Bed load extraction with vortex tubes

Bed load aggradations in the headrace channels upstream of a hydropower plant at the Limmat River had required intensive maintenance work. As the bedload transported to the powerhouse can cause both damages to turbines and operational disturbances those aggradations are undesired effects of flood events. Therefore, an alternative solution with vortex tubes was investigated in order to obtain an efficient and permanent supply of sediment to the residual flow reach. A vortex tube is a horizontal element below a channel with an intake slot along its longitudinal axis.

As vortex tubes are currently not part of a standard design to extract bedload upstream of power plants, a physical model (scale 1:35) was used to test three different tube configurations in terms of their extracting efficiency, hydraulic behavior and operational criteria.

The hydraulic tests showed that the principle of vortex tubes is suited for the extraction of transported sediment. The results demonstrated extracting rates over 95 % under appropriate hydraulic conditions. The extracting efficiency was particularly affected by tube discharge, vortex intensity, channel Froude number and sediment load. In addition, the tube geometry is dependent on sediment size, channel width and economical aspects.

The best results in terms of efficiency were obtained by the tube-type “Omega”. As exposition of the tubes leads to narrowing of the cross-section and flow separation, it is recommended to place the tube’s top edge in the same height as the bed level. For a bended open channel reach a deflection angle of 60° against the main flow direction gave the best results.