High Resolution Observations of an Outer-Bank Cell of Secondary Circulation in a Natural River Bend

Secondary circulation is a mechanism that increases vertical exchange between the horizontal layers in open channels. Vertical shear in curved flows induces secondary circulation in the plane normal to the primary flow direction. Models for secondary circulation have been developed that include the nonlinear advection terms in the momentum equations. A consequence of this is that secondary circulation deforms the vertical profile of the primary flow. This in turn reduces the vertical shear of the primary flow thus reducing the driving force of secondary circulation. Observations from a bend of moderate curvature in the Clutha river show flattening and deformation of the vertical profile of the primary flow due to the advection of momentum by secondary circulation. The observations are on a much larger river than previous studies so the increase in scale makes these observations robust. A counterrotating outer-bank cell of secondary circulation is associated with this deformation. A visual explanation for this outer-bank cell is proposed based around the forces of plane equilibrium and the deformation of the vertical profile of the primary flow. A suggested requirement for a counterrotating outer-bank cell is that the depth average velocity must be found at two depth locations on the vertical profile of the primary flow. The width of this outer-bank cell is twice that of previous flume experiments, perhaps due to outer-bank roughness. The strength of this outer-bank cell is approximately 0.5 of the main cell. Calculation of a bend parameter $\mathcal{B}=3.7$ classifies the curvature for this survey site as weak to moderate. The deformation of the vertical profile of the primary flow is greater than expected for a bend parameter of $\mathcal{B}=3.7$, perhaps due to the increase in size and strength of the outer-bank cell.