Dispersion experiments in UK coastal waters

Over the period 1968 to 1999, Brixham Environmental laboratory carried out dye tracing experiments to determine local mixing conditions at coastal and estuarine sires around the United Kingdom. Results from 25 different locat ions, and from five non-U.K. sites, have been analysed to estimate horizont al and vertical mixing coefficients. Analysis of the data from the 19 sites which had the most complete data sets showed that the minimum lateral disp ersion coefficient ranged from 0.003 to 0.42 m(2) s(-1) with a median value of 0.05 m(2) s(-1). Typical vertical diffusion coefficients ranged between 2.0 x 10(-4) to 110.0 x 10(-4) m(2) s(-1) and had a median value of 20.0 x 10(-4) m(2) s(-1). Comparisons of the values obtained have been made with mixing coefficients derived from earlier dye tracer studies in the North Se a, the Irish Sea and off the eastern seaboard of the U.S.A. A plot of minimum lateral dispersion coefficients against the corresponding vertical diffusion coefficients showed that the results fell into two dist inct bands, broadly represented by shallow or deep water. Within each band, the lateral mixing coefficients appeared to vary in inverse proportion to the vertical coefficients, as would be expected in shear dispersion in whic h mixing is restricted by the surface and seabed or pycnocline. Curves repr esenting this variation had different constants of proportionality for the shallow and deep waters. This was ascribed to a reduction in the curvature of the velocity profiles in the deep water areas, where stronger currents m ay have reduced the degree of stratification. Analysis of the full data set has not identified any generally applicable relationship between the mixin g coefficients and tidal currents or wind, however, relationships between t hese parameters could be found within specific survey areas. For example, l ateral diffusion coefficients in the Solent appeared to be correlated with the tidal current amplitude. (C) 2000 Academic Press.