Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The various consequences of climate change pose a significant threat to developments near the
coast. These threats include saltwater intrusion, coastal erosion and flooding. In the coastal context,
the climate change effect often raising the most concern is that of sea-level rise. Much work has
therefore been done on the linear setback caused by a rise in sea-level. In order to get the full
picture of possible changes caused by sea-level rise, the secondary effects of a rising sea-level also
need to be considered. Sea-level rise could cause changes to the nearshore wave climate and could
have impacts such as coastal erosion and changes to the coastline shape. The primary objective of
this study was therefore to investigate the effects of sea-level rise on the nearshore wave climate
and, consequently, the coastline stability.
Other consequences of climate change considered in this study include increasing average wave
heights and a rotation of offshore wave directions. The many headland-bay beaches on the South
African coastline are generally in a state of dynamic equilibrium and find their planforms based on
the local wave climate. Changes to the wave climate may therefore disrupt the equilibrium shapes of
these bays. This study was therefore also aimed at investigating the effects of the changes to the
wave climate on the stability of headland-bay beaches.
The three consequences of climate change expected to affect the nearshore wave climate were
identified as (1) sea-level rise; (2) an increased wave height; and (3) changing offshore wave angles.
Although changes to storm frequency and intensity are also possible, the impacts of these changes
were not studied.
In order to assess the impacts of the three considered changes on a typical headland-bay beach, two
numerical models were set up for Mossel Bay – a headland-bay beach on the southern coast of
South Africa. The modelling approach included a wave transformation model to calculate nearshore
wave climates from offshore data and a coastline model to assess the stability of the bay under the
changed nearshore wave climates.
The model results indicated that the rising sea-level alone would cause changes in the nearshore
wave direction. These changes were shown to alter the longshore sediment transport regime such
that rotations are expected in the south-western corner and eastern end of Mossel Bay. These rotations do not include the cross-shore effects of inundation and erosion, as suggested by models
such as the Bruun Model.
The results for an increased offshore wave height were inconclusive. The southerly rotation in
offshore wave angles was shown to affect the nearshore wave angles. These changes affected the longshore transport regime such that the outward sediment transports were reduced. A minor
accretion resulted in the centre of the bay for a 1° southerly rotation in offshore wave angles. For a
2° rotation, the extent of accretion increased and shifted towards the eastern end of the bay,
primarily due to the dominance of south-westerly waves in the local wave climate.
A valuable observation was made regarding the current stability of Mossel Bay. Inter-tidal reefs are
present along three sections of the bay. These reefs protect the coastline such that the current bay
shape contains sharp bends between the reefs. Under a rising sea-level, however, the effect of the
reefs will become less pronounced. If a water level should be reached where these reefs become
less significant, the planform of the bay is expected to smooth out through a significant
redistribution of sediment. This smoothing effect was shown to cause erosion of the coastline in the
order of 80m near the town of Klein Brak River. / AFRIKAANSE OPSOMMING: Die verskeie gevolge van klimaatsverandering bied ‘n merkwaardige bedreiging vir ontwikkelings
naby die kus. Hierdie bedreigings sluit die versouting van varswaterbronne, kuserosie en
oorstromings in. Vir kusgebiede is seevlakstyging gereeld die effek van klimaatsverandering wat die
meeste kommer wek. Dus is heelwat navorsing rakende die direkte erosie as gevolg van
seevlakstyging reeds gedoen. Om die volle beeld van die gevolge van ‘n stygende seevlak te verkry, is
dit egter nodig om ook die sekondêre effekte hiervan in ag te neem. Seevlakstyging kan
veranderinge in die golfklimaat naby die kus veroorsaak, en kan impakte soos kuserosie en
veranderende baaivorms tot gevolg hê. Die primêre doel van hierdie studie is dus om die effek van
seevlakstyging op die golfklimaat by die kus en gevolglik die stabiliteit van die kuslyn, te ondersoek.
Benewens die styging van die seevlak word die effekte van groter gemiddelde golfhoogtes en die
rotasie van diepsee golfrigtings ook in hierdie studie ondersoek. Die vele landpunt-baaie (headlandbay
beaches) op die Suid-Afrikaanse kus is meestal in ‘n dinamiese ekwilibriumtoestand, waarvan die
vorm deur die lokale golfklimaat bepaal word. Veranderinge aan dié golfklimaat mag dus die
ekwilibrium vorms van sulke baaie versteur. Hierdie studie het dus ook die stabilititeit van landpuntbaaie
onder ‘n veranderende golfklimaat ondersoek.
Die drie gevolge van klimaatsverandering wat verwag word om die golfklimaat naby die kus te
beïnvloed is geïdentifiseer as (1) seevlakstyging; (2) vergrote golfhoogtes; en (3) veranderende
diepsee golfhoeke. Veranderinge aan die frekwensie en intensiteit van storms is ook moontlike
gevolge van klimaatsverandering, maar die impakte hiervan is nie in die studie ondersoek nie.
Twee numeriese modelle is toegepas om die impak van die drie bogenoemde gevolge op Mosselbaai
– ‘n tipiese landpunt-baai aan die suidkus van Suid-Afrika – te ondersoek. ‘n Golfmodel is ingespan
om die golfklimaat naby die kus te bepaal waarna ‘n kuslynmodel gebruik is om die stabiliteit van die
baai onder die veranderde golfklimaat te ondersoek.
Die resultate van die studie dui daarop dat die golfhoeke naby die kus beïnvloed word deur
seevlakstyging. Daar is aangetoon dat dié veranderinge die langsstroomvervoer sodanig sal verander
dat kuslynrotasies in die suid-westelike hoek asook die oostelike rand van Mosselbaai verwag word.
Hierdie rotasies sluit nie die lineêre landwaartse verplasing van die kuslyn as gevolg van erosie en
oorstroming in nie.
Die effek van vergrote golfhoogtes kon nie met akkuraatheid ondersoek word nie. Daar is wel gevind
dat die suidwaartse rotasie van diepsee golfhoeke rotasies in die golfklimaat naby die kus
veroorsaak. Hierdie rotasies verander die langsstroom sedimentvervoer sodanig dat die uitwaartse sedimentvervoer verminder word en ‘n klein opbou van sediment in die middel van die baai vir ‘n 1°
diepsee rotasie verwag word. Vir ‘n 2° suidwaartse rotasie is daar ‘n groter opbou van sediment wat
verder ooswaarts veplaas is. Die ooswaartse veplasing is primêr ‘n gevolg van die oorheersing van
suid-westelike golftoestande in die golfklimaat.
‘n Waardevolle gevolgtrekking rakende die huidige stabiliteit van Mosselbaai is ook gemaak. Langs
drie gedeeltes van die Mosselbaaise kus word riwwe in die gebied tussen hoog- en laagwater
aangetref. Hierdie riwwe beskerm die kus sodanig dat skerp kinkels in die vorm van die baai tussen
die riwwe gesien kan word. Wanneer die seevlak styg, word die beskermende effek van die riwwe
egter minder doeltreffend. Indien ‘n watervlak bereik word waar dié effek genoegsaam verminder is,
word daar verwag dat die baai deur ‘n merkwaardige verplasing van sediment die kinkels sal uitstryk.
Deur hierdie proses word erosie in die orde van 80m naby die dorp van Klein Brakrivier verwag.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/79917 |
| Date | 03 1900 |
| Creators | Hugo, Pierre-Malan |
| Contributors | Toms, Geoff, 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 | Unknown |
| Type | Thesis |
| Format | xvi, 113 p. : ill. |
| Rights | Stellenbosch University |
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