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Cumulative effects of channel correction and regulation on floodplain terrestrialisation patterns and connectivity

Tena, A., Piégay, H., Seignemartin, G., Barra, A., Berger, J.F., Mourier, B., Winiarski, T.
Geomorphology 2020 v.354 pp. 107034
X-radiation, X-ray fluorescence spectroscopy, atomic absorption spectrometry, copper, floodplains, geographic information systems, hydrology, lead, orthophotography, sediments, surveys, France
One of the main drivers of overbank fine deposition and floodplain formation is the hydrological connectivity between the channel and the floodplain. Channel correction (i.e., groyne field construction within the main flow channel and secondary channel disconnections) and flow regulation can typically lead to a disconnection of riverine floodplains and disturbances that directly affect terrestrialisation. Channel correction and flow regulation can sometimes occur successively, and it is challenging to distinguish the roles of each. This work attempts to assess the respective effects of two phases of channel regulation (correction versus flow regulation) on floodplain terrestrialisation by comparing three bypassed reaches of the Rhône, France (Pierre-Bénite, Péage-de-Roussillon and Donzère-Mondragon). We applied a transversal methodology coupled with GIS analysis (old maps, Orthophotos, DEM's, etc.) to understand processes of channel-planform evolution, conducted a sediment survey (metal rod) to assess floodplain terrestrialisation, and performed sediment sampling (manual auger) to obtain surface sediment metal content levels (X-ray Fluorescence and Inductively Coupled Plasma Mass Spectrometry). We found a general trend of channel narrowing within the three reaches, among which approximately 40% was found to be associated with correction works while 20% was attributed to flow lowering caused by channel bypassing. The number of flowing channels in all sections declined significantly, and local anabranching reaches evolved into very stable single thread channels. Overbank sedimentation declined significantly over the period, with very high sedimentation levels observed immediately after correction works and with very low sedimentation levels observed after diversion. We also found overbank flooding (in the number of days per year) decreased while fine sediment thickness increased. Similarly, the highest concentrations of metals (Zc, Pb, and Cu) were found to be associated with a low connection frequency and vice versa. When similar 2-staged terrestrialisation patterns are observed in all three reaches, they differ in chronology and driving factors because of their longitudinal positioning and specific local conditions.