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Observatory of water, sediment and chemical fluxes in response to dam removal in the Selune River SELUNE

2012- 178 000 € for 2019-2021 Coordinator: UMR SAS SAS contact: Ophélie Fovet SAS research axis: Ressources website Keywords: suspended sediments, dissolved elements, geomorphology, dam removal, dam

Research

Physical obstacles such as dams influence fluxes of water, sediment and biogeochemical elements as well as the geomorphology. To identify, quantify and understand the consequence of dam removal in the Selune River, observations are being conducted that combine continuous monitoring of water, suspended solids (SS) and biogeochemical fluxes with geomorphological assessment. Fluxes are measured at two stations, one upstream and one downstream of the two dams, equipped with sensors for measuring discharge, turbidity, and physico-chemical parameters continuously. Weekly and storm samples were taken at both stations to measure concentrations of SS, phosphorus (P), dissolved nitrogen and silica (Si). Several variables are spatially analyzed (i.e. bankfull channel width, bank height, longitudinal profile, gravel bed grain size), along with the mobility of bed load, measured using RFID transponders.

Results for the pre-removal period:

Event dynamics of river concentrations between the two stations emphasized the buffering effect of the dams, with a retention of SS and P. Biological consumption in the reservoirs modifies the seasonal dynamics of Si and dissolved P concentrations but not that of nitrate concentration. The composition and granulometry of SS indicates a seasonal and biogenic origin downstream from the dams that differs from the SS that enters the dams. Annual biogeochemical loads increase as water flux increases, with higher loads downstream for dissolved elements and lower loads for SS and P. No downstream propagation signal from incision or paving could be detected despite high specific stream power (30-45 W.m-2) and a mean mobility of coarse sediments of approximately 1 m per year.

Publications

Fovet, O., Ndom, M., Crave, A., Pannard, A., 2020. Influence of dams on river water-quality signatures at event and seasonal scales: The Selune River (France) case study. River Res. Appl., 12. https://doi.org/10.1002/rra.3618

Basilico, L., Roussel, J.-M., Peress, J., Ronot, C. et Fraisse, S. 2021. Quand les rivières reprennent leur cours - Notes sur l’effacement de barrages et de seuils, sur la Sélune et ailleurs. Office français de la biodiversité. Collection Rencontres-Synthèse. 76 pages. Chp. 2.3 – La dynamique fluviale : effet des barrages et premières réponses.

https://professionnels.ofb.fr/sites/default/files/pdf/documentation/RS_2021_Effacement_Selune.pdf

Fovet, O., Ndom, M., Gilliet, N. and Crave, A. Understanding the effect of reservoirs on suspended sediments and biogeochemical fluxes over seasonal and storm scales. Poster at European Geosciences Union General Assembly 2018, Apr 2018, Vienne, Austria. Geophysical Research Abstracts, 20, pp.EGU2018-6470, 2018.

Martignac F., Le Gonidec C., Daroux A., Guillard J., Baglinière J.L., Grimaldi C., Crave A., 2014,

Rapport d’étape 2013, Rapport d’étape annuel-Programme Sélune : Phase pré-arasement – Volets biocénose et géomorphologie, flux sédimentaire hydriques et chimiques.

Le Gonidec C., Grimaldi C., Crave A., Rollet A-J., Beauchamp A., Lespez L., 2015, Rapport d’étape 2014, Rapport d’étape annuel-Programme Sélune : Phase pré-arasement – Volet A : Dynamique fluviale : Flux hydriques, sédimentaires et chimique et géomorphologie

Le Gonidec C., Grimaldi C., Fovet O., Vergnaud V., 2016, Rapport bilan 2013-2015 phase pré-arasement, Dynamique fluviale-Volet 1 : flux hydriques, sédimentaires et chimiques

Rollet A-J., Crave A., Beauchamp A., Lespez L., Le Gonidec C., 2016, Rapport bilan 2013-2015 phase pré-arasement, Dynamique fluviale-Volet 2 : Géomorphologie

Mamadou NDOM, Ophélie FOVET, Alain CRAVE et Nicolas GILLIET, 2018, Rapport bilan 2016-2017 phase pré-arasement, Dynamique fluviale-Volet 1 : flux hydriques, sédimentaires et chimiques

Mamadou NDOM, Ophélie FOVET, Alain CRAVE et Nicolas GILLIET, 2019, Rapport bilan 2017-2018 phase pré-arasement, Dynamique fluviale-Volet 1 : flux hydriques, sédimentaires et chimiques

Mamadou NDOM, Ophélie FOVET, Alain CRAVE et Nicolas GILLIET, 2020, Rapport bilan 2018-2019 phase « Travaux d’arasement », Dynamique fluviale-Volet 1 : flux hydriques, sédimentaires et chimiques

SAS staff involved

• Chercheur(e)s : Ophélie Fovet, Catherine Grimaldi (2012-2015)
• Field staff: Nicolas Gilliet (2012-2018), Mikael Faucheux (support)
• Laboratory staff: Béatrice Trinkler (2012-2020), Laurence Carteaux (2012-) Yannick Fauvel (2021-), Gaelle Hamelin (2021-)
• Project Engineer: Claude Legonidec (2011-2015), Mamadou Ndom (2017-2021), Vincent Stubbe (2021-)

Partners

• Alain Crave, OSUR, Géosciences Rennes, CNRS 
• Anne-Julia Rollet, OSUR, LETG Rennes, Université Rennes 2

Funding and Support

• Agence de l’Eau Seine Normandie