The purpose of this paper is to study the individual and combined effect of upstream planform curvature and difference in bed elevations at the tributary entrance to the confluence on the flow in the confluence hydrodynamics zone. To do this, flow at right-angled confluences with three planforms and four values of bed elevation discordance ratio (Δz_T/h_d) is simulated using a three-dimensional (3D) numerical model. Three confluence planforms include confluences with the (1) straight tributary canal (SC), (2) right bend (RB) and (3) left bend (LB) in the tributary. Four Δz_T/h_d values in the range [0.0, 0.5] include both concordant and discordant beds' confluences. Overall, nine cases with the straight main canal are considered. Special attention is paid to the flow deflection and flow separation zones since the former affects transfer of momentum from the tributary to the main canal and the latter affects transport capacity of the post-confluence channel. Comparison of the results reveals that the influence of RB in the tributary is practically negligible in comparison to the straight canal case. With the increasing difference in bed elevations between the tributary and main canals (Δz_T), the presence of LB strengthens 3D flow and the structure of the recirculation zone is destroyed.

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.

U ovom radu se za procenu protoka koriste metode Ekersa i razmene količine kretanja koje su do sada detaljno testirane na osnovu rezultata laboratorijskih ogleda u kanalima sa poprečnim presekom jednostavne geometrije (dvogubo pravougaono ili trapezno korito) ili na deonicama vodotoka sa uređenim koritom (dvogubo trapezno korito). Prema saznanjima autora, ove metode još uvek nisu u značajnijem obimu proverene u prirodnim koritima. Hidrološka stanica Bezdan na reci Dunav ima zadovoljavajući fond merenja protoka u domenu velikih voda i pruža mogućnost za poređenje i validaciju dve metode. Na ovoj hidrološkoj stanici mereni su protoci tokom tri poplavna talasa: u julu 2009. godine (dva merenja), junu 2010. (jedno merenje) i junu 2013. (četiri merenja). Svi izmereni protoci veći su od 6000 m3/s. Merenja je obavio RHMZ Srbije pomoću ADCP uređaja. Pošto ovaj uređaj tokom merenja snima dubine, moguće je rekonstruisati deo poprečnog preseka pod vodom u onom njegovom delu u kojem je čamac sa opremom mogao da obavi merenja (u plitkim zonama merenje je nepouzdano, a postoji i opasnost od oštećenja opreme). Delovi poprečnog preseka izvan osnovnog korita rekonstruisani su na osnovu satelitskih snimaka za poznate položaje mernih preseka. Na osnovu ovako određene geometrije korita za veliku vodu i zabeleženih vodostaja, izračunati su protoci primenom ove dve metode i upoređeni su sa merenjima iz 2013. godine. Imajući u vidu česte morfološke promene korita, izabrana je jedna godina i to ona sa najvećim brojem merenja.

European inland waterways are a key element of the European transport network and are mostly used for freight traffic. The hydrographic surveying is the basis for the maintenance of inland waterways and the establishment of safe navigation. A part of the European waterway corridor E80 consists of the rivers Danube, Sava and Tisza, which part of their course also flows through the Republic of Serbia. On these sections of the mentioned rivers, the competent state institutions are conducting a continuous campaign of hydrographic measurements, which, among other things, includes GNSS RTK positioning and depth measurement with a single-beam ultrasonic echo sounder. In this paper, the characteristics of these sections of rivers, the current methodology of depth measuring and positioning during depth measuring will be shown in details.