Abstract. The nature of abandoned channels' sedimentary fills has a significant influence on the development and evolution of floodplains and ultimately on fluvial reservoir geometry. A control of bifurcation geometry (i.e., bifurcation angle) on channel abandonment dynamics and resulting channel fills, such as sand plugs, has been intuited many times but never quantified. In this study, we present a series of experiments focusing on bedload transport designed to test the conditions for channel abandonment by modifying the bifurcation angle between channels, the flow incidence angles and the differential channel bottom slopes. We find that disconnection is possible in the case of asymmetrical bifurcations with high diversion angle (≥30∘) and quantify for the first time an inverse relationship between diversion angle and sand plug length and volume. The resulting sand plug formation is initiated in the flow separation zone at the external bank of the mouth of the diverted channel. Sedimentation in this zone induces a feedback loop leading to sand plug growth, discharge decrease and eventually to channel disconnection. Finally, the formation processes and final complex architecture of sand plugs are described, allowing for a better understanding of their geometry. Although our setup lacks some of the complexity of natural rivers, our results seem to apply at larger scales. Taken into account, these new data will improve fluvial (reservoir) models by incorporating more realistic topography and grain size description in abandoned channels.
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.
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