ABSTRACT
At one of the last free flowing sections of the Austrian Danube River, ongoing river bed incision leads to an unsatisfying situation for the adjacent national park and for inland navigation. Hence an integrative concept was developed for a restoration project, aiming to reduce bedload transport by a mix of measures, including the allowance of larger gravel within the natural grain size spectrum.
Prior to the implementation, a comprehensive monitoring was conducted over the last five years including recurring bathymetric measurements using both single and multi beam devices. As gravel-sheets in gravel bed rivers are rarely detected and described, an intensive study was performed supplementary to the frequent measurements, observing the gravel-sheet propagation by surveys conducted every ten minutes over several hours.
The measurements documented the ongoing incision but also revealed the occurrence of gravel-sheets, which strongly influence bedload transport at the Austrian Danube. The medium distance between the observed gravel dune crests lies at 10 m, the medium dune height is strongly dependent on Danube discharge history and varies between 5-7 cm and more than 30 cm. A hydrodynamic numerical model could also reproduce the movement of the gravel-sheets. Sorting processes lead to a high variability in sediment transport influenced by the transport of the bed forms. They were found to be active at much lower discharges than expected and were transported at very high speeds. The paper describes dune characteristics and their influence on bedload transport at the Danube.
