Medea Focus: Flash Floods

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). Flash floods are usually caused by heavy rainfall and often occur within 2 hours of the start of high intensity rainfall.

Floods are weather-related hazards and the most frequent and among the costliest and deadliest natural disasters worldwide. Disastrous flash-floods are much more frequent in some parts of the Mediterranean region than in the rest of Europe. This is due to the local climate, which is prone to short intense bursts of rainfall.

The reliefs surrounding the Mediterranean Sea force the convergence of low-level atmospheric flows and the uplift of warm wet air masses that drift from the Mediterranean Sea to the coasts, thereby creating active convection. In addition, population growth is particularly high along the Mediterranean coasts, leading to a rapid increase in urban settlements and populations exposed to flooding.

The flood regimes and the types of dominant flood generating rainfall events vary significantly along the coasts of the Mediterranean Sea. Damaging floods are mainly produced by:

  1. Short-lived (often less than 1 hour) strongly convective intense precipitation events (up to 180 mm/h in only 5 minutes) but limited total rainfall amounts (generally less than 100 mm). Such events have a limited areal extent (typically less than 100 km2) and generate local flash floods of small headwater streams. A typical example of such flash floods is the catastrophic flood that occurred in Algiers in November 2001.
  2. Mesoscale convective systems can produce stationary rain lasting several hours leading to rainfall amounts exceeding 200 mm in a few hours. In France, up to 700 mm of rainfall within 12 hours was locally recorded during floods in the Aude region in November 1999 and in the Gard region in September 2002. The areal extent of such events ranges from several hundred to several thousand km2. These events mainly occur in fall and affect the north-western coast of the Mediterranean Sea. The flash floods that occurred in Genoa, Italy, in October 1970 and in the Var region of France in June 2010 belong to this category.
  3. On some occasions, heavy and sustained rainfall may be part of a large scale perturbation lasting several days. In such cases, extreme rainfall accumulation may be observed locally: 1,500 mm over four days and a record of close to 1,000 mm in 24 hours in October 1940 in the eastern Pyrenees in France. These events cover a large area.

The purpose of the MEDEA project workshop is to identify capabilities gaps in the case of a very common hazard in the MEDEA region: Flash flood during a Mediterranean phenomenon.

Moreover the workshop allows a productive interaction on various common and recurrent identified gaps that have emerged through the working sessions. The gaps have been grouped under four topic areas, namely:

(a) Forecast & Alert;

(b) Rescue ;

(c) Lessons learned toward prevention and

(d) Lessons learned toward preparedness.

Based on the outcomes of the workshop, a concrete document summarizing the perceived problems and gaps that the practitioners come up against through their daily practice, will be developed. 

The MEDEA project workshop is restricted to a selected number of invited experts in order to ensure the maximum and most fruitful exchange among the participants.

Capability Gaps

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