AERIAL SURVEYS OF AXIAL CONVERGENT FRONTS IN UK ESTUARIES AND THE IMPLICATIONS FOR POLLUTION

In estuaries where strong tidal stirring prevails and vertical gradients are minimal, buoyancy forces are involved in driving a transverse circulation that produces a surface convergence line along the axis of the tidal flow during flood tides (Simpson & Turrell, 1986). Extending continuously many km along the axis of the estuary the feature acts to concentrate foam, flotsam and detritus. During the ebb a weak divergence deposits much of the material on the banks, which under normal riverine outflow leads to a net upstream transport of surface material. Aerial surveys of 26 UK estuaries during a period of spring tides in September 1985 revealed strong evidence of the presence of such fronts in 15 cases, with a further 6 identified during field observations. The relative abundance of such features and their associated secondary flows have important implications for estuarine pollution.