MEASUREMENTS OF THE SEMI-DIURNAL DRAG COEFFICIENT OVER SAND WAVES

Simultaneous measurements of tidal flow and pressure gradient over a 1 0 km distance have been used to estimate the frictional drag coefficie nt over sand waves in the southern North Sea. The measurements were ma de over a 27-day period in October-November 1988 at 52 degrees 10'N, 3 degrees 46'E, within a held of essentially two-dimensional sand waves approximately 3 m high and of wavelength 250 m. The M(2) drag coeffic ient for depth-averaged flow normal to sand wave crests is found to be 2.95 x 10(-3), in good agreement with values used in numerical models of the legion. The uncertainty in this value is estimated to be of th e order of 10%, primarily due to possible errors in the phase of the f low relative to the pressure gradient. The time series of daily-averag ed 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 coeffi cient rather than the increase predicted by most wave-current interact ion theories and some previous observations. It is suggested that this behaviour is caused by stratification near the bed due to sediment re suspension under high waves. Predictions using the GLENN and GRANT (19 87, Journal of Geophysical Research, 92, 8244-8264) theory provide qua litative support for this hypothesis, though the magnitude of the pred icted effect is smaller than observed. The relative insensitivity of t he drag coefficient to wave conditions suggests that incorporating sim ple wave-current algorithms into numerical models may be misleading.