Analysis of dike breach sensitivity using a conceptual method followed by a comprehensive statistical approach to end up with failure probabilities 4 th International Symposium on Flood Defence, Toronto, Canada P. Peeters 1, R. Van Looveren², L. Vincke³, W. Vanneuville 1 and J. Blanckaert 2 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

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

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

Failure mechanisms (of earth dikes)

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

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

1 e Orientating (geotechnical) calculations (1) Conceptual method 2 e 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!) Comparison of calculation methods Sensitivity-analysis of model parameters Outcome: selection of calculation methods & list of (more) sensitive variables

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 (1) Conceptual method

eg. Erosion inner slope Based on literature and expert judgement (1) Conceptual method Assessment of failure index for overflow and wave overtopping F 1, erosion inner slope Revetment type Overflow (l/m/s) GrassGeotextile Open concrete blocks Open stone asphalt < – 102 (*) – 501 (*)2 (*)22 > 5001 (*) 2 (*) Diminish by 1 if an irregular crest is suspected.

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 (1) Conceptual method

eg. Piping Based on Bligh formula and expert judgment (1) Conceptual method Assessment of Failure Index for piping F 5, piping L d /dH (*) Presence of (coarse) sand beneath the dike? < 4  4 and < 18  18 No222 Possible122 Yes012 (*) Neglecting thickness of clay layer

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 (1) Conceptual method Mechanical properties for different fill and foundation materials  unsat (kN/m³)  sat (kN/m³) E (MPa) c (kPa)  (°) Clay Loam Sand Cover Under-consolidated clay- rich layer

eg. Inner slope failure By expert judgment: FOS ≤ 1.15 => Failure Index = Failure Index = Failure Index = 2 FOS > 1.50 => Failure Index = 3 (1) Conceptual method Assessment of Failure Index for inner slope failure F 3, inner slope failure Slope Height > 5 and  7 m (*) 16:412:410:48:46:4 Clay3 (**)2 (**)1 (**)00 Loam3 (**)2 (**)1 (**)00 Sand3 (**)1 (**)000 (*) Difference between crest level and land-side ground level (**) Diminish by 1 if aggravating factors are suspected.

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 (1) Conceptual method Assessment of residual strength for erosion inner slope Core materialSignificant wave height (m) Flow velocity (m/s) Residual strength Clayey  0.65  2 Yes Loamy  0.45  1 Yes Sandy + top layer  0.20  0.5 Yes

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

Failure index of a dike Breaching is more likely where low Failure Index is obtained!

(2) Assessing dike failure probability

Example Failure Index for different failure mechanisms Failure probability of different failure mechanisms 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 Failure Index Erosion inner slope2 Erosion outer slope2 Inner slope failure2 Outer slope failure0 Piping2 Microstability (inner slope)2 Microstability (outer slope)2 Probability (year) Erosion inner slope> Erosion outer slope> General slope failureno results yet Piping> Microstability (inner slope)> Microstability (outer slope)~ 2 Recently this dike segment suffered from macro(in)stability of the outer slope!

Complementary use of both methods Conclusions 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

ABOVE BELOW THE LEVEL OF WATER WITH A PROBABILITY OF FLOODING (i.e. a dike) “Lawrence Weiner” Thanks Questions, suggestions, …