Abstract
Regional storm tidal levels of the South African coastline are investigated by means of a calibrated and validated numerical model. The model was developed utilizing the shallow water hydrodynamic model, Delft3D. This model was coupled (online) with a non-stationary spectral wave model (developed in the Simulating WAves in the Nearshore (SWAN) numerical code). A local, 4.4 km version of the Unified Model was applied as atmospheric forcing for the coupled system. The models presented in this study form part of the operational marine forecasts of the South African Weather Service, Wave and Storm Surge model. The operational protocol and model calibration and validation are presented via statistical correlations with measured water levels at six South African coastal locations. The main calibration parameters and thus physical drivers were winds, atmospheric pressure and waves. The validated numerical model is used to provide an experimental physical description of South African storm surge characteristics, per coastline. The dominant driver of South African storm surge is winds associated with mid-latitude cyclones. Further novelty in the present study is the quantification of the relative contribution of extreme storm wave set-up to the South African storm surge signal. This wave set-up contributes approximately 20% of the total surge signal in the southwest, with winds contributing approximately 55%. The importance of the continental shelves is also elucidated concerning the frictional shoaling effects of the long surge wave propagation.
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Acknowledgements
The authors would like to thank the South African Navy Hydrographic Office (SANHO) for providing recorded sea surface elevations and high-resolution coastal bathymetry required for model calibration and validation. We acknowledge Transnet National Port Authorities (TNPA) for the directional wave rider buoy data used in the present study. We would also like to acknowledge the use of NCEP Reanalysis data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at https://www.esrl.noaa.gov/psd/. Thank you for making these data freely available. Finally, this work is based on the research supported in part by the National Research Foundation of South Africa (Grant Numbers: 116359).
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Dr. Christo Rautenbach wrote the manuscript. He formulated the research questions and the resulting methodology. He designed and ran the calibration and validation simulations and co-developed most of the post-processing scripts, statistics and figures. Me. Tania Daniels processed the Unified Model outputs and executed their statistical validation. She also wrote and contributed towards the atmospheric descriptions. Mr. Marc de Vos pre-processes the raw water level data obtained from SANHO and wave data obtained from TNPA. Mr. Michael A. Barnes helped with the post-processing of the model results and figure production. He also helped in scheduling and running the considerable amount of calibration simulations. All the authors contributed towards the general discussions and reasoning pertaining to the main research results and conclusions presented here.
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Appendix
Appendix
Final model validation results for Richards Bay during a July 2017 and b August 2017. Each upper panel indicates the modelled and measured water levels on the left axis, after the tidal signals have been filtered out. On the right-hand axis, the prevailing atmospheric pressure close to Richards Bay is provided. Each lower panel indicates the associated near shore UM wind speeds together with their directions as magenta, True North arrows
Final model validation results for Port Elizabeth during a July 2017 and b August 2017. Each upper panel indicates the modelled and measured water levels on the left axis, after the tidal signals have been filtered out. On the right-hand axis, the prevailing atmospheric pressure close to Port Elizabeth is provided. Each lower panel indicates the associated near shore UM wind speeds together with their directions as magenta, True North arrows
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Rautenbach, C., Daniels, T., de Vos, M. et al. A coupled wave, tide and storm surge operational forecasting system for South Africa: validation and physical description. Nat Hazards 103, 1407–1439 (2020). https://doi.org/10.1007/s11069-020-04042-4
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DOI: https://doi.org/10.1007/s11069-020-04042-4