Measurements of the semi-diurnal drag coefficient over sand waves

Simultaneous measurements of tidal flow and pressure gradient over a 10 km distance have been used to estimate the frictional drag coefficient over sand waves in the southern North Sea. The measurements were made over a 27-day period in October–November 1988 at 52°10′N, 3°46′E, within a field of essentially two-dimensional sand waves approximately 3 m high and of wavelength 250 m. The M 2 drag coefficient for depth-averaged flow normal to sand wave crests is found to be 2.95 × 10 −3, in good agreement with values used in numerical models of the region. The uncertainty in this value is estimated to be of the order of 10%, primarily due to possible errors in the phase of the flow relative to the pressure gradient. The time series of daily-averaged semi-diurnal ( Z 2 ) drag coefficients over the 27-day period shows surprisingly little correlation with nearbed wave orbital velocities. The time of highest waves is associated with a decrease in drag coefficient rather than the increase predicted by most wave-current interaction theories and some previous observations. It is suggested that this behaviour is caused by stratification near the bed due to sediment resuspension under high waves. Predictions using the Glenn and Grant (1987, Journal of Geophysical Research, 92, 8244–8264) theory provide qualitative support for this hypothesis, though the magnitude of the predicted effect is smaller than observed. The relative insensitivity of the drag coefficient to wave conditions suggests that incorporating simple wave-current algorithms into numerical models may be misleading.