Browsing by Author "Wahl, Johan Louw"
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- ItemEvaluation of storm surge components at Saldanha Bay(Stellenbosch : Stellenbosch University, 2016-12) Wahl, Johan Louw; Schoonees, J. S.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.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.