Flood of 23 November 2019 on the French Riviera: a comparison of discharges estimated by image analysis and hydrological modelling

The short and heavy rainfall events observed on the French Riviera often lead to flash floods on coastal catchments: during the flood of 2 October 2015, a peak discharge between 185 and 295 m(3)/s was estimated on the Brague at Biot at 8:00 PM, whereas the flow rate was about 1 m(3)/s at 5:30 PM there. While measurements of such discharges are very important (for statistical flood analysis, flood modelling, design of hydraulic structures), they are dangerous when they require an operator to handle an instrument in or near the river. Alternative methods can be used, such as video analysis. In this work, we applied two different image velocimetry (LSPIV) algorithms (Fudaa-LSPIV and OpyFlow) to several videos of the 23 November 2019 floods within the Brague catchment, in order to estimate discharge values. The discharge values obtained were then compared with the values observed on the available gauging stations, as well as to the results of rainfall-runoff modelling on the same sections. The LSPIV estimates, discharge simulations and observations are consistent across the studied sections, showing the interest of combining such different and independent techniques to estimate flood discharge values.