Characteristics of ocean gravity waves off the Cape South West coast

Shillington, Frank Alan

Characteristics of ocean gravity waves off the Cape South West coast

Master Thesis

1974

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

Ocean gravity waves have been recorded near Melkbosstrand with a Wemelsfelder float type wave recorder on a sea tower situated in water 13m deep and one Kilometre offshore. The half hour records, taken twice daily between July 1972 - June 1973, have been analysed in a format uniform with that of Draper (1966). Measurements have been made from the wave records to check the value of the statistical ratios of Longuet- Higgins (1952) and Draper (1963). Ratios of maximum wave height to root mean square amplitude are lower than the theoretical values and reasons for this are discussed in the light of the records being taken in shallow water. Ratios of maximum wave height to the average upper one tenth wave height and average upper one third wave height agree closely with the theoretical values. Spectral analysis using the methods of Blackman and Tukey (1958), with 36 degrees of freedom and 100 lags have been applied to half hour records digitized once per second. Frequency, time diagrams contoured at equal energy values have been constructed for the period May and part of June 1973 on the basis of twice daily spectral measurements at 5 mHz intervals. The wave events, which include generation of swell from near South Georgia, (54°S, 37°W), have been compared with the South African Weather Bureau synoptic charts in order to identify generation centres. Repeated energy values near the 50 mHz band seem to be generated by winds of a lower velocity than required by Moskowitz (1964). Predominant siting of fetches at distances of the order of 1000 nautical miles accounts for the bulk of wave generation between latitudes 40° - 50°S. One storm appears to have originated near the Drake passage, but no events have been detected with waves passing through the passage. Analysis of wave spectra from several storms generating maximum wave heights over 5 metres, show that fully developed seas are seldom, if ever, present in Cape waters.

Keywords

Oceanography

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