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REMOTE MEASUREMENT OF THE RIVER FLOW SURFACE VELOCITY AT HIGH WATER WITH LARGE SCALE PARTICLE IMAGE VELOCIMETRY METHOD
Abstract
Measurements of high water in rivers are an important element of all collected stream discharge information. It is necessary to have such information in real time during floods because of the danger caused by inundations. Another aspect is the relatively large discharge volume of high water compared to medium and low flow periods. On the other hand, measuring the river discharge during high water with the traditional methods with propeller or floats is often constrained by the manpower availability for that precise time and the higher difficulties related to the measurement itself. Therefore, it is necessary to analyse and experiment with alternative remote and non-contact methods for measuring velocities and calculating discharge during high water. These include methods based on measuring the surface velocity of the flow in a cross section with video surveillance, recording and analysis of video frames – Large Scale Particle Image Velocimetry (LSPIV) method. This paper presents the first experiments of non-contact surface water velocity calculation in Bulgaria using the LSPIV method applied to a section of Varbitsa River near the town of Kardzhaly. The application uses a statically mounted video camera to record natural floating tracers in a flow area up to hundred square meters. The video clip is analyzed with Fudaa-LSPIV software to calculate surface velocity field and stream discharge of the river flow at a certain moment. We describe the process of site preparation, the necessary instruments and video clip processing methodology. The described method permits to implement a cheap and robust way of water surface velocity and stream discharge determination at remote locations where a video camera can be installed, maintained, and remotely accessed. The results suggest that LSPIV is accurate and well-suited for flow fields with small, medium, and high velocities only when there are clearly visible floating tracers. LSPIV showed the potential to become competitive method compared to conventional velocity measurement with propeller or floats. Moreover, it is shown that under certain limits, the measured maximum surface water velocity is strongly related to the stream discharge through the area of the cross-section and the mean velocity. Assuming that the river cross-section profile remains relatively stable, two preliminary investigated regressions were used: first, between the gauge height (stage) as independent variable and the area of the cross-section; and second, between the maximum and mean water velocities. The knowledge about these relations allows for the measurement of maximum surface velocity to be converted to stream discharge flowing through the profile in quasi-real time.
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