Estimates of Form Drag from Drifter Tracks

Estimates of the barotropic component of the form drag due to tidal flow past a sharp headland are presented. The calculation uses GPS drifter data obtained over a nine-day period in June 2002, at Three Tree Point (TTP) in Puget Sound, WA. Ten drifters were drogued at 20 meters depth and seeded throughout the flood tide near the headland. Daily drifter tracks were time-referenced to maximum flood and all tracks were then combined in order to map the surface height deformation around TTP. The surface height field is estimated (to within a constant) using the horizontal momentum equations. The Lagrangian acceleration and Coriolis terms are measured directly by the drifters. The baroclinic contribution to the pressure gradient is negligible because the drogues are shallow relative to the bathymetry. The frictional contribution is also negligible, because the drogues are below the surface boundary layer, leaving only the barotropic pressure gradient to balance the acceleration. The resulting surface height anomaly field may be used with bathymetric information to arrive at estimates of the form drag. Variations in the form drag along TTP, and in time, are discussed. The net form drag is at least an order of magnitude greater than the frictional boundary layer drag in this location.