Thesis (MEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The Touw River estuary and Wilderness coastal lakes is a sensitive system from a flooding and ecological viewpoint
and, therefore, careful consideration is placed on the hydrodynamics and salinity levels within the system. The estuary
consists of a “temporary open/closed” estuary, where during closed mouth conditions, the sand bar at the estuary
mouth is artificially managed in an attempt to reduce flood water levels in the system. The reason behind this
management strategy is the construction of residential property along the flood plains of the estuary and coastal lakes,
which in the past, had been exposed to regular cycles of inundation during flood events. In an attempt to reduce flood
water levels in all water bodies and hence reduce the risk of inundation, a management policy was formulated. The
past and present management plan is to maintain the sand bar at Touw estuary mouth, during closed mouth conditions,
at an elevation of between +2.1m to +2.4m MSL, based on proposals made by the CSIR in 1981.
Recent flood events, after the implementation of the management policy, still occasionally result in significant
inundation of residential property, which has raised concern for some interested parties over the effectiveness of the
management strategy. Furthermore, a growing concern was also evident over the long term wellbeing of the system
from an ecological viewpoint. Historical data shows significant changes in salinity levels since the implementation of
the management strategy which could impose negative long term effects on the system.
In this study, numerical models were consequently constructed and applied in order to analyse the effectiveness of the
current management policy and recalculate flood water levels under a number of proposed scenarios. Long term
salinity changes were also analysed in an attempt to better understand salinity propagation throughout the system,
using extreme hypothetical cases.
Through the analysis of the simulation results, it was concluded that flood water levels in the Touw estuary were
almost completely dependent on the size of the Touw River flood and the initial height of the sand bar at the estuary
mouth. Whereas, water levels in the coastal lakes are almost entirely dependent on the quantity of runoff into the lakes
and their initial water levels. The current management plan, involving only artificial manipulation of the sand bar at
the estuary mouth, therefore has a fairly insignificant effect on flood water levels achieved in the coastal lakes.
Furthermore, it was concluded that the construction of the preparatory channel is a vitally important aspect of the
current management plan and that skimming of the sand bar alone is ineffective to completely mitigate the risk of
residential inundation along the banks of the Touw River.
The salinity modelling study provided a first indication of the salinity characteristics within the system. It was found
that the penetration of seawater into the system was less prominent as the water bodies became further removed from
the ocean and that a direct relationship was evident between the volume of direct freshwater inflow to a water body
and the degree of salinity variation in that specific water body. In water bodies with high volumes of direct freshwater
inflow such as the Touw estuary, a large degree of salinity variation is evident. However, in water bodies with no
freshwater inflow, such as Rondevlei, salinity levels remain more stable and are less likely to fluctuate. / AFRIKAANSE OPSOMMING: Uit ‘n vloed- en ekologiese oogpunt is die Touwsriviermonding en Wilderniskusmere ‘n uiters sensitiewe stelsel en
daar is dus deeglike oorweging gegee aan die hidrodinamika en soutvlakke in die stelsel. Die monding bestaan uit 'n
"tydelike oop / geslote" monding, en tydens geslote mondtoestande word die sandbank by die riviermond kunsmatig
beheer in 'n poging om vloedwatervlakke binne die stelsel te verminder. Die rede vir hierdie strategie is omdat baie
residensiële eiendomme langs die vloedvlaktes van die monding en kusmere gebou is, wat in die verlede aan 'n
gereelde siklus van oorstromings blootgestel is tydens vloede. In 'n poging om vloedwatervlakke in al die watermassas
te verminder, en sodoende die risiko van oorstroming te verminder, is 'n bestuursbeleid geformuleer. In beide die
vorige en die huidige bestuursplanne is die sandbank in die Touwsriviermond tydens geslote mondtoestande in stand
gehou op 'n hoogte van tussen 2,1 m en 2,4 m MSL, gebaseer op die voorstelle wat deur die WNNR in 1981 gemaak
is.
Onlangse vloede wat plaasgevind het na die implementering van die beleid, het steeds van tyd tot tyd gelei tot
noemenswaardige oorstromings van residensiële eiendomme, en kommer is uitgespreek deur 'n paar belanghebbende
partye oor die doeltreffendheid van die strategie vir die bestuur. Daar is verder kommer uitgespreek oor die
langtermyn welstand van die stelsel uit 'n ekologiese oogpunt. Historiese data toon 'n beduidende verandering in
soutvlakke sedert die implementering van die bestuurstrategie met ‘n negatiewe langtermyn uitwerking op die stelsel.
In hierdie studie is daar derhalwe numeriese modelle opgestel en toegepas ten einde die doeltreffendheid van die
huidige bestuur van die beleid te bepaal, asook om die vloedvlakke te herbereken en te analiseer na aanleiding van 'n
aantal voorgestelde scenario's. Langtermyn soutgehalte veranderinge is ook ontleed in 'n poging om die soutgehalte
verspreiding deur die hele stelsel beter te verstaan, deur gebruik te maak van uiterste hipotetiese gevalle.
Deur die ontleding van die simulasie resultate, is daar tot die gevolgtrekking gekom dat vloedwatervlakke in die
Touwsrivier-monding byna heeltemal afhanklik was van die grootte van die Touwsrivier vloed en die aanvanklike
hoogte van die sandbank by die riviermond. Watervlakke in die kusmere is egter byna heeltemal afhanklik van die
hoeveelheid afloop na die mere en die aanvanklike watervlakke. Die huidige bestuursplan, wat slegs ‘n kunsmatige
manipulasie van die sandbank by die riviermond behels, het dus 'n redelik onbeduidend invloed op die
vloedwatervlakke wat in die kusmere bereik is. Daar is verder tot die gevolgtrekking gekom dat die konstruksie van
die voorbereidende kanaal 'n uiters belangrike aspek van die huidige bestuursplan is, en dat die afskraping van die
sandbank alleen oneffektief sou wees om die risiko van residensiële oorstroming langs die oewer van die Touwsrivier
uit te skakel.
Die soutgehalte modelleringstudie verskaf 'n eerste aanduiding van die soutgehalte eienskappe binne die stelsel. Daar
is gevind dat die penetrasie van seewater in die stelsel minder prominent was as in die watermassas verder van die see
af, en dat daar 'n duidelike direkte verband is tussen die volume van die varswater wat direk invloei na 'n watermassa
en die mate van soutgehalte variasie in daardie spesifieke watermassa. In watermassas waar hoë volumes varswater
direk invloei soos die Touwsrivier-monding, is 'n groot mate van soutgehalte variasie sigbaar. In die watermassas waar
geen varswater invloei nie, soos die Rondevlei, bly soutvlakke meer stabiel en is minder geneig om te wissel.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/85775 |
| Date | 12 1900 |
| Creators | Donald, Ian R. |
| 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 | xiv, 151 p. : ill. |
| Rights | Stellenbosch University |
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