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

  • W. R. Turrell
  • J. H. Simpson

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.

Keywords

Secondary Flow Transverse Velocity Flood Tide Capillary Wave Diagnostic Model 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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References

  1. Hansen DV, Rattray M (1965) Gravitational circulation in straits and estuaries. J Mar Res 23:104–122Google Scholar
  2. Nunes RA (1982) Dynamics of small scale fronts in estuaries. Ph D Thesis, University College of North Wales, UKGoogle Scholar
  3. Nunes RA, Simpson JH (1985) Axial convergence in a well mixed estuary. Estuarine Coastal Shelf Sci 20(5):637–649CrossRefGoogle Scholar
  4. O’Farrell SP (1983) The axial convergence in the Conwy and Mawddach estuaries, Gwynedd. MSc Thesis, University College of North Wales, UKGoogle Scholar
  5. Prych EA (1970) Effects of density differences in lateral mixing in open channel flows. Keck Lab Hydraul Water Resour, Calif Inst Tech Rep KH-R-21Google Scholar
  6. Simpson JH, Nunes RA (1981) The tidal intrusion front: an estuarine convergence zone. Estuarine Coastal Shelf Sci 13:257–266CrossRefGoogle Scholar
  7. Simpson JH, Turrell WR (1986) Convergent fronts in the circulation of tidal estuaries. In: Wolfe (ed) Estuarine variability. Estuarine Research Federation, pp 139–152Google Scholar
  8. Smith R (1977) Long term dispersion of contaminants in small estuaries. J Fluid Mech 82:129–146CrossRefGoogle Scholar
  9. West JR, Knight DW, Shiono K (1984) A note on flow structure in the Great Ouse estuary. Estuarine Coastal Shelf Sci 19:271–290CrossRefGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1988

Authors and Affiliations

  • W. R. Turrell
    • 1
  • J. H. Simpson
    • 2
  1. 1.Department of Agriculture and Fisheries of ScotlandMarine LaboratoryAberdeenScotland, UK
  2. 2.Department of Physical OceanographyUniversity College of North Wales, Marine Science LaboratoriesAngelsey, GwyneddUK

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