Author(s): Johanna Ficsor; Alexandra Gradwohl-Valkay; Ervin Pirkhoffer; Szabolcs Czigany

Linked Author(s): Johanna Ficsor,

Keywords: Bedload entrainment; Bedload texture; Hydromorphology; Drava River

Abstract: Nature conservation and landscape management need to know possible changes in a riverbed. The morphodynamics of alluvial rivers are adjusted by the mobilization of bed material. However, the details of mobilization of mixed-texture bed materials at low flows, which are becoming more common due to climate change, are still unclear. The hydrological impacts of global climate change have been described for the Drava watershed too. Climate change affects sediment transport indirectly through water flow, modifying physical, chemical and biological properties of the riverbed. Our research aims to map the morphological state of the River Drava in the Hungarian-Croatian section between the Mura confluence and Drávaszabolcs, where the upstream part of this reach is typically in a natural state, but the downstream part is heavily modified. This 161-km-long alluvial reach of the River Drava was surveyed in 2019. A monitoring campaign was launched to study channel morphology and bedload entrainment dynamics with regard to texture at cross sections on average 5.55 km apart. A sonar, an ADCP and a Helley-Smith bedload sampler were mounted on a double-hull vessel for the surveying. The results of the survey was compared with the results of previous research and hystorical data, as well. To understand the mediated development processes, the survey was compared with historical data and examined the relationship between the official midline (from 2013) and the midline based on a sonar bed survey (from 2019). By comparing the midlines, the locations and direction of the typical bed movements of the last seven years were determined. The result of the research shows a sudden fining between river cross-sections of 170 km and 175 km from the mouth. The fraction of d60 was found to be finer than in 2003 and 2012 for the upstream stations of Botovo and Bélavár, and showed a good correspondence with the records of the Barcs and Drávaszabolcs stations. The temporal fining and higher entrainment rate may be due to (i) changing climate of the catchment, i.e. diminishing flow between the monitoring dates (2003, 2012 and 2019); (ii) reduced armouring, (iii) variability of cross-sectional position of sampling points and (iv) the different mesh size of the employed bedload samplers. This research can be a basis for predicting future expected changes. Our reach-scale results may be relevant for the alluvial sections of other alpine and subalpine, partially channelized rivers of similar size, flow dynamics and mixed bedload.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221830

Year: 2022