Vízügyi Közlemények, 2003 (85. évfolyam)

1. füzet - Rövidebb tanulmányok, közlemények, beszámolók

156 Tímár Gábor — In the flat-land regions of the river basin the geological formations were subject to substantially accelerated compaction due to the excessive exploitation of artesian waters and the hydrocarbon resources and this resulted in the subsidence of the ground level to varying extent at different location; — On the mountainous and hilly parts of the catchment large scale deforestation has accelerated the time of concentration of floods and disrupted the natural sediment discharge balance of the rivers; — The disruption of the sediment-discharge balance and the termination of the natural morphological adaptability of the rivers, which latter was causcd by the river regulation works, initiated the deposition of sediments. This sedimentation process was confined into the floodplain within the flood-levees, and by the flood levees; — The excessive growth of the vegetation on the flood-plain, along with various man-made establishments built there, resulted in the substantial decrease of the hydraulic smoothness of the floodplain. In the vicinity of Szolnok the flood-channel could discharge, in the year 2000, 200 m

3/s less flow than what was conveyed at the same water level in 1970 (Figure /.). The question is: what has caused this rise of the water level? A part of the alteration of the discharge-water stage relationship was due to the reduction of the hydraulic smoothness of the floodplain, which latter was caused by the denser vegetation coverage and sediment deposition. Thus the water conveyance capacity of the flood-channci was reduced by 100 mVs. The other 100 m-Vs capacity loss is due to geological and neotectonic processes. At Szolnok the ground-level subsidence amounts to about 4.3 mm/year, while downstream of this site, at Tiszakécske and Tiszasas it is only 1.3-1.5 mm/year (Figure 2.). The combined effect of these subsidences brought, during 30 years, the water level gauge of Szolnok 100 mm nearer to the sea level than the position of the gauge of Tiszakécske. Due to this uneven subsidence the slope of the flood-channel upstream of Szolnok was increased, while it was decreased downstream of the city (Figure 3.). This results in a backwater effect in the river reach upstream of Szolnok. Along the river reach of about 60 km length between Szolnok and Tiszakécske the slope is 0.06 m/km (Figure 3). As according to the discharge formula of Chezy the flow is proportional to the square-root of the slope, this slope condition explains about 30 m

3/s capacity loss (if we assume this simple model to be valid for this river reach). Sediment deposition is a well known process, which becomes especially dangerous at sites where the river was building its alluvial cone, before the commencement of the river regulation activities. Data of the digital terrain model of the Tisza river reach upstream of Tokaj (Figure 4.) indicate that the highest point of the flood-plain can be higher than the terrain on the protected side of the levee by as much as 4 metres. The levees, bordering the floodplain, protrude from the terrain of the protected side, but so does, by several meters, also the terrain level of the floodplain (due to the intensive sedimentation within the floodplain). The essential features of the proposed long term river management system (which would better suit to the geological-hydrological processes) is than one should find a solution which would assure the following conditions: Either it will decrease the height of the flood hydrographs in a given section of the river, or vice verse, it would assure the conveyance of higher discharges at the same water level also securing the spreading of the sediment carried by the flood. This latter should not result, due to sediment deposition, in rapid and large rise of the terrain level of the floodplain in comparison to that of the protected side. The first element of this solution should be the widening of the floodplain on the expense of the protected side to as far as the "low flood plain", the border of the meander zone (Figure 5 ). The increasing of the average hydraulic smoothness of the expanded floodplain (or the