Dutch approach to coastal flood hazard

Joost Beckers

Deltares, The Netherlands

January 28, 2013, NRC Workshop on PFHA, Rockville MD
The Netherlands
January 29-31, 2013

Flood of Feb 1st 1953

PFHA Workshop, Washington DC 3
• Improvement of flood protection

• Closing of estuaries with dams and 

storm surge barriers 

• Formal safety standards in terms of 

probabilities

Response to the 1953 Flood
Coastal defenses:

1/4000 - 1/10.000 per year

Estuaries:

1/2000 per year

River levees:

1/1250 per year
 Done every 5 years

 Performed by Water boards

 Tools and methods prescribed by Ministry of Infrastructure 

 Central role for probabilistic methods

Water Act (1990): periodic safety assessment

Levees

Dunes

Hydraulic structures
statistics

statistics

Safety assessment – probabilistic framework

7

loads on 

levee

river discharge

sea water level

wind

hydrodynamic 

model

failure model

levee 

resistance

soil cohesion

crest level

slope angle

probability 

of failure
Example: Coastal sea defense

Failure mechanism: wave overtopping

Load = water level and wave conditions

Resistance = height and profile of the levee

Aim: determine the probability of a 

critical overtopping discharge of >1 l/m/s

Required input: 

- Water level statistics

- Wind statistics (direction and speed)

- Wave model for transformation of wind to nearshore waves
Water level statistics at gauging stations

GPD on POT

10-4

5 m

95% uncertainty

Range of 

Observations
‘Modified’ Regional Frequency Analysis

Variation to RFA for a non-homogeneous region:

GPD shape parameter may vary, but follow a coherent spatial pattern

Constraints on shape parameter, combine with hydrodynamic modeling 

and expert judgment
Hydrodynamic modeling

Use Delft3D model simulations of ‘superstorms’ to:

• Improve the consistency between stations

• Get surge levels at other locations

• Verify the feasibility of extreme surge levels
Wind statistics

Wind statistics: 

• Potential wind speed at 10m

• Conditional on wind direction (12 sectors)

• GPD on POT

• PW-Moments

• ‘Modified’ RFA
Nearshore wave conditions

• SWAN (Simulation WAves Nearshore) 

 Delft University model for wave generation and propagation

• Translate offshore wind to nearshore wave conditions 

 Significant wave height Hs and spectral wave period Tm-1,0

• Use nested grids from full North Sea down to resolution of 25 m. 

http://www.swan.tudelft.nl 
Calculate failure probability

Water level

Wind speed

Failure

Non-failure

Integrate over all 

combinations that 

lead to failure

-4 -2 0 2 4 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

x

1

f(x1

)

-4 -3 -2 -1 0 1 2 3 4 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

x

2

f(x2

)

Critical overtopping

discharge
Probabilistic models – Hydra family

Different models for different water systems

and associated forcing variables:

• Coast

• River

• Lake 

• Tidal river into sea

• River into lake
Dealing with uncertainty (1)

Aleatory uncertainty: natural variability

Epistemic uncertainty: model and statistical uncertainty

First generation of Hydra models (1990’s): 

For the safety assessment:

Use best estimates of hydraulic load

Disregard epistemic uncertainty

For (re)design of flood protection: 

Apply safety margins for model uncertainties

And margins for sea level rise and land subsidence. 

Any newly built structure will pass the safety assessment
Dealing with uncertainty (2)

New generation of Hydra models:

Epistemic uncertainties as random variables in the probabilistic model 

Example: uncertainty in water level frequency curve

Roscoe and Diermanse, Coastal Engineering 58 (2011) 1023–1033

Statistical uncertainty 

included in the AEP

Increase of 10-4 AEP

surge level of 50 cm
Effect on failure probability

Dune erosion model

At 10-4 level, increase in retreat distance: 10-20m

Roscoe and Diermanse, Coastal Engineering 58 (2011) 1023–1033

Failure probability 

increases to 2*10-6

Dune would still pass 

the safety assessment

Contribution of 

statistical uncertainty ~10%
Summary and conclusions

• Flood risk management based on AEP’s of 1/10,000 can be done

• Large uncertainty in extrapolation of frequency curves

• Use (M)RFA and hydrodynamic modeling for spatial coherence and 

improve general confidence in the frequency curves

• Current safety assessment procedures are based on best estimates 

of exceedance frequencies of the hydraulic load

• Probabilistic models are being developed that allow for a Bayesian 

approach. Informative for:

• Probability of failure according to Bayesian interpretation

• Contributions of uncertainties to failure probability

• Relevance for the decision at hand
Thanks for your attention 

Deltares, Delft, The Netherlands

Joost Beckers 

+31-88 335 8336

joost.beckers@deltares.nl
Extra slide: Stochastic storm modeling

Random variables are depression track, speed, size, depth

Probability distributions based on historical storms

Hydrodynamic modeling produces surge levels

Compare results with statistical extrapolation

Simulated results

Observations

Work by Mathijs van Ledden (Royal Haskoning, Delft University) Matthijs de Jong (Delft University), 

Kees den Heijer (Delft University ,Deltares) and Pieter van Gelder (Delft University)