The effect of sea level rise on flood levels in the Great Brak Estuary: assessing the adequacy of a 5 m setback line

Du Pisani, Julia (2015-03)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Global warming will result in a sea level rise of between 0.25 and 0.82 m by 2090, as well as an increase in intensity and frequency of both extreme sea level and extreme rainfall events. In consequence, low-lying areas will be permanently inundated, extreme waves will penetrate further inland and flood intensity and frequency will increase. Estuaries are subject to the effect of both extreme sea levels and extreme floods and water levels in estuaries are expected to increase, under both open and closed conditions. As a response to expected higher flood levels, setback lines have been legislated in South Africa. For cases where a flood level study has not been undertaken, a minimum setback line at the 5 m above mean sea level (MSL) contour is prescribed in terms of the National Environmental Management Act (Act 107 of 1998). This study assessed the adequacy of the 5 m setback line, under the effects of climate change, for Great Brak estuary. Local features of the Great Brak estuary may influence flood levels. Specifically, the lagoon of the Great Brak estuary, below the N2 Bridge, is small at 1.1 x 0.7 km. Further, it is constrained at the upstream end by road and rail embankment, and on the left bank by steep slopes. A sand barrier at the mouth is at times breached, both naturally and artificially. Artificial breaching is initiated when the sand barrier is between 1.5 and 2.0 m high, or when a flood is forecast. The barrier has previously reached 2.7 m, higher than the still water level of the sea, which has not exceeded 2 m above MSL. There is a populated island about 180 m upstream of the mouth. The greater extent of the island is below 2.5 m above MSL. Mike11 software was used to generate flood levels on which the conclusions of this study are based. The study determined that the influence of the increased sea levels does not extend much beyond the N2 Bridge. This may be a peculiarity of the Great Brak estuary, due to the influence of the three bridges and the road and rail embankments. For the scenario where Mean High Water Springs coincides a with an extreme sea storm and there is a 100-year riverine flood, the flood level in the estuary is 3.16 m at the mouth, increasing to 4 m upstream of the N2 bridge. In the scenario where the barrier height was raised to 4 m above MSL, the flood levels were 4.52 m downstream of, and 5 m upstream of, the N2 Bridge. Extensive inundation of properties in the floodplain and on the Island will occur, as well as the inundation of the N2 embankment. The probability of such an extreme sea level event occurring at the same time as peak runoff of a 100-year riverine flood is unlikely. It is the conclusion of this study that, for the Great Brak River, the 5 m setback line, as prescribed, is sufficient for an extreme situation where a future 100-year flood coincides with the MHWS coincides and an extreme sea storm raising the sea level to 2.65 above current MSL.

AFRIKAANSE OPSOMMING: Aardverwarming sal lei tot 'n styging van seevlakke van tussen 0.25 en 0.82 m teen 2090, sowel as 'n toename in intensiteit en frekwensie van beide stormseevlak en reënval. Gevolglik sal laagliggende gebiede permanent oorval word, stormgolwe verder in die binneland dring en vloed intensiteit en frekwensie toeneem. Riviermondings is onderhewig aan die effek van beide hoë seevlakke en vloede. Om die negtaiewe effekte van hoër vloedvalktes te bekamp word ‘n minimum terugsetlyn van 5 m bo seevlak voorgeskryf, in terme van die Wet op Nationale Omgewingsbestuur (Wet 107 van 1998). Hierdie is van toepassing waar ‘n vloedlyn studie nie onderneem is nie. Hierdie studie beoordeel die geskiktheid van die 5 m terugsetlyn, onder die invloed van klimaatsverandering, vir Groot Brak rivier monding. Plaaslike kenmerke van die Groot Brak monding mag vloed vlakke beïnvloed. Spesifiek, die Groot Brak monding meer het ‘n oppervak van net 1,1 x 0,7 km; is in die stroomop rigting beperk deur pad en spoor walle; en word op linkeroewer deur steil hellings vesper. Die sandversperring by die word kunsmatig oopgemmak wanneer die sand versperring tussen 1,5 en 2,0 m hoog is, of wanneer 'n vloed voorspel word. Hierdie sandversperring het al voorheen 2.7 m hoogte beriek, hoër as die 2 m maksimum historiese stilwater vlak van die seë. Daar is 'n bevolkde eiland sowat 180 m stroomop van die mond. Die die eiland is meestelik onder 2.5 m bo seevlak. Mike11 sagteware is gebruik om vloed vlakke, waarop die bevindinge van hierdie studie gebaseer is, te bepaal. Hiedie studie bevind dat die effek hoër voedvlakke trek nie veel verder stroomop as die N2 brug, oontlike weens die voorkoms van die drie bruë. In die geval waar ‘n uiterste seëstorm terselfde tyd voorkon as die lente hoogwater gety endie 100 jaar river vloed, sal die watervlak in the mondingsmeer tot 3.16 m bo huidiglike seëvlak styg by die mond, en tot 4 m bo huidiglike seëvlak by die N2 brug. In die geval waar die sandversperring by the riviersmond 4 m verhoog is, sal die watervlak in the mondingsmeer tot 4.5 m bo huidiglike seëvlak styg by die mond, en tot 5 m bo huidiglike seëvlak by die N2 brug. Faktore nie in ag geneem in hierdie studie sluit in die uitwerking van die verhoogde afloop, sediment verandering en die effek van windgolwe oor die ondingsmeer. Wydverspreide vloeding van ontwikkelde areas aangrensend to vloedvlakte sal voorkom, insluitend die oorstroing van die N2 padwal. Die waarskynlikheid is klein dat ‘n uiterste seëstorm terselfde tyd voorkom as the lengte hoogwater gety en die 100 yaar rivier vloed. Dit is dus die gevolgtrekking van hierdie studie dat die 5 m terugsetlyn soos voorgeskryf, voldoende is vir Groot Brak rivier vir so ‘n uiterste geval.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97078
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