# Estimation of Changes in Sediment Transport along the Free-Flowing Middle Danube River Reach
###### Abstract
[5] extended hydrological analyses to the entire free-flowing Middle Danube River reach (from Dunaijvaros--rkm 1581 in Hungary to Novi Sad--rkm 1255 in Serbia) following the reports on water intake problems at different locations, out of which the Bezdan pumping station which supplies the Danube-Tisza-Danube irrigation and drainage canal network in Serbia [6]; and decreasing inundation frequencies in the nature reserve Kopacki rift on the right bank floodplain in Croatia [7] were outstanding. The analysis covered a 70-year-long period from 1951 to 2019. During the first 30 years of this period, many river training works were completed along this reach to facilitate navigation, thus contributing to setting up one of the ten Transeuropean transportation corridors--the Danube-Rhine corridor. Despite the obligation of all Danubian countries to regularly monitor the river channel for operational purposes, there were only a few attempts to analyse the sediment budget along the river e.g., [8] until the Interreg project \"Danube sediment\" [9]. The project focussed on the adjustment of the sediment monitoring methodologies and the data in the Danube River basin. This paper aims to present a comprehensive study of the sand bed free-flowing Middle Danube River reach, with hydrological and morphological analyses and the indirect estimation of the average multiannual sediment transport along the study reach. Given the fact that the suspended sediment monitoring on the Hungarian sub-reach is relatively scarce in space and time [3] and that there was no sediment monitoring at all on the Serbian side [3], the estimation of the sediment transport is, for now, based on analyses of the bathymetric data of surveillance cross-sections from the Directorate for Inland Waterways in Serbia and the Lower Danube-Valley Water Authority in Hungary. This procedure is standard in estimating reservoir sedimentation processes, e.g., the reservoir sedimentation at Shumburit Dam in Ethiopia [10]. A similar analysis was performed along the gravel bed reach of the Danube River between rkm 1800 and rkm 1826 for the period 1992-2001 by [PERSON] and [PERSON] [11]. [PERSON] and [PERSON] [12] demonstrated the advantages of a morphological method based on available field surveys in evaluating the performance of different bed load transport formulae over larger spatial and temporal scales (decadal-scale) on a 65 km long gravel-bed reach of the Fraser River. [PERSON] et al. [13] used bathymetric data collected in two years bounding 30 years (1977 and 2006) to analyze riverbed deformation, i.e., cumulative volume change in the channel of the largest Mississippi Delta distributary--the Atchafalaya River. Analysis of the bathymetric data revealed, among other facts, a discrepancy between the transport capacity of the flow and the inflow of sediments [13]. Sediment transport can also be successfully estimated from dune-tracking methods based on bathymetric surveys of channel longitudinal profiles recorded with a certain time shift [14; 15]. Recently, [PERSON] and [PERSON] [16] suggested a modified method based on routing local erosion and deposition volumes. It is applicable to channels with a complex bedform morphology when it is difficult to delineate individual bedforms. However, the conventional dune-tracking method and its modifications require on-purpose bathymetric surveys. This is why it is not possible to use these methods for the estimation of sediment transport in less monitored rivers like the free-flowing Danube River reach, as on-purpose surveys have not been performed there. The research questions to which the answers are sought in this paper are as follows:  
1. Are the previously observed decreasing trends in water levels stagnating? Did they revise, or are they continuing after 30 years from the initial studies? 2. Is there a correlation between the observed minimum water level and the recorded incision of the riverbed at gauging stations? 3. What are the long-term sediment transport characteristics along the investigated reach?