Floods in France: Key figures

Different types of flooding
 

• river flooding 
• surface runoff 
• coastal flooding
• groundwater flooding 
• hydraulic structure failures
• other types, such as lake flooding, glacial lake outburst flood (GLOF), stormwater flooding, etc.
   

Different types of floods often tend to be interconnected.For example, surface runoff can lead to river flooding. Coastal flooding may cause or worsen river flooding, while groundwater levels may rise due to river flooding. Hydraulic structure failures can also cause or worsen river flooding or coastal flooding. Conversely, when exceptionally high river water levels exceed the capacity of a hydraulic structure, this may result in partial or total failure of that structure.

Except in cases of hydraulic structure failure, these phenomena are triggered by extreme weather conditions. Heavy or long-lasting rainfall, can lead to surface runoff, river flooding and rising groundwater flooding. Strong energy swell waves and/or high sea levels may also cause coastal flooding, thunderstorms, storms, as well as cyclones in the French Overseas Territories. 
Moreover, underwater earthquakes, which may results in a tsunami, also give rise to coastal flooding.

Floods can vary widely in intensity and duration. They may affect one municipality or span a large drainage basin, or even several basins, and may last for a few hours up to several months. 
In mountainous or Mediterranean areas, floods often occur quickly, while water level tends to rise more slowly in plains.

Certain factors can increase flood intensity: soil conditions may accelerate surface runoff, particularly when ground is frozen, very dry or, on the contrary, saturated with water. Floods can become more severe due to spring snowmelt, a failure of flood protection structures, leading to debris build-up that obstructs water flow. Structural vulnerability of buildings especially single-storey houses can also be considered as an aggravating factor. 
 

Here are a few examples of major flood events which have occurred in France over the past decades:
 

Floods in the Aude department, October 15, 2018: along an axis running from the Cabardès region to the Corbières mountain range, following extremely heavy rainfall - up to 300 mm in six hours - a severe flood affected the Aude River and its tributaries rising in the Montagne Noire, the Trapel and the Orbiel, particularly around Trèbes. The Orbiel reached a peak discharge of 490 m³/s, while the Aude in Trèbes reached 1,700 m³/s. The Aude water levels recorded in Trèbes and Puichéric were similar to those observed during the historic flood of 1891. The flood wave then moved downstream, inundating the low-lying plains of the Aude.
 

Floods of May-June 2016: for several weeks, the Centre and Île-de-France regions experienced repeated rainfall events, making May 2016 the wettest month in those regions since weather monitoring began roughly 150 years ago. Heavy rain fell on already waterlogged soils, increasing surface runoff and causing river levels to rise sharply. The tributaries of the Seine in Île-de-France, the Loing and the Ouanne, as well as some tributaries of the Loire, suffered by exceptionally severe flooding for that time of year. A red flood warning was issued for the upper Loing section – the Ouanne on May 31, as well as the lower Loing section, on June 1. Within the Seine-Normandy basin, only the rivers of the Loing basin and the Seine and the Marne tributaries in the Île-de-France region reached exceptionally high levels, although the extreme flood did not spread across the entire basin. 
In Paris, the flood caused damage to buildings, riverside roads closed, some public transport services, including the RER C line were supended. 
The Seine peaked at 6.10 meters on June 3, 2016, at the Austerlitz gauging station, well below the historic 8.62 meters level in 1910. 
 

Floods in the Alpes-Maritimes, October 3-4, 2015: on the evening of October 3, the municipalities along the coastal corridor between Mandelieu-la-Napoule and Nice were struck by an intense thunderstorm, with rainfall exceeding 100 mm in 1 hour and 150 mm in 2 hours. The flooding was mainly caused by surface runoff in urban areas. The disaster claimed 20 lives and property damage was estimated at approximately 1.2 billion euros. 
 

The Xynthia storm hit France on February 27 and 28, 2010, resulting in significant human and material damage, due to coastal flooding worsened by dike breaches. Four departments were severely affected. 3 million euros was allocated to help flood victims.