# A Modeling of Human Reliability Analysis on Dam Failure Caused by Extreme Weather
## 4 Bayesian Network Construction
### Accident Event Tree  
When studying dam failure accidents caused by extreme weather, it is first necessary to study the path of dam failure in this context and then consider the human error in each step. The idea of tracing in the CREAM method [41] is used to find the root cause of it and trace to obtain the PSFs. Flood overtopping is the most important cause of dam failure in China, accounting for about half of all historical breaches. From the historical dam-break database of Nanjing Hydraulic Research Institute, after entering the 21 st century the proportion of dam failures caused by insufficient flood relief capacity has decreased significantly, mainly due to emphasis on the safety management of reservoirs and extensive removal and reinforcement projects. At the same time, the proportion of dam failures due to excessive flooding has increased, from 12.26% of the total number of dam failures from 1954 to 1999 and 39.22% from 2000 to 2021, which is three times as much as before. This is attributed to the high incidence of extreme weather, the increased frequency of excessive flooding, and sometimes droughs that lead authorities to violate the rules of overstocking to alleviate drought conditions, which makes them prone to dam failures after heavy rainfalls. The overtopping of a dam is often the result of a combination of causes that can lead to and develop into a dam collapse. Through the study of dam failure path and mechanism, it can be obtained that under the over-standard flood, if the reservoir exists in the current situation of insufficient flood resistance, a gate that cannot be opened, scheduling and utilization errors and other problems, it is more likely to develop into a dangerous situation. The event tree for dam failures due to over-standard flooding during extreme weather is shown in Figure 10.  
Dam failure occurs as a result of a combination of internal weaknesses and external loads in a number of possible ways. Since more than 90% of the dams in China are earthrock dams [42], the event tree is drawn mainly based on their failure patterns and paths. For a particular reservoir, the dam characteristics and internal weaknesses are different, with different failure modes and damage paths; therefore, analysis is still needed when applied to specific reservoirs. For reservoir distresses under excessive flooding, the distresses are first categorized into whether or not they have been successfully dealt with. If they are not properly dealt with before a flood arrives, instead of meeting the flood with a higher storage level, the probability of a dangerous situation is extremely high, and there is a high probability of dam failure. The causes of dam failure in reservoirs were categorized as downstream slope landslide, upstream slope landslide caused by rapid decline in water level after opening the gates, normal decline but insufficient elevation of the dam, gate breakdown resulting in the inability to properly lower the flood water through insufficient gate height, a rise in water level after inadequate discharge via the spillway, destruction of the spillway by washout,  
Figure 10: The event tree for dam failures due to over-standard flooding during extreme weather. cracks, and seepage and pipe surges. Intervention success/failure branches were added to all dam failure pathways, which is an important component of human reliability, but as it is not the focus of this thesis, it will be explored with further details in future studies. In using the event tree to determine the probability of dam failure due to each factor, there are three issues that need to be addressed. 1. It is necessary to identify the possible routes of dam failure, without missing the main routes, often requiring the help of dam experts who are familiar with dam conditions;
2. It is necessary to determine the various possible loads and their frequencies, often requiring more detailed information on reservoir utilization and water levels;
3.