Airborne Image Velocimetry Measurements at the Hydropower Plant Schiffmühle on Limmat River, Switzerland

Abstract

Airborne Image Velocimetry (AIV) measurements of a large surface velocity field were performed on the upstream and downstream river reaches of the hydropower plant (HPP) Schiffmühle on river Limmat, Switzerland. The averaged river discharge was 71.5 m3/s and slightly fluctuated ±0.5 m3/s during the measurements. The overarching goals of the study were to evaluate efficiency of fish guidance structures with horizontal bars installed at the turbine intake of the HPP and to assess the effect of the HPP operation on the downstream fish habitat. To this end, both AIV and 3D Acoustic Doppler Current Profiler (ADCP) measurements were conducted. The AIV setup consisted of a quadrocopter DJI Phantom 4 Pro+ incl. camera costing below 2,000 Euro and seeding particles of ecofoam chips costing around 150 Euro. Surveying flights were performed on a reach length of about 500 m with 4K video resolution at 23.976 frames per second. The total video data set comprised 23,600 image frames. To minimize computational time for the subsequent image analysis, a reduced data set of 579 image pairs each with an internal time shift of 0.2085 s was chosen. Image ortho-rectification was performed by photogrammetry software PhotoScan with ground control points taken from an existing 3D terrain model. The MATLAB-based open source software PIVlab was used to determine the time averaged surface velocity field. Despite small areas with misleading and unreliable results, the main flow characteristics match with those measured quasisimultaneously by ADCP.