Evaluation of storm surge components at Saldanha Bay

[摘要] 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 valuesfor 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 datafrom TNPA. Weather data was used for the comparison with water level data. Data sets spanapproximately 5 years in total (January 2010 to March 2015) although large and frequent gaps arepresent. The time series analyses included a Fourier analysis, cross correlation analyses, regressionanalyses and filtering. Correlations were sought out between wind and pressure, and the measured waterlevels, 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 usinganalytical calculation techniques as well as a numerical model. Analytical techniques used include theformulae as prescribed by Bretscheider and Kamphuis. The numerical modes made use of DHI's MIKEHD module. Furthermore, NCEP data was sourced, validated and used for the calculation of extremestorm surges.The study results indicate that values for wind setup and pressure setup at Saldanha Bay are in the orderof 31 to 64mm and 91-268mm respectively. The extreme value analysis performed on the NCEP datasuggests that the maximum expected values for wind setup and pressure setup are 163mm and 386mmrespectively. From the results it is concluded that the wind setup component of storm surge is smallenough 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 couldreach 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 andsouthwestern coast. In cases (possibly outside the coastal area considered in this study) where detailedcalculations are needed, the appropriate guidelines of this study may be taken into account. On themethods for calculating storm surge, it was found the MIKE 21 model yields good results for staticconditions, but that very little of the expected dynamic effects of storm surge is revealed. The accuracyof the analytically calculations was not conclusively determined. However, there was strong evidenceto suggest that, provided the assumptions and limitations of these formulae are respected, the analytical formulas provide satisfactory results.

[发布机构] Stellenbosch University