Challenges of Decision Making in the Context of Uncertain Forecasts in France

Abstract

Flood is a major risk in France and an operational hydrometeorological organization (a team of 450 employees for national coordination and local services) is in place since 2003 for surveying the main river courses, about 22,000 km which can be damaged by flash flood, fluvial flood, and coastal flood hazard. Besides the building of a national network of hydrometric station, with real-time access of data on the Internet, the services are responsible for flood vigilance, over the next 24 h, and for more detailed forecasts at shorter lead-time. The information is disseminated since 2006 on the www.vigicrues.gouv.fr website. Hydrological and hydraulic deterministic models are used to increase the forecast lead-time for some 500 hydrometric stations located close to vulnerable flood areas. Within the set of information integrated into the operational procedure, ensemble meteorological forecasts are used by the hydro-forecasters to evaluate rainfall distribution in the following days. Besides those routine activities, the European Flood Awareness System (EFAS) platform, the ensemble based forecasting system, has been tested since 2009 for some 60 flood events to study how the system can be best used within the French flood vigilance and warning procedure. During the first years of the evaluation, the results stayed quite difficult to be accounted for in the operational activities, as the number of false alarms and missed events was too high. Since 2013, the tendency is completely different, with the production of Flash Flood reports for watersheds smaller than 3,000 km². The localization of the impacted area and the timing is better foreseen; this tendency should be even improved when real-time discharges of main gauging stations will be transferred to EFAS.