Assessment of methods used in 1D models for computing bedload transport in a large river: the Danube River in Slovakia

Comprehensive measurements of bedload sediment transport through a section of the Danube River, located approximately 70km downstream from Bratislava, Slovakia, are used to assess the accuracy of bedload formulae implemented in 1D modelling. Depending on water discharge and water level, significant variations in the distribution of bedload across the section were observed. It appeared that, whatever the water discharge, the bed shear stress τ is always close to the estimated critical bed shear stress for the initiation of sediment transport τcr. The discussion focusses on the methods used in 1D models for estimating bedload transport. Though usually done, the evaluation of bedload transport using the mean cross-sectional bed shear stress yields unsatisfactory results. It is necessary to use an additional model to distribute the bed shear stress across the section and calculate bedload locally. Bedload predictors also need to be accurate for τ close to τcr. From that point of view, bedload formulae based on an exponential decrease of bedload transport close to τcr appear to be more appropriate than models based on excess bed shear stress. A discussion on the bedload formula capability to reproduce grain sorting is also provided.