The Seismic Hazard Assessment For Cascade Hydropower Station System

By taking the cascade hydropower station system(CHSS) in the mainstream of Dadu River(MSDR) as a case study and considering the change of seismic background after Wenchuan 8.0 earthquake in 2008, this paper investigate the influence of complicated geoseismic environment upon the seismic hazard of CHSS. The paper also make some supposes and simplifications to cascade hydropower station dam and CHSS. By considering the additional hazard from upstream dams upon the nearby downstream dam and taking all the cascade hydropower station dams as one whole system, the paper submits a new method to assess seismic hazard of CHSS.The paper fully collects available investigation data of modern destructive earthquakes in Southwest China and establishes the attenuation relations for seismic intensity and acceleration motion on bed rock by regression and conversion. The paper deeply absorbs the new data and results of the seismic hazard assessment for significant construction programs in recent years. The paper also takes advantage of the new information and knowledge of Wenchuan 8.0 earthquake. Then the paper carefully study the seismotectonics background and seismicity features around MSDR. According to the characteristics of regional seismotectonics and seismicity, the paper divides out three seismic belts in relative smaller spatial scale, i.e. Southern Ganshu-Northern Sichuan belt, Sichuan-Yunnan-block-area belt and Sichuan Basin belt. The paper also calculates out the seismic parameters of all the seismic belts at the same time. The paper further indicates that small-belt scheme shows better performance in presenting actual regional seismicity level and possible earthquake action for anti-collapse design of buildings.The paper creatively proposes a bisite seismic hazard analysis method to calculate the exceedance probability of two nearby sites located in a same seismotectonics environment and suffered seismic acceleration exceeding their given values within a same earthquake. This new method is an effective tool for determining the dangerous segment of a linear spread system and calculating the additional seismic hazard from its neighbor site upon a given site.The paper analyses the seismic hazard components of a dam and find out that the united seismic hazard is composed by self seismic hazard and additional seismic hazard. The self seismic hazard is the exceedance probability which seismic acceleration great than its design acceleration for a dam in a give seismic background. The additional seismic hazard comes from upstream dam seismic hazard and is passed by the catastrophic overflow flood of upstream dam failure. Considering of reservoir capacity, the paper leads in a flood adjustment coefficient to measure the reservoir ability of bearing and reduction the overflow flood from upstream. Furthermore, the paper detailed study out the source and calculation method of additional seismic hazard. The paper then establishes a method to assess the seismic hazard for CHSS by simplifying CHSS as a series system. Moreover, the weak link in CHSS can be determined by its united seismic hazard.There are many factors that play important role on seismic risk of CHSS. The paper only focuses its aim on the dam failure caused by earthquake and mainly takes account of three failure modes, i.e. dam structure failure, arch collapse and overtopping, which may directly cause catastrophically sudden, rapid, and uncontrolled release of impounded water. Based upon the achievement of dam antiseismic design target, regulation and dam failure probability caused by earthquake in USA dams, and also upon the fact that the annual dam failure probability in China is the same as that in USA, the paper derives out the conditional failure probability of P(F|E)=3.137×10-2 when dam is suffered a seismic load over its design parameter. Then the paper gives out a preliminary seismic risk assessment for CHSS.Taking CHSS in MSDR as a case study, the paper finds out that the seismic hazard for each dam in present seismic background is somewhat different from its original design value. From the dam united seismic hazard, the paper points out the most dangerous dams in MSDR are as follows in order:Tongjiezhi, Shaping, Gongzhui, Laoyingyan, Longtoushi, Busigou, Dawei, Yinliangbao, et al. The paper further finds out that the additional seismic hazard has significant effect on the seismic hazard assessment for CHSS and then must be taken into account. It is the most effective path to enhance the seismic safety of CHSS by promoting the seismic design parameters of a few weakest dams.