Concave‐bank benches and associated floodplain formation

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Abstract
This paper describes the morphology, sequential development and general sedimentology of concave‐bank benches on the Murrumbidgee River of southeastern Australia, and also notes their important role in floodplain formation on certain meandering rivers in western Canada. Benches form against the concave bank (cut‐bank) of abruptly curving bends immediately upstream of the point of maximum curvature. As a result of flow deflection against the upstream limb of the convex bank, the channel widens here and produces a zone of expanded flow facilitating flow separation near the upstream limb of the concave bank. Sedimentation within this zone starts with a longitudinal‐shaped bar of medium sand forming a platform isolated even at low flow by a narrow secondary channel against the concave bank. Aggradation of the longitudinal‐shaped bar with fine sand, mud and organic matter permits the establishment of trees. Further sedimentation, particularly around the young trees, results in the formation of a fully developed bench isolated by the secondary channel from the remainder of the floodplain only during high flows. Observations on confined meandering rivers in western Canada provide evidence of substantial floodplain formation by concave‐bank bench accretion, a process distinctly different in character to the more familiar mechanism of lateral point‐bar accretion. Furthermore, the preservation of abundant organic debris means that extensive bench deposits may be a source of locally useful natural gas from within floodplain sediments.