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Analysis of dike breach sensitivity

Analysis of dike breach sensitivity. using a conceptual method followed by a comprehensive statistical approach to end up with failure probabilities. P. Peeters 1 , R. Van Looveren², L. Vincke³, W. Vanneuville 1 and J. Blanckaert 2

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Analysis of dike breach sensitivity

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  1. Analysis of dike breach sensitivity using a conceptual method followed by a comprehensive statistical approach to end up with failure probabilities P. Peeters1, R. Van Looveren², L. Vincke³, W. Vanneuville1 and J. Blanckaert2 1 Flanders Hydraulics Research, Flemish Government, Berchemlei 115, Antwerp 2140, Belgium 2 International Marine and Dredging Consultants, Wilrijkstraat 37-45, Antwerp 2140, Belgium 3 Geotechnical Division, Flemish Government, Tramstraat 52, Gent 9052, Belgium 4th International Symposium on Flood Defence, Toronto, Canada

  2. www.watlab.be

  3. 1. Probability 3. Damage calculations 2. Flood modelling Water level Flemish Risk Methodology (Vanneuville et al) Water management today: limit the damage 4. Risk = Σ Probability x Damage eg. Actualised Sigmaplan (Flood protection plan for tidal reach of Scheldt river)

  4. Overflow Geotechnical failure Flooding caused by Probability of exceedence Probability of flooding

  5. Failure mechanisms (of earth dikes)

  6. In-depth diagnosis  Evaluate breach sensitivity of a dike Pragmatic approach  ?? • Enormous amount of data required • Currently not available in Flanders • Extensive field surveys necessary • Multiple survey & calculation methods • Expensive and time consuming UK – Fragility curves GE – FORM-ARS approach NL – Stochastic subsurface model • Rapid diagnosis • Identification of weaknesses • Using readily available data • Understandable • Reducing diagnostic work load

  7. Evaluation of failure mechanisms Conceptual method (1)Rapid identification critical sectors without missing out possible weaknesses Restricting in-depth diagnosisin space and timeHistorical research, (expert) visual inspection, geotechnical and geophysical exploration, … Restricting probabilistic approach in space and timeAssessing dike failure probability (2) using site specific (geotechnical) data  reducing uncertainties!

  8. (1) Conceptual method 1e Orientating (geotechnical) calculations • Comparison of calculation methods • Sensitivity-analysis of model parameters Outcome: selection of calculation methods & list of (more) sensitive variables 2e Weighting driving and resisting forces • Using literature threshold values (eg. Maximum tolerable flow velocities) • Based on numerous (geotechnical) calculations • For typical dike configurations • Only varying (more) sensitive parameters • Less sensitive parameters set worst-case Outcome: Safety assessment in terms of Failure Indexes (low Failure Index breaching is more likely!)

  9. (1) Conceptual method eg. Erosion inner slope Based on orientating calculations with Manning formula (overflow) and Schüttrumpf formulas (wave overtopping), steepness and height of the land-side slope considered of minor importance  only function of revetment type & overflow

  10. (1) Conceptual method eg. Erosion inner slope Based on literature and expert judgement Assessment of failure index for overflow and wave overtopping (*) Diminish by 1 if an irregular crest is suspected.

  11. (1) Conceptual method eg. Piping Based on orientating calculations with Sellmeyer formula: thickness of covering clay layer (at ground level) and of sandy aquifer beneath the dike considered less influential  Bligh formula is suggested

  12. (1) Conceptual method eg. Piping Based on Bligh formula and expert judgment Assessment of Failure Index for piping (*) Neglecting thickness of clay layer

  13. (1) Conceptual method eg. Inner slope failure Numerous orientating calculations using PLAXIS: crest width 5m, drained situation, 0.5m cover in case of sandy dike, phreatic line assumed Mechanical properties for different fill and foundation materials

  14. (1) Conceptual method eg. Inner slope failure By expert judgment: • FOS ≤ 1.15 => Failure Index = 0 • 1.15 < FOS ≤ 1.30 => Failure Index = 1 • 1.30 < FOS ≤ 1.50 => Failure Index = 2 • FOS > 1.50 => Failure Index = 3 Assessment of Failure Index for inner slope failure (*) Difference between crest level and land-side ground level (**) Diminish by 1 if aggravating factors are suspected.

  15. (1) Conceptual method eg. Residual strength Only assessed when Failure Index = 0 • General slope failure and piping: no residual strength • Other failure mechanism: if yes, Failure Index is augmented to 0.5 Assessment of residual strength for erosion inner slope

  16. (1) Conceptual method Failure Indexes from tables • Combining readily available variables Driving forces (GIS-based) Resisting forces (GIS-based) Aggravating factors (field expertise)

  17. (2) Assessing dike failure probability

  18. Scheldt river Tidal range of 6 m Crest at AD +10 m Groundlevel at AD +5 m Outer slope 16:4 Inner slope 12:4 Example Failure Index for different failure mechanisms Failure probability of different failure mechanisms Recently this dike segment suffered from macro(in)stability of the outer slope!

  19. Conclusions Complementary use of both methods • Rapid identifications of potential weak links • Failure probabilities at locations with low failure indexes and/or high damage costs • Reducing diagnostic work load • From rapid diagnosis to in-depth diagnosis • Input for prioritising in-depth dike diagnosis • Input for flood risk analysis • Input for upgrading works

  20. ThanksQuestions, suggestions, … ABOVE BELOW THE LEVEL OF WATERWITH A PROBABILITY OF FLOODING (i.e. a dike) “Lawrence Weiner” patrik.peeters@mow.vlaanderen.be

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