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Flash floods and Urban flooding

The definition of a flash flood

Although there is no single definition, a flash flood is usually defined as a sudden flood in a small catchment area (usually less than 1000 km2), occurring within 6 hours or less of the causative event (heavy rain, dam break, levee failure, rapid snowmelt or glacier-outburst flood) and often within 2 hours of the start of high intensity rainfall. Flash floods are usually caused by heavy rainfall that can either be local, affecting only one or two catchments, or more extended, producing flash floods as part of the framework of a major flood event (3). 



Flash floods caused on average 50 casualties per year in Europe, 70% of the total number of deaths due to floods (4). The fatalities mainly occur in countries surrounding the Mediterranean Sea, where a large population density exists at the coastal areas due to the important urbanization processes in this area during the last decades (2).


The island of Madeira was struck by a flash flood on 20 February 2010, with 270 mm, and locally even up to almost 400 mm of rain falling in one day, leading to 46 deaths and widespread damage in the capital Funchal (1).


The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Portugal.

  1. Lourenço and Rodrigues (2015)
  2. Llasat et al. (2010), in: Pino et al. (2016)
  3. Llasat et al. (2016)
  4. Barredo (2007)

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