Author(s): Paul J. Kinzel, Carl J. Legleiter, Jonathan M. Nelson, Jeffrey S. Conaway
Linked Author(s): Paul Kinzel
Keywords: Infrared imaging, particle image velocimetry, remote sensing, river discharge, surface water hydraulics
Abstract: Thermal infrared cameras with high sensitivity to medium and long wavelengths can resolve features at the surface of flowing water that arise from turbulent mixing. Images acquired by these camera can be processed with particle image velocimetry (PIV) or similar optical flow software to compute surface velocities based on the displacement of thermal features as they advect with the flow. We conducted a series of field measurements to test this methodology for remote sensing of surface velocities in rivers. We positioned an infrared video camera at multiple stations across bridges that spanned five rivers in Alaska: Chena, Knik, Matanuska, Montana and Salcha. Simultaneous non-contact measurements of surface velocity were collected with a radar gun. In situ velocity profiles were collected with ADCP�s. Infrared image time series were collected at a frequency of 10Hz for a one minute duration at a number of stations spaced across each bridge. Commercial PIV software used a cross-correlation algorithm to calculate pixel displacements between successive frames, which were then scaled to produce surface velocities. A blanking distance below the ADCP prevents a direct measurement the surface velocity. However, we estimated surface velocity from the ADCP measurements using a program that normalizes each ADCP transect and combines those normalized transects to compute a mean measurement profile. The program extrapolates this profile to the surface and in so doing so provides a velocity index, which is the ratio between the depth-averaged and surface velocity. For the rivers in this study, the velocity index ranged from a commonly used value of 0. 85 to 1. 0. Average extrapolated ADCP surface velocities were are in good agreement with average infrared PIV calculations (<10% difference). Average radar and PIV measurements were are also similar (<10% difference). The infrared PIV methodology provides a more spatially explicit depiction of surface velocity than radar and a characterization of turbulent fluctuations. If applied to a river reach, this kind of thermal imaging can provide ancillary benefits such as characterizing macro-scale hydraulic features and identifying surficial thermal interfaces
Year: 2017