Morphodynamisme d'un tronçon de Loire bourbonnaise depuis le XVIIIe siècle / Changing dynamics of the Loire river in Burgundy (France) during the last 200 years

Résumé En Fr

This paper is part of an ecological study designed to outline a conservation plan for a natural wetland area located in the Loire alluvial plain of Burgundy (Sologne bourbonnaise). In this part of the river (upstream from Decize), the meandering characteristics of the channel system are very different from those that occur downstream from its confluence with the Allier river. After a brief synopsisof the geographical characteristics of the Loire valley in Burgundy, the changing dynamics of the Loire along a 40 kilometre stretch of river are studied through an analysis of channel shifts and meander evolution between 1967 and 1992. The intensity and occurrence of floods recorded at Never s from 1852 to 1985 are presented. They provide a good idea of channel evolution in natural conditions prior to the construction of the Villerest dam. A perceptible decrease in the average wavelength (13 m), and an increase of the curve index can be distinguished, but generally, variations in meander morphometry have been very small during the 25 years of interest. The relative meander stability was compared with river activity as inferred from temporal changes in sand bank surfaces. The results show that the active channel surface decreased by 13 % between 1967 and 1992. This is far from insignificant and is clearly indicative of a change in stream energy. This analysis is supplemented by the more detailed morphodynamic study of a small area ( "Г île de la Crevée ") spanning a period of more than 230 years. From 1760 to 1852, lateral shifting of the river bed, in relation to major flooding, was considerable. Both of these spatio-temporal studies testify to a recent stabilisation of channel systems in the Loire. The situation since 1967 is largely stable. This is ascribable to a decline in major flooding events and is reflected in the near stability of meanders, with a

Après un bref rappel des caractéristiques géographiques ligériennes dans le secteur concerné, une étude d'un tronçon de 40 kilomètres de lit fluvial a permis de mettre notamment en évidence les divagations du fleuve dans le lit mineur, l'évolution du méandrage en 25 ans (1967-92) et d'apprécier l'activité de la Loire. Par ailleurs, une seconde approche morphodynamique, s'étalant sur une période de plus de 230 ans, a été réalisée sur un secteur de dimension réduite : l'île de la Crevée. Ces études spatio-temporelles mettent en exergue, l'une et l'autre, un ralentissement récent de l'activité de la Loire dû à l'absence de grandes crues et se traduisant par une quasi-stabilité des méandres, avec pour conséquence une fermeture du lit par l'extension de la ripisylve.