US Army Corps of Engineers

BUILDING STRONG®

Probabilistic Flood Hazard Assessment

Panel 4 – Flood Induced Dam and Levee Failures

Risk Informed Decision Framework for Dam And Levee Safety

Rockville, MD

30 January 2013
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Why?

 Decisions will be made

►With incomplete information

►In a constrained environment

 Risks are not intuitive

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Issue 

Evaluation

Action 

Justified

Modification Study

Implementation

How?

Operation and 

Maintenance

Monitoring and 

Instrumentation

Inspection

Safety Issue

Risk Assessment

Risk 

Characterization

► Routine Activities

► Non-Routine Activities

 Portfolio Risk 

Management

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Who?

Leaders

Managers

Analysts

•Understand basis for decisions

•Communicate risk

•Understand risk concepts

•Make important decisions

•Conduct risk analysis

•Defend recommendations

Where 

decisions are 

made

Where organizational use of risk usually starts

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 What decision are we making?

 Who is making the decision?

 How will the decision be made?

► What criteria will be used?

► What are the key factors?

 What information is available?

► Will more information improve the decision?

 What are the consequences of a poor decision?

Decision Driven

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Risk Informed Decisions

 Characterize risk

 Identify uncertainties

 Recommend actions

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Components of Risk

Risk = f(Hazard, Performance, Consequences)

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Hazard –

Magnitude, Frequency, Duration, etc

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 How does the system 

perform

 What can lead to failure

 How credible and likely 

is it

Performance –

Potential Failure Modes Analysis

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Failure Mode - Description

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 A large flood raises the river to unprecedented levels which imparts 

high water pressures in an exposed basal sand/gravel layer through 

an outcrop in the river channel or bridge pier penetrating to the sand 

layer. The sand layer is continuous beneath the alluvial clays in the 

foundation of the levee and outcrops on the land side in a low sump 

or ditch. A path for unrestricted water flow through the foundation 

develops. Internal erosion begins by movement of soil into the 

sump or ditch on the land side of the embankment and progresses 

towards the river by backward erosion beneath a foundation clay 

layer capable of forming a roof. Erosion progresses to the river and 

water entering the piping channel erodes and expands the pipe until 

eventually the embankment sloughs into the void and breaches 

causing uncontrolled flooding. This is most likely to occur at a 

pumping station where there are exposed channels on both the river 

side and land side.
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Failure Mode - Evidence

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Flood Fighting Full Scale Tests

Geomorphology Incidents Indicators

Investigations
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Failure Mode – Key Factors

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 More Likely (Adverse)

► Geomorphology indicates three point bars extending through the 

foundation

► Sand and gravel layers exist in the foundation

► Critical gradient may be low for fine sand

► Numerous sand pockets are possible in the foundation due to 

meanders of the river and depositional environment

 Less Likely (Favorable)

► No seepage has been observed through the known sand layer

► Average gradient is low due to long distance between source 

and exit

► Lower sand layers in foundation soils are unlikely to daylight on 

protected side
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Failure Mode - Event Tree

Expert Elicitation

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Consequences

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 People and property at risk
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Consequences

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 Warning and evacuation
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Consequences

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 Life loss
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Risk Characterization –

Tolerable Risk Guidelines

Evidence:

No observed seepage or erosion 

at lower flood events

Continuity is questionable (riverside to land-side)

Moderately long seepage path

Relatively short duration of 

loading

Opportunity to intervene

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Risk Characterization –

Inundation Scenarios

Leveed Area Residual 

Inundation Risk

Levee System 

Incremental Risk

Levee Overtopping 

Without Breach Risk

Overtopping Without 

Breach

Overtopping with Breach

Malfunction of Levee 

System Components

Breach Prior to 

Overtopping

(Non-breach Risk) 

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Risk Characterization -

Classification

Moderate 

Risk High Risk High Risk Very High 

Risk

Very High 

Risk

Moderate 

Risk

Moderate 

Risk High Risk High Risk Very High Risk

Low Risk

Moderate 

Risk

Moderate 

Risk High Risk High Risk

Very Low 

Risk

Low Risk

Moderate 

Risk

Moderate 

Risk High Risk

Very Low 

Risk

Very Low 

Risk

Low Risk

Moderate 

Risk High Risk

Likelihood

Consequences

Levee Safety Action Classification

Urgency of Action Actions Characteristics

Very High

Actions 

recommended 

for each class.

Likelihood of inundation 

with associated 

consequences 

characterizes each 

class.

High

Moderate

Low 

Normal

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Investment in Knowledge

Uncertainty

 This is what we are reasonably certain of

 This is what we think is likely but is by no means proven

 This is what we think is unlikely but still possible

The known knowns

The known unknowns

Unaware of risk,

cannot estimate

Risks well understood, 

can be estimated

Aware of risk, 

difficult to estimate

The unknown unknowns

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Value in Reducing Uncertainty

High Impact

High Uncertainty

Low Impact

Low Uncertainty

High Impact

Low Uncertainty

Low Impact

High Uncertainty

Uncertainty

Decision 

Impacts

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Recommended Actions

 This is what we are doing (or not doing) to 

reduce the risks and/or uncertainty that remains

1. Estimate of current risk.

2. Risk reduction and management 

measures implemented.

a. Actions that reduce likelihood

b. Actions that reduce consequences

3. Trend due to aging and wear and tear.

4. Trend due to proper maintenance, 

repairs, and operations

5. Trend resulting from development

6. Trend resulting from continued risk 

management activities

Moderate 

Risk High Risk High Risk Very High Risk

Very High 

Risk

Moderate 

Risk

Moderate 

Risk High Risk High Risk Very High Risk

Low Risk Moderate 

Risk

Moderate 

Risk High Risk High Risk

Very Low 

Risk Low Risk Moderate Risk

Moderate 

Risk High Risk

Very Low 

Risk

Very Low 

Risk Low Risk Moderate Risk High Risk

Likelihood

Consequences

1

3

6 4 5

2

b

a

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Making the Case

 Interpretation by the experts

► No simple numerical solutions

 Supports a credible decision

► Consistency between risk estimate, 

evidence, and recommendations

 Answers the ‘So What’ question

You want me 

to spend how 

much?

Why should I 

believe you?

How do 

you know 

that?

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1. Do your homework with literature, experts, and users

2. Let the problem drive the analysis

3. Make the analysis as simple as possible, but no simpler

4. Identify all significant assumptions

5. Be explicit about decision criteria and policy strategies

6. Be explicit about uncertainties

7. Perform systematic sensitivity and uncertainty analysis

8. Iteratively refine the problem statement and the analysis

9. Document clearly and completely

10.Expose to peer review

Closing Thoughts

Morgan, M. G. and Henrion, M., (1990). Uncertainty: a 

guide to dealing with uncertainty in quantitative risk and 

policy analysis, Cambridge University Press, New York

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Closing Thoughts

 Theory and calculations are not 

substitutes for judgment, but are the bases 

for sounder judgment. (Peck)

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