Great Brak Island - measures for flood defence

Viljoen, Johannes Theodorus (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: This thesis firstly presents the analysis of large flood events, causing low-lying property in and around the Great Brak estuary to be inundated. The nature of several South African estuaries renders adjacent properties vulnerable to river and ocean related flooding. An important objective of the study is to develop a feasible form of flood defence for the Island, a residential development situated in the lower estuary basin, 250 m from the estuary mouth. The Island regularly experiences inundation due to high water levels in the estuary which have partially driven the need for the artificial manipulation of the estuary mouth berm using millions of litres of potable water. The lowest property on the Island is situated at +2.2 m MSL whereas the highest flood level ever recorded in the estuary was +2.9 m MSL. This extreme water level was achieved by an equivalent 1 in 10-year flood, flowing into the estuary and coinciding with a closed estuary mouth. Research into climate change predictions for regional overland precipitation trends and for sea level rise have been included in the study. The sea level is predicted to rise by about 1 m in the year 2100 whereas the frequency of extreme floods is set to increase for the catchment area in the next century. Large storm events may cause direct wave attack on the shorefront properties of the Island due to the rising sea levels and the increase in storminess. Much higher extreme water levels are therefore expected in the estuary and a form of flood defence is therefore sought; also, to reduce the current need for “wasteful” water releases to artificially induce open mouth conditions. Various possible measures for flood defence have been identified for application either directly around the Island, in the surf-zone to dissipate wave energy or upstream of the estuary, at the Wolwedans Dam to attenuate large floods from rainfall in the catchment. A Multi-Criteria Analysis approach has been followed to objectively identify the preferred flood defence measure. The evaluation criteria were based on the hydraulic -, environmental-, and economic performance of the proposed flood defence measure. The Multi-Criteria Analysis identified a combined solution of an Armoured Dike- and Rock Revetment structure directly around the Island, to be the preferred solution. The proposed solution was conceptually designed for various lifetimes and extreme flood conditions to find the least expensive option. An order of magnitude cost estimate for the construction of the proposed solutions was derived, which formed the basis of comparison to the costs foreseen if no flood defence measure is implemented (the Do-Nothing alternative). The study found that the proposed solution becomes economically feasible if designed for a lifetime of 33 year and more. The most attractive solution was found to be the combination of a +3.0 m MSL crest level Armoured Dike and a +5.0 m MSL crest level Rock Revetment, which is estimated to cost R95 million to construct.

AFRIKAANSE OPSOMMING: Hierdie tesis bied eerstens ‘n analise van groot vloede aan, wat veroorsaak dat laagliggende eiendomme in en rondom die Groot Brak strandmeer oorstroom word. Die aard van verskeie Suid Afrikaanse strandmere, insluitend die Groot Brak strandmeer, maak aangrensende eiendomme kwesbaar vir rivier- en oseaanverwante oorstromings. ‘n Belangrike doel van die studie is om ‘n haalbare manier van vloedverdediging vir die Eiland te ontwikkel, wat ‘n residensiële ontwikkeling in die strandmeer is, ongeveer 250 m van die see af. Die Eiland beleef dikwels oorstroming as gevolg van hoë watervlakke in die strandmeer, wat gedeeltelik die behoefte aan kunsmatige manipulasie van die mond dryf, deur middel van miljoene liter drinkbare water uit die Wolwedans Dam te gebruik. Die laagste eiendom op die eiland is geleë +2.2 m bo seevlak terwyl die hoogste vloedvlak ooit bereik in die riviermond aangeteken is as +2.9 m bo seevlak. Hierdie ekstreme watervlak was veroorsaak deur ‘n ekwivalente 1 in 10 – jaar vloed wat saam met ‘n toe mondkondisie gepaart gegaan het. Navorsing oor streeks voorspellings vir klimaatsverandering vir oorlandse neerslagneigings en vir seevlakstyging is by die studie ingesluit. Daar word voorspel dat die seevlak met ongeveer 1 m sal styg teen die jaar 2100 en dat die voorkoms van uiterste vloede in die opvangsgebied sal toeneem. Groot toekomstige storms sal veroorsaak dat golwe die Eiland kan bereik as gevolg van stygende seevlakke en die toename in stormagtigheid. Daar word dus baie hoër watervlakke in die strandmeer verwag en ‘n metode van vloedbeskerming word dus benodig; ook om die huidige verkwistende water loslatings, om die mond kunsmatig oop te spoel, te verminder. Verskeie moontlike maatreëls vir vloedverdediging is geïdentifiseer vir aanwending direk rondom die Eiland, in die brander sone om golfenergie te demp en stroomop van die riviermond, by die Wolwedansdam om groot vloede te demp wat veroorsaak word deur reënval. ‘n Multi-Kriteria-Analise benadering is gevolg om die beste vloedverdediging maatreël objektief te identifiseer. Die evalueringskriteria is gebaseer op die hidrouliese-, omgewings- en ekonomiese prestasie van die voorgestelde vloedverdedigingsmaatreël. Die Multi-Kriteria-Analise het ‘n gekombineerde oplossing van ‘n bewapende dyk en ‘n ruklipkeermeer vir aanwending direk rondom die Eiland, geïdentifiseer as die voorgekeurde oplossing. Die voorgestelde oplossing was ontwerp vir verskeie leeftye en uiterste vloedtoestande om die goedkoopste oplossing te vind. ‘n Ordegrote-kosteberaming vir die konstruksie van die struktuur is gemaak, wat die grondslag van vergelyking gevorm het met die koste wat voorsien is as geen vloedverdediging geïmplementeer word nie (die Niks-Doen alternatief). Die studie het bevind dat die voorgestelde oplossing ekonomies haalbaar raak as dit ontwerp word vir ‘n leeftyd van langer as 33 jaar. Die mees aantreklike oplossing was geïdentifiseer as die kombinasie van ‘n +3 m bo seevlak kruinhoogte bewapende dyk en ‘n +5 m bo seevlak kruinhoogte ruklipkeermuur.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102783
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