TIDALLY-INDUCED FLOW STRUCTURE OVER INTERTIDAL FLATS

Previous investigations have assumed chat a significant linear relatio nship between u(z)-lnZ is sufficient to define a logarithmic velocity profile given by the von Karman-Prandtl equation. However;;, it is dem onstrated here that such a criterion alone is insufficient. An interna l consistency analysis is developed to establish realistic logarithmic velocity profiles, in the estimation of boundary layer parameters ass ociated with intertidal flat environments. The analytical procedure in cludes: (i) onward linear regression, to establish the relationship be tween apparent u(z), values and near-bed current speeds, u(z), (here, the observed u(50),, is used); (ii) an assessment of the constant and correlation coefficient within the u-u(z),, relationship; (iii) calcu lation of z(0) and C-50 values, on the basis of the slope of the regre ssion line; and (iv:) undertaking a comparison between the derived z(0 ), and C-50,, and those obtained using the instantaneous velocity prof iles. The internal consistency is shown to be enhanced if the data are filtered, before a more detailed analysis. This approach is applied t o the analysis of 192 current velocity (gradient rig) data sets collec ted from the intertidal flats of the Loughor Estuary and Swansea Bay ( South Wales), and The Wash (eastern England). Overall, less than 40% o f the data sets are logarithmic in character, with variations between the various stations. Nevertheless, boundary layer parameters were det ermined for most locations. Non-logarithmic profiles on intertidal fla ts are considered to be caused by: (i) rotary tidal currents; (ii) win d effects; (iii) wave action and other short-period oscillations; and (iv)topographically-induced secondary flows. Measurement errors can le ad also to deviations from an 'idealized' logarithmic profile.: (C) 19 98 Academic Press Limited.