The latest developments in sedimentary geology

The Loire, like other major rivers in Western Europe, has undergone numerous developments linked to its use (navigation, flood protection, sediment extraction, etc.), which have significantly altered it in morphological and functional terms. From the perspective of sedimentary processes, the construction of large dams upstream (Grangent and Villerest on the Loire, and Naussac on the Allier) and the installation of groynes along almost the whole of the Loire’s middle and lower reaches have deprived it of a significant sediment supply. Furthermore, the virtually continuous embankment restricting its lateral movement and the massive sediment extraction in the low-flow channel have led to significant morphological changes, the most noticeable evidence of which is the downward erosion of the main channel. It is this transformation that particularly caused such significant events as the collapse of the Wilson Bridge in Tours in 1978.

An increased risk of flooding

The extent of erosion increased following the end of the Second World War due to the extraction of sediment to supply reconstruction projects. The volumes extracted between Nevers and Angers have been estimated at 6.4 million tonnes per year (Gasowski, 1994), whilst natural bedload sediment transport was estimated at 300,000 tonnes per year upstream of the confluence with the Maine (Le Guern, 2021). The deepening of the main channel led to the side channels becoming cut off, compounded by the closure of the central section due to vegetation encroachment. This increasing plant cover is diminishing the diversity of habitats of enormous interest and raising the flooding risk.

Ensuring morphological and ecological river dynamics

Against the current backdrop of climate change, the effects of the cut-off side channels are becoming critical, particularly as low-water levels are expected to last longer and become more severe. Understanding how the river has evolved is therefore essential for adapting management policies and ensuring both morphological and ecological vitality and the maintenance of a low risk of flooding.
Recent technological advances, such as topo-bathymetric LiDAR, enable comprehensive, precise description of aquatic ecosystems and provide continuous morphological data between terrestrial and aquatic habitats, something that was a challenge until quite recently.

Based on Alex Andréault’s thesis ‘Vertical structure of a multichanneled river […] Implication in the morphodynamics and lateral connectivity of the Loire’