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Complex hydro- and sediment dynamics survey of two critical reaches on the Hungarian part of river Danube

Detailed hydrodynamic survey of two critical river reaches has been performed from hydro- and sediment dynamics points of view, in order to explore the main features, moreover, provide calibration and verification data to related 3D flow and sediment transport modelling. Special attention has been paid to compare moving and fix boat measurement modes for estimating various flow and large-scale bed form features, resulting in recommendations e.g. on the time period needed in stationary mode operation to obtain sufficiently stabilized average velocity profiles and related parameter estimations. As to the study reaches, the first comprises a 5 km long sandy-gravel bed reach of river Danube located in Central-Hungary, presenting problems for navigation. As a conventional remedy, groyne fields have been implemented to make and maintain the reach sufficiently deep, navigable even in low flow periods. As is usually the case, these works resulted in rather complex flow characteristics and related bed topography at places. The second site is another 5 km long reach of river Danube, close to the southern border to Serbia. There the river presents navigational problems similar to the previously mentioned reach, however, having entirely sand bed conditions, abundant in a variety of dunes, especially in the shallower parts. In both study reaches ADCP measurements were done with around 2.5 Hz sampling frequency both in moving boat operation mode providing overall, though locally moderately representative picture, and in fixed boat mode at a considerable number of selected verticals with 10 minutes long measuring time.