EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Low-cost river discharge measurements using a transparent velocity-head rod

Aurélien Despax, Jérôme Le Coz, Francis Pernot, Alexis Buffet, and Céline Berni
Aurélien Despax et al.
  • INRAE, RiverLy, Lyon, France (

The common streamgauging methods (ADCP, current-meter or tracer dilution) generally require expensive equipment, with the notable exception of volumetric gaugings and floats, which are however often difficult to implement and limited to specific conditions. The following work aims at testing and validating a reliable, easy-to-deploy and low-cost gauging method, at a cost typically below 40 € each.

The “velocity-head rod” firstly described by Wilm and Storey (1944), made transparent by Fonstad et al. (2005) and improved by Pike et al. (2016) meets these objectives, for wading gauging with velocities greater than 20 cm/s typically. The 9.85 cm wide clear plastic rod is placed vertically across the stream to identify upstream and downstream water levels using adjustable rulers. The difference in level (or velocity head) makes it possible to calculate the average velocity over the vertical, using a semi-empirical calibration relationship.

Experiments carried out in INRAE’s hydraulic laboratory and in the field have enabled us to find a calibration relationship similar to that proposed by Pike et al. (2016) and confirm the optimal conditions of use. The average deviation to a reference discharge has been found to be close to 5 % except for very slow-flow conditions. The influence of the width of the rod on the velocity-head was studied in the laboratory. The uncertainty of the velocity due to the reading of water levels has been estimated. It increases at low velocity due to decreasing sensitivity, and increases at high velocities due to water level fluctuations that are difficult to average.

Several improvements were tested in order to facilitate and improve the measurement operations, without increasing the cost too much: magnetic ruler, removal of a graduated steel rule (expensive), plastic ruler with water level and velocity graduations, reading the depth with another ruler, spirit level, electrical contact (so the operator has not to bend to the surface of the water). An operational procedure and a spreadsheet for computing discharge are proposed. The method being extremely simple and quick to apply is well suited for rapid estimates of flow (instead of floats), training or demonstrations, citizen science programs or cooperation with services with limited resources.

Acknowledgments: The authors thank Q. Morice, J. Cousseau, Y. Longefay (DREAL) who were involved in this study by carrying out field tests.

How to cite: Despax, A., Le Coz, J., Pernot, F., Buffet, A., and Berni, C.: Low-cost river discharge measurements using a transparent velocity-head rod, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5684,, 2020


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  • CC1: Comment on EGU2020-5684, Robin Pike, 28 Apr 2020

    Great slide deck Jerome! We are very happy to see this tool and equation further verified by your team.  Independent verification important as we have encountered some disbelief among hydrometry professionals that something so simple, can provide results that are useful (and in many instances very accurate). Like every hydrometric tool, this method has it’s place and optimal conditions.  


    Some comments on the slide deck:

    Slide 5: One of our observations is that delta h is not only a factor of the super-elevated head on the upstream side of the rod, but also from the extended depression formed in the lee that increases the total delta H (hence the need for the calibrations in field and lab to known velocities).


    Slide 9: One operational comment, for VH > 50mm (which is beyond your graph axis limits) it is virtually impossible (and dangerous) to hold the rod perpendicular to flow, without it first being set into the streambed.  In Pike et al. 2016, we did not attempt calibrations of the mTVHR beyond 120 mm VH /~ 1 m/s as we found that operator conditions at highest velocities became dangerous when instrument kicked back if the streambed anchor was lost.   It only took a couple of sore shins to learn of the importance of setting the instrument firmly on the streambed.  


    Slide 10:  Sensitivity of instrument width to the calibrated relationship is something we pondered, and I am happy to see this slide.  We thought width might affect the formation of the depression on the lee of the instrument and hence the calibrated equation.   Pike et al. 2016 stuck with the 9.85cm width as provided in Fonstad et al. 2005 as this was a good width for field use. In our field trials, we observed that 9.85 cm width was a good at taking representative velocities in our mid-section panels, similar to our comparative standard.  I postulate that wider rods > 10cm may have issues in representativeness to other instruments if they are measuring different (wider proportion of the) velocity panel that is comprised of perhaps two different flow velocities (i.e., seams).   Additionally, we can to realize the application of the instrument may be greater for very small streams ( < 2m wetted width) and wider instruments would unnecessarily reduce the number of panels that could be measured with the instrument.  


    Slide 13:  Awesome slide.  This really shows the sweet spot of the method.

    • AC1: Reply to CC1, Aurélien Despax, 01 May 2020

      Thanks for your comments and appreciation, Robin. It's a great tool and we relied a lot on your advances and those made earlier by Mark Fonstad.

      Slide 5. Right, the depression on the lee of the rod contributes to observed DH higher than the actual velocity head, hence the need for a correction. However, the depression seems not to be large enough to explain the full difference.

      Slide 9. Agreed. The idea behind this laboratory test was to check whether the flow contributing to the observed superelevation extended over the full flow depth, or mainly in the top of the water column. It showed that the flow has to be blocked over the full depth to create the correct water superelevation. However, the tested configurations with 20% or 60% of free flow under the rod are not realistic situations in the field. Given the results, it would be interesting to test the effect of small underflows that may occur when the rod is set on top of gravels, for instance.

      Slide 10. Yes, wider rods would bring practical issues, especially for narrow streams. We hoped that wider rods would be more sensitive to slow velocities (<20 cm/s) but our findings suggest that we'd need much wider, and certainly too wide rods for this. We also tried to find less streamlined designs (like concave, semi-circular rods) but the sensitivity was not significantly increased.

      On the other hand, narrower rods (micro-rods?) may be advantageous for very narrow streams. Our data are not high quality but they suggest that a twice narrower rod would have a similar velocity rating as the 9.85 cm standard.

      We have several perspectives of disseminating the tools to non-hydrometry-specialist professionals, especially some irrigation system managers and some river board technicians. I'm sure it has the potential of extending the range of discharge data, especially for improving water resource allocation and drought management.

      • AC2: Reply to AC1, Aurélien Despax, 01 May 2020

        Sorry, I realise I'm usurpating Aurelien's identity while replying to your comment, Robin... This is Jérôme Le Coz.

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