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Pré-Publication, Document De Travail (Preprint/Prepublication) Année : 2023

Dike Fields as Drivers and Witnesses of 20th Century Hydrosedimentary Changes in Highly Engineered Rivers

Résumé

Many large European and American rivers have been channelized in the 19 th century and since then feature Dike Fields (DFs) forming engineered alluvial margins. Drivers and witnesses of contemporary geomorphological and ecosystem changes, these initially aquatic DFs have for the most been filled with fine sediments and become terrestrial. On the Rhône River (France) which has not only been corrected but also equipped with numerous dams (mid-20 th century), we studied the terrestrialization ( i.e., transformation of aquatic areas in terrestrial ones) in two types of DFs: open fields (groyne fields) and closed fields (groyne fields closed by a longitudinal dike). A classification of spatio-temporal terrestrialization patterns (5 types) has been obtained under GIS thanks to aerial photographs and completed by ground penetrating radar surveys to characterize the sediment structural organization of the deposits. It highlights local specificities (inherited forms) within a generalized trajectory of fluvial disconnection. Studying the evolution of the water lines and riverbed elevation allowed to emphasize the control factors and the associated forcings leading to terrestrialization. During phase 1 (reach only channelized – 1890s to 1970s), it is 47% of the closed fields areas which have been terrestrialized and 16% for open fields. Since the incision is not very pronounced on the reach, it appears to be mainly due to accretionary processes as a result of lower shear stresses within the DFs. The terrestrialization from phase 2 (channelized and bypassed reach – 1970s-2000s) corresponds to 32% of the areas of closed fields and 51% of open fields. A cross-validation between the planimetric approach and a lateral connectivity model shows that dewatering caused by the flow diversion has provoked the emersion of almost the half of the DF extent on the upper – and most impacted – part of the reach (75% of the total terrestrialized area). In terms of fluvial rehabilitation, to understand the DFs trajectories provides new insights to guide future restoration design in line with the societal stakes and the current hydrological conditions. Strategical DF reconnections (removing or lowering dikes) could support the river to gain space and recreate hydrological connectivity gradients favorable to habitat diversity that it is currently unable to create or maintain on its own.
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Dates et versions

hal-04061245 , version 1 (06-04-2023)

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Gabrielle Seignemartin, Brice Mourier, Jérémie Riquier, Thierry Winiarski, Hervé Piégay. Dike Fields as Drivers and Witnesses of 20th Century Hydrosedimentary Changes in Highly Engineered Rivers. 2023. ⟨hal-04061245⟩
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