Collaborative Research: Combined Waves and Currents over Multi-Scale Topography: From Boundary Layer Dynamics to Parameterization

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Multiscale bottom topography is ubiquitous in coastal systems. Interactions of surface waves and currents with this topography cause spatial patterns in pressure, velocities, and turbulence production that result in bottom drag, drive mixing, and dissipate wave energy. In most wave and circulation studies these processes are not resolved and are represented in bulk friction parameters, typically derived empirically. Dynamics of wavy flows over multiscale topography are not well understood and there is currently no method for computing friction parameters for waves and currents a priori from multiscale topography properties. This project will examine interactions of surface waves and currents with multiscale bottom topography, to investigate the controlling dynamics and develop appropriate bottom friction parameterization schemes, using modeling of flow over idealized and natural coral reef topography together with theoretical development. Implications for drag, wave dissipation and mixing will be addressed yielding improved understanding and modeling of reefs and similar systems. The project will inform interdisciplinary coral reef work through the PIs’ involvement with the Moorea Coral Reef LTER, and support an early career PI, a post-doc and a PhD student, and provide six undergraduates with research experiences. The project team will also develop and implement new K-12 outreach and education activities. In previous work, the PIs investigated boundary layer dynamics over topography characterized by a single length scale across a parameter range typical of reefs. Those results, along with analyses of reef topography, show that a range of topography length scales (cm - m) are likely to contribute substantially to bottom friction. This project will investigate how different topography length scales act together in multiscale topography to determine dynamics of the combined current and oscillatory flow, total drag on currents, and dissipation of wave energy. Using dynamical regimes for single-scale topography as a guide, the work will investigate the physics of combined waves and currents over multiscale topographies spanning different regimes (inertia-, drag-, stress-dominated) by conducting a series of computational fluid dynamics simulations in which key parameters (wave properties, current, topography length scales scaling properties) are systematically varied. Simulations will include topographies composed of superposed discrete length scales, surfaces with a continuous range of length scales, and reef topographies. Simulations (using OpenFOAM with LES closure) will resolve flow patterns down to roughness element scales. A spatially- and wave- ensemble-averaged Navier-Stokes framework will be applied to simulation results to analyze effects of roughness-element-scale processes on oscillatory and steady flow dynamics, and mechanisms by which energy is lost from waves and current will be quantified. These analyses will form the basis for new parameterizations for wave dissipation and drag on currents over multiscale topography that represent smaller-scale dynamics and can be incorporated into wave and circulation models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.