Assessing the national flood risk in the Netherlands

Assessing the national flood risk in the Netherlands

Assessing the national flood risk in the
Netherlands
Wouter ter Horst
Dr. Jan Stijnen
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Outline
• Introduction
• Methodology
• Capita selecta
– Spatial correlation in failure probability calculations
– Assessing consequences of a flooding: economic
damage and fatalities
– Combining failure probabilities and consequences
using flood scenario’s
• How results can be used
• Questions / discussion
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Introduction
A fully probabilistic risk analysis
... to quantify flood risks
• Economic risk
• Fatality risk
...for all major levee systems
• 60% of Dutch territory
• 3600km of levees
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Methodology: quantifying flood risks
1.
Divide levee system (dike ring) into smaller sections
2.
Quantify failure probability per section
3.
Define flood scenarios and compute their probabilities
4.
Estimate consequences per flood scenario
5.
Combine scenario probabilities and consequences
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1. Probabilities per dike section
3. Risk
Economical
Individual
2. Consequences per breach location
Group
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Spatial correlation
• First step: assess the failure probability Pf of a cross section
with a FORM routine or other techniques (Monte Carlo –
Numerical Integration)
• Spatial correlation is taken into account:
Pf cross section
Pf dike section
Pf dike ring
• Spatial correlation is described by the following correlation
function:
 k (x)  c ,k
 x 2 
 (1  c ,k ) exp   2 
 dk 
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where Δx is the distance between two cross-sections, ρc,k is the
intrinsic correlation, and dk the correlation length. This function is
given for every parameter.
Combining probabilities of dike sections
Failure of system, when weakest section fails (series system).
Suppose N dike sections with strength characteristics (R) and load
conditions (S)
Coefficient of correlation between dike sections:
Strength dominated failure mechanism
(i.e. piping) : σR >> σS  ρ ≈ 0
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Load dominated failure mechanism (i.e.
overflow) : σR << σS  ρ ≈ 1
 S2
 2
 S   R2
N
Pf   Pi
i 1
Pf  max  P1...PN 
Assessing consequences
• Assessment of damage and fatalities for
various breach locations
• Taking into account evacuation effectiveness
(see next slide)
• Dose-response functions ‘translate’ inundation
depth into relative consequences
– % of total value lost by flood
– % of exposed population killed
(mortality function)
– Mortality also depends on
flow velocity and rise rate
Flood
simulation
Evacuation
analysis
Value/Population at risk
Evacuation
effectiveness
Exposed assets/population
Water depths, flow
velocities, rise rates
Dose-response
functions
% Damage/
Mortality rates
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Damage/fatalities
Assessing consequences
Effectiveness of a evacuation mainly based on two
elements:
1. Forecasting uncertainty
2. Preparedness / execution
In the Netherlands the evacuation rate is estimated on
75% for riverine areas and 15% for coastal areas.
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Combining failure probabilities and consequences
using flood scenario’s
• Scenario based approach is used to address the flood risk:
each scenario describes a situation where one or multiple dike
breaches haven taken place
• Each flood scenario has a certain probability of occurrence. The
sum of all possible scenarios equals the total failure probability
of the dike ring
– Relief effects are taken into account
– All flood scenarios can be considered as independent events
• Each flood scenario has a certain amount of consequences in
terms of fatalities and economic damage
• For each flood scenario the flood risk can be obtained by
multiplying the probability of occurrence and the amoint of
consequences.
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• The total flood risk is the sum of the flood risk of all individual
flood scenarios
Piping / seepage
Failure probability
(per year)
Erosion by waves
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Combining failure probabilities and consequences
using flood scenario’s
Dike breach at Vossendijk
(highest failure probability)
Damage:
0.8 – 1.4 bill. €
Fatalities:
20 - 160
Dike breach at R’dam-Beverwaard
(largest consequences)
Damage:
6.0 – 7.0 bill. €
Fatalities:
200 – 3.500
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Combining failure probabilities and consequences
using flood scenario’s
• Taking into account most important scenario’s
–
–
–
–
–
–
–
–
Dike
Dike
Dike
Dike
Dike
Dike
Dike
…
1
2
3
…
1 & Dike 2
1 & Dike 3
1 & Dike 2 & Dike ...
• Numerous possible scenario’s
– Selection of most important scenario’s
– Covering large part of the overall failure probability
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Combining failure probabilities and consequences
using flood scenario’s
Scenario
Failure
Probability
Damage
[mln euro]
Fatalities
Econ. Flood
risk / scen
[mln euro]
Fatal. Risk /
scenario
1
3.20E-04
1,280
800
4.10E-01
2.56E-01
2
5.80E-04
70
65
4.06E-02
3.77E-02
3
4.20E-05
4,010
900
1.68E-01
3.78E-02
…
…
…
…
…
…
Total
1.01E-03
1,000
237
1.0
0.24
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Expected values
per flood
Total flood risk in economic
damage & fatalities
How can the results be used
1.
Standard setting
2.
Evaluating strategies 
How do the alternatives
influence the level of risk?
3.
Prioritizing interventions 
How to allocate limited
resources?
4.
(Re)direct research efforts  Dominant uncertainties?

Is it safe enough?
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Measures to reduce flood risk
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Further reading
• Papers:
– Quantifying Flood Risk in the Netherlands by Dr. R.B.
Jongejan and B. Maaskant
– The VNK2-Project: A fully probabilistic Risk Analysis
for all major levee systems in the Netherlands by Dr.
R.B. Jongejan et al
– The importance of domino effects in flood risk
assessments by W.L.A. ter Horst and R.B. Jongejan
• VNK – Method in brief
• VNK – Final report
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Contact information
Wouter ter Horst
E-mail: Wouter.terhorst@hkv.nl
Mobile: +31 6 13 744 533
Jan Stijnen
E-mail: Jan.Stijnen@hkv.nl
Mobile: +31 6 10 380 992
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