The Dwarskersbos, South Africa local tsunami of August 27, 1969: field survey and simulation as a meteorological event

Emile A. Okal*, Johan N J Visser, Coenraad H. de Beer

*Corresponding author for this work

Research output: Contribution to journalArticle

16 Scopus citations

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 N101∘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.

Original languageEnglish (US)
Pages (from-to)251-268
Number of pages18
JournalNatural Hazards
Volume74
Issue number1
DOIs
StatePublished - Jan 1 2014

ASJC Scopus subject areas

  • Water Science and Technology
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)

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