Measurement of Tidal Form Drag Using Seafloor Pressure Sensors

As currents flow over rough topography, the pressure difference between the up- and downstream sides results in form drag—a force that opposes the flow. Measuring form drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography. An array of bottom pressure sensors was used to measure the tidal form drag on a sloping ridge in 200 m of water that forms a 1-km headland at the surface in Puget Sound, Washington. The form drag per unit length of the ridge reached 13 104 N m21 during peak flood tides. The tidally averaged power removed from the tidal currents by form drag was 0.2 W m22, which is 30 times larger than power losses to friction. Form drag is best parameterized by a linear wave drag law as opposed to a bluff body drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography. Maximum turbulent kinetic energy dissipation rates of 5 3 1025 W kg21 were measured with a microstructure profiler and are estimated to account for 25%–50 % of energy lost from the tides. This study is among the first to measure form drag directly using bottom pressure sensors. The measurement and analysis techniques presented here are suitable for period-ically reversing flows because they require the removal of a time-mean signal. The advantage of this technique is that it delivers a continuous record of form drag and is much less ship intensive compared to previous methods for estimation of the bottom pressure field. 1