DRAG AND SEDIMENT DISPERSION OVER SAND WAVES

Estimates of the drag coefficient over sand waves during calm weather in the southern North Sea have been obtained from measurements of the water slope and currents at different heights (z) above the sea-bed us ing the log profile and momentum balance methods. An observed phase di fference between principal terms in the momentum balance equation is e xamined theoretically. Drag coefficient estimates are found to agree b roadly with previous studies. Owing to bedform asymmetry, average drag coefficient values obtained at z = 1 m (C-100) are found to be 0.0021 and 0.0029 for flood and ebb tides respectively. Systematic changes i n bed roughness are not detected. Using a momentum balance approach, t he average drag coefficient value (C-d) at z = 10 m is found to be 0.0 056. Changes in 10-min average C-d values over sand waves during the t idal cycle are found to be small with bedform asymmetry having no dete ctable effect. Correlation between C-d and C-100 is found to be poor a nd separation of skin friction and form drag terms is not possible wit h existing measurements. The inclusion of form drag in C-100 values at the present site leads to over-estimation of the bed shear stress (ta u) available to mobilize and transport sediment. Mobile sediment, dete cted through the use of tracers and a transmissometer, was not found t o have any measurable effect on either C-d or C-100 in calm weather co nditions. (C) 1995 Academic Press Limited