Abstract
We investigate the hitherto unexplained wave which inundated the village of Dwarskersbos, South Africa, in the early hours of August 27, 1969, in the absence of any seismic disturbance or major meteorological storm. A field survey, based on the interview of nine elderly witnesses still residing in the community, documented maximum run-up of 2.9 m, concentrated on an extremely short segment of coastline, less than 2 km in length. These characteristics are incompatible with generation by a seismic source (which, at any rate, should have been felt by the population). A landslide source, located at the only canyon featuring a steep enough ocean floor, is also ruled out since a numerical simulation fails to reproduce the concentration of the wave at Dwarskersbos. By contrast, the wave can be explained as a “meteo-tsunami” resulting from resonance between a meteorological squall propagating at 18 m/s in the azimuth \(\hbox {N101}^{\circ }\hbox {E}\) and a gravity wave propagating in the shallow waters off the eastern shore of St. Helena Bay. This is confirmed by numerical simulation under the formalism of Proudman (Dynamical oceanography. Methuen, London, 1953), which provides a satisfactory model of the distribution of run-up along the beach.
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Acknowledgments
We thank Wilhelm van Zyl of the Marine Geoscience Unit, Council of Geoscience, for a digitized map of the bathymetry of St. Helena Bay, and Mr. Theuns Smit of Dwarskersbos for access to a 1967 land-surveyor’s map of the future town layout. Mr. Nikos Kalligeris helped in the initial aspects of the landslide simulation. Discussions are also acknowledged with Dr. Nasr-Eddine Taibi during the RASMER meeting in Zeralda, Algeria, in June 2013. Constructive comments on the original version of the paper by Editor Alexander Rabinovich and two anonymous reviewers are gratefully acknowledged. We are grateful to Emily Wolin for help with the final production of the manuscript, especially Fig. 3.
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Okal, E.A., Visser, J.N.J., de Beer, C.H. (2014). The Dwarskersbos, South Africa local tsunami of August 27, 1969: field survey and simulation as a meteorological event. In: Vilibić, I., Monserrat, S., Rabinovich, A.B. (eds) Meteorological Tsunamis: The U.S. East Coast and Other Coastal Regions. Springer, Cham. https://doi.org/10.1007/978-3-319-12712-5_14
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