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Title: Calculating erosion rates of river bank sediment by combining field measurements of erodibility parameters and small-scale topographic features – A case study at the Danube River

Author
item PFEMETER, MARTIN - University Of Natural Resources & Applied Life Sciences - Austria
item KLOSCH, MARIO - University Of Natural Resources & Applied Life Sciences - Austria
item Langendoen, Eddy
item HABERSACK, HELMUT - University Of Natural Resources & Applied Life Sciences - Austria

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/25/2015
Publication Date: 9/8/2015
Citation: Pfemeter, M., Kloesch, Langendoen, E.J., Habersack 2015. Calculating erosion rates of river bank sediment by combining field measurements of erodibility parameters and small-scale topographic features – A case study at the Danube River. Proceedings WMHE 2015, September 8-10, 2015, Brno, Czech Republic, pp. 77-86.

Interpretive Summary: Bank protection measures are being partially removed along a 30 km reach of the Danube River between Vienna, Austria and Bratislava, Slovakia, which flows through the Donau-Auen National Park, in order to restore near-bank aquatic habitat. The resulting bank erosion rates should reshape the current steep bank profile into a more gently sloping bank profile, but could have adverse impacts on the intense navigation along this reach of the Danube River. During 2013 and 2014 scientists from the USDA-ARS Watershed Physical Processes Research Unit (WPPRU) in collaboration with researchers of the University of Natural Resources and Life Sciences (Vienna, Austria) have characterized the bank material resistance to erosion using a jet test device manufactured by WPPRU, and have developed a detailed three-dimensional topographic model of the bank surface along a 2 km section of the reach. These data were used to calculate potential erosion rates of the bank material using the 2009 discharge record. Sandy soil layers reached erosion rates up to 6.5 cm/h, the critical shear stress of some sandy silt layers (higher than 12 N/m²) where never exceeded. Results of this study can be used by the Austrian government to assist in forecasting the possible effects of the evolution of the bank profile on navigation on the Danube River along the studied section.

Technical Abstract: This paper examines the application of a method for calculating fluvial erosion on river banks. In the investigated area the determination of potential erosion rates are essential to estimating the initiated river widening processes and their effect on navigation. A mini-jet device was employed, for in situ measuring of the erodibility of river bank sediments with respect to shear stresses induced through the jet-flow on the bank surface. The obtained data was then used to calculate the parameters of a commonly used excess shear stress equation to compute bank erosion rates, the critical shear stress and the erodibility coefficient, for a variety of Danube sediments. The actual shear stresses on the river banks were calculated according to the method of Kean & Smith (2006a). The method accounts for the effects of eddies in the lee of small scale topographic features in reducing the shear stress acting on the bank surface. This implies an accurate representation of the bank topography as well as its mathematically defined simplification. The law of the wall and the boundary layer theory are the bases for the calculation of the ‘skin drag,’ which is the part of the total shear stress responsible for fluvial erosion and is therefore used in the excess shear stress equation for determining erosion rates. Based on a natural discharge hydrograph potential erosion rates were calculated. While sands reached rates up to 6.5 cm/h, the critical shear stress of some sandy silts (higher than 12 N/m²) where never exceeded. Furthermore, relations between erodibility parameters as well as dependencies of the erodibility on the grain size distribution were found.