Author(s): Michael Tritthart, Christoph Hauer, marlene haimann, Helmut Habersack
Keywords: Suspended sediment; 3D numerical modelling; Severity index; River flow; Reservoir flushing;
Abstract: Reservoir flushing or controlled drawdown are common strategies to keep a reservoir operational but cause high suspended sediment concentrations in the river reach downstream that may be harmful to a river’s biota and are therefore usually legally regulated. However, field measurements show that there exists a considerable variability of suspended sediment concentrations over a river’s cross section. Aim of the study is to employ a modelling framework to determine this variability throughout an entire river reach and assess its ecological implications.
The study was conducted at a 2.5 kilometer long reach of the river Inn in Austria. This stretch of the river consists of two sub-reaches, one being a winding type while the other is predominantly straight. Cross-sectional measurements of flow and suspended sediment concentrations were conducted using an ADCP device to provide data for model calibration and validation. The numerical modelling suite RSim-3D/iSed was used to compute the three-dimensional flow field and related suspended sediment concentrations.
In both sub-reaches a decrease of the suspended sediment concentrations was found from the center towards the banks and the surface. However, while the winding sub-reach additionally exhibited a larger variability in concentrations following a redistribution of sediments in the cross-section by secondary flow processes, the concentrations were more homogeneous in the straight section. An ecological assessment integrating the results over time showed that there may exist refugial habitats for biota, especially fish, even for overall higher concentration values. This knowledge and the opportunities to use this novel model for the ecological impact assessment of reservoir operation is of great importance in terms of future hydropower management.