Response of gravel-bed rivers to changing sediment supply
Friedl, Fabian Weitbrecht, Volker
Many Swiss rivers are heavily affected by various river engineering measures and exhibit ecological deficits. Sediment input from upstream is limited by hydropower plants, sediment retention basins and other river training measures, resulting in a lack of bed load and morphological degradation. Therefore, nearly 500 hydropower plants and other constructions have to be remediated in Switzerland till 2030, based on the adapted Water Protection Act.
The cantons have to define the restoration objectives for the rivers and the type and extent of measures for the affected structures. One key parameter for bed load restoration is the „required total bed load“. This is the minimum mean annual bed load volume that is required so that an unconfined river can develop near-natural bed forms and associated habitats for different species. While numerous interactions between bed forms and bed load transport have been described in literature, there are no quantitative criteria to determine the required total bed load for practical application.
The VAW was commissioned by FOEN to conduct physical model investigations at a scale of 1:50 to investigate the response of gravel-bed rivers to changing sediment supply. The main goals are to investigate the influence of added sediment on bed forms and on the morphology and to find a relation between sediment input and deposition geometry, namely area, volume, and thickness.
As we are using a hybrid approach, numerical simulations were performed in parallel. The software BASEMENT was used to simulate the development of the river bed.
Sediment replenishment by gravel deposits and lateral erosion in Swiss lowland rivers – Physical experiments
Friedl, Fabian Weitbrecht, Volker
Many Swiss rivers are heavily affected by various river engineering measures and exhibit ecological deficits. Sediment input from upstream is limited by hydropower plants, sediment retention basins and river training measures, resulting in a lack of bed load and morphological degradation. A possibility to improve the sediment balance is sediment replenishment. The source of sediment can be enhanced bank erosion or artificial gravel deposits, which are placed in the river. Pilot projects in Switzerland have shown that the behaviour of artificial gravel deposits and their impact on the river system are not clearly predictable. The main process to entrain sediment from both sources is bank erosion. The mechanisms influencing bank erosion are not entirely known and insufficiently included in most numerical models and therefore not necessarily well predictable.
The goals of this study are to (i) study the erosion process of gravel deposits, (ii) study induced bank erosion, (iii) improve the understanding of fluvial bank erosion in case of non-cohesive, granular material, (iv) provide fundamentals to implement and improve bank erosion in numerical models.
Physical experiments are conducted in a trapezoidal channel with a length of 35 m, a bed width of 2 m and a bed slope of 0.17% in the laboratory. Froude similarity has been used to model the Reuss River at a scale of 1:25. Tests are conducted to investigate the influence of the governing parameters, namely the grain size distribution, bulk density, geometry of the deposit and hydraulic load. The investigated hydrological conditions range between a mean annual discharge and a flood with a return period of 30 years (HQ30). Lateral bank erosion rates and the rate of sediment supply to the downstream river reach are determined. Existing equations for lateral bank erosion are evaluated.
As we are using a hybrid approach, numerical simulations were performed in parallel. The bank erosion process was implemented and enhanced in the software BASEMENT to simulate the erosion of the gravel deposits.