Altimetry comparison, backward wave refraction and extreme wave analysis of NCEP data: Cape Point to Richards Bay

[摘要] ENGLISH ABSTRACT: Offshore wave climate along the south and east coast of South Africa has been investigatedin this thesis. The design waves are needed to understand the offshore wave climate andnecessary for the design of structures in this region. The wave condition data was producedby the WAVEWATCH III numerical model, i.e. NCEP data. The ultimate aim of the study was to derive extreme wave estimates off the South African coastline and assessits accuracy through comparison of various datasets. Comparisons of Satellite Altimetrydata with NCEP and wave buoy data at two locations along the south coast (Cape Pointand FA-Platform) was performed. Thereafter, wave conditions, obtained from 'backward'refracting nearshore data to the offshore NCEP locations, were performed in an attempt tovalidate NCEP data for East London and Richards Bay. Two approaches were conductedin order to backward refract, namely; Snell's Law (serving as a first estimate) and SWAN(fully describing the wave processes). The SWAN approach achieved offshore conditionsby running a numerical model of various input conditions and creating a lookup table or matrix by means of an interpolation function using those general input conditions and outputresults. The methodology is important as neither the SWAN outputs nor the NCEPdata could be regarded as 'ground truth'. An Extreme Wave Analysis was performed forthe offshore NCEP data at six locations along the south and east coast of South Africa.An Exponential, Gumbel and 3-parameter Weibull distribution was conducted, where it was found that the latter was a best fit for the South African waters. An objective and conservative automated Peaks-over-Threshold (POT) value technique was used and wave heights for 1 in 1, 5, 10, 30, 50 and 100 year return periods with associated directions were obtained. Lastly, an analysis of the different distributions as well as percent exceedancegraphs of the peak period per site was performed and compared.

[发布机构] Stellenbosch University