Rhodamine WT tracer experiments to check flow measurements in sewers
Résumé
Flow meters are widely used in urban hydrology to assess discharges, volumes and transport of pollutant loads but may deliver wrong or biased results if they are not checked appropriately. In order to help operators to check flow meters and their discharge values, an operational protocol based on Rhodamine WT tracer experiments is proposed, tested and applied to various case studies. It includes a detailed uncertainty assessment, the reduction of the uncertainty in the injected mass of tracer and a complete data processing. Using Rhodamine WT as a tracer offers the following advantages: i) low injection volume and mass, allowing tracer experiments even for high flow conditions, ii) absence of Rhodamine WT in wastewater, which ensures low and stable background signal, iii) on-line data acquisition at short time step with a portable fluorimeter. Tests show that the protocol provides accurate flow measurements when compared to reference values (electromagnetic flow measurements and salt tracer experiments), with repeated tracer injections giving discharge values with relative standard uncertainties of approximately 5%. Field applications confirm it is an efficient approach to improve the quality of flow measurements in sewers. After an in situ flow meter is checked for various values of discharge, a correction function can be established if necessary for each specific measurement site if there is no alternative solution to improve the measurements by changing the location and position of the sensor or by replacing it by a more appropriate technology. In its present state, the protocol can be applied as a routine method. As a complement, CFD modelling is applied to one of the case studies to explain the causes of bias in flow meter measurements.