Form drag due to flow separation at a headland

Observational and model estimates of the form drag on Three Tree Point, a headland located in a tidal channel of Puget Sound, Washington, are presented. Subsurface, Three Tree Point is a sloping ridge. Tidal flow over this ridge gives rise to internal lee waves that lead to wave drag and enhanced mixing. At the same time, horizontal flow separation produces a headland eddy that distorts the surface height field in the lee of the point. Two observational methods for estimating the portion of the form drag associated with deformation of the surface height field, referred to here as the “external” form drag, are also introduced. Drogued drifters and ship-mounted acoustic current profiles from different days are used to indirectly map the flood-tide surface height field. Data are derived from a depth shallow enough that baroclinic pressure gradient forcing may be neglected, and yet deep enough that wind stress may also be ignored. This leaves an approximate balance between the acceleration and surface height pressure gradient, permitting, in this case, two independent estimates of the surface height (to within a constant). These fields are used to calculate the external form drag at the headland. Drag estimates from both observational datasets agree well. External form drag decreases offshore of the headland as expected, and is highly dependent on tidal phase, with maximum drag leading peak flood currents by 1–2 h at this location. Form drag is much larger than model estimates of the frictional drag, implying that it is the dominant mechanism extracting energy from the barotropic tide. A kinematic argument is also presented to show why the external form drag should increase in importance relative to the frictional drag as the topographic slope and tidal excursion increase.