Author(s): Roman Dunst; Mario Klosch; Eddy J. Langendoen; Helmut Habersack

Linked Author(s): Roman Dunst, Helmut Habersack

Keywords: River restoration; Morphodynamics; Bank erosion; Bar; Numerical modelling

Abstract: As the channelization of rivers led to significant adverse impacts on ecosystems, caused channel incision and often increased flood risk, restoration became an often-taken counteraction in the course of the 20th century, with widening being a popular measure to allow for lateral erosion and therefore more natural-state morphodynamics. However, there is still a lack of tools for the planning of counter measures. In a companion paper, a tool was developed, which provides information on the established width after widening, the required amount of bedload to re-establish a more natural morphology, and the bedload supplied to downstream reaches. However, the tool remained untested to natural conditions. This paper deals with the verification of the model assumptions based on field data and shows the results of a first application at the Drava River, Austria. For this, spatial data of a mid-channel bar, recorded over the period of one year, is used to verify the assumed geometrical shape of a mid-channel bar in all three dimensions. Cross-sectional analyses were conducted to verify the approximation of the bar transect by a cosine-function; the validity of the elliptic plan shape was assessed by calculating the resulting width with given area, volume, height and aspect ratio using the model formulas. The validity of the overall three-dimensional shape in respect to various alternative geometrical forms was evaluated by hypsometric analyses. Then, the model was applied at the Drava River to the restoration site Kleblach-Lind, using spatial data surveyed seven times from May to October 2009, which provided the geometrical input parameters. Furthermore, Jet-Tests were conducted at the study site to derive the critical bank shear stress. Starting from an already existing mid-channel bar as initial condition, the model was tested whether it can reproduce the bar and bank evolution over the abovementioned period. The focus lied on a close fit in terms of widening and sediment consumption, but also on a realistic description of the sequencing of erosion and aggradation processes as a consequence of the flow. The analyses of the geometrical parameters indicate that the cosine function and the elliptic bar shape serve as a good approximation of the surveyed bar geometry. The model showed a high potential in serving as practical tool in the planning process of restoration measures, as it delivers a good estimation of required space and sediment of the reach in dynamic equilibrium based on a limited number of input variables.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221114

Year: 2022