Vide et al. (2023) Large wood debris that clogged bridges followed by a sudden release. The 2019 flash flood in Catalonia.

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2023
• Date: 2023-03-30
• Authors: Juan Pedro Martín Vide, Allen Bateman, Marc Berenguer, Carles Ferrer‐Boix, A. Amengual, M. Campillo, Carles Corral, María Carmen Llasat, Montserrat Llasat-Botija, Santiago Gómez-Dueñas, B. Marín-Esteve, F. Núñez-González, Arnau Prats-Puntí, R. Ruiz-Carulla, R. Sosa-Pérez
• DOI: 10.1016/j.ejrh.2023.101348

Research Groups

• Department of Civil and Environmental Engineering, Technical University of Catalonia – BarcelonaTech, Spain
• Grup de Meteorologia, Departament de Física, Universitat de les Illes Balears, Palma, Mallorca, Spain
• Department of Applied Physics, University of Barcelona, Spain

Short Summary

This study reconstructs the catastrophic October 2019 flash flood in the Francolí River, Catalonia, focusing on the impact of large wood debris (LWD) at bridges. It demonstrates that the sudden release of an LWD jam at one bridge generated a destructive surge, leading to two fatalities and exacerbating downstream inundation.

Objective

• To reconstruct the October 2019 flash flood in the Francolí River, documenting rainfall, flood marks, channel changes, log drift, and woody debris at bridges.
• To demonstrate that the flood wave caused by the collapse (detachment) of a large wood debris jam at a bridge increased the peak discharge and exacerbated flood damages downstream, leading to fatalities.

Study Configuration

• Spatial Scale: Francolí River basin (838 km² catchment area) in Catalonia, Iberian Peninsula, including specific towns (L’Espluga de Francolí, Montblanc) and river reaches. Detailed analysis of the uppermost 100 km² of the basin and specific bridges (A, B, C, D, E, F, G, H).
• Temporal Scale: The flash flood event occurred on October 22-23, 2019. Fieldwork was conducted from October 2019 to March 2020. Rainfall data covered 00:00 on October 22nd to 12:00 on October 23rd.

Methodology and Data

• Models used:
  • Hydrological model (rainfall-runoff and flood routing) specifically developed for the study.
  • SCS abstraction method (curve number CN) and SCS unit hydrograph.
  • Kinematic wave model for flood routing.
  • 1D hydraulic models (Hec-Ras) for steady flow analysis.
  • 2D hydraulic models (IBER) for steady and unsteady flow analysis (depth-averaged Saint-Venant equations).
  • Application of various hydraulic principles: critical flow, Bernoulli equation, discharge equations for weirs and sluice gates, energy loss at bridge piers/decks, mass and momentum conservation in rapidly varied unsteady flow, drag force on obstacles, dam-break flow theory, and open channel surge theory.
• Data sources:
  • Extensive field work: topographical surveys of maximum water level marks, 3D drone flights (3 cm/pixel resolution) supported by ground topography, pre-event 2 m × 2 m Digital Elevation Model (DEM) for channel change analysis, direct observations, photographs, video recordings, witness interviews, damage reports, and casualty reports.
  • Rainfall data: 186 automatic rain gauges (Meteorological Service of Catalonia - SMC) and 9 rain gauges (Meteoprades amateur association).
  • Weather radar observations (SMC radar network, 4 single-polarization, C-band radars).
  • Combined rainfall reconstruction using kriging with external drift (1 km spatial resolution, 30 min temporal resolution).
  • Flow gauging stations from the Catalan Water Authority (Station A covering 348 km², Station B covering 823 km²).
  • National Centers for Environmental Prediction (NCEP) reanalysis for meteorological features.
  • Forestry Administration data for tree density and volume rates.

Main Results

• The flash flood was characterized by intense rainfall, peaking at almost 300 mm in a few hours (up to 40 mm in 30 min).
• The uppermost 100 km² of the basin produced discharges up to 700 m³/s, with local specific discharges reaching 50 m³/s/km². The peak flow at the river mouth (Station B) was 871 m³/s.
• The main river channel widened more than tenfold (from 2-5 m to 15-90 m), acting like a bulldozer and mobilizing an estimated 21,500 trees (6240 m³ of wood) from 70.6 hectares of devastated land. Significant scour (0.5-2 m deep, ≈16,850 m³ volume) was observed in alluvial beds.
• Three bridges failed (A, B, E), but a cascading failure was not proven.
• Bridge C was overtopped and initially clogged by large wood debris. The subsequent sudden release of this jam generated a destructive surge (estimated at approximately 1090 m³/s, 2D model result: 1064 m³/s at bridge D), which overtopped bridge D and caused two fatalities.
• Bridge F, located downstream, became heavily clogged by woody debris, worsening the inundation in the adjacent neighborhood f, where flood levels reached 2.50-2.75 m above ground.
• The flood wave velocity accelerated from 3.8-4.5 m/s upstream of bridge C to 10.2 m/s downstream, consistent with the theory of open channel surges caused by a sudden release.
• The event resulted in six fatalities and total damages of approximately 44 million euros.

Contributions

• Provides a comprehensive, multi-faceted reconstruction of a catastrophic flash flood event, integrating extensive field data, hydrological modeling, and advanced 1D/2D hydraulic simulations.
• Offers novel insights into the dynamic interaction between large wood debris, bridge hydraulics, and flash flood severity, specifically demonstrating the destructive potential of sudden wood jam releases.
• Quantifies the extreme geomorphic impacts of such floods, including massive channel widening and sediment/vegetation mobilization, highlighting the "geomorphic flood" nature in the uplands.
• Emphasizes the critical vulnerability of vegetated, torrential basins with narrow bridges to catastrophic floods, providing a warning for flood planners.
• Integrates citizen science and local witness accounts into the scientific reconstruction, enriching the understanding of the event's impacts and perceptions.

Funding

• UPC’s contribution: Contract CTN2000029 of the “Agència Catalana de l′Aigua”.
• UB’s contribution: AGORA project, funded by the "Agència Catalana de l′Aigua".
• UIB’s research: Ministerio de Ciencia, Innovación y Universidades (CGL2017-82868-R and PID2020-113036RB-I00/AEI/10.13039/501100011033 research projects), partially supported by European Regional Development Funds.

Citation

@article{Vide2023Large,
  author = {Vide, Juan Pedro Martín and Bateman, Allen and Berenguer, Marc and Ferrer‐Boix, Carles and Amengual, A. and Campillo, M. and Corral, Carles and Llasat, María Carmen and Llasat-Botija, Montserrat and Gómez-Dueñas, Santiago and Marín-Esteve, B. and Núñez-González, F. and Prats-Puntí, Arnau and Ruiz-Carulla, R. and Sosa-Pérez, R.},
  title = {Large wood debris that clogged bridges followed by a sudden release. The 2019 flash flood in Catalonia.},
  journal = {Journal of Hydrology Regional Studies},
  year = {2023},
  doi = {10.1016/j.ejrh.2023.101348},
  url = {https://doi.org/10.1016/j.ejrh.2023.101348}
}