When geomorphological field data and systemic analysis help refine the uncertainties of numerical hydrometeorological models in extreme values. Case study: The catastrophic flood event of October 14-15, 2018, in the Aude watershed (southern France)

A detailed systemic analysis of the hydrometeorological event of Oct. 14-15, 2018, made it possible to identify, for the first time, all the predisposing, triggering, and aggravating/mitigating factors that explain the genesis, impacts, and management of the event in the Aude watershed. Attention then focused on one of the parts of the watershed that received the highest cumulative rainfall, namely the Rieu Sec watershed, a right-bank tributary of the Orbiel River in the Minervois. It has been shown that the Oct. 2018 event was extraordinary in several respects (meteorology; hydrology; geomorphology; sediment budget; specific degradation rate; impact on riparian vegetation; recurrence interval estimated at 204-378 yrs). A model of small, mountainous watersheds (< 40 km2) functioning is proposed, in which high-magnitude/low-frequency flood events play a key role. The significant variability in precipitation observed from one sub-watershed to another complicates the management of rainfall and floods. The challenge for the future lies in improving flood forecasting, and the effective dissemination of these forecasts.