Plastic waste as a persistent contaminant of our environment is a matter of increasing concern due to the largely unknown long-term effects on biota. Although freshwater systems are known to be the transport paths of plastic debris to the ocean, most research has been focused on marine environments. In recent years, freshwater studies have advanced rapidly, but they rarely address the spatial distribution of plastic debris in the water column. A methodology for measuring microplastic transport at various depths that is applicable to medium and large rivers is needed. We present a new methodology offering the possibility of measuring microplastic transport at different depths of verticals that are distributed within a profile. The net-based device is robust and can be applied at high flow velocities and discharges. Nets with different sizes (41 µm, 250 µm, and 500 µm) are exposed in three different depths of the water column. The methodology was tested in the Austrian Danube River, showing a high heterogeneity of microplastic concentrations within one cross section. Due to turbulent mixing, the different densities of the polymers, aggregation, and the growth of biofilms, plastic transport cannot be limited to the surface layer of a river, and must be examined within the whole water column as for suspended sediments. These results imply that multipoint measurements are required for obtaining the spatial distribution of plastic concentration and are therefore a prerequisite for calculating the passing transport. The analysis of filtration efficiency and side-by-side measurements with different mesh sizes showed that 500 µm nets led to optimal results.

The objectives of this research are to verify a numerical sediment transport model based on field data from a reach of the Austrian Danube River, to identify the influence of non-uniform bed-load transport modelling and the selected bed schematization on the simulation results and to investigate the cause and the numerical reproducibility of spatio-temporal variations found in field data. Simulation results based on variants of the transport equation by Meyer-Peter and Müller were compared to basket sampler and radio tracer stone observations. The best agreement was obtained for a non-uniform formulation with an exchange layer, while the uniform formulation of the transport equation underestimated the transport rates for discharges lower than the mean flow since it predicts a later onset of bed-load transport. The consideration of an exchange layer in the numerical code led to the prediction of bed-load sheets due to sorting waves, allowing the model to account for the spatio-temporal variability of field data.

As is widely known, the groyne layout affects the three-dimensional flow situation and sediment budget in groyne fields. Lack of knowledge exists concerning the evolution of hydrodynamics and morphodynamics in the main channel of rivers due to distinct groyne layouts. Accordingly, the sensitivity of the stream flow and the river bed in the fairway to groyne geometry were investigated with the help of a three-dimensional hydrodynamic model and a sediment transport model by considering eight variants and seven discharges in a straight reach of the River Danube. The variation of crest elevation and length indicated highest sensitivity, especially at modelled discharges for which the largest geometrical changes due to the modification of the groyne layout were implemented. Within a curved reach larger effects for length variation and smaller ones for the variation of crest elevation were found. These findings can be incorporated into future research and planning of groyne layouts.