Evaluation of storm surge components at Saldanha Bay

Wahl, Johan Louw (2016-12)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Extreme sea levels are made up of many different components – the most noteworthy of these include tides, tsunamis, seiches, wave setup, runup and storm surge. The latter is often considered to be the most significant contributing component. Accordingly, some of the most severe coastal floods globally have been as direct result of storm surge. Storm surge is the increase (or decrease) in sea levels resulting mainly from wind setup and the inverse barometer effect (pressure setup). Wind setup usually makes up the majority component. With this in mind, it is vast shallow coastlines in areas frequented by strong winds that are most vulnerable to storm surge. Many scholars have essentially ruled out the possibility of large storm surges along the South African coast on the basis that the coastal shelf is too deep and that winds are too moderate to agitate significant wind setup. As a result, storm surge heights for the South African coast, specifically, has never been thoroughly researched or documented. Yet, in the absence of definitive literature on this matter, storm surge is often quoted as the scapegoat for coastal damages. Furthermore, when it comes to engineering designs, indiscreetly estimated values are often used. This study, focussing on Saldanha Bay as a test case, gains knowledge into the actual range of values for storm surge to be taken into account in engineering designs. This is done primarily by means of time series analyses of available water level data. Methods by which to easily calculate such results are also looked at. These methods include analytical calculations and numerical modelling. The data available for this study included water level data (tide gauge) from SANHO and weather data from TNPA. Weather data was used for the comparison with water level data. Data sets span approximately 5 years in total (January 2010 to March 2015) although large and frequent gaps are present. The time series analyses included a Fourier analysis, cross correlation analyses, regression analyses and filtering. Correlations were sought out between wind and pressure, and the measured water levels, so as to attribute certain components of the total measured fluctuations to storm surge. Subsequent to the time series analysis, a hindcast of the storm surge components was done using analytical calculation techniques as well as a numerical model. Analytical techniques used include the formulae as prescribed by Bretscheider and Kamphuis. The numerical modes made use of DHI’s MIKE HD module. Furthermore, NCEP data was sourced, validated and used for the calculation of extreme storm surges. The study results indicate that values for wind setup and pressure setup at Saldanha Bay are in the order of 31 to 64mm and 91-268mm respectively. The extreme value analysis performed on the NCEP data suggests that the maximum expected values for wind setup and pressure setup are 163mm and 386mm respectively. From the results it is concluded that the wind setup component of storm surge is small enough to be ignored for most practical applications. An exception to this rule would apply to extreme shallow water bodies such as large lagoons and estuaries where it is estimated that wind setup could reach heights of up to 1.5m. The pressure component of storm surge is regarded as more significant and (for design purposes) may be assigned a fixed maximum value of 390mm along the entire south and southwestern coast. In cases (possibly outside the coastal area considered in this study) where detailed calculations are needed, the appropriate guidelines of this study may be taken into account. On the methods for calculating storm surge, it was found the MIKE 21 model yields good results for static conditions, but that very little of the expected dynamic effects of storm surge is revealed. The accuracy of the analytically calculations was not conclusively determined. However, there was strong evidence to suggest that, provided the assumptions and limitations of these formulae are respected, the analytical formulas provide satisfactory results.

AFRIKAANSE OPSOMMING: Ekstreme seevlakke bestaan uit verskillende komponente waarvan getye, tsoenami’s, “seiches”, golfopstuwing, oploop en stormopstuwing die vernaamste is. Laasgenoemde word allerweë as die belangrikste bydraende komponent beskou en sommige van die ergste kus-verwante vloede wat al wêreldwyd plaasgevind het, is ʼn direkte gevolg van stormopstuwing. Stormopstuwing is die toename (of afname) in seevlakke, hoofsaaklik as gevolg van windopstuwing en die omgekeerde barometer effek (lugdrukopstuwing). Windopstuwing is normaalweg die grootste komponent. Teen hierdie agtergrond is die groot, vlak kuslyne in gebiede waar sterk winde gereeld voorkom, besonder kwesbaar vir stormopstuwing. Baie navorsers het in die verlede die moontlikheid van groot stormopstuwings langs die Suid-Afrikaanse kus vir alle praktiese doeleindes buite rekening gelaat, aangesien die kusplat te diep is en die winde te matig, om betekenisvolle windopstuwing te veroorsaak. Die gevolg hiervan is dat die hoogte van stormopstuwing langs die Suid-Afrikaanse kus nog nooit deeglik nagevors en gedokumenteer is nie. Nietemin, ten spyte van die gebrek aan toepaslike literatuur, word stormopstuwing dikwels as die sondebok beskou wanneer dit kom by stormskade langs die Suid-Afrikaanse kus. Verder, wanneer daar ingenieursontwerpe in hierdie verband gedoen word, word daar dikwels, op onoordeelkundige wyse, van geskatte waardes gebruik gemaak. Hierdie studie, wat vir toets doeleindes op Saldanhabaai fokus, is ʼn poging om kennis in te win oor die werklike stel waardes wat vir stormopstuwing in berekening gebring behoort te word by die opstel van ingenieursontwerpe. Dit word primêr gedoen deur die ontleding van tydreekse wat uit die beskikbare watervlak data geneem word. Daar word gekyk na metodes waarvolgens hierdie tipe resultate redelik maklik bereken kan word. Hierdie metodes sluit onder meer analitiese berekeninge en numeriese modellering in. Die beskikbare data vir hierdie studie sluit in: watervlak data (getymeter) van SANHO en weer data van TNPA. Weer data is gebruik as deel van ʼn proses van vergelyking met watervlak data. Datastelle strek oor ongeveer 5 jaar in totaal (Januarie 2010 tot Maart 2015), maar is ongelukkig nie heeltemal volledig nie en bevat ʼn aantal opvallende gapings. Die ontledings van tydreekse sluit onder meer die volgende in: ʼn Fourier analise, ʼn kruiskorrelasie analise, ʼn regressie analise en ʼn filterproses. Daar is gesoek na korrelasies tussen wind, lugdruk en die gemete watervlakke met die oog daarop om sekere komponente van die totale gemete fluktuasies aan stormopstuwing toe te skryf. Die ontleding van tydreekse is opgevolg met ʼn naskatting van die stormopstuwing komponente, deur gebruikmaking van analitiese berekeningstegnieke, sowel as ʼn numeriese model. Van die analitiese tegnieke wat gebruik is, sluit die formules in wat deur Bretscheider en Kamphuis voorgeskryf is. Die Numeriese model wat gebruik is, maak gebruik van DHI se MIKE HD module. NCEP data is ook verkry, gevalideer en gebruik vir die berekening van ekstreme stormopstuwing. Die studieresultate dui aan dat waardes vir windopstuwing en lugdrukopstuwing by Saldanhabaai wissel van 31 tot 64 mm en van 91 tot 268 mm onderskeidelik. Die ekstreme waarde analise wat op die NCEP data toegepas is, dui op ʼn maksimum verwagte waarde van 163 mm vir windopstuwing en 386 mm vir lugdrukopstuwing. Vanuit hierdie resultate word die gevolgtrekking gemaak dat die windopstuwing komponent van stormopstuwing vir alle praktiese doeleindes klein genoeg is om geïgnoreer te word. ʼn Uitsondering op hierdie reël is wanneer daar met watermassas, soos strandmere en riviermondings, gewerk word waarin die water baie vlak is en waar die windopstuwing vlakke van so hoog as 1.5m kan bereik. Die lugdruk komponent van stormopstuwing word as meer betekenisvol beskou en vir ontwerpdoeleindes mag ʼn vaste maksimum waarde van 390mm hieraan toegeken word. Dit geld in die algemeen vir die hele suid- en suidwes kus. In gevalle waar gedetailleerde berekeninge benodig word (moontlik in ander kusgebiede as die area waarop daar in hierdie studie gefokus is), kan die betrokke riglyne van hierdie studie in aanmerking geneem word. Wat die metodes betref vir die berekening van stormopstuwing, is daar bevind dat die MIKE 21 model goeie resultate lewer vir statiese toestande, maar dat baie min van die verwagte dinamiese effekte van stormopstuwing na vore kom. Die akkuraatheid van die analitiese berekeninge kon nie bo alle twyfel vasgestel word nie. Daar was nogtans sterk aanduidings dat die analitiese formules wel bevredigende resultate lewer, indien dit met inagneming van die relevante aannames en beperkinge toegepas word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100097
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