ABSTRACT

This paper describes the methodology and selected results of a Joint Industry Project called WAX (West Africa Extremes) whose purpose was to develop estimates of extreme waves for the West Coast of Africa between 5° N and 15° S. Extremewaves there consist of swell propagated from distant sources associated with extra tropical storms in deep southern latitudes or from episodic enhancements of anti cyclonic southeast winds off South Africa. The basic steps of the hindcast method aredescribed: storm selection, specification of wind fields, application of a directional spectral wave prediction model, model validation and extremal analysis of results of hindcasts of the historical storm population selected. The validation consisted of a comparison of wave model predictions and measurements of significant wave height, peak spectral period and frequency spectra during several episodes of extreme swell conditions, selected from wave measurements recorded at a platform offshore Nigeria over a two-year period (1981-82) and over a 12-month period (1990-91) by a Wave rider buoy moored offshore Cabinda. It was found that despite the special difficulties of hind casting the wind fields of Southern Hemisphere (SH) storms caused by sparsity of meteorological data, the hindcast skill exhibited in these comparisons is comparable with the good agreement typically achieved in Northern Hemisphere (NH) basins with the hindcast methodology applied. The wave model used for these hindcasts has been well validated in NH basins mainly in extreme storm regimes where the source terms which model the physical mechanisms of wave growth and dissipation dominate the model solution. This SH validation in a swell dominated regime suggests that the wave propagation scheme used in the model is also quite accurate and that synoptic wind fields may bespecified with sufficient accuracy in the South Atlantic, at least for storms within the last two decades. The WAX extremes are derived from hindcasts of a population of 50 historical storms. The extremes are summarized for selected areas.

INTRODUCTION

The study described in this paper provides a new description of the extreme wave climate of the west coast of Africa, specifically the continental shelf between about 5° N and 15° S. This includes areas of current intense interest to the industryoffshore Angola, Benin, Cabinda, Congo, Equatorial Guinea, Gabon, Ghana, Ivory Coast, Nigeria and West Cameroon, and of course all areas of potential future interest within the study area. The study was carried out as a Joint Industry Project (lIP) called West Africa Extremes (WAX) which currently has 11 participants.

The extreme wave climate in the study area is quite unusual for a tropical oceanic region because tropical cyclones do not occur in the South Atlantic Ocean (SA). Extreme waves therefore are not generated locally but rather propagate to the area as swell, from distant sources of wave generation. Wave measurements have been made at some sites within the study area. In most instances these data are insufficient to allow reliable estimation of extremes. In a few areas, however, wave measurements have been made over several years and have been extrapolated to provide design estimates (Lebas et al., 1991; 1992).

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