58-75Vol 5No 22011
The Croatian-Hungarian border section of the Dráva River has been undisturbed for almost a century, and it is characterised by unique fluvial morphology (braided pattern and islands) supporting rich habitats and wildlife. However, during the last decades human impact became more and more intensive. Between 1975 and 1989 three water reservoirs wer...e built on the Croatian section of the river, just 16 km from the beginning of the border-section, altering the hydrology and the sediment characteristics of the river. On a local scale cut-offs, revetments and groynes were built. The aim of the study was to evaluate the effect of these human interventions. As the result of the alteration of the hydrology the channel pattern of the Dráva has been changing from braided to meandering, though on the upstream meandering part the territory and number of islands increased due to the drop of water stages. A cut-off and a groyne influenced only the morphology of a short section. As the result of the cut-off braided pattern became more pronounced, and the groyne caused intensive channel aggradation and gave way to lateral island development.
1-10Vol 3No 12009
Land-use changes and their effect on overbank sediment accumulation were investigated on thefloodplain of Middle-Tisza River. Military survey maps (1783, 1860, 1883 and 1890) and aerialphotos (1950, 1965, 1980 and 2000) were used to evaluate land-use changes and to calculate thevegetational roughness of the area. To determinate the rate of overban...k sedimentation sedimentsamples were collected from a pit, the grain-size, content of organic matter, heavy metal content (Pb,Cu, Zn, Ni and Cd) and pH were measured. Until 1950 meadows and pastures were typical on thefloodplain, gallery-forest was along the river, the oxbow-lake and the artificial levee. Notable landuse changes were detected in the second half of the 20th century, as the aerial photo taken in 1965shows extensive forestry in the area. These land-use changes affected the average vegetationalroughness, as it has been doubled since the disappearance of grasslands. Land-use changes highlyaffect the aggradation, as the increased roughness decreases the flood velocity on the floodplain,causing accelerated aggradation. Using Pb marker horizons and grain-size changes the studiedsediment profile was compared to dated profiles (Braun et al. 2003), thus, the sediment accumulationrate could be determined for the periods of 1858-1965 and 1965-2005. According to ourmeasurements the accumulation rate was doubled since 1965, very likely in connection with thedoubled vegetational roughness.
Active point bar development and river bank erosion in the incising channel of the lower Tisza river, Hungary
13-28Vol 13No 12019
The development of point-bars and bank erosion are critical near-bank processes, as they indicatethe sediment and hydraulic regime of a river system, thus, they refer to the equilibrium conditions ofa channel. However, throughout history, rivers have been modified for various benefits which changethe development of point-bars and the rate of bank ...erosion. In the Lower Tisza River (Hungary), riverregulations influenced the channel and floodplain development, altering the natural fluvial processes.The aim of the research was to determine the rate of near-bank processes and to make trajectory forfuture river evolution to support future engineering works. The bank erosion and point-bar developmentat human-influenced and freely meandering sections of the Tisza River were monitored since 2011.Behind a collapsed revetment, the bank erosion rate was 0.6 m/y, while at a freely meandering section itwas 2.3 m/y. The studied point-bars are located in revetted and freely meandering sections. Their surfaceeroded within the period. These intensive erosional processes refer to an incising meandering channel,which must be considered during future planning of in-channel structures (e.g. revetments, bridges),thus, geomorphic methods must be considered in any river engineering scheme.
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