Advances and challenges of the French operational flash flood warning system, Vigicrues Flash
Authors: Demargne Julie , Fouchier Catherine , Organde Didier , Piotte Olivier , Belleudy Anne 
Published: March 2021

&lt;p align=&quot;justify&quot;&gt;&lt;span&gt;Since March 2017, t&lt;/span&gt;&lt;span&gt;he French flash flood warning system, Vigicrues Flash, provides warnings for small-to-medium ungauged basins for about 10,000 municipalities to help emergency services better mitigate potential impacts of ongoing and upcoming flash flood events. Set up by the Ministry in charge of Environment, this system complements flood warnings produced by the Vigicrues procedure for French monitored rivers. Based on a discharge-threshold flood warning method called AIGA, Vigicrues Flash currently ingests radar-gauge rainfall grids at a 1-km resolution into a conceptual distributed rainfall-runoff model. Real-time peak discharge estimated on any river cell are then compared to regionalized flood quantiles (estimated with the same hydrological model). Automated warnings are issued for rivers exceeding the high flood and very high flood thresholds (defined as years of return periods) and for the associated municipalities that might be impacted. This service shares a web platform for the dissemination and communication of early warnings and hazard map displays with the APIC heavy rainfall warning service from M&amp;#233;t&amp;#233;o-France. &lt;/span&gt;&lt;/p&gt;&lt;p align=&quot;justify&quot;&gt;&lt;span&gt;To better anticipate flash flood events and extend the coverage of the Vigicrues Flash service, the hydrological modeling is being enhanced within the SMASH &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;S&lt;/span&gt;&lt;span&gt;patially-distributed &lt;/span&gt;&lt;span&gt;M&lt;/span&gt;&lt;span&gt;odelling and &lt;/span&gt;&lt;span&gt;AS&lt;/span&gt;&lt;span&gt;similation for &lt;/span&gt;&lt;span&gt;H&lt;/span&gt;&lt;span&gt;ydrology) &lt;/span&gt;&lt;span&gt;platform developed by INRAE (formerly Irstea). For the upcoming operational update of Vigicrues Flash, a simplified distributed hydrologic model is continuously run at a 15-minute time step and a 1-km resolution. It includes only 2 parameters per cell, controlling respectively a production reservoir and a transfer reservoir from the G&amp;#233;nie Rural (GR) conceptual models. Cross-validation and regionalization of these two parameters have been improved to better account for basins spatial heterogeneities while optimizing flash flood warning performance. Evaluation results for 921 French basins on the 2007-2019 period show improvements in terms of flash flood event detection and effective warning lead time. Current developments aim to integrate a cell-to-cell routing component and improve parameters estimation at the national scale with the variational calibration schemes recently developed on the SMASH platform by Jay-Allemand et al. (2020). Challenges of including high-resolution precipitation nowcasts and accounting for the hydrometeorological uncertainties via data assimilation and ensemble forecasting are also discussed based on ongoing SMASH research.&lt;/span&gt;&lt;/p&gt;&lt;p align=&quot;justify&quot;&gt;&amp;#160;&lt;/p&gt;&lt;p align=&quot;justify&quot;&gt;Jay-Allemand, M., Javelle, P., Gejadze, I., Arnaud, P., Malaterre, P.-O., Fine, J.-A., and Organde, D.: On the potential of variational calibration for a fully distributed hydrological model: application on a Mediterranean catchment, Hydrol. Earth Syst. Sci., 24, 5519&amp;#8211;5538, https://doi.org/10.5194/hess-24-5519-2020, 2020.&lt;/p&gt;