Flash flood in France

A total of 21 people died and nine others were reported missing. Thousands of people were evacuated from their homes and virtually all roads and railway lines were cut between Lyon and the south coast.

In total, up to 650 mm of rain fell over a period of 24 hours, the normal rainfall for a period of six months in this area. The Department of Gard was most seriously affected with 20 deaths, four people reported missing and many homes without electricity. Several thousand electricity pylons were torn down and more than 150 relay stations were flooded.

On 10 September, French Prime Minister Jean-Pierre Raffarin and French Minister of the Interior Nicolas Sarkozy toured the flood-hit parts of southern France, including visits to Nîmes, Orange and Avignon, which had been badly hit.

Rainfall totals reported in 24 hours were 98mm in Montélimar, 103mm in Orange and 140mm in Nîmes. A report of about 650mm in total at Anduze near Alès in the Department of Gard has been confirmed by Météo-France.

Meteorologists associate the torrential rainfall, which occurs regularly in this area, with the particular geographic situation of the coastal area of southern France, situated between the Mediterranean Sea and the Alps/Cévennes.

Three elements are essential for producing such flash floods:

1. An Atlantic depression that passes very slowly over northern or central France.
2. Strong southerly winds over south-eastern France carrying very moist air from the Mediterranean Sea.
3. Mountain barriers of the Cévennes and the Pre-Alps: the moist air coming from the south is forced to climb which leads to cloud formation and destabilisation of the atmosphere. In situations of high potential instability, with moist air in the lower levels and dry air at higher levels, the forced lifting can lead to the formation of severe thunderstorms that are continuously generated over the same region.

The people in southern France call this situation "pluies cévenoles", indicating that the rainfall is strongly linked to the lifting of the air masses by the mountain barrier. Depending on the wind direction, the strong rains either occur in the Cévennes and surrounding areas (south-easterly winds) or in the Pre-Alps (south-westerly winds).

An additional element for the floods is the season. Most of the flash floods in southern France occur in early autumn, when the Mediterranean Sea is still warm (sea surface temperatures in the Gulf of Lion around 22°C), and the soil is too dry to absorb the heavy rainfall.

A very similar rainfall event occurred 10 years earlier, on 22 September 1992, in Vaison-la-Romaine (France), in the southern foothills of the Alps not far from the Mediterranean coast (see Meteosat-5 animation below).

Several mesoscale convective systems generated a flash flood in and around the city of Vaison-La-Romaine causing 35 fatalities; the destruction of hundreds of houses, and an estimated property damage of nearly one billion EUR.

A Le Monde (French newspaper) article reported that the event was totally unexpected and that people involved in public safety (firemen, medical doctors) were themselves trapped by the flash flood. As a consequence of this disaster, Météo-France has improved the communication chains, which allow meteorologists to quickly alert the emergency services, and then the citizens.

One of the improvements is the vigilance map on the Météo-France homepage (see example map below), which gives a general overview of the risk level for strong wind, strong precipitation, thunderstorms, snow/ice and avalanches, and gives frequently (up to hourly) updated information on the weather situation at Department level. Several meteorological services, including Météo-France, are working on optimising this concept, to ensure that short-term forecasts are correctly transmitted, in real time, to the population.

Meteosat-6 images