Laboratory testing of a new thermal tracer for infrared-based PTV technique for shallow overland flows

The accurate measurement of velocities of shallow overland flows normally found in the field of hydrology is essential to understand the kinematics of these flows. In view of this, we have tested the capability of a new thermal particle tracer (cold oil droplets) under controlled laboratory conditions to measure the velocity of shallow overland flows (<2 mm depth) using the infrared-based particle tracking velocimetry (PTV) technique. Experimental apparatus comprised a soil flume 3 m long and 0.3 m wide with a feeder tank attached to the upstream end. Flow velocities acquired through infrared-based PTV were compared with corresponding values given by the volumetric discharge method, dye tracing, and conventional PTV (using polystyrene beads as tracer) for eleven flow conditions that combined different slopes (5, 7, 10 and 15%) and discharges (0.15 to 0.51 L s−1 m−1). The Reynolds number (569 to 1938) marked it as quasi-laminar flow. The results showed that acquired flow velocity given by through infrared-based PTV and conventional PTV techniques was closer to the mean velocity estimated by the volumetric discharge method than it was to dye tracing estimated velocity. The mean correction factor value (0.78) estimated by the infrared-based PTV technique was higher than the dye tracing estimated correction factor value (0.48). Furthermore, the detection of the leading edge of the dye tracer was difficult and the stagnation of polystyrene beads at lower discharges resulted in an underestimation of mean velocity. This research stresses the potential of using thermal imaging to estimate the velocity of shallow water bodies, because of the tracer's conspicuous visibility and independence from illuminating conditions.