Amengual et al. (2024) Hydrometeorological controls of and social response to the 22 October 2019 catastrophic flash flood in Catalonia, north-eastern Spain

Identification

• Journal: Natural hazards and earth system sciences
• Year: 2024
• Date: 2024-07-01
• Authors: A. Amengual, R. Romero, María Carmen Llasat, Alejandro Hermoso, Montserrat Llasat-Botija
• DOI: 10.5194/nhess-24-2215-2024

Research Groups

• Grup de Meteorologia, Departament de Física, Universitat de les Illes Balears, Palma, Mallorca, Spain
• Grupo de Análisis de situaciones Meteorológicas Adversas (GAMA), Departament de Física Aplicada, Universitat de Barcelona, Barcelona, Spain
• Department of Environmental Systems Science, Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland

Short Summary

This study comprehensively investigates the hydrometeorological controls and social response to the catastrophic 22 October 2019 flash flood in Catalonia, Spain, finding that local orography and dry initial soil conditions modulated the extreme event, while public perception of risk and understanding of warnings were notably low despite effective dissemination.

Objective

• To comprehensively investigate the hydrometeorological controls and social response to the 22 October 2019 catastrophic flash flood in the Francolí River basin, Catalonia, Spain.
• To identify the leading physical mechanisms responsible for the onset and evolution of the convective systems.
• To investigate the main features of the heavy precipitation event (HPE) over the Francolí catchment and assess basin response to the torrential rainfall.
• To quantify human behaviors, perceptions, and reactions during the flash flood and evaluate the effectiveness of current social protocols to improve future preparedness.

Study Configuration

• Spatial Scale: Francolí River basin (approximately 858 km²) in Catalonia, north-eastern Spain. Atmospheric model domain covers the western Mediterranean (horizontal resolution 4.5 km, equivalent to 3 km for a square mesh). Hydrological model uses a 25 m grid size.
• Temporal Scale: The study focuses on the 22 October 2019 flash flood event. Atmospheric simulations and radar-derived precipitation estimates cover 48 hours from 22 October 2019 00:00 UTC to 24 October 2019 00:00 UTC. Social response analysis covers the period from 21 October 2019 08:00 UTC to 24 October 2019 00:00 UTC.

Methodology and Data

• Models used:
  • Triangle-based Regional Atmospheric Model (TRAM) for mesoscale atmospheric simulation.
  • Kinematic Local Excess Model (KLEM) for hydrological response, using the Soil Conservation Service curve number (SCS-CN) method for runoff computation.
• Data sources:
  • ERA5 grid reanalyses for TRAM initial and lateral boundary conditions.
  • Barcelona Doppler C-band radar (AEMET) for Quantitative Precipitation Estimates (QPEs) (1 km spatial, 10 min temporal resolution).
  • Automatic pluviometers (SMC, AEMET, Meteoprades) and daily pluviometers (AEMET) for precipitation observations.
  • Automatic stream-gauge stations (Catalan Water Agency, ACA) for streamflow data (5 min temporal resolution).
  • Post-flood field observations and hydraulic modeling (Martín-Vide et al., 2023) for peak discharge estimations.
  • Citizen science campaign ("Floodup Francolí") involving post-event interviews to assess human behavior, perceptions, and warning effectiveness.
  • Socioeconomic data from news reports and compensation payments by Consorcio de Compensación de Seguros (CCS).
  • Laser imaging detection and ranging-derived Digital Elevation Models (Spanish National Geographic Institute).
  • Lithology information from the Geological and Mining Institute of Spain (IGME).
  • Land use maps from the CORINE Land Cover project (EEA).

Main Results

• A persistent south-easterly airflow, combined with local orography, was instrumental in triggering deep moist convection and a quasi-stationary convective train, leading to extreme precipitation over the Francolí catchment headwaters.
• The 48-hour accumulated radar-estimated precipitation exceeded 300 mm in the north-western mountainous headwaters (approximately 25 km²), with a maximum 10 min rainfall accumulation of approximately 21.0 mm and a total of 299.5 mm.
• The basin's hydrological response was significantly modulated by very dry initial soil moisture conditions, leading to a low runoff ratio (0.1 for the entire basin) despite high precipitation volumes. Early rainfall stages were crucial for moistening topsoil and reducing infiltration rates, enabling subsequent acute infiltration-excess runoff.
• The peak discharge at the Tarragona station reached 871.0 m³/s, increasing by more than 13 times in just 15 minutes, highlighting a very flashy basin response.
• Lag times were relatively long for smaller basin scales (up to 100 km²), exceeding typical European flash flood minimums due to initial soil moisture replenishment and runoff threshold exceedance. However, a sudden release of a wood debris jam at a bridge caused an anomalous surge, leading to exceptionally high channel velocities (10.2 m/s) and sharply reduced lag times for larger basin scales (>350 km²).
• The institutional warning cycle (organization, protection, prevention) was initiated 24-30 hours before the peak flow, operating at meteorological spatial and temporal scales.
• While warnings effectively reached over half the population in affected areas, one-third of respondents felt uninformed, and over half did not understand the warning levels. A significant majority (66%) did not perceive any threat to their homes or properties, and none felt their personal safety was at risk, despite the event causing six fatalities in the basin.

Contributions

• Provides a comprehensive, integrated analysis of both physical (hydrometeorological) and human (social response) dimensions of a catastrophic flash flood, addressing a knowledge gap in existing literature.
• Highlights the critical role of local orography and antecedent soil moisture conditions in modulating extreme flash flood events in the Mediterranean, including the non-linear basin response and the impact of rainfall variability.
• Quantifies the effectiveness and limitations of current warning systems and public risk perception during a real-world catastrophic event, revealing discrepancies between warning dissemination and public comprehension/risk perception.
• Identifies specific areas for improving social preparedness and mitigation actions, including enhancing observational networks, integrating advanced hydrometeorological ensemble prediction systems, revising urban planning, and conducting targeted public awareness campaigns.
• Offers insights into the complex interaction of woody debris with infrastructure during flash floods and its impact on flood dynamics, particularly on downstream flood velocity and lag times.

Funding

• Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades: TRAMPAS (grant no. PID2020-113036RB-I00/AEI/10.13039/501100011033)
• Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades: C3RiskMed (grant no. PID2020-113638RB-C22/MICIN-AEI/10.13039/501100011033)
• AGORA project, funded by the ACA of the Generalitat de Catalunya, with significant support from the Museu de la Vida Rural de l’Espluga de Francolí.

Citation

@article{Amengual2024Hydrometeorological,
  author = {Amengual, A. and Romero, R. and Llasat, María Carmen and Hermoso, Alejandro and Llasat-Botija, Montserrat},
  title = {Hydrometeorological controls of and social response to the 22 October 2019 catastrophic flash flood in Catalonia, north-eastern Spain},
  journal = {Natural hazards and earth system sciences},
  year = {2024},
  doi = {10.5194/nhess-24-2215-2024},
  url = {https://doi.org/10.5194/nhess-24-2215-2024}
}