Technical Workshop TW1Survey of International Dam Safety

Risk Approaches and Practices:NETHERLANDS

Hans Janssen & Niels RoodeRijkswaterstaat (RWS)

A logo ofyour

institution

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Context: The Dutch situation

Elevation < 5m MSL Flood prone

• 17 million people, 9 million are flood prone• Delta of Rhine/Meuse rivers, bordering North Sea• GDP 875 billion USD, spent on FRM 0,5 - 1 billion USD/yr• Levee systems protect 60% of Dutch territory

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The 1953 flood

• Lives lost: 1836• Homes lost: 100,000• Storm surge + high tide• Flood alerts were not

understood and telegrams came too late

• Levee maintenance had been insufficient (due to 1930’s depression & WWII)

• Too many organizations with overlapping tasks (public water boards, provinces, state and private water boards)

• Too few funds, insufficient tax base

11/7/17

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Response after 1953

1) Delta WorksShortened coastline and levee restoration, dams, surge barriers, overtopping under control

2) Better organizationDecrease # water boards, better oversight, maintenance and flood alerts, formal inspections

3) Better legislation and fundingStatutory flood risk standards and periodic assessments, structural national funding

4) Better informationRisk and science informed since 1960, monitoring, mapping, continuous improvement of models, statistics, consistency

11/7/17

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Purpose: inform decisions• Safety standard / ‘Tolerable Risk’ (national government)

(risk + cost/benefit political decision on tolerable risk)• Levee assessment ‘verdict’ (regional authorities + RWS)

(flood probability + ‘distance to safety standard’ + funding eligibility)• Prioritising + programming projects (Flood protection

programme)(‘distance to the standard’ or urgency)

• Design / upgrade (project level)(levee dimensions that comply with standard at end of life cycle)

• Operation and maintenance (regional authorities + RWS)(prioritisation of O&M measures)

Purpose and approachof risk assessment

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Approach: Levee safety risk management portfolio process

Riskstolerable when 

standards are met?

Update statutorysafety standards

No

Statutory safety assessment 

(standard testing)

Flooddefence meets 

standard?

National Flood Protection 

Programme

Restorationmeasures

Routine inspection and maintenance

Yes

No

Monitor risk Yes

• 2.300 km of primary levees

• Statutory safety standards (tolerable risk, 25-50yr cycle)

• Statutory safety assessment (test standard, 12yr cycle)

• Levees not meeting standard feed Flood Protection Programme

Purpose and approachof risk assessment

25-50 y

12 y

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Failure mode at location x

Overall ‘verdict’

Simple technicalassessment

Detailed technicalassessment

Adv. technicalassessment

Criteria met?

Criteria met?

Criteria met?

Subjectiveassessment

Positivejudgment?

Section failstechn. assessment

Section passestechn. assessment

Section failssubj. assessment

Section passessubj. assessment

Approach: Levee safety assessment process

• safety assessment: standard testing

• From coarse to fine• rules and expert

judgement: tell the story• Semi-probabilistic (safety

factors & design values)• Probabilistic techniques

from 2019 (both loads and strength)

• Sources (mandatory): standards, manuals, instruments

y

y

y

y

n

n

n

n

Purpose and approachof risk assessment

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• 50 primary levee systems (dike rings); 2300 km

• Dams, barriers, levees, dunes, hydraulic structures

• Statutory safety standards ranging from 1/250-1/10.000 per year

• Derived from CBA (1959)

• Exceedance probabilities (i.e. design loading conditions)

Use of RA in decision making:

Flood Defence Act prior to 2017

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New flood risk insights (VNK)

Domino effect influences risk

Coastline: low risk due to wide dunesand nourishments

Risk varies within and between dike ring areas

Rivers: piping underestimatedfailure mode

Source: VNK national risk analysis

high risk of fatalities

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Translating policy goals to safety standardTolerable fatality risk

10-5per yr

Tolerable economic risk

Economical risk

investment

€Total cost

Policy choice: choose the strictest per levee system

‘Economic optimization’

‘Compared to other risk domains and natural character of floods’

From risk to tolerable risk

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A new standard type

Risk standard (e.g. 10‐5 for a certain area)

Flood probability standard(e.g. 1/x for part of levee 

system)

Probability of hydraulic exceedance standard

(‘old practice’)

Tolerable risk considerations

Target tolerable risk (combination of probability and consequences), through prevention, spatial planning and/or disaster planning.(multilayer safety: find case by case optimal mix of measures to reach tolerable risk)

Target tolerable flood probability (specific requirement to levee performance: flood probability) where uncertainties in both hydraulic loads as geotech are incorporated. 

Target tolerable hydraulic load (specific requirement to levee performance given a probability of hydraulic exceedance. Geotech as safety factors. (indirect and less efficient assessment of flood probability)

Direct  derivation

From risk to tolerable risk

Indirect  derivationJP

Slide 11

JPFM3 wat wordt met deze laatste stap bedoeld?Janssen, Hans (GPO), 6/15/2017

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• New safety standards for specific levee stretches (no dike rings)

• Expressed as ‘flood probability’• Standards = ambition 2050• Total cost estimate: € 11-15 Billion• 60-70% in riverine area• Requires levee designs 20m-200m

extra width and, hence:• ‘Innovative’ piping measures

needed• In development:

1. new design guides; 2. new assessment tools 3. new programming/prioritisation tools; 4. the expertise to use them

New Water Act (from 1/1/2017)

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Stretches

Upgrade in %

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ResearchVNK / CBA

RiskInsights

Policy: new safety standards

Formal decision

Legal embedding

WBI2017

Under construction

Delivery of tools

Results

Programming and prioritisation

HWBP on basis of:

WTI2006 + Hybrid risk

tool

WTI2011 + VNK results

WTI 2017 results

3rd HWBPDesign & Execution

‘Old’ projectsHWBP2

1st

programmeOI2014

2nd

programmeOI2018

3rd

programmeOI2018?

2014

2017

2023

2029

Towards risk-informed decision making

Future

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Pro’s and Con’sNL: “rule based” decision makingStandardized decision making procedures (statutory standards, assessments, guidelines)

PRO’s+ Quality and consistency through

standardized rules and instruments

+ Legally arranged “Standard-driven” budgets (budget waiting for projects).

CON’s- Over-reliance on models/

automation- Loss of knowledge for

organisation (“tell the story”)

Advice for startersStart simple: expert based approach, case by case analysis, prioritize with risk, build capacity.

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Fundamentals of flood protection

Link between NL policy/legislation and guidelines/technical reports.