Analysis of dike breach sensitivityusing a conceptual method followed by a comprehensive statistical approach to end up with failure probabilities

4th International Symposium on Flood Defence, Toronto, Canada

P. Peeters1, R. Van Looveren², L. Vincke³, W. Vanneuville1 and J. Blanckaert2

1 Flanders Hydraulics Research, Flemish Government, Berchemlei 115, Antwerp 2140, Belgium2 International Marine and Dredging Consultants, Wilrijkstraat 37-45, Antwerp 2140, Belgium3 Geotechnical Division, Flemish Government, Tramstraat 52, Gent 9052, Belgium

www.watlab.be

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 diagnosisin 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!

1e Orientating (geotechnical) calculations

(1) Conceptual method

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!)

• Comparison of calculation methods• Sensitivity-analysis of model parameters

Outcome: selection of calculation methods & list of (more) sensitive variables

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0

dike height land side [m]

flow

vel

oci

ty [m

/s]

n=0.01;Q=600 l/s/m

n=0.01;Q=50 l/s/m

n=0.10;Q=600 l/s/m

n=0.10;Q=50 l/s/m

imdc 2007

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

F1, erosion inner

slope

Revetment type

Overflow (l/m/s)

Grass GeotextileOpen

concrete blocks

Open stone

asphalt

< 1 2 2 2 2

1 – 10 2 (*) 2 2 2

10 – 50 1 (*) 2 (*) 2 2

> 50 0 1 (*) 1 (*) 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

4,0

4,5

5,0

5,5

6,0

6,5

7,0

7,5

8,0

0 2 4 6 8 10 12 14 16 18

thickness sandy aquifer

dH

[m

]

imdc 2008

eg. Piping

Based on Bligh formula and expert judgment

(1) Conceptual method

Assessment of Failure Index for piping

F5, piping Ld/dH (*)

Presence of (coarse) sand beneath the dike?

< 4 4 and < 18 18

No 2 2 2

Possible 1 2 2

Yes 0 1 2(*) 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 18 18 3 5 25

Loam 18 18 5 3 27.5

Sand 17 20 25 0.1 30

Cover 20 20 15 5 30

Under-consolidated clay-rich layer

16 16 1 5 17.5

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

(1) Conceptual method

Assessment of Failure Index for inner slope failure

F3, inner slope failure Slope

Height > 5 and 7 m (*) 16:4 12:4 10:4 8:4 6:4

Clay 3 (**) 2 (**) 1 (**) 0 0

Loam 3 (**) 2 (**) 1 (**) 0 0

Sand 3 (**) 1 (**) 0 0 0(*) 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 material Significant 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)

(2)

Ass

essi

ng d

ike

failu

re p

roba

bilit

y

ExampleFailure Index for different failure mechanisms

Failure probability of different failure mechanisms

Scheldt river

Tidal range of 6 mCrest at AD +10 m

Groundlevel at AD +5 mOuter slope 16:4Inner slope 12:4

Failure Index

Erosion inner slope 2

Erosion outer slope 2

Inner slope failure 2

Outer slope failure 0

Piping 2

Microstability (inner slope) 2

Microstability (outer slope) 2

Probability (year)

Erosion inner slope > 1.000

Erosion outer slope > 90.000

General slope failure no results yet

Piping > 100.000

Microstability (inner slope) > 100.000

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 WATERWITH A PROBABILITY OF FLOODING (i.e. a dike)

“Lawrence Weiner”

Thanks

Questions, suggestions, …

patrik.peeters@mow.vlaanderen.be