Chris Hackney, Dan Parsons

The erosion of the outer-banks of meander bends is mediated by the form roughness of the bank topography, which has been shown to affect near bank three-dimension flow structures and shear stresses. As the scales of bank roughness is known to vary vertically from bank toe to bank edge variations in flow discharge are likely to driver changes in near-bank flow velocities and turbulent structures in the near-bank zone. However, to date field observations of three-dimensional flow in the near-bank zone across multiple discharges remain limited. Here, we report observations of three-dimensional flow characteristics in the near-bank zone of a meander bend at a range of discharge levels; low (9 m3 s-1), mid- (23 m3 s-1) and bankfull (90 m3 s-1). Our results reveal that the relationship between the presence of a secondary outer-bank flow cell (SOC) and magnitudes of turbulent kinetic energy (TKE) and shear stresses vary with discharge. At high flows, the presence of a SOC is related to increased TKE, whilst at mid-flow discharges SOCs are associated with reduced TKE compared to transects where SOCs are absent. At low flows there is no observed pattern between TKE and the presence/absence of a SOC. We attribute these patterns to the role that failed material at the bank toe plays in deflecting and modulating near-bank flows. This work highlights how important detailed flow data across multiple discharges is in terms of understanding flow through meander bends and thus the impact these flow processes will have on longer term bend morphodynamics.