Hidrológiai Közlöny, 2017 (97. évfolyam)
2017 / 3. szám - MANAGING ENVIRONMENTAL RISKS - FLOOD PROTECTION EXPERIENCES AND COOPERATION IN THE DANUBE RIVER BASIN - Engi Zsuzsanna: Flood hazard modelling of the River Mura based on the silting up processes of the inundation area
86 Hidrológiai Közlöny (Hungarian Journal of Hydrology) 2017. 97. évf. 3. sz. Flood hazard modelling of the River Mura based on the silting up processes of the inundation area Zsuzsanna Engi West-Transdanubian Water Directorate, Szombathely, Vörösmarty u. 2. (E-mail: engi.zsuzsanna@nyuduvizig.hu ) Abstract In my dissertation, the flooding and silting up processes of the inundation area of downstream sections of the meandering river were studied. The travel zones in the flood-plain forming during the travel time of the flood wave were determined by using 2D hydraulic modelling. The flooded status of the inundation area was compared during the flood peak and the falling limb of the flood wave in order to get information about the changes in the river bed and the silting up of the inundation area. The results of different research methods were compared (hydraulic modelling, geomorphological methods, sedimentological analyses, dating of the layers in the sediment sample) in order to evaluate the possible effect of the changes of discharge on the flood hazard due to climate change. The geomorphological characteristics of the meandering outfall river sections were studied. Morphometric inundation area parameters and the trend of long-term development of the meanders and the river were evaluated. Key words Hydraulic modelling, MIKE 21, flood hazard, silting up of the inundation area, geomorphology, morphometric analyses, development of meanders, dating of the sediment sample core, river Mura. INTRODUCTION In my dissertation, the flood hazard modelling was studied on the outfall stretches of the rivers (Figure 1), based on the silting up process of the inundation areas. The process of flooding and the conveyance of the inundation area were analysed and we tried to learn if the silting up data could be used for calibration and validation for hydraulic modelling. Comparisons were made between the flooding of the inundation area at different times, such as the rising and falling limbs of the flood wave. The propagation of flooding was analysed. In the research, various methods were used and the results were compared with the aim to evaluate the effect of flood events caused by climate change on flood hazard. Figure 1. The catchment area of the River Mura (adaptedfrom: Hydrological Study of the River Mura, 2012) The Mura originates in the mountains of Styria, 1,898 m above sea level. The total length of the river is 465 km, of which 295 km is in Austria, 35 km on the border between Slovenia and Austria, 98 km in Slovenia, and 30 km on the border between Croatia and Hungary. The total area of the river basin is 14 241 km2, of which about 10 200 km2 belongs to Austria, 1 400 km2 to Slovenia, 590 km2 to Croatia and 2 040 km2 to Hungary. Water regulation work was first started in the 18th century on the upper section of Mura (Hochenburg regulation between Graz and Cven river section). The river became rapid flow type due to the construction of 26 hydropower plants operating in chain (Kovacic el al. 2004, Balazic 2004). The natural sediment transport completely stopped between the upper and the middle, lowland-type river sections, which caused the morphological degradation and, consequently, the deepening of the riverbed (Hornich et al. 2004, Novak 2004). The flow was concentrated in the main riverbed, the connection with the side channels and branches was terminated, so flow velocity and tension in the riverbed increased. 40% of the discharge of the Mura was in the main riverbed, 40% was in oxbow lakes and 20% was still water (Novak 2004). The natural sediment transport cycle disappeared because the hydropower plants on the upper Mura completely dammed the above-mentioned section of the river in Austria. No natural processes of bar formation and or any significant hydromorphologi- cal changes happened. The riverbed became 1.2 m deeper on average, but it reached 2.28 m deepening at some places (Globevnik and Mikos 2009). Erosion of the bottom of the bed was happening at a lower rate due to the bigger width of the riverbed in the middle section of the Mura and on the Slovenian-Croatian border, so the deepening of the bed of the Mura was about 30-40 cm. The ground-water level decreased as a consequence of the deepening of the riverbed after radically cutting off the meanders ( Petkovsek and Mikos 2000, Hornich et al. 2004, Novak 2004, Ulaga 2005, Globevnik and Mikos 2009). Meanwhile, completely different problems arose in the part of the Mura on the Hungarian-Croatian border. The construction of dikes was finished in the 1970s and the floodplain between the dikes became 600-750 m wide. Agricultural developments were started according to the economic policy of the time. The areas along the river, together with the side branches, slowly lost their function and biodiversity decreased. Interfering with the natural status by deforestation, the drying of wetlands, changing land use and the coastal stripe have all contributed to the change of primary fields. New uses for land appeared: meadows, pastures and forest remnants. The newly established vegetation is full of foreign and invasive species that cause an
