Hydrodynamics and sedimentology of concave benches in a lowland river

This paper describes the hydrodynamics and sedimentation associated with concave-bank benches (concave benches) which are the fine-grained sediment deposits considered geomorphically and ecologically important in meandering rivers. Most notions previously associated with concave bench formation ignore hydrodynamic complexities and consequently fail to recognise the importance of bankfull flows: being commonly based on inference from stratigraphic profiling or casual observation of hydrodynamics. Concave benches are a depositional anomaly since their location in the channel is commonly associated with scour. Mature concave benches are formed by separation of the reverse flow (upriver flow) from the stem flow (downriver flow) in an abrupt bend whereby deposition is dominated by vertical accretion of suspended sediments. For this study two-dimensional hydrodynamic modelling in conjunction with the collection and analysis of contemporary sediments is used to demonstrate the persistence of reverse flow over concave benches for stages from incipient bench inundation to bankfull and greater. For two mature concave benches on the Ovens River, southeast Australia, bankfull flows were depositional with shear stresses commonly <0.5 N m(-2) incapable of scouring the bench surface. Localised scour is restricted to the streamward edge of the bench during the early and late phases of an inundation event. Discharges most conducive to deposition were found to be higher than anticipated, in the range of 40 to 80% of bankfull (benches initially inundated at 20% of bankfull), and could be responsible for bench formation. These findings highlight the potential negative impact of river regulation on channel morphology by reducing the frequency of flows forming in-channel benches and increasing the occurrence of destructive flows. (C) 2011 Elsevier B.V. All rights reserved.