Particle path characteristics at the large gravel-bed river Danube: results from a tracer study and numerical modelling

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A tracer study performed on a 3 km long reach of the Danube River in Austria is presented. Forty artificial stones of three different sizes (intermediate b-axis: 25 mm, 40 mm, 70 mm) were produced and a coded radio acoustic transmitter was implanted. The measurement system had to be improved to be applicable to large rivers with water depths up to 12 m. The positions of the stones were observed approximately once a week, depending on hydrology, over a period of at least one year by radio-tracking from a boat, including a 15 year flood event. Transport paths and velocities, as well as the incipient motion of bedload transport, could be monitored for the first time on a large gravel-bed river. The particle paths were found to be mostly bankline-parallel, even though the stones passed a 30° river bend. The median of the transverse particle displacement was found to be 4% of the longitudinal displacement. Calculations considering both transverse slope and transverse flow velocities showed transverse transport to be 6·6% of the longitudinal transport indicating that marginal lateral transport is mainly influenced by morphology. A three-dimensional (3D) numerical model using a stochastic particle tracing approach was validated with the data, indicating that the observed positions are well reproduced by the model. Within the observation period, 74% of all stones passed the reach. With more than 1000 detections, particle transport could be characterized by a mean travel velocity of about 10 m per day (variable for the different grain sizes); single tracer stones were transported up to 1000 m during a single flood event. Size-selective behaviour could be shown and the incipient motion of the large 70 mm gravel was detected at lower discharges than predicted by commonly used uniform bedload transport formulae.

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