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
87 Zsuzsanna Engi: Flood hazard modelling of the River Mura based on the silting up processes of the inundation area impenetrable barrier in the floodplain (Figure 2). The higher water level of floods is a consequence of this when the Mura enters the floodplain (Hercsel 2008, VIZITER V Consult and Láng 2012). 1580 m3/s discharge and 514 cm water level were measured on the Mura in 1972, but only 1200 m3/s discharge and 509 cm water level in 2005 (Engi et al. 2011). Figure 2. Overgrowth with low vegetation (A), medium vegetation (B), and high vegetation (C) on the basis ofLiDAR data in the experimental area 2 Furthermore, there was no complex water management due to the historical changes of the state borders in the 20th century (Tóth et al. 2013). The riverbed changes and floods require constant attention, which need has been further amplified by extreme climatic changes in the last decades. Almost 300 m3/s volumetric capacity has disappeared from the floodplain in the last 3-4 decades. The flood protection dikes are not able to cope with this tendency, but it would be too expensive to elevate them. Thus, the present capacity of the floodplain is not enough to contain the high discharge. METHODS, RESULTS AND CONCLUSIONS Sedimentological analyses of the samples and dating of the layers in the sample In order to determine the tendency of the silting up process of the inundation area, the sediment sample was taken from the oxbow lake Hosszúvíz (Nguyen et al. 2008, Braun et al. 2010, Korponai et al. 2010) {Figure 3). The layers of sediment relating to flood events could be distinguished on the X-ray image. Sedimentology analyses were also carried out and the grain size distributions of 195 sediment samples were analysed (de Boer et al. 1987, Blott and Pye 2001) (Figure 4). The sand fraction grains (the first three categories) were taken into account for the dating of the drill core. The increased velocity transports the larger grains during flood events. The sampling location has been an oxbow lake since about 1830. The remarkable floods of the river Mura, registered in recorded by the Water Directorate, have been attributed to the extremely high sand fraction values. On the grain size distribution curve, the sediment of floods from the years 1972 and 1998 could be identified very clearly. The sediment samples were dated and the speed of the silting up process of the inundation area was calculated (Engietal. 2016b). The value is 1.17 cm/year (Engi 2016) (Figure 5 and Table 1). Table 1. Connecting sample sections and ages Depth (cm) 10 cm 30 cm 50 cm 70 cm 90 cm Date (year) 1990 1973 1956 1939 1922 The method gives an adequate forecast for the future silting up process. The result was compared with similar data from other floodplains (Károlyi 1960, Szlávik 2001, Keesstra 2007, Szabó et al. 2008, Oroszi 2009, Sándor 2011). Hydraulic modelling - Flood simulation The experimental areas MIKE 2D FM modelling system was used for the hydraulic modelling. In the dissertation, the model was developed using DEM data from the Mura Hydrographic Atlas of 2014 and the hydrological data of the 2009 flood. The experimental area covers the Hungarian section of the River Mura (Figure 6). The model was built from 9 km to 48 km (Engi et al. 2016a, Engi 2016). The effect of flood propagation The effect of flood propagation on the inundation area was studied. The values of specific discharge and velocity were analysed for the whole inundation area and in detail in 20-60 points of monitoring profiles. The main direction of the flow and the depth of the water was also shown on maps using different layers (Figure 7). The results show separated zones of specific discharge and velocity with the values of 0.2 m3/s/m-0.8 m3/s/m and 0.2-0.8 m/s (Engi, 2016). The possible locations of meander cut-offs The studied river is of the alluvial type with developing meanders in spite of the standard bank protection. The possible locations of meander cut-offs were identified using separate layers for this purpose. The model forecasted the possible direction of the meander cut-off in a location which was evaluated in May 2016 (Engi 2016, Engi et al. 2016a) (Figure 8).
