Flash floods or sudden floods occur when rivers rise rapidly

Flash floods, also known as sudden floods or flash floods in English, are highly dangerous meteorological events. The reason lies in their rapid and violent nature, as well as their relatively unpredictable occurrence, which limits the time available to activate emergency protocols.

However, by understanding what they are and how they develop, we can implement mechanisms that help reduce their effects. After all, the more you know about your adversary, the more resources you can put into practice to confront it.

Flash floods: Knowing the enemy

According to the National Oceanic and Atmospheric Administration (NOAA), flash floods correspond to rapid increases in water levels in streams or rivers above the flood level. This phenomenon is common in mountainous areas subjected to intense rainfall or rapid snowmelt, as well as in normally dry regions. In fact, in Europe, the three areas with the highest potential for the occurrence of this phenomenon are (1):

  • The Mediterranean region, where usually dry riverbeds (ramblas) play a special role. Approximately 70% of the river systems in southern and eastern Spain are ephemeral streams (2).
  • The Mediterranean alpine region, characterized by rugged terrain.
  • The interior of continental Europe.

    However, it should be clear that a flash flood can occur almost anywhere, from natural environments to urban areas where the sewer system is overwhelmed by heavy rainfall.

    Why are flash floods so dangerous?

    Flash floods are, by nature, fast, destructive, and quite unpredictable. Typically, they occur within a period of less than 6 hours. However, this timeframe is influenced by the environment or the triggering event, as we will see later.

    Comparison of two photographs taken in a river channel in Zarvraggia, Switzerland, on July 18, 1987. The right image was taken 15 minutes after the left image, showing the advancing wave. Source: Swiss Federal Office for the Environment (extracted from APFM, 2007).

    The aforementioned characteristics result in very short response times. Hence, these sudden floods cause enormous material and often human losses. Examples of these consequences include the more than 3 billion euros of damages in the Aude region in 1999, or the 1.2 billion euros in losses caused by the flash flood in the French department of Gard in 2002, which we will discuss in more detail later.

    Factors and situations that increase the risk of flash floods

    Among the main circumstances and events that increase the risk of torrential flooding, we can distinguish:

    • Terrain topography, especially areas with steep slopes, which determine the speed at which water can flow.
    • Density of vegetation, which usually helps reduce the amount of water freely flowing over a surface.
    • Intense rainfall resulting from convective processes. The risk is increased if the area has accumulations of snow that undergo rapid melting or if there are artificial or natural dams that, when they break, release a large volume of water, such as in the case of glacial lakes.
    • Failures in hydraulic infrastructure leading to their rupture.
    • Soil absorption capacity, which may be saturated from previous rainfall. In relation to this factor, special mention should be made of the waterproofing effect often caused by forest fires or the presence of non-porous pavements that facilitate runoff.
    Difference between natural soil cover and various types of impermeable surfaces in urbanized areas. Source: Saraswat, Kumar & Mishra, 2016 (3)

    All these situations, which can occur individually or in combination, make the monitoring of watercourses a fundamental task, especially in high-risk areas.

    In this regard, surveillance programs such as Copernicus and the European Flood Awareness System (EFAS) have improved data availability. However, a significant portion of flash floods occur in small areas, less than 500 km2, with many basins not exceeding 100 km2, as noted by Penna, Borga & Zoccatelli (4). Therefore, in these areas, it is useful to complement satellite information with data from ground-based monitoring systems.

    This is precisely the role played by the surveillance networks installed by Arantec in the Garonne River and the Sió River basin. The increasing intensity of extreme weather events is making these systems a crucial element within emergency management strategies. This aspect was already highlighted, for example, in the European project ANYWHERE, in which Arantec participated. In addition to their early warning function, these networks also collect data that subsequently serve to analyze the specific characteristics of each event in greater detail. And this work, essential for a better understanding of how flash floods occur, is a necessity that, to this day, cannot be carried out in many areas due to a lack of monitoring (5).

    Flash floods that have made a significant impact: learning from mistakes

    The purpose of this section is not to dwell on morbidity but to learn. After all, episodes of torrential floods also offer valuable lessons, and their analysis contributes to the understanding of the circumstances that trigger them.

    So let’s take a brief look at some of these events that are already part of the collective history of many towns and cities.

    Montserrat (Catalonia), June 10, 2000

    On June 10, 2000, extremely heavy rainfall concentrated in the basins of the Llobregat, Besós, Francolí, and Riera de la Bisbal rivers caused severe flooding, landslides, and debris flows. The most notable damage occurred in the vicinity of Montserrat Monastery (720 m above sea level), where various infrastructure was destroyed by the force of the water, and 500 people had to be urgently evacuated.

    The rainfall recorded during this event stands out for its intensity, with numerous locations exceeding 100 mm in 1 hour and a maximum accumulated amount of 224 mm in less than 24 hours (6).

    As seen in the attached video, the phenomenon originated from a convective system detected on the night of June 9, which was reinforced by the presence of warm and humid air near the Earth’s surface.

    The event resulted in losses estimated at over 65 million euros and five fatalities.

    Gard Department (France), September 8-9, 2002

    This episode of flash floods originated as a result of a mesoscale convective system, considered in some forums as the “queen of storms,” which typically forms in the Mediterranean region.

    As detailed by Delrieu et al. (6), the information bulletins issued by Meteo France underestimated the amount of precipitation and inaccurately predicted the position of the storm, shifting it by 100 km, due to the low resolution of the information. In fact, the rainfall records associated with this event are among the highest in the region, with maximum values of 600-700 mm in 24 hours.

    These floods, as mentioned earlier, resulted in losses amounting to 1.2 billion euros and claimed the lives of 24 people.


    Flash floods are one of the most destructive phenomena, especially in the Mediterranean area. The speed at which they occur, their difficult predictability, and the force they unleash make satellite monitoring complemented by on-site basin-level control systems one of the most effective surveillance mechanisms.

    The solutions offered by Arantec do not stop floods, an option only achievable through costly structural measures. However, relying on non-structural measures such as our SmartyRiver can also help save many lives.

    Sources consulted:

    • (1) Norbiato, D.; Borga, M.; Degli Esposti, S.; Gaume, E.; Anquetin, S. (2008). Flash flood warning based on rainfall thresholds and soil moisture conditions: An assessment for gauged and ungauged basins. Journal of Hydrology 362(3-4), 274–290. https://doi.org/10.1016/j.jhydrol.2008.08.023
    • (2) Camarasa-Belmonte, A.M. 2021.Flash-flooding of ephemeral streams in the context of climate change. Cuadernos de Investigación Geográfica 47, http://doi.org/10.18172/cig.4838
    • (3) Saraswat, C., Kumar, P., & Mishra, B. K. (2016). Assessment of stormwater runoff management practices and governance under climate change and urbanization: An analysis of Bangkok, Hanoi and Tokyo. Environmental Science and Policy, Vol. 64, pp. 101–117. https://doi.org/10.1016/j.envsci.2016.06.018
    • (4) Penna, D., Borga, M., & Zoccatelli, D. (2013). 7.9 Analysis of Flash-Flood Runoff Response, with Examples from Major European Events. Treatise On Geomorphology, 95-104. https://doi.org/10.1016/B978-0-12-374739-6.00153-6
    • (5) Gaume, E., Bain, V., Bernardara, P., Newinger, O., Barbuc, M., & Bateman, A. et al. (2009). A compilation of data on European flash floods. Journal Of Hydrology, 367(1-2), 70-78. https://doi.org/10.1016/j.jhydrol.2008.12.028
    • (6) Llasat, M., Rigo, T., & Barriendos, M. (2003). The «Montserrat-2000» flash-flood event: a comparison with the floods that have occurred in the northeastern Iberian Peninsula since the 14th century. International Journal Of Climatology, 23(4), 453-469. https://doi.org/10.1002/joc.888
    • (7) Delrieu, G., Nicol, J., Yates, E., Kirstetter, P., Creutin, J., & Anquetin, S. et al. (2005). The Catastrophic Flash-Flood Event of 8–9 September 2002 in the Gard Region, France: A First Case Study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory. Journal Of Hydrometeorology, 6(1), 34-52. https://doi.org/10.1175/JHM-400.1
    • (8) Gaume, E., Livet, M., Desbordes, M., & Villeneuve, J. (2004). Hydrological analysis of the river Aude, France, flash flood on 12 and 13 November 1999. Journal Of Hydrology, 286(1-4), 135-154. https://doi.org/10.1016/j.jhydrol.2003.09.015

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