Three-dimensional Variability of Suspended Load Transport in Rivers and its Ecological Implications in Terms of Reservoir Flushing and Controlled Reservoir Drawdown

For hydropower plants strategies such as sediment sluicing or flushing are necessary to keep the reservoir operational; however, the amount of sediment released is limited due to ecological and thus legal constraints. High sediment concentrations may be harmful to biota populating the river, particularly as function of time and concentration. However, cross-sectional variability in the transport process is caused by interactions between sediment pick-up and deposition at the river bed, combined with a variability of velocity field and turbulence in the flow continuum. This fact was so far neglected in the legal restrictions of fine sediment releases.

Hence, a model-based investigation of the three-dimensional variability of suspended load concentrations in an alpine river was performed, followed by an ecological assessment taking also the duration of occurrence of the corresponding suspended load concentrations into account. The study was conducted at a 2.5 kilometre long reach of the river Inn in Austria. This stretch of the river consists of two sub-reaches, one being a winding type with higher bed slope conditions, small water depths, thus larger flow velocities up to the point of transition to a supercritical flow regime, while the other is a predominantly straight section with smaller slopes, larger water depths and slower flow velocities. Near the downstream model boundary, a gauge provides continuous water level and turbidity measurements; moreover, 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 suspended sediment concentrations for three discharges from low flow to flood conditions.

In the straight reach a high suspended sediment flux between the river bed and the water column was found to result in larger concentrations near the river bed, while in the winding reach higher concentrations are present near the water surface, with relatively smaller concentrations near the bed. Combined with the analysis of the duration of exposure to suspended sediments during the passage of a sediment wave, these results show that even under relatively high suspended load conditions in the river there may exist regions of smaller flow velocities and suspended sediment exposure which could serve as refugial habitats for biota - especially fish. 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.