Bedload infilling and depositional patterns in chute cutoffs channels of a gravel‐bed river: The Ain River, France

• Published in: Earth surface processes and landforms 2022-02, Vol.47 (2), p.459-476
• Main Authors: Szewczyk, Leo, Grimaud, Jean‐Louis, Cojan, Isabelle, Piegay, Herve
• Format: Article
• Language: English
• Subjects: abandoned channels; Accretion; Alluvial channels; Alluvial plains; Bed load; bedload fill; Channel bends; channel plug; Channels; Deposition; Downstream; Downstream effects; Earth Sciences; Flow separation; Fluvial deposits; Geomorphology; Gravel; Hydrology; Plugs; River channels; River discharge; River flow; Rivers; Sciences of the Universe; Sediment transport; Stratigraphy; Surveys; Thickening; Water flow
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.5260

Although abandoned channels are common and identifiable features in alluvial plains, their detailed internal architecture remains overlooked, particularly their coarse permeable compartment, with implications for underground water flow. The actively shifting gravel‐bed Ain River (France) provides an opportunity to study the geometry and architecture of bedload deposits (plug) during channel disconnection, in relation to river discharge and planform evolution. In this study, combined geomorphic and grain‐size surveys were conducted on the bedload deposits associated with the closure phase of four cutoff channels. We find that bedload accumulates mainly through (i) the initiation of a bar at the mouth of the abandoned channel in the flow separation zone, which reduces flow connectivity; (ii) lateral accretion of coarse‐grained bars, resulting in channel narrowing downstream of the initial upstream bar; and (iii) merging of coarse‐grained longitudinal bars—anchored to the initial plug bar. The channel plug progrades downstream until occupying ca. 0.5–0.7 times the channel surface and then starts thickening. We also find that channel plug volume is controlled by sediment supply and channel inherited topography, which are partially controlled by the dynamics of neighbouring channel bends. A model for coarse‐grained channel plugs formation and architecture is proposed from abandoned channels along the actively shifting gravel‐bed Ain River (France). Bedload deposits accumulate in abandoned channels mainly through initiation of a bar in the flow separation zone limiting discharge and lateral accretion bars resulting in channel narrowing. Finally, merging longitudinal bars contribute to the channel closing. The channel plug reaches its maximal extension before thickening by bars accretion. Its geometry is controlled by sediment supply and channel geometry.