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On the reef scale hydrodynamics at Sodwana Bay, South Africa

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Abstract

The hydrodynamics of coral reefs strongly influences their biological functioning, impacting processes such as nutrient availability and uptake, recruitment success and bleaching. For example, coral reefs located in oligotrophic regions depend on upwelling for nutrient supply. Coral reefs at Sodwana Bay, located on the east coast of South Africa, are an example of high latitude marginal reefs. These reefs are subjected to complex hydrodynamic forcings due to the interaction between the strong Agulhas current and the highly variable topography of the region. In this study, we explore the reef scale hydrodynamics resulting from the bathymetry for two steady current scenarios at Two-Mile Reef (TMR) using a combination of field data and numerical simulations. The influence of tides or waves was not considered for this study as well as reef-scale roughness. Tilt current meters with onboard temperature sensors were deployed at selected locations within TMR. We used field observations to identify the dominant flow conditions on the reef for numerical simulations that focused on the hydrodynamics driven by mean currents. During the field campaign, southerly currents were the predominant flow feature with occasional flow reversals to the north. Northerly currents were associated with greater variability towards the southern end of TMR. Numerical simulations showed that Jesser Point was central to the development of flow features for both the northerly and southerly current scenarios. High current variability in the south of TMR during reverse currents is related to the formation of Kelvin-Helmholtz type shear instabilities along the outer edge of an eddy formed north of Jesser Point. Furthermore, downward vertical velocities were computed along the offshore shelf at TMR during southerly currents. Current reversals caused a change in vertical velocities to an upward direction due to the orientation of the bathymetry relative to flow directions.

Highlights

  • A predominant southerly current was measured at Two-Mile Reef with occasional reversals towards the north.

  • Field observations indicated that northerly currents are spatially varied along Two-Mile Reef.

  • Simulation of reverse currents show the formation of a separated flow due to interaction with Jesser Point with Kelvin–Helmholtz type shear instabilities along the seaward edge.

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Acknowledgements

We thank the iSimangaliso Wetland Park Authority for approving and supporting this research project in the World Heritage Site. We thank Petro and Christo van Jaarsveld from the SeaXplore Dive Centre for their generous support of fieldwork including deployment and retrieval of instruments. Finally, the authors are grateful to the South African Weather Service for the wind data provided.

Funding

DDS thanks the eThekwini Municipality for the financial support of a sponsored Chair in Civil Engineering at UKZN, which includes bursary funds and running costs to support AD and CW during their PhD studies. JP is sponsored by the National Research Foundation UID: 127361

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Authors and Affiliations

  1. Environmental Fluid Mechanics Lab, University of KwaZulu-Natal, Durban, South Africa

    Atish Deoraj, Calvin Wells, Justin Pringle & Derek Stretch

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Contributions

The topic of study was conceptualised by DS and JP. Development and deployment of the tilt current meter for field measurements were undertaken by CW. The numerical model was developed by AD with assistance from DS, JP and CW. The first draft of the manuscript was written by AD and CW. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Correspondence to Justin Pringle.

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Deoraj, A., Wells, C., Pringle, J. et al. On the reef scale hydrodynamics at Sodwana Bay, South Africa. Environ Fluid Mech 23, 1261–1282 (2023). https://doi.org/10.1007/s10652-022-09896-9

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