ABSTRACT
This paper identifies the potential of airborne video recordings to estimate surface velocities, illustrated by a monitoring project at Thur River, Switzerland. The complex flow situation along a 70∘ river bend is documented and analyzed prior and after revitalization measures from 2013 until 2020 . Due to the river width of 40 m, the demanded Particle Image Velocimetry (PIV) recordings of floating tracer particles have to be taken from a helicopter at a height of 50 m. Within first experimental runs plywood chips with an edge length of 60mm have been added as tracer particles from a bridge located 700 m upstream of the measurement area. Challenging for the image correlation within the PIV analysis procedure are the unsteady camera shakes due to the inevitable non-stable position of the helicopter. To correct this influence, an algorithm has been developed that performs a back transformation. It uses information of predefined geo-referenced riparian control points which are automatically object-detected on the video. The images are filtered by a three-step methodology to finally obtain binary images, in which only seeding particles are visible. PIV analysis is conducted by the open source software PIV lab. First results indicate that this new type of airborne PIV system is capable to describe the mean flow field quite well. The flow is clearly directed to the outer bend with maximum velocities up to https://www.w3.org/1998/Math/MathML"> 2 m / s https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq12990.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> . Comprehensive measurements documenting the as-is-state will be performed in spring 2014 before the revitalization works are started.
