Probabilistic design and risk analysis for the Duong river dike in the Red River delta in Vietnam

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Traditionally, the design of sea and river dikes in Vietnam is primarily based on a water level with a particular frequency of being exceeded. The frequencies of exceedence of design levels and governing discharge are widely regarded as constituting a standard for the safety of the region protected by the dikes and are interpreted in terms of inundation probabilities. However, this is not fully correct since the inundation probability is not only determined by the frequency of exceedence of design levels, but it is also caused by other mechanisms below that design levels. Normally, the dike crest exceeds the design water level to a certain extent, thus, the probability of overtopping is smaller than the design frequency. However, some parts of the dike may already be critically loaded before the design water level is reached. Water logging may lead to slide planes through the dike or piping may undermine the body of the dike, with sudden failure as a consequence. In short, there are other failure mechanisms that can lead to flooding of the protected area than overtopping. In fact, if all possible causes of dike failure at high water level such as overtopping, piping, macro instability, and micro instability etc. could be listed and the associated probabilities of their occurrence be ascertained, then, in principle, the probability of inundation could be calculated. As a next step, a new approach to safety of the design of dikes, embankments, and other flood defenses was developed. This is the probabilistic design method. The probabilistic approach aims to determine the true probability of flooding of a river system and to judge its acceptability in view of the consequences. In this approach, the stochastic character of the various load and strength parameters is taken into account and the design is based on an analysis of failure probabilities. This new approach is called "risk-based" approach. During this study, the probabilistic design method is applied for a case study in Vietnam, which is the dike improvement for the right Duong River dike in the Red River Delta in Vietnam. Due to lack of time, only the failure probabilities caused by overflowing, piping, and sliding mechanisms and the failure probability caused by hydraulic structures have been calculated. For each mechanism, uncertainties for various variables are considered by mean of statistica I tools. The failure probability of each mechanism was determined based on the defined reliability function and, as aresuit, the inundation probability of the dike was calculated by combining all the mechanism failure probabilities. By considering the risk and the cost for the improvement, the optimization of the dike design is reached. From the economic point of view, the optimal dike crest elevation is the point corresponding with the minimum total cost. Following this study this leads to a crest level that can withstand a 1/500-year flood approximately.