IAHS2022-383, updated on 23 Sep 2022
IAHS-AISH Scientific Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Towards a hydrological consensus about the 2nd – 3rd October 2020 ALEX storm event in the French “Alpes Maritimes” region

Frederic Pons1, Laurent Bonnifait1, David Criado1, Olivier Payrastre2, Felix Billaud3, Pierre Brigode4, Catherine Fouchier5, Philippe Gourbesville6, Damien Kuss7, Nathalie Le Nouveau8, Olivier Martin9, Stan Nomis10, Emmanuel Paquet11, and Bernard Cardelli12
Frederic Pons et al.
  • 1Cerema, Aix-en-Provence, France
  • 2GERS-LEE, Univ Gustave Eiffel, IFSTTAR, Bouguenais, France
  • 3REGIE EAU D’AZUR, Nice, France
  • 4Université Côte d’Azur, CNRS, OCA, IRD, Géoazur, France
  • 5INRAE, RECOVER, Aix-Marseille Université, Aix-en-Provence, France
  • 6Université Côte d’Azur, Polytech Nice Sophia, Nice, France
  • 7ONF/RTM, Chambéry Cedex, France
  • 8MNCA, Service Gemapi, Nice, France
  • 9DREAL PACA, Service Hydrométrie, Aix-en-Provence, France
  • 10SMIAGE, Pôle étude et conception, Nice, France
  • 11EDF-DTG, Grenoble, France
  • 12DDTM des Alpes-Maritimes, Nice, France

After having swept over western France, the ALEX storm led to an exceptional Mediterranean rainfall event which hit the “Alpes Maritimes” region during the night of the 2nd to 3rd October 2020. The rainfall accumulations observed on 12 to 24 hours durations were unique in this region, with a record of 663mm in 24h (EDF raingauge, Les Mesces).

Form West to East, several valleys, mainly those of Tinée, Vésubie, and Roya were affected by major floods, landslides, sediment transport and geomorphological changes. The hydrometric network was almost destroyed. The human and material damages were considerable, with many fatalities and missing people, several villages largely destroyed, and important destructions of communication and transport networks.

A lot of technical post-flood surveys were launched by national authorities to gather a detailed knowledge of the event characteristics, with regard to rainfall accumulations, water discharges, description the torrential phenomena, and inventory of damages. This communication is focused on the question of water discharges.

National and local authorities and organisms, universities and companies, were involved in different post-flood surveys aiming at gathering information on the peak discharges and the hydrographs of the floods, for their own needs and/or within structured programs (Administrative survey, HYMEX research project www.hymex.org).

Several kind of discharge field estimations were provided using field survey measurements, satellites images, post-event Lidar data, combined with hydraulic estimations based on hydraulic formulas, and 1D/2D hydraulic models. Several teams also applied hydrological models based on radar quantitative precipitation estimates, to calculate hydrographs at different basins outlets.

To combine and draw a uniform synthesis of all these results, a consensus exchange was launched to share the knowledge gathered by the different data providers. The objective was to compare, assess, and propose common intervals of peak discharges in the different impacted valleys. We also evaluated for each valley the return period of the final interval of discharge established by the consensus.

The final product is an official administrative document, established at the end of October 2021 by the French state authorities, providing the peak discharge values to be used for post flood studies, reconstruction, and prevention measures.

How to cite: Pons, F., Bonnifait, L., Criado, D., Payrastre, O., Billaud, F., Brigode, P., Fouchier, C., Gourbesville, P., Kuss, D., Le Nouveau, N., Martin, O., Nomis, S., Paquet, E., and Cardelli, B.: Towards a hydrological consensus about the 2nd – 3rd October 2020 ALEX storm event in the French “Alpes Maritimes” region, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-383, https://doi.org/10.5194/iahs2022-383, 2022.