Form Drag and Mixing Due to Tidal Flow past a Sharp Point

Abstract

Barotropic tidal currents flowing over rough topography may be slowed by two bottom boundary–related processes: tangential stress of the bottom boundary layer, which is generally well represented by a quadratic drag law, and normal stress from bottom pressure, known as form drag. Form drag is rarely estimated from oceanic observations because it is difficult to measure the bottom pressure over a large spatial domain. The “external” and “internal” components of the form drag are associated, respectively, with sea surface and isopycnals deformations. This study presents model and observational estimates of the components of drag for Three Tree Point, a sloping ridge projecting 1 km into Puget Sound, Washington. Internal form drag was integrated from repeat microstructure sections and exceeded the net drag due to bottom friction by a factor of 10–50 during maximum flood. In observations and numerical simulations, form drag was produced by a lee wave, as well as by horizontal flow separation in the model. The external form drag was not measured, but in numerical simulations was found to be comparable to the internal form drag. Form drag appears to be the primary mechanism for extracting energy from the barotropic tide. Turbulent buoyancy flux is strongest near the ridge in both observations and model results.