Collaborative Research: Wave driven flow through a shallow, fringing reef

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Coral reefs face two significant long-term challenges to their existence: (1) Ocean acidification, which hinders the ability of corals to calcify; and (2) Ocean warming, which leads to coral bleaching. The Ofu reef (American Samoa), home to corals that seem to have exceptional bleaching resistance, is the location of active research into how corals may be able to meet those challenges biologically and biogeochemically. Given that both carbon system and temperature dynamics on the reef depend on flow, the physical insights and quantitative results from the data and modeling of this study should provide significant input to studies of ecology and biogeochemistry at Ofu and other similar reefs. In addition, the project will advance the technology for mapping reefs at high resolution. The project will involve several undergraduate and graduate students, as well as a postdoctoral scientist, providing unique material for university courses. The results of this study will be brought to the general public through a series of short online videos, produced to explain the link between hydrodynamic processes and reef health.The goals of this project are: (1) Test the concept that the relative importance of friction in the lagoon and friction on the reef crest determines the structure of the overall circulation; (2) Test the hypothesis that drag coefficients related to local, spatially averaged, flows can be estimated from data on reef topography; (3) Carry out a quantitative evaluation of the dynamics of Ofu reef flows using a depth-averaged circulation model based on detailed bathymetry and estimates of spatially variable drag parameterizations. The primary activities of this project would be: (a) Mapping of reef bathymetry at decimeter scale using an Unmanned Aerial Vehicle carrying cameras and novel image processing of the resulting images; (b) Deploying a set pressure loggers, temperature loggers, acoustic Doppler current profilers and a meteorological station for a 2-km long section of the Ofu reef for two one-month periods and analyzing the results of these observations to estimate flow properties such as drag; (c) Idealized and realistic modeling studies carried out using a three dimensional model. The design of the instrument array in combination with the mapping and modeling is intended to both test the conceptual circulation model and to evaluate how drag on different parts of the reef varies with overall depth, flow and with geometric parameters describing the physical structure of the reef.