Tips for non-contact stream gaging using captured video
The United States Geological Survey (USGS) and Environment and Climate Change Canada (ECCC) are exploring the possibility of using large-scale particle image velocimetry (LSPIV) to obtain measurements of surface velocity in streams and rivers. It is envisioned that LSPIV could be a valuable tool for measuring discharge when traditional measurement techniques are not possible, for verification of theoretical measurements, or as a “backup” to direct measurements of discharge. For example, this method might be especially suited to streams that experience very rapid changes in stage (and discharge), such as those that experience flash flooding. LSPIV may also be used to measure surface velocities for model calibration or other hydraulic studies.
The following guidelines have been developed based on the recommendations and experience of staff at the USGS, the ECCC and researchers at the Center for Water Research and Technology (CETA) of the National University of Cordoba, Argentina. These guidelines are expected to evolve as we learn from our experiences.
An example field note and a handy on-site video checklist have been provided and should be completed for measurements to be submitted to the Surface Velocity Work Group:
Images illustrating the application of Large Scale Particle Image Velocimetry. a) Mean displacement fields of the water surface of the Picso River, Pisco, Peru. b) Mean displacement fields of the flow over the dam spillway of Arroyo Corto River, Córdoba, Argentina
Any camera sensor can be used that has video resolution larger than 640 x 480 Pixels (includes most smart phones). If higher definition settings are available, use them. Video should be captured at a rate of 15 frames per second (fps) or faster. For high velocity flows (a dam spillway for example), even higher video fps settings/capabilities are preferred and potentially necessary. Cameras capable of rates upward of 60 fps are becoming commonplace consumer products. Videos captured should be as stable as possible, either by supporting the camera on/against a fixed object, such as the railing of a bridge or ideally by using a tripod. Panning and zooming with the camera should avoided.
Example set up where the digital camera is supported on a railing of a lock gate.
Lenses that distort the image should not be used, such as a wide angle lens. When using a zoom lens, be careful not to use the wide-angle setting. In the event that such a lens is used, it is important to record and save the lens type information with the data, so that corrections for distortion can be made during post-processing.
Identify a suitable location for obtaining video of the flow. Such a location should include 4 readily visible fixed reference (control) points. The distances between the control points can be measured either at the time the video is recorded, or during a subsequent visit after the water has receded. Video may be obtained from a bridge (Figure 3) or from the shore (Figure 4), being careful to avoid wake effects from piers.
Single frame of a video recorded from a bridge where the control points were rocks
Single frame of a video recorded from shore where the control points are tree trunks
Capture video over the entire width of the measurement cross section from as high an angle (close to 90°) as possible including banks from both sides in the image (Figure 5 and 6). Error in the LSPIV-processed results increases as the angle between the camera and the water surface decreases to 0° (at water surface level).
Single frame of a video recorded from a bridge where both banks are visible. Notice that the perspective of this video is neither oblique or normally oriented to the flow, but is instead at about a 45° angle to flow. This is not optimum, but valid results can still be obtained.
Single frame of a video recorded from a lock where both banks are visible.
Single frame of a video recorded where banks are not visible and no reference points.
Snapshot of a video recorded from a lock including the reference points.
Snapshot of a video recorded from shore including the reference points (CP1 through CP4 ).
Avoid reaches that are known to be susceptible to scour and fill as the streambed elevations cannot usually be measured at the time of the filming.
Avoid reflections, shadows, and sparkling patterns on filmed surface, such as those shown in figure 10. Polarizing lens filters can be used to mitigate or remove surface reflections.
Single frame of a video recorded from a bridge where reflections, large shadows, and sparkling patterns are observed.
Record the location coordinates of the video. Ideally this would be the latitude/longitude obtained from an accurate GPS. However, very accurate descriptions of the measurement location would also be acceptable.
Record the exact date and time of the recording, using the time zone of the recorder at the site (if obtained from a streamgaging station).
Results of the LSPIV analysis process during a flash flood event in the San Antonio River, Córdoba, Argentina. a) Snapshot at the beginning of the event. b) Results of LSPIV processing at the beginning of the event. c) Mean flow velocity time evolution of the recorded event. d) Discharge time evolution of the recorded event and trendline.
A. Patalano, C. M. Garcia, W. Brevis, T. Bleninger, N. F. Guillén, L. Moreno, A. Rodriguez, (2015), “Recent Advances In Eulerian And Lagragian Large‐Scale Particle Image Velocimetry”, 36th IAHR World Congress, The Hague, Netherlands, June 2015.
Results of the LSPIV process for many flow conditions in the San Antonio River, Córdoba, Argentina. a) Results of LSPIV processing at the beginning of an event. b) Velocity profile of the corresponding event. c) Discharge results from different conditions versus water surface elevation. ADCP results and Water National Institute (INA) rating curve are also plotted.
N. F. Guillén, A. Patalano, C. M. García, “Validación de la Técnica PIV a Gran Escala (LSPIV) para la estimación de caudales en ríos de montaña” in English: “Validation of LSPIV for flow discharge measurements in mountain rivers”, IV Simposio Sobre Métodos Experimentales En Hidráulica, La Plata, Argentina, March 2015.
Frank L. Engel
5563 De Zavala, Ste. 290
San Antonio, TX 78249
Robert R. Holmes, Jr.
1400 Independence Road
Mail Stop 100
Rolla, MO, 65401-2602
Head, Hydrometric Monitoring Technology Unit
Water Survey of Canada - Relevés hydrologiques du Canada
C. Marcelo Garcia
National University of Cordoba, Argentina
National University of Cordoba, Argentina
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