Risk Analysis On Embankment Dam Breach Caused By Seepage Through Embankment And Overtopping In China

The number of embankment dams in China is large and the proportion of dam breach is high.The cases of dam breach caused by seepage through embankment and overtopping account for a very high proportion.With the continuous development of the economy and society,people are getting harder to withstand the serious consequences brought by dam breach.Based on this,this paper aims to analyze the risk of embankment dam breach caused by seepage through embankment and overtopping.The main contents include:Nine cases of dam breach caused by seepage through embankment and nine caused by overtopping will be selected.Through the analysis of breaching process and the causes,the risk sources and potential failure modes of the two conditions are initially summarized.The common risk sources of seepage through embankment are from three hidden dangers.They are the internal part of dam,dam end and nearby flooding facilities such as spillways.Potential risk sources of the internal part are the nature and filling quality of the materials for building dam,the validity and interconnection of all parts of structure of the dam,animals’caves and plant roots.Potential risk sources at the dam end are the nature and filling quality of the materials for building dam,the treatment of the fractured slopes at the two ends of the dam,the slope and flatness of the abutment profile,and the connection between the dam and the side slope.And potential risk sources that may exist next to spillway facilities such as spillway are compaction density,the impermeability of parts contacting the dam,and stability of the dam foundation.The potential failure mode of seepage through embankment is:normal incoming water（not overtopping）-initiation of erosion-continuation of erosion-forming a seepage channel-dam breach.The common risk sources of overtopping are excessive rainfall,the side slope stability of the reservoir area,and flood water caused by dam breach in the upstream.The potential failure mode is:the excessive water stream-insufficient ability to discharge-overtopping-dam breach.For dam breach caused by seepage through embankment and overtopping,it is assumed that dam breach must happen once the water is excessive（overtopping）.By dividing the process of dam breach caused by seepage through embankment into four stages—the initiation of erosion,the continuation of erosion,the expansion into a channel and the formation of a breach,with the mechanism of each stage and the factors inducing them,an event tree model to evaluate the probability to occur is constructed and the evaluation method of conditional probability is given.The case study of Gouhou dam,which had been breached,and Nuozhadu dam built recently are selected.The results are:（1）the failure probability of the Gouhou dam from the initiation of erosion to dam breach is 0.19;（2）four load states are divided according to the upstream water levels of different return periods,to calculate the failure probability of the Nuozhadu dam separately.Thus,the annual failure probability of the Nuozhadu dam is 4.76×10^-7,and the numerical simulation results of simplified seepage are used to test the plausibility.The risk analysis framework for dam breach caused by seepage through embankment and overtopping is constructed.According to the existing research results,the methods for calculating the consequences of dam breach and the criteria for assessing the risk level are preliminarily proposed.Thus,it is determined that the Nuozhadu dam meets the acceptable risk criteria,and further calculation of the consequences of dam breach is not needed to assess its risk level.