Page - 271 - in Book of Full Papers - Symposium Hydro Engineering

based on 40000 stochastic simulations. The results are shown in Table 3. Table 3 Probability under different scenario for cascade reservoirs Reservoir Probability 1st(A) 2nd(B) 3rd(C) 4th(D) Maximum stage not exceeding the initial stage (top level of flood control or normal storage level) 0.99785 0.99775 0.999175 0.99745 Maximum stage exceeding stage of check criterion for flood control 7.5×10-5 2.0×10-4 5.0×10-5 1.75×10-4 Maximum stage overtopping 2.5×10-5 0 0 5.0×10-5 4.2. ANALYSIS (1) In the view of dam safety itself, in flood season, dam failure is frequently caused by overtopping. The risk will be unacceptable, if risk probability of overtopping exceeds a certain threshold. According to the statistics of dam failure events between 1950s and 1990s caused by flood related overtopping issues, the threshold or acceptable risk probability will be 1.0×10-4 [4]. However, based on the statistics of data after 1990s [6], the critical value decreases to 9.47×10-5 that becoming stricter. In the case study, the risk probability of overtopping for 1st to 4th reservoirs are 2.5×10-5, 0, 0, and 5.0×10-5 respectively, which are all below the threshold or acceptable risk probability for flood control and satisfy the demands for dam safety. (2) In the view of flood control capability of dam, when flood of check standard occurs, the spillway works with designed maximum discharge to ensure that stage stops rising and not exceed check flood level. In the case study, the check standard of flood control for all the four reservoirs is with a return period of 5000 years, which means a probability of 2.0×10-4. Based on the statistics of 40000 stochastic simulations, the probabilities of exceeding check flood level for 1st to 4th reservoirs are 7.5×10-5, 2.0×10-4, 5.0×10-5, and 1.75×10-4 respectively, which doesn’t exceed check standard and satisfy the requirements for flood control capability of dam. (3) In the view of flood control for downstream regions protected by dam, during flood season, if water level of reservoir maintains lowering than the initial stage, the spillway works to perform flood regulation and reduce outflow comply with the safe and multipurpose requirements of downstream regions. In the case study, the probability not exceeding initial stage is approximately 99.75% for the four reservoirs, which indicates that when flood with a return period less than 400 years occurs, flood regulation works and makes the downstream regions safe and 271