Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The Western Cape has been subject to extreme rainfall storms in the last decade, which has
been responsible for millions of Rands worth of damage to public infrastructure and human
settlements. Current climate model projections under climate change suggest that the Western
Cape can expect greater rainfall intensities, with magnitude of rainfall events increasing, but the
frequency of occurrence of events decreasing. The aim of this research is to assess if historical
short duration rainfall data provides any evidence to support increasing intensities in rainfall in
the Western Cape.
Short duration rainfall data (< 24 hours), consisting of digitised autographic- and automatic
weather station data (AWS), was selected from the South African Weather Services’ database
for the Western Cape and wider South African region, in order to combine the data for an
extension of the effective record length. Numerous difficulties were encountered with the data
that required the application of editing and quality control procedures. The digitised autographic
data contained many errors and generally compared poorly to standard gauge daily rainfall
totals of the same period. After the application of editing and quality control procedures, seven
stations were selected and their autographic and AWS data was combined for further detailed
analysis. Analysis was divided into two sections, one for the analysis of the magnitude of the rainfall,
the other for the frequency of occurrence of rainfall events. For the magnitude analysis, nonstationary
extreme value theory was applied by implementing a parametric and non-parametric
non-stationary approach to both the generalised extreme value distribution (GEV) with an annual
maximum series, and a generalised Pareto distribution with a peaks over threshold series.
The parametric approach entailed fitting linear models to the parameters of the extreme value
distributions, and the non-parametric approach tested if return levels of the distributions remained
constant over a moving window period. The frequency analysis entailed the recording
of the annual number of rainfall events exceeding predetermined threshold values.
For the magnitude analysis, the parametric approach only produced two stations with significant
non-stationarity and only for the GEV, whilst the non-stationary approach produced results that
can indicate non-parametric behaviour, but this was most likely because of the combination of
autographic and AWS data. The frequency analysis produced results that indicated no observable
results for some stations, while others produced trends that are consistent with a change
from the autographic to AWS data.
This lead to the conclusion that with the available data, no clear or significant evidence supporting
increasing intensities or any other change in short duration rainfall was found regarding the
magnitude and frequency of occurrence of rainfall events. / AFRIKAANSE OPSOMMING: The Wes-Kaap het die afgelope dekade deur uiterse storms deurgeloop wat miljone Rande se
skade aan publieke infrastruktuur en nedersettings veroorsaak het. Huidige klimaatsmodel resultate
wat klimaatsverandering inkorporeer, beraam dat die Wes-Kaap hoër reënvalintensiteite
kan verwag, waar die grootte van die en reënvalgebeurtenisse toeneem, maar die frekwensie van
die reënvalgebeurtenisse afneem. Die doel van hierdie navorsing is om te bepaal of historiese
kortduurtereënvaldata in die Wes-Kaap enige toenemende reënvalintensiteite aandui.
Kortduurtereënvaldata (< 24 uur) wat uit versyferde autografiese- en automatiese weerstasie
(AWS) data bestaan, was uit die Suid Afrikaanse Weerdienste se databasis gekies vir stasies in
dieWes-Kaap en die wyer Suid Afrikaanse gebied, sodoende om die effektiewe rekordlengte te
verleng. Redigering- en kwaliteitsbeheerprosudures moes toegepas word na ’n aantal probleme
met die data ontdek was. Die versyferde autografiese data het baie foute bevat en oor die
algemeen swak vergelyk met die standaard daaglikse totaalmetings van dieselfde periode. Na
die redigering- en kwaliteitsbeheerprosudures toegepas was, was sewe stasies gekies en hul
autografiese en AWS data was saamgestel vir verdere analise. Die analise het uit twee afdelings bestaan: die analise van die grootte van die reënvalgebeurtenisse,
en die frekwensie van die reënvalgebeurtenisse. Nie-stasionêre ekstreemwaardeteorie
was toegepas op die grootte van die reënvalgebeurtenisse deur gebruik te maak van parametriese
en nie-parametriese nie-stasionêre metodiek. Die veralgemeende ekstreemwaardeverdeling
(AEW) met ’n jaarlikse maksimum reeks, en die veralgemeende Paretoverdeling met ’n
pieke bo drempelwaarde reeks was gebruik. Die parametriese metode het bestaan uit die passing
van lineêre modelle aan die parameters van die estreemwaardeverdelings. Die nie-parametriese
metode het getoets of die terugkeervlakke van die verdelings konstant gebly het oor ’n bewegende
vensterperiode. The frekwensie analise het bestaan uit die aantekening van die jaarlikse
reënvalgebeurtenisse wat bepaalde drempelwaardes oorskry.
Die parametriese metode het net twee beduidende nie-stasionêre staties opgelewer, uitsluitlik
vir die AEW. Die nie-parametriese metode het resultate gegenereer wat moontlike niestasionarieit
aandui, maar dit is heel moontlik as gevolg van die samestelling van die autografiesen
AWS data. Die frekwensie analise het óf geen sigbare neiging vir die resultate vir ván stasies
gegenereer nie, óf het resultate gegenereer wat aandui op die samestelling van die autografiesen
AWS data.
Dit het tot die gevolgtrekking gelei dat daar geen aanduiding in die kortduurtereënvaldata
bestaan wat toenemende reënvalintensiteite ondersteun nie vir beide die grootte en frekwensie
van reënvalgebeurtenisse.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71900 |
| Date | 12 1900 |
| Creators | Burger, Gysbertus Johannes |
| Contributors | Du Plessis, J. A., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | [175] p. : ill. |
| Rights | Stellenbosch University |
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