0 max) did not increase. This non-proportional rise of water levels, being independent of the flow, called the attention to the decrease of the flow carrying capacity of the flood-plain (the flood-flow channel). More and more observations confirmed the fact that in the Middle-Tisza (at Szolnok) the flood channel has a decreasing flow carrying capacity. In this region neither the natural conditions of the river channel allowed the release of the highest floods. These earlier floods had broken out below Tokaj and reached the Tisza channel again via the rivers Hortobágy, Berettyó and Körös. The peaking water level of the high flood of 2001 was successfully decreased at Szolnok by cutting the high banks and releasing flows through an emergency flood release canal constructed on the flood-plain. The highest floods have increased the level of the bank-line dike (high banks), thus increasing the mid-flow channel and affecting the flow carrying capacity of the flood-plain and also the sediment load carrying capacity of the river. Results of flood flow velocity and sediment load measurements confirmed that the maximum sediment load occurs before the river entered the flood plain (Figure 7.). Investigations carried out during the cut-off of the bank-dike proved that the two high floods of the turn of the Millennium have together increased the height of the dike by 0.3 m. The age of sediment deposition was determined with the help of radioactive sediments, fairly accurately (Figure 11.). The still ongoing river bend development process of the Middle-Tisza river increases flood-sediment deposition, even when the sediment load washed away from the catchment had not arrived to the area. Comparison of the levelling data of the flood-channel cross section (form levee to levee) indicates the transversal migration of the main channel, the transformation of channel morphology and the asymmetry of the bank-lines. The length of the channel of the River Tisza was increased by 16 km due to the river-bend development within the flood plain. It is considered a proven fact that the deposition process of the flood plain still continues due to the river-bend development and bank-bar building and the oxbow lakes of the flood-plain act as sediment traps. Due to these identified and proven processes it is not sufficient to characterise the rate of flood-plain sediment deposition, along the full river length, with a single average value that corresponds to a given year or given flood. It is the conviction of the authors that the decrease of the flow-carrying capacity of the flood-plain (flood-channel) plays a role in the increase of Hmax, which is equally important to that of the anthropogenic influence in the mountainous regions of the catchment. * * *
