Tidally-induced Flow Structure Over Intertidal Flats

Previous investigations have assumed that a significant linear relationship betweenuz-lnZis sufficient to define a logarithmic velocity profile given by the von Kármán–Prandtl equation. However, it is demonstrated here that such a criterion alone is insufficient. An internal consistency analysis is developed to establish realistic logarithmic velocity profiles, in the estimation of boundary layer parameters associated with intertidal flat environments. The analytical procedure includes: (i) onward linear regression, to establish the relationship between apparentu*values and near-bed current speeds,uz(here, the observedu50is used); (ii) an assessment of the constant and correlation coefficient within theu*–uzrelationship; (iii) calculation ofz0andC50values, on the basis of the slope of the regression line; and (iv) undertaking a comparison between the derivedz0andC50and those obtained using the instantaneous velocity profiles. The internal consistency is shown to be enhanced if the data are filtered, before a more detailed analysis. This approach is applied to the analysis of 192 current velocity (gradient rig) data sets collected from the intertidal flats of the Loughor Estuary and Swansea Bay (South Wales), and The Wash (eastern England). Overall, less than 40% of the data sets are logarithmic in character, with variations between the various stations. Nevertheless, boundary layer parameters were determined for most locations. Non-logarithmic profiles on intertidal flats are considered to be caused by: (i) rotary tidal currents; (ii) wind effects; (iii) wave action and other short-period oscillations; and (iv) topographically-induced secondary flows. Measurement errors can lead also to deviations from an ‘ idealized ’ logarithmic profile.