30 years ago: the Vaison la Romaine disaster

In autumn, the south of France is used to being regularly affected by episodes of bad weather known under the term “Cévennes episodes” which occur when heavy rains block durably on the departments of the Cévennes, or “d’ Mediterranean episodes” when this type of bad weather occurs, in the same way, in all the south-eastern regions. These Cévennes phenomena are usually intense but that of Vaison la Romain was exceptional in more ways than one and the causes of the tragic results of this historic episode are to be found in the conjunction of several factors.

Vaison la Romaine: an aggravating weather context

After a stormy summer, autumn 92 was particularly rainy in France. There are several “Mediterranean” and Cévennes episodes in September, October and November. The episode of Vaison la Romaine intervened on September 21 and 22: in addition to the Cévennes, diluvian stationary storms broke out on both sides of the Rhône valley and caused an exceptional flood on a Provençal river – the Ouvèze – due to remarkable rainfall totals.

The initial weather scenario is fairly standard for the season. With the arrival of the cold front of a disturbance from September 21 by the Massif-Central, the winds oriented to the south-east bring up warm air saturated with humidity from the Mediterranean: this is the usual process. the establishment of a “Mediterranean episode”. At the same time, a depression located in the north of the Bay of Biscay brings cool oceanic air down to the west of France.

This strong thermal conflict, reinforced by a water temperature that is still warm at this time of year in the Mediterranean (24°C) makes the air mass very unstable (heat and humidity supply), an instability that is increased tenfold by another aggravating factor: the digging of a secondary depression in the Gulf of Lion.



Credit: The Weather Channel

Timeline of a disaster

As of September 21, a weather alert bulletin is issued. That day, the arrival of the cold front caused an active Cévennes episode, which mainly affected the north of Hérault and Gard, the south of Lozère and Ardèche. There is 448 mm of rain in Caylar in the Hérault, the equivalent of 2 and a half months of precipitation (a record). As thunderstorms pass, violent gusts of wind are recorded in Nîmes-Garons (155 km/h).

On September 22, this still violent stormy system over the Ardèche and the Gard is gaining momentum by slowly shifting to the east of the Rhône, where between 200 and 300 mm of rain will be recorded in 4 to 5 hours ( for a monthly average of about 80 mm in the region).

From 10 a.m., the firefighters asked for the evacuation of a campsite located at the confluence of the Ouvèze and the Lauzon, upstream from the Roman bridge. Most campers refuse to evacuate. On Vaison La Romaine, the storms then turn to disaster.

Two stormy episodes separated by a lull of about an hour give rise to very heavy rainfall, exceeding 3 mm/minute (3 liters of water per m 2 /minute). Between 11 a.m. and 12 p.m. the first storm broke. It is followed by a second storm which bursts between 1 p.m. and 4 p.m., with rains which intensify. Water quickly invades the streets of the city.



Credit: The Weather Channel

At 3 p.m. a first mudslide of 50 cm submerged the municipal campsite then at 4 p.m. a second torrent of water and mud carried away everything in its path, including caravans. The raging waters of the “flash flood” reach 17 meters in height at the bottleneck of the Roman bridge, rising 2 meters above the bridge deck. We note 179 mm at Vaison-la-Romaine but 240 mm at Mollans-sur-Ouvèze and 300 mm at Entrechaux, on the upstream part of the catchment area. In Orange (25 km from Vaison), there is “only” 51 mm, which illustrates the very great spatial variability of the amounts of rain that can occur during such an episode.

At 5 p.m., the ORSEC plan was triggered and first aid arrived. The decline begins in the evening and very quickly the observation of the first damage is essential: devastated campsite, houses built in the major bed of the river completely damaged, hundred of houses near the Roman bridge destroyed, parapets of bridges washed away, sites damaged Gallo-Romans, and above all a very heavy human toll to deplore.

If the weather context was very unfavorable that day, it is not enough on its own to explain the violence of the phenomenon, the causes of which are also linked to the location of Vaison la Romaine.

Steep slopes and dense hydrographic network

Indeed, Vaison la Romaine is located in a hilly area, transition between the wide plain of the Rhône and the reliefs of Mont Ventoux and the Baronnies. The Ouvèze, 93 km long, drains a catchment area of ​​880 km2 including 580 km2 upstream of Vaison. The total elevation of the river reaches 880 m from its source in the Chamouse mountain and its confluence with the Rhône. The watershed of the Ouvèze which stretches widely to the east corresponds to a mid-mountain basin with reliefs culminating at more than 1000 meters in altitude, with fairly steep slopes. Moreover, the hydrographic network of the Ouvèze is very hierarchical with many small side rivers which descend from the surrounding hills or mountains. The Toulourenc, a tributary of the Ouvèze, descending from the northern slope of Mont Ventoux, has a more mountainous hydrological regime, and more reactive to torrential floods. It flows into Vaison La Romaine at the entrance to the town.

That day, the combined contribution of the tributaries of the Ouvèze (Toulourenc, Aigue Marce, Grozeau and Lauzon) already swollen by heavy rainfall transformed the ancient bridge into a bottleneck. The flood was rapid and brutal with a peak flow close to 1200 m3/s whereas the ten-year flood had been around 300 m3/s for a usual average flow of 6 to 10 m3/s.

Example of a flash flood

https://www.youtube.com/watch?v=tsI7V5_-bE4

An exceptional phenomenon that Vaison had already experienced

The flood of the Ouvèze was characterized by its short duration and its remarkable intensity. In 4 hours it fell between 150 and 300 mm over a large part of the watershed and only 5 hours elapsed between the flood and the beginning of the decline. The ancient city had already experienced similar episodes in the past. Texts recall that floods had occurred in the 19th century (1840, 1868 and 1886) but also in 1616 when the water also passed over the Roman bridge.

Episodes of heavy autumn rains are one of the characteristics of the Mediterranean climate and affect all the bordering countries. The recurrence of these episodes is even described as the “Mediterranean monsoon” by geographers.

In recent years, their occurrence has become less frequent, but in the decade 1990 – 2000, real disasters occurred, often devastating, deadly and very costly.

Vaison la Romaine will forever be marked by this event with dramatic consequences. Although the reasons for this disaster are first and foremost natural (weather and geography), we cannot exclude an amplification of the phenomenon linked to anthropogenic causes (modes of land use and management of riverbeds ). And if the number of victims and the cost that such disasters entail must remain in the collective memory, it is also to remind us that man has an essential role to play for a better coexistence between his concerns and the whims of lady nature.

Leave a Comment