Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In the context of rapidly expanding cities, it is imperative that urban planning in South Africa has
sufficient guidance regarding stormwater and river corridor management, in order to provide
solutions that address issues of flood risk and the environmental health of river systems.
Attenuation of stormwater runoff, the focus of this study, is one of the most important structural
mechanisms used for the mitigation of many of the negative impacts caused by uncontrolled urban
runoff. Typically, it involves the use of attenuation ponds or wetlands, which temporarily store
runoff during a storm and release flow downstream at a reduced rate so as to mimic natural flow
patterns.
The focus of urban stormwater management and flood control has historically been on the
protection of human life and property. However, in recent decades, through growing environmental
awareness and the advancement of the concept of sustainable development, urban stormwater
management has become a growing field of research worldwide, with a broader focus which
considers not only flood control, but also water quality, aquatic biodiversity and the amenity value of
urban drainage systems. Flood attenuation controls are becoming more widely used within South
African urban areas, primarily due to policies or legislation brought into effect by local authorities.
However, there is often little understanding regarding the positive and perhaps negative effects that
these attenuation controls are having on receiving watercourses downstream.
Three case studies were assessed by means of stormwater modelling simulations to evaluate various
flood attenuation practices which are currently in use in South Africa. Two of the study areas, the
Mosselbank River Catchment and the Bayside Canal Catchment, were selected in areas of Cape
Town where future development has been proposed by spatial planners. The third study area, the
Upper Kuils River Catchment, was evaluated in terms of the performance of existing attenuation
facilities in an area which is already almost completely developed. The study found that attenuation
facilities constructed with a single culvert-type outlet structure, designed to reduce flows during
large storm events, do not mitigate the impact of post-development runoff occurring during lower
recurrence interval storm events. Attenuation facilities with multi-stage outlet structures were
found to be much more effective at mimicking pre-development flow during a range of storm
events. It was also found that because attenuation does not reduce post-development runoff
volumes to pre-development levels, but merely reduces peak flow rates, the cumulative runoff from
multiple attenuation controls across a large (>30 km2) urban catchment resulted in higher runoff
peaks in downstream watercourses.
The study concluded that more widespread use of stormwater Best Management Practices (BMPs)
and Sustainable Drainage System (SuDS) controls allows a greater portion of runoff to infiltrate,
resulting in less runoff volume and therefore reduced peak flows downstream, especially during low
recurrence interval storm events. In addition, the study recommended the use of detailed
catchment-wide stormwater modelling to understand specific catchment dynamics holistically, thus
increasing the potential for designing effective attenuation controls in urban stormwater systems. / AFRIKAANSE OPSOMMING: In die konteks van die vinnige tempo van stedelike uitbreiding, is dit noodsaaklik dat stedelike
beplanning in Suid-Afrika plaasvind met in aggenome van voldoende riglyne vir die bestuur van
stormwater en rivierkorridors, ten einde oplossings te vind vir die kwessies van vloedrisiko en die
omgewingsgesondheid van rivierstelsels. Vloedvertraging, wat die fokus van hierdie studie is, is een
van die belangrikste strukturele meganismes wat gebruik word vir die verligting van talle negatiewe
impakte wat veroorsaak word deur onbeheerde stormwaterafloop in stedelike gebiede. Tipies
behels dit die gebruik van vloedvertragingsdamme of vleilande, wat afloop vertraag tydens 'n storm
en dus vloei stroom-af teen 'n verlaagde tempo uitlaat met die doel om natuurlike vloeipatrone na
te boots.
Die fokus van stedelike stormwaterbestuur en vloedbeheer was in die verlede hoofsaaklik op die
beskerming van lewe en eiendom gefokus, maar het egter die afgelope dekades verskuif na water
gehalte, die biodiversiteit van waterekosisteme en die geriefswaarde van stedelike
dreineringstelsels. Hierdie verskuiwing van fokus is weens die groeiende omgewingsbewustheid en
die bevordering van die konsep van volhoubare ontwikkeling wat wêreldwyd 'n groter
navorsingsgebied geraak het. Vloedvertraging beheermeganismes word al hoe meer algemeen
gebruik in Suid-Afrikaanse stedelike gebiede, hoofsaaklik as gevolg van die beleide of wetgewing wat
deur plaaslike owerhede in werking gestel is. Daar is egter dikwels min begrip vir die positiewe en
moontlike negatiewe gevolge wat hierdie vertragingsmeganismes op stroom-af sisteme het.
Drie gevallestudies is geëvalueer deur middel van numeriese modelstudies wat verskeie benaderings
van vloed beheer, wat tans in Suid-Afrika gebruik is, in ag neem. Twee van die studie areas, naamlik
die Mosselbank en die Bayside-kanaal opvanggebiede in die Kaapse metropool, is gekies in areas
waar toekomstige ontwikkeling in die vooruitsig gestel is deur stadsbeplanners. Die derde studie
area, die opvangsgebied van die bolope van die Kuilsrivier, is in terme van die prestasie van
bestaande stormwater infrastruktuur in 'n gebied wat reeds byna heeltemal ontwikkel is,
geëvalueer. Die studie het bevind dat vloedvertragingsfasiliteite met 'n enkele duiker
uitlaatstruktuur, wat ontwerp is met die doel om die vloeispitse tydens groot storms te demp, nie
die impak van die na-ontwikkeling afloop, wat gedurende storms met laer herhalingsinterval
voorkom, verminder nie. In terme van vloedvertragingsfasiliteite met 'n veelvuldige uitlaatstruktuur,
is dit bevind dat voorontwikkelingsafloop tydens 'n reeks van groot en kleiner storms veel meer
effektief nageboots word. Daar is egter ook bevind dat die demping van die vloedspitse nie die naontwikkeling
afloopvolumes verminder tot voorontwikkelingsvlakke nie, maar slegs tot die
vermindering van maksimum snelhede lei. Die gevolg is dat die totale afloop van ‘n kombinasie van
‘n aantal vertragingsdamme oor 'n groot (> 30 km2) stedelike opvanggebied ‘n hoër spitsvloei in die
stroom-af riviere tot gevolg het.
Die studie het bevind dat die wydverspreide gebruik van bestebestuurspraktyke (BMPs) en
volhoubare stedelike dreineringstelsels (SuDS) tot die infiltrasie van ‘n groter gedeelte van die afloop
lei, wat laer afloopvolume en dus verminderde spitsvloei stroomaf tot gevolg het, veral gedurende
storms met ‘n lae herhalingsinterval. Daarbenewens word die aanwending van gedetailleerde
modellering van stormwatersisteme binne die groter opvangsgebied aanbeveel ten einde ‘n meer
holistiese begrip van spesifieke aspekte van die opvangegebied dinamika, om sodoende die
potensiaal vir die ontwerp van effektiewe vloedvertragingskontroles in stedelike stormwaterstelsels
te verbeter.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96774 |
| Date | 03 1900 |
| Creators | Hotchkiss, Timothy Stephen |
| Contributors | Basson, Gerrit R., 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 | 151 pages : illustrations, maps |
| Rights | Stellenbosch University |
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