Design sea levels for Southern Africa : a probabilistic approach

Abstract

Includes bibliographical references.This thesis describes the development of a methodology for the quantitative assessment of design sea levels for southern Africa. In order to achieve this objective it was necessary to establish which ocean processes affected sea level in the sub-continent and develop a probabilistic model for the combination thereof. The methodology, is used to characterize regional design sea levels in terms of the west, south and east coasts. A site specific application is undertaken to demonstrate the model's capabilities with regard to the design of depth limited structures. The ultimate objective of this study is to provide a practical approach to the quantification of the sea level component of loading in the full probabilistic design assessment. Data analysis considers all available sea level and wave data for three ports around the coast. A three parameter threshold analysis technique is used to define independent identically distributed events. The distinction between the major processes affecting sea levels in southern Africa may be related to the differences in both the time and space scales of their response to the forcing mechanisms. The data analysis procedure is used to defined the primary statistical characteristics of the observed events in each data set as they relate to sea level. A stochastic simulation model is developed which reproduces a synthetic hourly sea level record displaying the same statistical characteristics as the observed data. Annual maximum values are extracted from the model output with a view to estimating extreme sea levels. The model may be run over any number of periods until satisfactory convergence in the results is obtained. The theoretical basis of the model is described and the results compared with the Gumbel method. A regional assessment of design sea levels for southern Africa indicated that the south coast experienced larger fluctuations in the stochastic component of sea level than the east and west coasts. Sea levels throughout the sub continent are primarily affected by tide, shelf waves, wind waves and edge waves. These processes were found to be statistically independent of one another for the areas evaluated. Design sea level would appear to be determined by a combination of a number of moderate magnitude events rather than one single process. The application of the model illustrates the importance of considering both wave height and sea level conditions as stochastic variables for the design of depth limited structures. The relative influence of stochastic sea level is shown to increase from deep to shallow water