Collaborative Research: Relating Topographic Complexity and Circulation Patterns on Coral Reefs from Colony-Scale to Reef-Scale

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Many important coastal marine systems either grow on or form complex topography that varies continuously over a wide range of spatial scales. For example, on coral reefs, topography varies from branch (centimeter) to patch (10-100 meters) to reef (several kilometers) scales. In the coastal ocean, water motion is forced past topography at a wide range of time scales by surface waves, tides and slowly varying currents. Understanding how these flows interact with complex multi-scale topography is critical for predicting circulation patterns in shallow coastal systems like reefs. At present, the physics of these interactions is not included explicitly in observational efforts or modeling studies and one of the biggest challenges for predictive modeling of circulation in coastal systems like reefs is a lack of methods for a priori estimation of drag and mixing parameters from topography spatial statistics. This project will provide a theoretical and conceptual framework for flow over complex topography in the coastal ocean that includes the physics of interactions of currents and waves with multi-scale topography. New parameterizations for flow over complex topography will be developed for use in ocean circulation models to improve their predictive ability in important coastal systems like coral reefs. Improved predictions of circulation over reefs should then lead to better estimates of cross-reef exchange and transport of nutrients and contaminants, as well as a better description of larval retention, dispersion and connectivity. The field study will be conducted at the NSF-supported Moorea Coral Reef Long Term Ecological Research (MCR-LTER) site which brings together many leaders in coral reef science. Through planned interactions with MCR-LTER and French/Tahitian researchers the results will directly reach a multi-disciplinary international audience. A PhD student will be trained in physical oceanographic field work, modeling, and theory, and gain interdisciplinary and international experience by interacting with the MCR-LTER. At least nine undergraduate students will conduct independent studies and write honors theses related to the project.In this project, a theoretical and conceptual framework will be developed for the interaction of currents and waves with multi-scale topography. A set of novel field measurements and simulations will examine how flow with different time scales interacts with topography characterized by different obstacle size, spacing, and patchiness. Topography spatial statistics will be computed from high-resolution satellite bathymetry and 3D scanning sonar measurements. Spatial variability in currents on the reef at scales from 0.2-500 meters will be quantified using a nested sampling array. Field measurements over constructed geometries will investigate how natural flow that varies at a range of time scales interacts with bottom topography with different length scales. Numerical modeling of steady and unsteady flow over idealized reef geometries and real reef segments will examine dynamics of flow-topography interactions across the range of currents, waves, and topography length scales at the field site.