Weak interactions between wind waves and tides and the drag coefficient in a tidal flow

A 1.5-level model of the oscillatory turbulent bottom boundary layer (BBL) is used to study interactions between waves and low-frequency tin particular, tidal) motions. It is shown that, under the conditions typical of shallow continental shelves, interactions between wave and tidal motions are weak in the sense that oscillations of the bottom-friction stress at the wave and tidal frequencies can be regarded, to a first approximation, as being uncorrelated. In turn, it follows that a linear superposition of the solutions corresponding to each of these individual motions provides a reasonable accuracy of reproducing the BBL characteristics during a tidal cycle. A formula describing the variation of the drag coefficient in a tidal flow under the action of wind waves is obtained. The verification of this formula from the CODE-1 and CODE-2 data and also from the results of measurements made in Marsden Bay indicates that the predicted and observed values of the drag coefficient are in better agreement than could be expected taking into account a wide scatter in experimental estimates.