Introduced by Fonstad et al. (2005) and modified by Pike et al. (2016), the transparent velocity-head rod is an inexpensive, easy and quick to use streamgauging tool, providing reliable discharge measurements under the right application conditions (velocity greater than 20 cm/s over most of the cross-section, operator stable within the flow). The principle is simple: the water level difference between upstream and downstream faces of a board blocking the flow increases with the velocity-head related to the depth-average flow velocity. Using a semi-empirical calibration, the velocity can be calculated from the water level difference. So, discharge can be measured like with a conventional current meter on a wading rod, and more quickly. This calibration is valid up to 120 cm/s approximately.
The model developed by INRAE in 2020 (Le Coz et al., 2021) is a little more expensive (~88 € excl. taxes each to manufacture a batch of 30 units) but it is still relatively easy to build and significantly improves ergonomics and measurement. Comparison experiments with reference measurements performed in a hydraulic laboratory and at different field sites (Pernot, 2018) confirmed the semi-empirical calibration relationship established by Pike et al. (2016), and discharge measurements are generally within 10% of the reference discharge. However, operator effect related to the handling of the rulers and the reading of the water level difference can lead to larger deviations, which requires training and attention at this stage of the measurement process.
As the method is extremely simple and quick to apply, it is well suited for rapid discharge estimates, training or demonstration, citizen science programmes or cooperation with services with limited resources and/or not specialised in hydrometry. We distribute the instrument, a procedure and a spreadsheet for calculating velocities and discharge. We help the deployment of the technique by different operators (scientists, environmental agencies, river boards, irrigation boards, agricultural agencies, …). Do not hesitate to contact us if you are interested! (contact: J. Le Coz, M. Lagouy).
Fonstad, M., Reichling, J., Van de Grift, J., 2005. The transparent velocity-head rod for inexpensive and accurate measurement of stream velocities. Journal of Geoscience Education, 53(1):44–52.
Le Coz, J., Lagouy, M., Pernot, F., Buffet, A., Berni, C. (2021). Jaugeages à bas coût par perches transparentes (Low-cost streamgauging using transparent velocity-head rods), SHF, Colloque Hydrométrie 2021, Montpellier, 23-24 novembre 2021.
Pernot, F., 2018. Elaboration de systèmes de jaugeage à bas coût (Development of low-cost streamgauging systems), Mémoire de stage de fin d’étude. Irstea / Polytech Nice-Sophia, 48 p.
Pike, R., Redding, T., Schwarz, C., 2016. Development and testing of a modified transparent velocity-head rod for stream discharge measurements. Canadian Water Resources Journal, 41(3):372–384.