Morphological Evolution of the Lower Tisza River (Hungary) in the 20th Century in Response to Human Interventions

To understand the hydro-morphological responses of a river to engineering works is a key to future sustainable engineering projects. The Tisza River in the Carpathian Basin has been the subject of human interventions in the 19th-20th centuries, with regular monitoring since the first intervention. Therefore, it provides a unique opportunity to understand the long-term responses of a lowland river on engineering works. Here the interventions include the construction of embanked levees, artificial meander cutoffs, construction of revetments and groynes, and construction of lock-dams, whose differing effects can be analysed in detail. The Lower Tisza River, Hungary was shortened by up to a third of its 89-km length during late-19th-century regulation works. Simultaneously, it was disconnected from its 10-20 km wide natural floodplain, which was restricted to 1-5 km. Revetments were constructed along 25% of the banks since the early 20th century. This paper evaluates the effects of these regulation works on the Lower Tisza channel based on channel-surveys (1891, 1931, 1961, 1976 and 1999). Based on the results, the channel responded to every stage of the regulation works by adjusting its cross-sectional area, thalweg depth and bankfull width. From 1891 to 1961, the cross-sectional area of the channel increased in response to the artificial meander cutoffs through rapid incision and channel widening. The construction of revetments and groynes distorted this morphological development. After their construction, the cross-sectional area of the channel reduced by up to 50% between 1961 and 1976. The channel experienced incision and narrowing within this period. Whilst the morphological response of the river to cutoffs was improved flood conveyance, the revetments drastically decreased the flood conveyance; therefore, the flood levels increased without an increase in discharge, so the revetment constructions indirectly increased the flood risk.