Despite relatively low mean flow rates, the Allier is by no means a tranquil river. Indeed, the diverse climatic influences affecting its many sources generate a high degree of seasonal and year-to-year variation in its flow rates, with seriously low water levels contrasting with floods that sometimes reach catastrophic proportions.
The Allier’s hydrological regime is of rain-fed type, with water levels high in winter and low in summer.
But floods can start to occur in autumn. This is often the case with Cévennes flooding, caused by rain in the Cévennes typical of the upper catchment area, which can be particularly heavy. Oceanic floods, generally occurring in winter or spring, resulting from rainfall of oceanic origin, only affect the downstream part of the catchment area.
Mixed floods due to a combination of both can be exceptionally severe, as in 1866.
The Allier’s floods can exert a very strong influence on those of the middle Loire.
The last major flood, on 4 and 5 December 2003, with an estimated frequency of the order of thirty years, reached 970 m3/s at Vieille-Brioude. It was 2½ times greater at the same spot in 1866.
In contrast, before the Naussac dam was brought into service, extremely low water levels could be seen in dry periods, for example in 1949, when flow rates at Vieille-Brioude and Bec d’Allier were 0.5 m3/s and less than 6 m3/s respectively.
FLOODS THAT ARE HARMFUL IN ONE PLACE BUT NECESSARY IN ANOTHER
Human developments in areas liable to flooding are often vulnerable to natural hazards, and severe flooding then becomes a source of problems for our society. But floods have positive effects too, especially for ecosystems, as they are intensive, structure-changing dynamic processes that can rejuvenate ageing habitats, create extensive beaches and cause drastic erosion, among other consequences. So floods are necessary to maintain the rich ecology of hydrosystems. They also contribute to restoring alluvial water tables.