Study of some Loir river sections (downstream Loir Basin) as a means to determine the dynamics of a low energy, harnessed river
Détermination des conditions de la dynamique fluviale d'une rivière aménagée de basse énergie, à partir de secteurs du Loir (Bassin de la Loire aval)
Résumé
The experiments carried out for two years on three sites (Ruillé, Bruant, and Cherré) of a single stretch of the Loir river (rate of flow: 24 m3/s; mean width: 40 m; mean depth: 2.5 m), a weak energy stream whose channel slope never exceeds 1‰, allowed for the conditions of the river’s dynamics to be determined. Two surveys were conducted between 2013 and 2015: riverbank recession and bathymetry measurements using 3D terrestrial LIDAR and multibeam sounder respectively. A new method suitable for processing and interpreting data acquired in moderate erosive zones (thus applicable to any other weak energy river) has been developed and applied to the surveys, in order to assess the changes occurring from one survey to the other. Those changes are local and the resulting riverbank shapes diverse. Our major findings are the following: (i) some local shore line recessions ranging from 0.80 to 1.40 m in length, which appeared in the course of a few months during the high-water period (in the absence of major floods), (ii) Some minor changes in the riverbed floor, including a deepening of 1.5 to 1.8 m in depth between permanently submerged shallow areas. The solid load carried by the stream channel, mainly made up of fine-textured materials (fine sand and silt), is released downstream. Coarse materials (cobbles of 5 to 9 cm in length), that sometimes jut from the banks, cover the bottom of the riverbed where they form a thick layer. Erosion is thus restricted to riverbanks.
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