Contenu du sommaire : Sur la séquence orageuse de novembre 1999

The torrential rains of the 12 and 13 november 1999 over the Languedoc-Roussillon region were directly related with two very strong Moving Polar Highs (MPHs), the one located first over the Atlantic Ocean and then over the Western Mediterranean, and the other over Central Europe and then over the Eastern Mediterranean. The narrowing of the low pressure trough between the southern part of this two MPHs concentrated the very high Mediterranean energy and channelled it towards the Gulf of Lion. The low- level collision, along a meridian convergence line, of this very wet and rapid easterly streamflow and a part of the MPH which had followed an Atlantic path and had been diverted towards southwest France by the Cantabric-Pyrenean range, provoked a violent ascent and heavy rains with dramatic consequences.

The article analyzes both the climatological and the meteorological aspects of the rainfalls of the 12 and 13 november 1999. By placing the accumulation of rainfall in a regional and historical perspective, confirmation is given of the exceptional nature of the rainfall in the Aude valley and surrounding river basins. The extent of the areas affected, the interdependence between the heavy rainfall and the relief, and the somewhat unusual atmospheric conditions renders this event a precedent, making it necessary to reconsider some of the accepted notions concerning strong Mediterranean storms.

The hydrological analysis of the floods of the 12, 13 and 14 novembre 1999 which affected the Agly, Aude, Orbieu and Tarn river basins shows the importance of the specific flow rates when the flood is at its heights (from 0,5 to 3,6m3/s/km2 and maximum flows of 1100 m3/s on the Agly and 2000 m3/s on the Aude). Such flow rates are not exceptional on the coastal rivers of the Mediterranean. Acceptance of their importance means reassessing protective measures which until now had been judged adequate.

Lying at the foot of the Pyrenees and bordering the Mediterranean, the Salanque plain has experienced a high level of urbanization since 1975. This is due to important migratory movements. The expansion of urban areas has been accompagned by major hydraulic works such as a dam to limit floodin and dykes to canalize the river Agly until it reaches the sea. However, such constructions where designed only for floods of average importance and the major flooding of the Agly on the 12 and 13 november 1999 showed their limits. The river overflowed in numerous places, while at Saint-Laurent-de-la- Salanque, the dyke was breached, leading to the rapid evacuation in the middle of the night of numerous inhabitants. This event, together with testimonies from the past, show quite clearly that along the coastal plains of the Mediterranean, the destructive force of floods will never be brought entirely under control. Greater account needs to be taken of the physical environment in regional development strategies.

When those responsible for managing natural hazards talk about riverside residents they send to present general cases often little related to the actual, more complex territorial realities. In adopting an overall view of hazard perception, we present various results of a study undertaken amongst riverside residents along to rivers in the urban area of Lyon. The river is but one territorial element, and the importance which is accorded to it can be modified by both individual and collective events. Protection systems ca be either material or symbolic, helping to make living conditions more tolerable in the case of a flood. If such features are ignored in discussions with riverside residents, the information provided by those responsible for managing hazards is likely to prove of little use.

The bad weather of the 12 and 13 november 1999 badly affected the departments of the Aude, Tarn, Hérault and a part of the Pyrénées- Orientales. In human terms this was one of the worst natural disasters to have affected France over the last thirty years. Nine months after the events, the complex range of factors involved in the efficient relaunching of the affected local economies makes it difficult to evaluate the term consequences of the disaster. Transforming into reality the numerous promi-ses of help made just following the storms, is a key condition to enable the rapid restructuring of the regional economy.

In law the definition of zones liable to flooding takes on various forms depending on how the limits of the flooded area is determined. First, the limits may be observed, and are thus based on the perception of the existing situation. Alternatively, they can be established by historical research or imagined by scientific modelling of the future. Finally, they can be duplicated on the basis of pre-existing limits or invented to resolve a particular legal problem. All these methods lead to the definition of a space with a legal legitimacy, but which often ignores the reality of the river's floodplain. This is because the definition is based on a compromise between the protection of property rights and the social acceptability of the risk.

The dynamics of the weather are determined by both local and distant factors. The recent evolution of the weather can be analysed according to two methods. The first, statistical and probabilistic, is founded on an analysis of observational data and gives only limited and uncertain results ; the second analyses the evolution of climatic parameters in a serological unit, namely the North Atlantic Ocean within which France is located. In this unit, controlled by the dynamics of MPHs, and due to Arctic cooling, the weather has become more violent since the 1970s and natural hazard have increased. It is a reality (operating in the opposite direction to the hypothetical global warming) which needs to be taken account of seriously in prevention programmes against the consequences of more violent weather.