Tarolli et al. (2012) Analysis of flash flood regimes in the North-Western and South-Eastern Mediterranean regions

Identification

• Journal: Natural hazards and earth system sciences
• Year: 2012
• Date: 2012-05-03
• Authors: Paolo Tarolli, Marco Borga, Efrat Morin, Guy Delrieu
• DOI: 10.5194/nhess-12-1255-2012

Research Groups

• Department of Land, Environment, Agriculture and Forestry, University of Padova, Legnaro, Italy
• Department of Geography, The Hebrew University of Jerusalem, Mount Scopus, Jerusalem, Israel
• Laboratoire d’etude des Transferts en Hydrologie et Environnement, Grenoble, France

Short Summary

This study analyzes flash flood regimes in the North-Western and South-Eastern Mediterranean regions, revealing significant differences in flood peak-catchment area relationships, seasonality, and rainfall-runoff characteristics driven by distinct climatic and hydrological controls. It highlights that the South-Eastern region experiences shorter, lower-depth rainfall events with more variable runoff ratios, and a more pronounced decrease in unit peak discharge with increasing catchment size.

Objective

• To characterize the flash flood regimes in two contrasting Mediterranean areas: the North-Western Mediterranean region (Catalonia, France, Northern Italy) and the South-Eastern Mediterranean region (Israel), focusing on flood peak distribution, seasonality, and rainfall-runoff properties.

Study Configuration

• Spatial Scale: Two distinct Mediterranean regions: North-Western (Catalonia, Southern France, Northern Italy) and South-Eastern (Israel). Catchment areas analyzed range from tens of square kilometers to approximately 2000 km².
• Temporal Scale: Flash flood events collected over the last 5 to 6 decades for primary data analysis (99 events). Detailed rainfall-runoff analysis for 13 selected major flash flood events.

Methodology and Data

• Models used: No explicit hydrological simulation models were used for flood generation; analysis relied on empirical relationships (e.g., envelope curves for peak unit discharge, Qu = qrAβ) and Budyko’s climatic classification scheme for catchment characterization.
• Data sources:
  • Primary data (99 events): Occurrence date, location, peak discharge (m³ s⁻¹), and catchment area (km²).
  • High-resolution data (13 selected events): High-resolution rainfall data (radar and raingauge data, including amateur and bucket analysis) and streamgauge data for runoff response.
  • Rainfall estimation methodology: Specifically devised for radar and raingauge data, including collection of metadata, analysis of detection domain and clutter, corrections for range-dependent errors, and optimization via radar-raingauge comparisons.
  • Event selection criterion: Flood-generating rainfall exceeding a 50-year return period for at least some duration.

Main Results

• Seasonality: Flash floods in the North-Western Mediterranean region predominantly occur between August and November, while in the South-Eastern region, they tend to occur between October and May, reflecting different synoptic conditions.
• Flood Peak-Catchment Area Relationship: The envelope curve for the South-Eastern region exhibits a more pronounced decrease in unit peak discharge with increasing catchment size (scaling exponent β = -0.62) compared to the North-Western region (β = -0.4). This difference is attributed to storm coverage effects and hydrological characteristics, including potential channel losses in semi-arid/arid areas.
• High Intensity Events: A higher proportion of high-intensity events (characterized by reduced peak discharge qr > 70 m³ s⁻¹ km⁻²⁽¹⁺β⁾) was observed in the South-Eastern region (27%) compared to the North-Western region (11–23%), clustering in semi-arid and arid areas of Israel.
• Rainfall Characteristics for Extreme Events: For selected major events, rainfall duration was generally shorter and rainfall depth lower in the South-Eastern region compared to the North-Western region.
• Runoff Ratios: Event runoff coefficients were relatively low in both regions (mean of 0.33 for North-Western, 0.32 for South-Eastern), consistent with previous European flash flood studies. However, variability was higher in the South-Eastern region (range 0.17–0.61) than in the North-Western region (range 0.12–0.44).
• Runoff Generation Controls: No clear relationship was found between runoff ratio and rainfall depth, indicating a major influence of rainfall intensity and initial wetness conditions on runoff generation for these events.

Contributions

• Provides a comprehensive comparative analysis of flash flood regimes in two climatically and hydrologically distinct Mediterranean regions using a homogeneously collected dataset.
• Quantifies regional differences in the scaling of flood peaks with catchment area through envelope curves, offering insights into the underlying hydrological and storm coverage controls.
• Characterizes the rainfall and runoff properties of extreme flash flood events, highlighting regional disparities in storm duration, depth, and runoff ratio variability.
• Emphasizes the critical role of rainfall intensity and initial soil moisture conditions over total rainfall depth in flash flood runoff generation across both regions.
• Compiles and analyzes a significant database of flash flood events, contributing valuable data for future flood risk assessment and management in the Mediterranean.

Funding

• EU RTD Project HYDRATE (European Commission, Sixth Framework Programme, Contract No. 037024)

Citation

@article{Tarolli2012Analysis,
  author = {Tarolli, Paolo and Borga, Marco and Morin, Efrat and Delrieu, Guy},
  title = {Analysis of flash flood regimes in the North-Western and South-Eastern Mediterranean regions},
  journal = {Natural hazards and earth system sciences},
  year = {2012},
  doi = {10.5194/nhess-12-1255-2012},
  url = {https://doi.org/10.5194/nhess-12-1255-2012}
}