An ADCP large-scale international intercomparaison: Sault-Brénaz 2023

In the last decades, Acoustic Doppler Current Profilers (ADCP) has become the most widely used tool for measuring discharge of rivers and canals. Discharge is a key information for many risk studies, structures dimensioning and even impact assessments. These values must therefore be correctly estimated. Quality assurance and quality control (QA/QC) procedures have been established to ensure that hydrological services share the best practices. Also, to ensure that ADCP tools are working properly, services has to regularly check that the equipment is properly calibrated.

The lack of references value most of the time makes the task difficult. To make sure that ADCP is properly calibrated, interlaboratory testing are frequently organized. Large-scale intercomparaisons are particularly interesting because of the diversity of models and practices but it also makes them more complicated to organize. The Sault-Brénaz intercomparaison was definitively a big one with more than 120 European participants with 16 RiverPro, 15 M9, 15 StreamPro and 12 RS5 for a total of 160 measurements with 1870 transects among 4 sessions. Due to hydrological conditions, the protocol had to be adapted. Measurements took place on small straight canal of the Rhone river with a discharge of around 2m³/s.

Following QA/QC procedures, participant had to post-process their data with the QRevInt open-source software. QRevInt provides many quality filters and computes uncertainty following OURSIN method. Then, to compute interlaboratory results, the QRame software has been used. This open-source software has been developed to apply QRevInt with default settings to a set of ADCP discharge measurements and to retrieve post-processed discharge and uncertainty results. When the dataset is actually an ADCP interlaboratory experiment, the empirical discharge uncertainty, for a given number of transects taken in the average, can be computed by application of the standard interlaboratory method.

Results show that discharge varied slightly over time, particularly between sessions. To exploit further all the discharge results, different approaches to homogenizing data were tested. This issue of varying discharge over time is a common issue for interlaboratory experiments. A generalizable solution would enable experiments in extended conditions. Also, interlaboratory experiments permit to validate uncertainty computations. The greater the number of intercomparisons and the wider the measurement conditions, the more robust uncertainty models will be.