Vízügyi Közlemények, 2002 (84. évfolyam)
3. füzet - Rövidebb tanulmányok, közlemények, beszámolók
468 Kiss T.— Sipos Gy. — Fiala К. into the floodplain, between the levees. Thus all the sediments that had been spread before over the huge flood basins are being, today, deposited onto this narrow floodplain, in between the levees. The depth of the sediment of the floodplain of Mindszent (river kilometres 212—216) have been measured since the autumn flood of 1998 by direct methods, including the physical characteristics of the deposits. The up-silting effects of three large floods were investigated: the first two ones jointly, while that of the record-breaking springtime flood of 2000, separately. The spring flood of 2001 was very high, but passed through swiftly, and thus one could not measure the deposited thin layer of sediment. The objectives of the research discussed in this study were as follows: — What is the depth of the sediment deposits, which were caused by the recent floods? — How did the high-water channel cross-section change upon the effect of these deposits? — What is the destiny of these deposited sediments? — What is the extent of bank-erosion on this given reach? Where are the most intensively eroded parts? What part of the eroded material gets back into the channel? Detailed géomorphologie maps were prepared for the area (Figures 1—2), showing the main surface-forming processes and formations. The comparison of this morphologic map to the data of the recent deposit accumulation indicates the relationship of these processes and the morphology of the floodplain. The mass-transport formations of the flood-channel allow the estimation of the further destiny of the sediment deposited on the banks. The results can be summarized in the following way: — The depth of the deposits created by the floods investigated ( 1998—2001 ) was found as 20.5 mm, in average. The actual deposit depth varies, however, over wide ranges. The bulk of the sediment was deposited in the 10-20 m wide zone along the river bank (of depths of 0.1-0.70 m) onto the point-bars, bank-bars and fluvial ridges ( Table II., Figure 3.) These differences can be explained by the different hydrological properties of the floods investigated, namely, that they inundated the flood-plains for different periods of time. While the first two floods covered the terrain for nearly 4 months, the another two ones lasted only for 1.2—2 months. — The borderline of the sandy sediment on the river bank is drawn by the 5 mm accumulation iso-line (Figure 4. ). On areas characterized by lesser deposition one finds mostly silt and clay deposits. The importance of this latter might rise in situations when the floods arrive with pollutants, as these are mostly attached to suspended sediment particles. Thus the most endangered area is the one between the mentioned 5 mm iso-line and the flood levees. — The géomorphologie situation and the surface-forming processes basically determine the rate of deposition and the further destiny of the sediment within the flood-plain. The near-bank zone, characterized by the most intensive sediment accumulation, is not only building actively up, but also gets eroded. Thus the flood-accumulated deposits will get back into the channel within a few years. Consequently the above mentioned average deposition of 20.5 mm, does not mean that the flood-channel will be decreased by corresponding value. — Out calculations indicate that the cross-section area of the flood channel was decreased by the 1998-1999 floods by 0.89%, while the spring flood of 2001 resulted only in an 0.09% decrease. These can be considered overestimated values, however, since the mean new deposits, which will be compacted as the time proceeds. We have corrected the values accordingly, disregarding the sediment accumulation of the near-bank zone. The corrected estimate was 0.3% decrease of the flood-channel, caused by the floods of 1998-2000. This decrease cannot be alone blamed for the record breaking flood peak levels of the recent years.
