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Probabilistic Assessment of Overtopping Reliability of a Dam

Probabilistic Assessment of Overtopping Reliability of a Dam. Melih YANMAZ and M. Engin GÜNİNDİ Middle East Technical University Civil Engineering Department Ankara-Turkey. A ssessment of various reliability levels under different combinations of random design parameters.

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Probabilistic Assessment of Overtopping Reliability of a Dam

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  1. Probabilistic Assessment of Overtopping Reliability of a Dam • Melih YANMAZ and M. Engin GÜNİNDİ • Middle East Technical University • Civil Engineering Department • Ankara-Turkey

  2. Assessment of various reliability levels under different combinations of random design parameters. • Probability-based safety levels of existing dams may be evaluated against various tendencies. • The adequacy of old dams needs to be checked with respect to the current safety and conformity degree of measures to be implemented Probabilistic Design/Analysis

  3. Important if pronounced variations occur in • aging and deterioration of constructional materials • deficiencies due to structural, hydraulic, and geotechnical aspects • changes in watershed characteristics • alteration of operational policies • inability of old design methods to consider random nature of loading and resistance parameters Probabilistic design and operation approaches provide more realistic information for site-specific conditions. Probabilistic Analysis of Existing Dams

  4. Accurate flood modelling by the jointconsideration of flood peaks, volumes, and durations Foundations of the modelling Sackl and Bergmann (1987), Goel et al. (1998), Escalante-Sandoval and Raynal-Villaseñor (1998), Yue (2000), Yue (2001), Yue et al. (2001), Yue (2002), Yue and Rasmussen (2002), Michele et al. (2005), Smith et al. (1982) Multivariate Flood Frequency Analysis

  5. A good distribution for hydrologic events having skewed distributions, such as flood peak discharge and flood volume Five-parameter bivariate gamma distribution

  6. g=M2/S2 (Scale parameter of marginal gamma distribution) β=M/S2(Shape parameter of marginal gamma distribution) M=mean S=standard deviation The PDFs of marginal distributions of X and Y

  7. Frequent floods; loss of lives and damage to facilities It is an elongated basin: A=219.6 km2 and hmax=2340 m. Existing structural measures be supplemented by an upstream flood detention dam at 2511 m upstream of the Black Sea shoreline *Design of the dam *Check its adequacy against overtopping induced by flood action Case Study- Fol Creek Basin in Turkey

  8. Flood Frequency Analysis

  9. Uni-variate Flood Frequency Analysis

  10. Gamma marginals are used for Qp and V Kolmogorov–Smirnov test is applied. The critical value = 0.290 at a= 0.05. The KS test statistics:0.198 for Qp O.K.0.204 for V Bi-variate Flood Frequency Analysis

  11. V V Q Q Multivariate Analysis of Fol Creek Data

  12. Limits for the Data to be Generated • Definition of rm=VD/QD

  13. Generation of Equal Return Period Curves Achieved by a code written in MS Excel medium

  14. rmin A B C D E rmax

  15. Comparison of Qp values

  16. Generation of hydrographs Aldama and Ramirez (1999) technique tp=2V/3Qp tb=3tp

  17. Generated hydrographs for Cases A-E

  18. The cases A and E represent two extreme conditions within the limits of upper and lower bounds of Qp and V pairs under the return period of 100 years. Hydrograph A has a high Qp but small tb Hydrograph E has a small Qp but large V and tb. Greater overtopping risk Release of high outflows during a long duration. Duration of downstream inundation increases as the outflow experiences the cases from A to E.

  19. hc hs hm K Spillway h D Computation of Overtopping Reliability

  20. Overtopping reliability=P(hc>hm) K=3 m, D=1 m, L=40 m hs=25 m hi=3.2 m Hydrograph shapes are expressed by Eq. (14)

  21. Reservoir routing is carried out using Monte Carlo technique with 20000 cycles

  22. Stage hydrographs

  23. Deterministic versus probabilistic stage hydrographs

  24. Variation of the deterministic SF against V*=V/Vd

  25. Variation of reliability against V*=V/Vd

  26. Conclusions • a of an earth-fill flood detention dam is investigated. • The design Qp and V are determined using a bivariate gamma distribution with five parameters. • Equal return period curves (Qp vs V) are generated. • A number of hydrographs under Tr =100 yearsare obtained using the technique proposed by Aldama and Ramirez to observe their effects on a • An overflow spillway 25 m high and 40 m long is proposed. Flood frequency analyses are performed for univariate and bivariate cases.

  27. Conclusions-contd. • The average Qpof different PDFs in univariate analysis < the lower limits of Qpranges of the bivariate analysis for all return periods. • This result implies that the deterministic approach yield underestimates in the design compared to the probabilistic approach. • Various possible hydrographs tested under Tr=100-year yielded relatively high values of a • Therefore, the flood detention dam is effective. • The effects hi, cov, PDF on h and various relevant dimensions of the dam on a may also be examined

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