Vízügyi Közlemények, 2000 (82. évfolyam)

2. füzet - Rákóczi László: A Duna-meder sorsa Szap és Szob között

A Duna-meder sorsa Szap és Szob között 279 and resulted in a number of graphs (Figures 2-4). The numerical values of these estimates are hardly comparable with each other, although they unambiguously prove that the rate of dredging was several times higher than the natural bed-load of the River Danube. The in-time distribution of the homes built in Hungary in the period 1966-1990, using the prefabricated highrise residential building technology, is very similar to that of Figure 2., proving that channel dredging was made mostly for construction material purposes. The channel changes of the Danube reach between Szap and Szob were decisively defined by the extremely high bank- and channel erosion of nearly point-like character, which resulted in the discharge of several hundred-thousand cubic-meter sandy gravel into the main channel, at about river kilometre 1811. This penumenon was caused by the tangentially connecting tailwater canal of the Bös/Gabcikovo Hydropower station, the high energy flow of which carried away the material of smaller and larger sandy-gravel bars, which latter were earlier bypassed by the main-stream which run along the left bank (Figure 5.). The river Danube transports this scoured material towards the downstream in the form of large sandy-gravel bars, the moving of whose can be well traced on the basis of detailed channel surveys made in 1992 and 1997. This has caused temporary navigation problems at the bridge of Medve and in the vicinity of Nagybajcs. A part of the moving sand-bars halted at Vének, further deteriorating the conditions of the ford of Csicsó, and forming two further fords in the vicinity of Gönyü. The impact of industrial gravel dredging on the low water levels of the Danube is well reflected by the changes of the lowest navigation water levels and regulation levels (levels DB), as reported by the International Danube Commission. The rising and falling of these water levels is proportional to the sedimentation and scouring of the channel. The effect of industrial gravel dredging resulted on the low-water level lowering in comparison with the surface profile DB 75, which was valid in the period 1966-1975. Therefore the Danube Commission defined new water levels in 1984 for the period 1976-1995, using the discharge rating curves of 1970 (DB 95). As indicated by figures 6/a and 6/b the surface profile DB 95 stays below DB 75 along the whole river reach examined and the difference approaches 1.0 metre at river kilometre 1770. Upon the effect of increased gravel dredging the demand of the Hungarian and Czechoslovak construction industries could not be met by the middle eighties. On the basis of hydrometric measurements the experts of the two countries defined (in 1984 and 1990) so callcd "working water levels" (marked as MVSZ 84 and MVSZ 90 on the figures). These surface curves follow the channel-subsiding effects of the dredging much better and differ with more than 1.0 metre from DB 95 at the most intensively dredged sections. The surface curve marked as KV 97 on the figures corresponds to a low-water surface profile survey results, made in 1997, thus reflecting the state which was reached after the termination of industrial dredging. The curve KV 97 runs lower than MVSZ 90 along the section between Szap and Medve, but rises above this at Nagybajcs and between Vének and Gönyü. This indicates that the scoured material left the vicinity of Szap and reached Gönyü in an elongated shape. The two surface curves run nearly together from this point downstream, until r.km 1720, indicating that no significant changes of the channel is encountered along this river reach. Between Esztergom and Szob the curve KV 97 exceeds again the curve MVSZ 90 by several dm. This upsilting indicates the effect of the sand-bars of Helemba and the rocky ford of Dömös. As the extent of the falling of low water levels is only proportional to the changes of the bottom water level, but is not necessarily the same, the extent of the latter was estimated in Figures 7/a-c. This curves show the difference of the deepest points of the standard Danube cross sections (called VO sections), published in the Danube Atlases of 1971 and 1996. It can be seen, that channel subsidence was more frequent than up-filling. The former reaches several metres magnitude at some locations, while channel rise of considerable extent can only be observed at Medve, Nagybajcs and in the vicinity of Vének. This latter indicates the likely state of the sand-gravel bars, five years ago, which resulted from the right bank erosion of Szap.