Contenu du sommaire : Risques naturels dans le couloir rhodanien : les excès pluviométriques

The southern area of France regularly receives heavy Mediterranean rainfall, chiefly during the autumn in September and October. This occurs particularly over the Cévennes and to a lesser extent over the eastern Pyrenees and the department of Alpes-Maritime. Over other, generally less exposed, regions disasterous events are linked to sudden flooding. This type of episodic occurrence contrasts with normal Mediterranean rainfall ; it is short-lived associated with more intense storm activity and results in a rapid rise in water levels, with damage focused on a particular, vulnerable point.

Presentation of annual precipitation maps of the Rhône- Alpes region : quantiles and the frequencies with which certain thresholds are exceeded.

Following the disaster of Vaison-la-Romaine, the question of an abrupt climatic change has again been raised. However, contrary to widespread opinion, the present pattern of rainfall is not exceptional and the succession of catastrophic floods recalls the period 1928-40. Since 1954 a continuous rise in pressure - more regular since 1975 - has been linked with a decrease in rainfall : a strong inverse correlation is observed between surface pressure and rainfall.

Unlike other regions in south eastern France, the French Riviera did not experience catastrophic rainfall in the autumn of 1992. Whether measured on an hourly or daily basis, intensities were nowhere exceptional. However, the frequent occurrence of heavy falls resulted in a very high total rainfall constituting an absolute

The METEOSAT 5 satellite imagery shows the movement of cloud formations over France and the adjacent area of the Atlantic between 21 and 23 September 1992. Analysis shows that the heavy rainfall over the Ouvèze river basin represents only an epiphenomenon of a much wider and more comprehensive system.

Exceptional precipitation (up to 300 mm within four hours) hit the South of France on 21 and 22 September 1992 and provoked dramatic flooding of certain tributaries of the Rhône, such as the Ouvèze and the Ardèche rivers. Rainfall characteristics are described and meteorological conditions of this phenomenon are analysed.

Diluvian rains related to the meteorological patterns of 21 and 22, 26 and 27 September 1992, were the result of the rapid eastward motion of two, early and strong MPHs having a direct meridional path over the Atlantic Ocean. Their power allowed them to move in the form of an airmass over the Iberian relief towards the western Mediterranean, and then to force a strong southerly, warm and moist airstream towards the southern part of France, where highlands amplified the stormy nature of disturbances. The month of September 1992, was characterized by a large number of MPHs with strong meridional paths. These dramatic events are connected with a climatic change governed by a cooling trend over the Arctic Ocean area.

This paper presents some methods used to estimate flows with incomplete data collected immediately after a flood. It highlights the difficulties of such a calculation and shows how to reconcile different methods which seem contradictory. The flood at Vaison-la-Romaine, south of France, on 22 September 1992, is presented as an example.

During the month of September 1992 a storm sequence of considerable intensity (nearly 300 mm of rainfall in 24 hours) was focused over the eastern border of the Massif Central between Les Vans and Cheylard, an area 70 km long and 30 km wide. The rainfall rapidly produced violent flooding of the Ardèche and its tributaries : the waters of the small Fontolière river rose 2 metres in 15 minutes, with an observed flow of 800 m3/s ; the Ardèche itself rose 7 metres at Pont-de-Labeaume. Although the human and material consequences were less serious than those produced by the flooding of the Ouvèze at Vaison- la-Romaine, the flooding of the Ardèche and its tributaries damaged river banks, bridge parapets, camp sites and supermarkets in the area above Aubenas, and killed 4 people. This event forms part of a much more widespread meteorological phenomenon which concerned the Mediterranean region of France at this period.

The storms and floods of 22 September 1992 in the southern part of the Drôme department and the northern and central western part of the Vaucluse department, severely damaged crops as well as soils, perennial plantations and farm buildings ; the cost of rectifying the damage increases these regions present economic difficulties.

The catastrophe which occurred at Vaison-la-Romaine after the heavy downpours of 21 and 22 September 1992 focused public sympathy on this commune in the department of Vaucluse. However, a socio-economic assessment of this event shows that damage was severe in other areas, particularly in three departments of the Rhône valley, and affected both public and private property. Five months later it is difficult to evaluate the long-term consequences of these events due to the numerous factors which influence the economic revival of the disaster-stricken regions.

On 2 June 1992 the Envers torrent (in the commune of Sollières-Sardières in Savoie) experienced a new flood with a heavy torrential charge. Material damage was relatively limited, but debris blocked the river bed of the Arc causing the formation of a dam lake. Action by the Department's engineering services enabled the rapid emptying of the lake. Heavy rainfall provokes the formation of a torrential lava, although the basic condition for this to occur is the availability of loose material. The Savoie Service for the Restoration of Mountainous Terrain considers it is impossible to carry out further corrective work on the river basin itself due to the extreme instability of the slopes. As a result it is proposed to create a depositional platform on top of the dejection cone.