Browsing by Author "Van der Spuy, Danie"

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    Improved statistical flood frequency estimation approach for South Africa
    (Stellenbosch : Stellenbosch University, 2023-12) Van der Spuy, Danie; Du Plessis, Jakobus Andries; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The performance of the most frequently used flood frequency probability distributions in South Africa, using the current statistical approach, were reviewed and all tend to perform poorly when lower probability frequency events are estimated, especially where outliers are present in the dataset. This can, essentially, be attributed to the challenge to analyse very limited ‘samples’ of annual flood peak populations, which are an unknown. As a result, outliers were inadequately 'managed' by attempting to 'normalise' the flood peak dataset, which concealed the significance of the observed data. Thus, by considering the outliers properly, the research was undertaken with the aim to improve the current statistical approach and to develop a more stable and consistent probability distribution. The re-evaluation of the plotting position was considered as a precondition to the research. The plotting position concept was initially applied more than a century ago. Since then, many alternative plotting position techniques have been developed that, practically, do not differ much from each other. The ineffective management of outliers is regarded as the main reason for the inconsistency in flood frequency results. Exploiting a more pragmatic approach, managing outliers by using Z-scores, an alternative plotting position technique was developed, referred to as the 'Z-set' plotting position. The main advantages of the Z-set plotting position technique, distinguishing it from the other plotting position techniques, is its consistency, the more sensible inclusion of outliers and a more homogeneous appearance. The Z-set plotting position technique also proved to be less susceptible to different record lengths than the existing plotting position techniques, which may encourage more consistent choices of appropriate probability distributions. The methodology followed in the development of the proposed new probability distribution, named IPZA, might be considered as unconventional, but the multiple regression approach was used to accommodate the strongly skewed data, often associated with annual flood peak series. The main advantages of the IPZA distribution, is its consistency, the simplicity of application (only one set of frequency factors for every parameter, regardless of the skewness), the integrated handling of outliers and the use of conventional method of moments, eliminating the need to adjust any moments. The performance of the Z-set plotting position and the IPZA probability distribution exceeded initial expectations. The results of both are more consistent than and, by taking outliers into account, appear to be more sensible than existing plotting positions and probability distributions. It is concluded that the Z-set plotting technique and the IPZA probability distribution should be used as valuable additions to the existing set of decision-making tools for hydrologists/engineers performing flood frequency analyses. Evidently, this does not exclude the use of existing probability distributions. It is sound practice to use more than one probability distribution to assist in making a more informed scientific decision.