Alterations of channel parameters in response to river regulation works since 1840 on the Lower Tisza River (Hungary)

In the last few years an increase in the frequency and magnitude of floods was detected on the Tisza River, endangering large areas of Hungary. The causes of these record floods were complex, including both natural and human induced factors. This paper focuses on river management works and their effect on planimetric and cross-sectional parameters, with special attention to the flood conductivity changes to the river channel. During 19th century river regulation works, half of the total length of the Tisza River was altered by cut-offs, while in the 20th century mostly revetments and groynes were constructed. Subsequently, horizontal and vertical channel parameters have changed considerably due to semi-natural bed processes. In order to reveal changes, hydrological map series (1842, 1890, 1929, 1957, 1976 and 1999) and cross-sectional surveys from the same dates were analysed. Prior to the intensive human interventions (before 1890s) the river's course was highly sinuous with some very sharp bends. Due to cut-offs both the length and sinuosity of the Tisza River decreased by 35%, while the lengths of straight sections and the river's slope doubled. As a consequence the river incised by up to 3.8 m until the 1929 survey, resulting better flood conductivity, which improved flood safety. In the 1920s river management favoured bank stabilisation in order to stop the lateral migration of the channel. Despite these measures, meander development has continued, however, in a distorted manner. This is reflected by the opposing processes of lengthening centre-line on the one hand and gradually decreasing radius of curvature on the other. These processes can be explained by the continuous development of natural point-bars on the convex bank, and the lack of lateral retreat on the concave stabilised bank. The width of the river decreased by 17–45%, while its mean and maximum depth increased by 5–48%. The area of cross-sections influenced by revetments decreased by 6–19%, resulting in a 6–15% decline in flood conductivity. The non-stabilised sections were influenced by upstream revetments. Therefore, their parameters show similar changes, but with a smaller rate. At present, the flood conductivity of the channel is worse than it was in its natural state. In all, it was found that the ongoing process of cross-sectional distortion is a significant factor in increasing flood stage and hazard, and high floods can be expected more frequently in the future partly due to this factor.