| Summary: | Thesis (MEng (Civil Engineering))--University of Stellenbosch, 1999. === 100 leaves single printed pages, preliminary pages and numberd pages 1.1-9.1.Includes bibliography. List of figures, tables, symbols and acronyms. Scanned with a HP Scanjet 8250 Scanner to pdf format (OCR). === ENGLISH ABSTRACT:
A tropical cyclone is a non-frontal, synoptic scale, low-pressure system over tropical or subtropical
waters with organised convection and a definite cyclonic surface wind circulation. The system varies in size between a hundred and a few thousand kilometres in diameter with high
winds circulating around a central low pressure. The process of bringing the lower atmospheric
layers into thermodynamic equilibrium with the warm tropical waters add the energy to the
atmosphere and lower the surface pressure. If favourable climatic conditions exist this leads to
the formation of a warm core vortex, which can develop into a tropical cyclone. The occurrence
of tropical cyclones follows seasonal variations, the tropical cyclone season for the Southwest
Indian Ocean being between November and March. The occurrences peak along the Southern
African East Coast between Mid-January and Mid-February.
The data on the location and intensity of tropical cyclones along the Southeast Africa coastline
were obtained from the Joint Typhoon Warning Centre and span the period between 1848 and
1999. The available data before 1945 consist of tropical cyclone tracks that influenced
populated areas or were encountered by ships. It was assumed that a number of tropical
cyclones before 1945 were not recorded and therefore data collected before 1945 were
disregarded in the analysis. The development of radar in 1945 significantly improved the
detection of tropical cyclones. Some of the tropical cyclone tracks recorded between 1945 and
1956 contain information about the intensity of the tropical cyclone. Since the dawn of the
satellite age in the mid 1980's, the detection of tropical cyclones and intensity measurements
have improved vastly.
Monte Carlo simulation techniques were used to create long term data series based on the
available measured data. Statistical distributions were fitted for each characteristic describing
the tropical cyclone at its nearest position to the site under investigation.
Tropical cyclones frequently occur along the Southern African East Coast. The region where
more than one tropical cyclone per 100 years can be expected is bordered by latitudes 2.5°S to
32.5°S. The design parameters for structures in these regions should provide for the influence
that a tropical cyclone will have on the site. The occurrence rate and expected maximum
intensity of tropical cyclones with a 1DO-year return period vary with latitude along the Southern
African East Coast. The maximum number of tropical cyclones in a 1DO-year period occurs at
latitude 15°S with an expected number of tropical cyclones of 157.2 per 100 years. The
maximum expected tropical cyclone intensity in a 100-year period is 143.5 knots at latitude 17.5°S. === AFRIKAANSE OPSOMMING: Tropiese siklone is nie-frontale laagdrukstelsels wat hulle ontstaan het oor tropiese en subtropiese
oseane. 'n Stelsel bestaan uit 'n sentrale laagdrukstelsel met sirkulerende winde daar
om. 'n Sikloon se deursnee kan wissel van 'n honderd tot 'n paar duisend kilometer. 'n
Laagdrukstelsel ontstaan as gevolg van 'n termodinamiese wanbalans tussen die atmosfeer en
die warm oseaanwater in die trope. Indien die benodigde atmosferiese toestande heers kan die
laagdrukstelsel in 'n tropiese sikloon ontwikkel. In die Suidwestelike Indiese Oseaan vorm
tropiese siklone tussen November en Maart. Die meeste siklone kom hier voor vanaf middel
Januarie tot middel Februarie.
Data is verkry vanaf die "Joint Typhoon Warning Centre" vir die Suidwestelike Indiese Oseaan
en strek vanaf 1848 tot 1999. Die data voor 1945 verteenwoordig slegs die tropiese siklone wat
bewoonde areas of skeepsvaart beinvloed het. Daar is aangeneem dat 'n betekenisvolle getal
van die tropiese siklone voor 1945 nie gedokumenteer is nie en derhalwe is slegs data van
sikloon voorkomste na 1945 gebruik in die studie. Vanaf 1945 het die ontwikkeling van radar die
opsporing van siklone in onbewoonde areas moontlik gemaak. Die gebruik van weersatelliete
vanaf die middel 1980's het die kwaliteit van die data nog verder verbeter.
Monte Carlo simulasie tegnieke is gebruik om langtermyn data vir spesifieke posisies langs die
kus te genereer. Statistiese verdelings is gepas op die eienskappe wat die sikloon beskryf
wanneer dit die naaste posisie aan die terrein bereik. Die passing van die verdelings is gedoen
op die beskikbare historiese data. Die verdelings is dan gebruik om langtermyn data stelle te
skep vir die terrein.
Tropiese siklone kom gereeld in die Suidwestelike Indiese Oseaan voor en beinvloed die Suid-Afrikaanse
Ooskus. Meer as een tropiese sikloon kan elke 100 jaar verwag word in kusgebiede
tussen breedtegrade 2.5° S en 32.5° S. Die ontwerpe vir strukture in die gebied moet dus
voorsiening maak vir die invloed van tropiese siklone. Die voorkoms en intensiteit van tropiese
siklone varieer met breedtegraad langs die Suid-Afrikaanse Ooskus. Die meeste siklone word
verwag by breedtegraad 15°S met 'n gemiddelde van 157.2 siklone per 100 jaar. Die mees
intensiewe siklone kom voor by breedtegraad 17.5°S met 'n verwagte 1:100 jaar intensiteit van
143.5 knope.
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