Suspended sediment and associated radioactivity fluxes during an exceptional flood. The Case of the Rhône river (SE France) in December 2003

Titre du congrès :The Fluvial System - Past and present Dynamics and Controls Ville du congrès :Bonn Date du congrès :16/05/2005

At the beginning of December 2003, the Rhône River recorded one of its biggest floods since at least 150 years. In the lower part of the river, the peak flood reached 11 000 m3 s'1. Geomorphological and radioecological conséquences of a such event were investigated using measured and modelled fluxes and total export of the suspended sédiment and associated artificial radionuclides. Results pointed out the importance of large floods in the annual suspended sédiment load as 3.60 106 tons of silts, 0.85 106 tons of sands and 0.83 106 tons of clays were transferred towards the coastal environment. Thèse solid load were found to be lower than those expected and suggested that previous floods which occurred on the river and on its main tributaries during the last décade hâve probably led to the évacuation of available sédiment in the channels and on their banks. Moreover, the 137Cs activities measured within the suspended load were estimated to 15.2 ± 3.4 Bq kg'1 which corresponds to characteristic levels of the suspended sédiments from the Rhône catchment area and demonstrate that nuclear installations located along the Rhône valley do not significantly contribute to any increase in 137Cs activities of waters during the flood. The total 137Cs particulate export amounted 77 + 16.4 GBq and was mainly associated to the silt fraction that contribute to around 70% of 137Cs total export. This study also highlighted the non negligible part of radioactivity supported by the sandy fraction (9%) and permit to suppose that the bed load could be an additional pathway for the radioactivity associated to the sands exported through the fluvial System. Finally, we underline that floodplain deposits could be a useful tool to assess trace levé! of radioactive contamination in waters.