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Spatial patterns and temporal dynamics of fine sedimentation in restored floodplain channels (Rhône River, France): actual trends and assessment of their potential persistence as aquatic habitat.

Abstract : The design of floodplain channel restoration projects is poorly informed by theoretical or empirical experiences, even if we observe an increased number of such projects in recent decades. Moreover feedbacks regarding their hydromorphological adjustments are rarely shared, despite the importance of such processes in controlling the persistence of floodplain channels as aquatic habitats. In this study, we used between five and seven monitoring campaigns, ranging from 7-15 years after restoration, to describe spatial patterns and temporal dynamics of fine sedimentation in 16 floodplain channels of the Rhône River (France), which were restored between 1999 and 2006. During surveys, water depth and fine sediment thickness were measured longitudinally along the centerline of channels. We also quantified the upstream overflow frequency and magnitude (i.e. maximum shear stress) and the maximum magnitude of backflow events to explore statistical relationships between forms and processes and identify geomorphic types and laws. We found a high variability of average fine sedimentation rates both in space (i.e. between channels) and time (i.e. from restoration to the final survey). Overall, average rates ranged from 0 cm to 40.3 cm.yr-1, with a maximum of 90.3 cm.yr-1 observed in a particular side-channel. Scouring processes were detected in all floodplain channels, except one. Several channels exhibited a significant decline in sedimentation rates over time, whereas others maintained rather constant rates. For the 16 studied channels, six channels exhibit no dependence upon the time span since restoration (i.e. self sustainable channels) and the 10 others are time-dependant (i.e. they tend to accumulate more and more fine sediment deposits as post-restoration time elapses). Among these 10 channels, four are high life-duration channels (i.e. more than 100 years), three are intermediate life-duration channels (i.e. likely from about 40 to 80 years) and three others are low life-duration channels (i.e. likely from about 15 to 50 years). We also demonstrate that the propensity of restored floodplain channels to sequester fine-grained deposits can be well predicted using simple hydrological and/or hydraulic metrics (i.e upstream overflow frequency, maximum shear stress and maximum magnitude of backflow events). These metrics reflect the control exerted by the geometry of floodplain channels (i.e. morphology of the upstream alluvial plug, slope conditions) on their flooding regime. By allowing the quantification of how technical decisions relative to the design of channels can affect their propensity to accumulate fine sediments, empirical relationships developed in this study are of great interest for managers. The associated estimates of potential life span ranges can be very useful to provide a design life estimate for constructed floodplain channels and to guide project design for future restoration efforts in the Rhône River.
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https://hal.archives-ouvertes.fr/hal-01402006
Contributor : Jérémie Riquier Connect in order to contact the contributor
Submitted on : Thursday, November 24, 2016 - 10:34:54 AM
Last modification on : Monday, August 30, 2021 - 1:24:02 PM

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  • HAL Id : hal-01402006, version 1

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Jérémie Riquier, Hervé Piégay, N. Lamouroux, Lise Vaudor. Spatial patterns and temporal dynamics of fine sedimentation in restored floodplain channels (Rhône River, France): actual trends and assessment of their potential persistence as aquatic habitat.. "Towards the best practice of river restoration and maintenance", Sep 2016, Krakow, Poland. ⟨hal-01402006⟩

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