Changes of cross-sectional morphology and channel capacity during an extreme flood event, lower Tisza and Maros rivers, Hungary

When examining the characteristics of individual floods Hungarian researchers primarily investigate hydrological and hydraulic processes, whilst the relation between flood events and morphological changes of the river-bed are widely ignored. The present research quantifies the morphological changes of two cross-sections of the lowland reaches of the River Tisza and its tributary, the River Maros, during a high magni-tude flood which occurred in spring 2000. During the flood several key morphological cross-section variables (mean depth, channel bed eleva-tion, maximum depth, cross-sectional area and channel capacity) were monitored. Relationships between these data and daily river stage height series of the flood and specific stream power were determined. Results suggest that the identified morphological changes highly affect the channel capacity of the two cross-sections during the flood event. The channel capacity changes (9-10%) were almost identical for both study sites. However, different morphological processes characterised the two cross-sections. We found that morphological parameters de-pend not only on the actual stream power, but the available amount of sediment for transport, the rate of stage and stream power change

Modelling the hydrological effects of a levee failure on the lower Tisza river

Along the Lower Tisza River (Hungary) the water level of the floods reached new record stages in 1998 and 2006, resulting in 80 cm increase in the peak flood level since the “great flood of 1970”. Due to the gradual weakening of the levee-system caused by the several long-lasting floods, the question has arisen, that as in case of a levee breach or failure how would it modify the hydrological parameters of the river. The aim of the research is to create a hydrological model to analyse the effects (as stage reduction, slope and stream power) of two different levee breaches: one happening before the peak of the flood and another at the time of the flood level. The simulated levee breaching happened on the Tisza River at Mindszent, and the data-set of the 2006 flood was used for the modelling (at that time no levee failure happened in Hungary, and it was the greatest flood in history)

Centurial Changes in the Depth Conditions of a Regulated River: Case Study of the Lower Tisza River, Hungary

The Tisza River is the largest tributary of the Danube in Central Europe, and has been subjected to various human interventions including cutoffs to increase the slope, construction of levees to restrict the floodplain, and construction of groynes and revetments to stabilize the channel. These interventions have altered the natural morphological evolution of the river. The aim of the study is to assess the impacts of these engineering works, employing hydrological surveys of 36 cross sections (VO) of the Lower Tisza River for the years of 1891, 1931, 1961, 1976 and 1999. The changes in mean depth and thalweg depth were studied in detail comparing three reaches of the studied section. In general, the thalweg incised during the studied period (1891-1931: 3 cm/y; 1931-1961: 1.3 cm/y and 1976-1999: 2.3 cm/y), except from 1961-1976 which was characterized by aggradation (2 cm/y). The mean depth increased, referring to an overall deepening of the river during the whole period (1891-1931: 1.4 cm/y; 1931-1961: 1.2 cm/y; 1961-1976: 0.6 cm/y and 1976-1999: 1.6 cm/y). The thalweg shifted more in the upper reach showing less stabile channel, while the middle and lower reaches had more stable thalweg. Although the cross-sections subjected to various human interventions experienced considerable incision in the short-term, the cross-sections free from direct human impact experienced the largest incision from 1891-1999, especially along the meandering sections

Jelenkori folyóvizi felszínformáló tevékenység és a szabályozások hatásainak vizsgálata a Tiszán és a Maroson = Fluvial processes and the effects of river regulation works on the Tisza and Maros Rivers

A kutatás célja a meder és a hullámterek fejlődésének vizsgálata. Terveink szerint a méréseket a Tiszán és a Maroson végeztük volna, de később kiterjesztettük a Hernádra és a Drávára is. A mederfejlődést a meder morfometriai paramétereinek hosszútávú alakulása alapján mértük, és vizsgáltuk befolyásoló tényezőit is (vízállás-tartósság, vízjárás változás, partfal-állékonyság, növényzet, tektonizmus és emberi létesítmények). A kanyarulatfejlődést részletesebben vizsgáltuk, ugyanis a Tiszán egyre szűkebb és kisebb kanyarulatok alakultak ki, rontva a meder vízszállító képességét. A mederszelvény és a vízsebesség profilok alakulását árvizekkor és egy hidrológiai év során vizsgáltuk, megállapítva, hogy a meder a legmélyebb az áradó időszak végén, míg tetőzéskor megkezdődik az akkumuláció, amit az izotachok fel- és leszállása is alátámaszt. A hullámterek változásainak vizsgálatakor az akkumuláció mértékének meghatározására illetve az ezt befolyásoló tényezők vizsgálatára koncentráltunk. Megállapítottuk, hogy minden árhullám egyedi módon viselkedik, igen eltérő mennyiségű üledéket halmoz fel, bár a felhalmozott üledék mintázata és szemcseösszetétele hasonló. Hosszútávú akkumulációt az üledék vizsgálatával és DDM segítségével mértünk. Az akkumulációt befolyásoló növényzeti érdesség alakulását a 2006-os árvíz során sebességprofilok felvételével és hosszú távon is vizsgáltuk, megállapítva, hogy ez az egyik olyan kulcsfontosságú tényező, ami az árterek vízvezető képességét rontja. | The aim of the research was to measure the rate of active fluvial processes. Measurements were carried out on the Tisza, Maros, Hernád and Dráva Rivers. The long-term channel development was studied by morphometric parameters and its influencing factors were also evaluated (i.e. stage frequency, regime alterations, bank stability, vegetation, tectonism and human activity). Meander development was studied in details, as the bends of the Tisza tend to be narrower and smaller decreasing the water transport capacity of the channel. The cross-sectional changes and velocity profiles were evaluated during floods and within a hydrological year. Scour and velocity is the greatest in the end of the rising stage of a flood, but during the peak-flood period accumulation is dominant as the velocity profile shifts upwards. During the analysis of floodplain development the process of aggradation and its influencing factors were studied in details. Each flood is unique, as the amount of deposited sediment varied within great scale, however, the pattern and grain-size distribution of the sedimentation was similar. Long-term floodplain accumulation was measured using sediment profiles and DTM. Vegetational roughness as an influencing factor was evaluated by velocity measurements during the 2006 flood and by patch analysis. We concluded that vegetational roughness is the most important factor in decreasing the flood conductivity of the floodplains

Szabályozások hatására bekövetkező morfológiai változások a Tisza és a Maros alsó szakaszán = Morphological alterations due to river regulation works on the lower sections of Tisza and Maros rivers

The regulation works of the last 150 years have drastically altered the morphology of Hungarian lowland rivers. The aim of the present study is to compare the answers of the Maros and Tisza Rivers given on the cut-offs and revetment constructions. The cut-offs on Tisza were made in the 19th century, and by the beginning of the 20th century the parameters of the channel became similar as they were earlier, suggesting equilibrium state and roboust answer. The local revetments built in the twentieth century restricted the possibility of lateral erosion, thus the channel deformed, pointing towards disequilibrium state. The channel pattern of the meandering Maros was radically changed in the ninteenth century, and as an answer braiding pattern was developed, thus the river gave a sensitive answer. During the last 50 years the channel is getting narrower especially on the braided sections, suggesting that the braids are declining and the river is going to return to its original pattern