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School of Geography and Geosciences, University of St Andrews, Fife, KY16 9ST, Scotland
Estuarine fronts may develop in several locations, across the water body as transverse, halocline-related features, as overbank spills, or where tidal flats release waters into principal tidal channels. In many estuaries longitudinal fronts originate at headlands or are associated with major sandbanks (Bowman 1988). Commonly a front separates a nearshore water mass from that of the main flow of tidal waters. In the marginal waters flows are slower than in the main channels, and suspended sediments and pollutants released to these marginal zones do not mix freely with the principal tidal waters entering and leaving the estuary. These nearshore waters also often differ in temperature from the channel waters (Anderson et al. 1992). It is suggested that the linear convergent frontal systems provide the ideal sites for the accumulation of sediment, and exert some control on the distribution of longitudinal estuarine sand banks. Some, but not all of these are associated with headlands. The geomorphological evolution of the sand bank systems within the upper and middle reaches of the Tay Estuary, Scotland, are discussed in relation to present and past locations of longitudinal fronts. The Middle Bank, off Dundee, is seen to occur between the lines of longitudinal fronts formed during the flood and ebb phases of the tide. The Naughton Bank, off Balmerino is related to an ebb phase headlnad-induced flow separation. In the seaward reaches longitudinal fronts locate the outer margins of the tidal flats and become axially convergent in the Tayport-Broughty Ferry narrows.
