Analysis Of The Overtopping Failure Probability In Cascade Reservoirs And Its Emergency Response Strategies

Predicting current and future group-dam risk continues to be a major challenge for hydrologists, flood modelers, as well as water engineers and managers. The complex and dynamic nature of group-dam risk challenges lead to the establishment of risk assessment methodologies and their modeling components, such as the overall failure probability evaluation in cascade reservoirs. Due to the inherent complexity of the relevant processes and the multi-aspect nature of the problem, the overall failure probability evaluation in cascade reservoirs is also highly complex and subject to substantial uncertainty, namely epistemic (reducible) and aleatoric (irreducible), emerging from different sources and processes. However, trying to capture all the complexity is neither pragmatic nor feasible given the constraints we might be under. Hence, we focus on the group-dam overtopping failure risk probability analysis in extreme conditions.To achieve this goal, overarching objectives were subdivided into four supporting objectives:(1) analyzing the dam overtopping failure risk under consideration of the upstream dam-break flood; (2) exploring the effect of risk transmission and risk accumulation in cascade reservoirs; (3) quantitatively analyze the overall failure probability and identifying the weak reservoirs; (4) founding the multi-reservoir emergent water dispatches among model for key projects in the basin. This paper is unique in that it tries to quantify the overtopping failure probability in cascade reservoirs and its emergency response strategies. Main results and conclusions include:1) The risk evaluation method proposed by Bureau et al, [2003] is improved in order to analyze the Seismic hazard and total risks of the cascade dams in the upper Dadu River, i.e., Xiaerxia, Bala, Dawei, Busigou, Shuangjiangkou. Also, considering the potential hazard may cause loss of life, destroy infrastructures and properties, impede economic and social activities, and so on, the eventual dam-collapse accompanying dam-break flood and its routing process has computed and validated by using the software pack of DB-IWHR model and proposed formulas. As a result, we found that (1) the overall risk level of the dams in the maximum historic earthquake condition ranked moderate, which was consistent with the actual situation. However, the TRF of Xiaerxia- and Shuangjiangkou reservoir based on the multiple fault activity ranked superior, which should be seriously considered for the construction safety; (2) the dam-break flood can be predicted with reasonable accuracy by DB-IWHR model fed with the input values of the key model parameters are within well-understood ranges.2) A new risk model is presented to evaluate the dam overtopping failure fuzzy risk probability under consideration of the upstream dam-break flood. Unlike other techniques previously proposed in the literature, the suggested approach is used the trapezoidal fuzzy number to show the complicated relationship between the fuzzy interval of risk indicator and failure probability and Shuangjiangkou reservoir is taken as a case study. The results indicate that (1) the analysis model based on trapezoidal fuzzy number can describe the uncertainty of the complicated risk analysis system efficiently and accurately; (2) although the failure risk probability for the dam overtopping under consideration of the upstream dam-break flood is higher than the acceptable maximum probability at high water levels, establishing systems for early warning and lowering the water level can significantly reduce the risk and ensure the reservoir's safety operation.3) A group-dam risk estimation model for the failure of the cascade reservoirs has been established based on the assumption that the outflow had no impact on the downriver dam's safety if the reservoir operation normally. Here we showed how to deliver information about overtopping risk among the reservoirs and derived the general formula for calculating the single earth dam overtopping failure probability. Also, the case of Shuangjiangkou reservoir and its upstream three reservoirs is implemented for quantitatively analyzing the overall failure probability. The evaluation results reveal that (1) the group-dam risk estimation model can effectively reflect the effect of risk transmission and transmission in cascade reservoirs and accurately evaluate the overall failure probability, which can provide a reference for the reservoir dam safety management on Dadu River; (2) the potential threats ranked as Busigou-, Dawei-, Bala- and Shuangjiangkou reservoir, of which the failure risk probability for the Dawei-and Busigou reservoir under different confidence a and water level z are higher than the acceptable maximum probability, should be analyzed with high priority and redesigned to increase the safety of the embankments and their appurtenant structures, if necessary.4) The emergency scenarios and its corresponding operation rules for Xiaerxia-and Shuangjiangkou reservoir are suggested in extreme conditions, such as flood, seismic activity, the reservoir self-failure and so on. In accordance with the characteristics of the study area, three scenarios were established:(1) only the Xiaerxia reservoir appearing the emergency; (2) only the Shuangjiangkou reservoir appearing the emergency; (3) the Xiaerxia reservoir dam-break. Meanwhile, an interactive reservoirs jointly emergency operation system is developed for analyzing the potential hydrological effect of each of the three scenarios. Results indicate that both Xiaerxia- and Shuangjiangkou reservoir can resist the overtopping failure risk at scenario 1 and scenario 2, and the outflow will affect the safety of the embankments; while the Shuangjiangkou reservoir has high-risk class of the overtopping in scenario 3 and it will directly cause damage to the downstream channel.5) An Emergency Knowledge Management System (EKMS) is presented in this study based on the case-based reasoning techniques, which could warn quantitatively for the similar reservoir problems and develop a solution strategy by adapting expert's knowledge, experience and successful solutions. In order to improve the accuracy and efficiency of the emergent rescue operations, a database related to reservoir emergency management case is established for the case retrieval, which could resolve the representation, organization and index of the typical hazards cases. Furthermore, based on the characteristics of new problem and the task of the emergent rescue, the strategy of the case modification and study is presented for increasing the adaptability of the database and providing the new solving thought or method more rational and comprehensive.Finally, the summary and further directions are given.