The Measurement and Modelling of Axial Convergence in Shallow Well-Mixed Estuaries

Abstract

An axial foam line observed on the flood tide in the Conwy estuary of North Wales indicates the presence of a two-celled, density-driven convergent flow system. The flows are maintained by the lateral salinity gradients established by the interaction of the shear of axial velocity with the longitudinal salinity gradients. A diagnostic model has been proposed by Nunes and Simpson (1985) to predict the pattern and magnitudes of these flows using a steady state balance between the lateral pressure gradients and frictional forces.

In order to test this model, a boat-mounted spar system equipped with four vanes, capable of measuring down to 4 m below the surface, has been used to directly determine details of the transverse circulation. By slowly traversing the estuary section, quasi-synoptic pictures of the transverse velocity field have been obtained. In an alternative mode, measurements have been made at a fixed position to examine the variability of the flow. The results confirm the existence of a full-depth, two-celled, flow system, and, when real salinity gradients are used as inputs to the model, the magnitudes of the secondary flows predicted agree well with the measured values.

It was observed that during the flood the vertical shear of the axial flow in mid-channel was generally small and the velocity profile exhibited a sub-surface maximum at times. This behaviour, which contrasts sharply with the strong vertical shear ( ~ 0.07 s⁻¹) on the ebb flow, is probably the result of advection of momentum by the transverse flow.