Author(s): Stefan Giehl

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Keywords: Floodplain restoration; River training; Retention; 2D hydrodynamic simulation; Historic river;

Abstract: This case study of an alpine river contributes to the assessment of retention potential of natural rivers. The Inn River in southeastern Germany flowing from the Alps northwards has been modified and channelized during the last two centuries. In the selected stretch of 52 kilometers in the Alpine foothills, the river, which had originally braided channels and wide floodplains, was transformed by river training into a channel almost completely impounded by run-of-the-river hydropower plants. The width of the inundated area for retention was reduced from several kilometers to hardly 200 to 800 meters. To conduct the study, two-dimensional hydrodynamic models of these two different states were set up: one representing today’s river and one a historic state around the year 1800. Based on the terrain of the first model and historical maps from the first half of the 19th century, the position of the river channel without any major man-made alterations was reconstructed for the second model. Available data found in archives was used to estimate the bathymetry. Flood events typical for this alpine region with different magnitudes ranging from 10 to 100 years return period were simulated with both models to show the effect of river training on flood discharges. The results confirm that the potential of natural flood retention with regard to peak discharge attenuation decreases significantly for flood scenarios of high magnitudes whereas flood wave deceleration was significant in all scenarios. The findings have to be furthermore evaluated with a greater set of flood events.

DOI: https://doi.org/10.3850/38WC092019-1852

Year: 2019