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Stellenbosch University (2015)

Risk based dam safety in Namibia : a quantitative approach

Cloete, Gert Christiaan

Titre : Risk based dam safety in Namibia : a quantitative approach

Auteur : Cloete, Gert Christiaan

Université de soutenance  : Stellenbosch University

Grade : Doctor of Philosophy (PhD) 2015

Résumé
A flood event in the town of Mariental, in 2006, raised a sudden awareness regarding the state of dam safety in Namibia. Although damage was caused at the town, the flood was not extreme ; it was approximately a one in fifty year event. The concern, however, was the increase in risk imposed on the town due to the temporary malfunctioning of the back-up power system : should the secondary back-up system also have failed, the embankment could have overtopped with subsequent failure ; a catastrophe. The Rational Quantitative Optimal (RQO) approach, presented in this dissertation, provides a robust risk evaluation model which produces a definitive result for the reduction of risk from the overtopping of earth-fill dams. The model is based on principles of risk, but an assessment of a portfolio of dams provides discrete optimal results, not expressed in terms of probability. All the steps that the methodology comprises have been developed exhaustively and propose to address concerns raised by dam owners and decision makers regarding risk-based dam safety : a transparent framework for decision making related to public safety, which will also appeal to the technically minded portfolio manager looking for a purely quantitative procedure to assist in the decision making process. The RQO process is applied mechanistically, not requiring judgement from the decision maker. It thus addresses the concern raised by dam owners regarding the probability of risk assessment being judgmental. Risk in this dissertation is associated with embankment dams and concomitant external erosion, which globally is the single largest cause of failure of these dams. This specific failure mechanism, in particular, is a threat in Namibia, since other mechanisms, such as internal erosion, poses very little risk to the type of embankment dams typically found in Namibia. Therefore, for practical purposes, the extreme flood hydrology in Namibia is revisited and applied to real dams in the RQO model. Extreme flood hydrology in Namibia has, for the past thirty years, largely been based on the South African Department of Water Affairs Technical Report 137 (TR 137) of 1988 ; This report proposes an empirically established upper limit of flood peaks, called the Regional Maximum Flood (RMF), which is associated with an annual recurrence interval of 10 000 years, as shown in this study from probabilistic analysis which included palaeoflood data. The updated flood model incorporates thirty years of additional systematic data, as well as palaeoflood data that has resulted from a new approach. The new data have provided an increase in the K‒value boundaries for some of the regional flood zones. A revised graphical distribution of the K‒value zones for Namibia is presented and is proposed as a replacement for the current model.

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