# Estimation of Changes in Sediment Transport along the Free-Flowing Middle Danube River Reach
###### Abstract  
The subject of this study is an approximately 300 km long Middle Danube River reach that spans from river kilometer (rkm) 1581 in Hungary to (rkm) 1255 in Serbia. The observed drying of floodplains in Hungary some thirty years ago initiated the hydrological studies. However, problems related to the navigation route maintenance of the Danube River and those in the water supply of irrigation and drainage canal networks are now present in the whole free-flowing middle Danube region. The study aims at investigating the correlation between the observed water level decrease and recorded incision of the river bed at gauging stations and the indirect estimation of the long-term sediment transport along the sand-bed reach based on the surveillance cross-sections' data collected during regular monitoring surveys on the navigable Danube. It starts with hydrological analyses of the 70-year-long time series of water level and discharge yearly data and continues with morphological and correlation studies. It ends with the estimation of sediment transport. The decreasing trend in water levels and the increasing trend in cross-sectional areas are persistent. There is a linear correlation between the two. Longitudinal changes in sediment transport indicate the existence of both degrading and aggrading riverbed reaches. low water levels; incision of riverbeds; sand bed; surveillance cross-sections; indirect estimation of sediment transport +
Footnote †: journal: Physics Letters B  
0023  
131051331  
of the free-flowing Middle Danube River reach (from Bezdan to Novi Sad in Serbia, a part of which is a boundary with Croatia). There has been no sediment monitoring at all along this stretch of the river, not only during the crisis in the 1990 ies [3]. The situation is similar to bed load monitoring in the Hungarian reach, where the bed load is sampled only in the gravel bed stretches at the most upstream gauging stations (GSs) of the country, while data from suspended sediment sampling, which is more frequent than bed load sampling, are of questionable quality because (1) the sample is usually not related to flood events when the suspended sediment concentrations are the highest, (2) the cross-section is not usually surveyed during the measurement campaign and (3) the discharge is not usually measured simultaneously with suspended sediment sampling. In contrast, water level monitoring was not and is not susceptible to the crisis to the same extent as the sediment monitoring, and there are only a few gaps in the stage time series. Owing to the relationship between changes in the water level (stages) and morphological changes in the riverbed (either the incision or aggradation of the channel), trends in the time series of the stage data might be an indicator of trends in riverbed changes. Riverbed deformation results from an imbalance in the sediment transport capacity of the flow and sediment supply. Since water stage monitoring is simpler and cheaper than bathymetric surveys and sediment monitoring, analyzing its trends is a convenient initial step in the assessment of the average multiannual change in sediment transport. The first sign which initiated analyses of long-term trends in the water stage and discharge data in the free-flowing Middle Danube River reach was an alarming drying of floodplain forests in the Gemenc area in Hungary in the late 1980 ies and early 1990 ies. The first analyses covered the time series from 1901 to 1992 at 17 GS along the Hungarian reach and 8 GS downstream of Budapest, including Dunaujvaros at river kilometer (rkm) 1581, Dunafoldvar (rkm 1560), Paks (rkm 1531), Baja (rkm 1479) and Mohacs (rkm 1447) [4]. The results have shown a decreasing trend of water levels at all these gauging stations. However, there was no attempt to correlate it with the suspected incision of the riverbed, most probably initiated by river training activities, mostly carried out in the second half of the 19 th century and the first half of the 20 th. Recently, [PERSON] et al.