This study will explore the physics of "form drag," and its importance to the dissipation of tidal energy in coastal regions. Form drag is that part of the total drag experienced by a fluid flow that arises from pressure differences across an obstacle, in this case rough topography. Form drag is typically associated with the generation of large, coherent flow features, such as tidal eddies and internal waves. The study therefore focuses in part on the physical mechanisms governing the creation and decay of these eddies and waves. The importance of this type of drag has come to be seen as a leading candidate for the dissipation of tidal energy in the deep ocean, whereas in the coastal ocean the more classical type of drag, bottom boundary layer friction, is assumed to be of greater importance. However, the PIs' recent work on tidal flow past the Three Tree Point headland in Puget Sound, WA demonstrated that form drag was, by at least a factor of 20, the dominant mechanism extracting energy from the tides in that region of the Sound. They will continue that line of work, with the overall goal of predicting the net drag (and loss of energy) a current will experience when flowing over the complex topography of any coastal region. In addition the PIs seek to understand the dynamics and dissipation mechanisms of tilted eddies in a stratified fluid, which are key processes in tidal dissipation. The current study has two main objectives. The first is to perform further analysis of existing observations, along with new numerical simulations and lab experiments, to determine the tilted eddy potential vorticity evolution mechanisms. The second component is a numerical investigation of form drag with the goal of developing a practical method of (i) its calculation in complex channel shapes, and (ii) its parameterization in numerical models.
Broader Impacts This study will help to improve the predictive skill of coastal numerical circulation models, by increasing our understanding of the effects of unresolved rough topography. Numerical models are important tools used both for conceptual understanding of the ocean and for predicting outcomes of practical management decisions. Improvements to model accuracy will have general scientific and societal benefits through such uses. This study will also contribute to education through the involvement of two graduate students and three undergraduates.
