Flooding in the Vallée des Baux: from field observations to wetland restoration

The Vallee des Baux during the floods of winter 2003-04

The Vallée des Baux, Provence, is an east-west basin just south of the Alpilles Massif of south-east France that holds the reclaimed ancient marshes of Baux and Arles, covering some 3000 hectares. Both the A Rocha France study centre and its study area lie in the Vallée des Baux.

From 1 to 3 December 2003 more than 250 mm of rain fell in the area of the Alpilles Massif and the Vallée des Baux. The soils were already saturated and the water in this drainage basin, an area of around 200 sq km, ran straight off. The canal which drains the Vallée des Baux, and its remaining 150 ha of marshes, were unable to handle the amount of water and widespread flooding occurred, both here and downstream in the historical city of Arles.

By 5 December an area of around 2700 ha was flooded. The valley continued in a flooded state throughout the winter and some areas remained under water until June 2004.

Members of A Rocha France studied this event. The flooded area was mapped on detailed topographic maps for 15 separate time periods. Depths of water were determined on the basis of natural flood meters (trees, posts, stakes). In two places, bridges acted as improvised storm hydrographs and allowed estimates of the water flow through the area.

The floods clearly had a social and economic impact. They were also significant environmentally in several ways:

Negatively, many animals drowned in the freezing water: terrestrial reptiles, small mammals and insects were particularly affected. Many birds left the area; these included not just land-loving birds such as sparrows and Red-legged Partridges (Alectoris rufa) but also shallow wetland species. Indirectly the Green Woodpecker (Picus viridis), feeding on terrestrial ants, showed a population decrease of 30%. This is particularly important as this species is the single hole excavator for many important cavity nesters of this region (European Roller Coracias garrulous, owls and bats).

Positively, the return of water to the old marshes attracted a host of birds. Within weeks, the traditional waterbirds of the Vallée des Baux had returned and the regular A Rocha bird surveys show that, compared to the preceding three winters at least, many species were present in appreciably greater numbers. Northern Lapwing (Vanellus vanellus) and Mallard (Anas platyrhynchos) numbers rose dramatically with a peak number of 4000 lapwings (a figure of regional importance); and there were increased numbers of Grey Herons (Ardea cinerea) and Black-winged Stilts (Himantopus himantopus) in the marsh. Furthermore, many species previously rare in the valley began to appear such as Common Cranes (Grus grus), geese, and Greater Flamingos (Phoenicopterus ruber).

Map showing the extent of 2003-04 flooding. The darker the area, the longer the water was present

Finally, this study highlights the importance of the Vallée des Baux as a flood buffer. Calculations suggest that the whole of the valley held around 33 million cubic metres of flood water. This shows the wisdom of keeping the valley un-urbanised. Gathering all stakeholders, A Rocha is helping to develop a more sustainable agricultural and multi-usage project for this land, including marshland restoration to enhance this buffering effect.

Project leader: François Tron. For further infomation contact france@arocha.org