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Risk Analysis for Irrigation Dam Safety
2015. 04. 15Gun Heo
Outline
• Introduction
• Literature Review
• Research Plan
• Summary
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Background
Fig. Teton Dam failures (05.07.1976 A.M. 11:15, 11 Fatalities )4
Fig. Mississippi levee failures by Hurricane Katrina(08. 2005, 1800 Fatalities)
Background
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• Prior to these tragedies occurred in the U.S., they used to manage their dams without priorities
• USBR, USACE recognized need to implement risk analysis following failure of Teton dam and levees in New Orleans
• Tried to find efficient way to manage lots of dams with limited budget and developing risk analysis for dam safety
Background
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< Sandae Dam > < Gyeyeon Dam >
< Naeduck Dam > < Bookgun Dam >
Background
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Suggestions(for Prioritizing)
• More than 3,000 dams
• Pre-screening need
• PFM analysis is essential
• Concentrate on the High Risk Dams Phase 1
Hazard Classification for pre-Screening
Phase 2 Screening Risk Analysis for Prioritizing PFM Analysis
Phase 3 PFM Analysis for Prioritizing Risk
Phase 4 Prioritizing Risk of Dams and Rating
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Suggestions(for Prioritizing)
Phase 4 Prioritizing Risk of Dams and Rating
Where our efforts should be focused
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Challenges
• But several challenges are on this suggested procedure
• At phase 2, Evaluating risk of lots of dams consistentlywith this spread sheet is not easy, specially internal erosion is the most difficult failure mode
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Internal Erosion /Subjective probability
• One of the leading causes of failure of embankment dams has been internal erosion(47% of failures due to internal erosion)
• Estimating Probability of Internal Erosion is Subjective Probability since Site Specific
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Internal Erosion /Subjective probability
• Subjective probability makes difficult to maintain consistent priority for large inventory of dams
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Consistent Methodology
• Need a consistent methodology to evaluate many High hazard dam safety.
• Be simple, quick and easy to implement
• Avoid subjectivity and unnecessary bias
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Research Topic
• Develop a simple methodology evaluating risk by internal erosion for numerous Korean irrigation dams
• And suggest more detailed and improved process for the screening portfolio risk analysis by improving Phase 2
Phase 1Hazard Classification for pre-Screening
Phase 2 Screening Risk Analysis for Prioritizing PFM Analysis
Phase 3 PFM Analysis for Prioritizing Risk
Phase 4 Prioritizing Risk of Dams and Rating
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General Categories of Internal Erosion
• Internal Erosion through Embankment
• Internal Erosion through Foundation
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General Categories of Internal Erosion
• Internal Erosion of Embankment into Foundation
• Internal Erosion into/along Structures
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General Types of Internal Erosion Mechanisms
• Piping Mechanism
Piping Mechanism Schematic from van Beek et al. (2010) 18
General Types of Internal Erosion Mechanisms
• Stoping Mechanism (Backward erosion)
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General Types of Internal Erosion Mechanisms
• Scour Mechanism ( contact erosion )
Contact erosion process from ICOLD, 2012
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General Types of Internal Erosion Mechanisms
• Suffusion/Suffosion Mechanism
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• Now, How to evaluate the probability of failure of embankment dams by internal erosion?
• Event tree(USBR, USACE)• Statistical analysis(UNSW)• Anchor points(DEFRA)
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Event Tree• Event trees depict a logical and/or chronological
sequence of events or conditions
(Decompose complex events
into simple events that areeasy to understand and for which probabilities can
be estimated)
• Risks are estimated by mathematically combining the branch probabilities
∏ = 0.00108
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Event Tree of Internal Erosion(USBR)- Reservoir at or above threshold level
- Initiation – Erosion starts (Historical performance)
- Continuation – Unfiltered or inadequately filtered exit exists
- Progression – Continuous stable roof and/or sidewalls - Progression – Constriction or upstream zone fails
to limit flows - Progression – No self-healing by upstream zone
- Unsuccessful detection and intervention
- Dam breaches(uncontrolled release of reservoir)
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Event Tree of Internal Erosion(USBR)- Reservoir at or above threshold level
- Initiation – Erosion starts (Historical performance)
- USBR thinks,“Laboratory testing of small specimens to develop erosion properties may not be representative of the weak link or true condition in the large embankment-foundation system.”
- So, they are using historical rate of initiation of internal erosion.
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Historical rate of initiation(USBR)- Sum all dam-years of operation at
USBR(12,000)- Number of incidents with particle transport(4)
Table. Proposed Best Estimate Values of Annual Probabilities of Initiation of Internal Erosion by Category
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Event Tree of Internal Erosion(USACE)- Reservoir loading (at or above threshold level) - Flaw exists – Continuous crack, high permeability zone,
zones subject to hydraulic fracture, etc. - Initiation – Particle detachment (erosion starts)
- Continuation – Unfiltered or inadequately filtered exit exists
- Progression – Continuous stable roof and/or sidewalls - Progression – Constriction or upstream zone fails
to limit flows - Progression – No self-healing by upstream zone
- Unsuccessful detection and intervention
- Dam breaches(uncontrolled release of reservoir)27
Event Tree of Internal Erosion(USACE)- Reservoir loading (at or above threshold level) - Flaw exists – Continuous crack, high permeability zone,
zones subject to hydraulic fracture, etc. - Initiation – Particle detachment (erosion starts)
- USACE suggestssome analytical methods and tests(complex and not easy to use) to aid in making reasonable probability estimates.but all final probabilities are estimated using team elicitation procedures based upon the evidence.
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Utilization of Event Tree method - It needs lots of energy and information
to evaluate the probabilities.
- Event tree method is good for the evaluating probability of individual dam
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Statistical analysis(UNSW)• A statistical analysis of embankment dam failures and
accidents is thoroughly presented in UNSW, based on a large database of embankment dam incidents
• Assumption of UNSW method, “It is reasonable to make estimates of the relative likelihood of failure of embankment dams by piping from the historic frequency of failures”
• The likelihood of failure of a dam by piping is estimated by adjusting the historical frequency of piping failure by weighting factors.
• Weighting factors are dam zoning, filters, age of the dam, core soil types, compaction, foundation geology, dam performance, and monitoring and surveillance
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Statistical analysis(UNSW)
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Statistical analysis(UNSW)
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Statistical analysis(UNSW)
- Calculate the weighting factors WE, WF, and WEFfrom Tables respectively, - The weighting factors are obtained by multiplying the individual weighting factors
WE = WE(filt) × WE(cst) × WE(cc) × WE(obs) × WE(mon)
- Obtain the annual likelihood of failure by piping, Pp,by summing the weighted likelihoods of each of the modes:
Pp = WE Pe + WF Pf + WEF Pef
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Anchor points(DEFRA)
-Base probabilityGet the base probability(default value) for the Best and Worst condition dam by historical performance data
-Current condition scoreDepends on the seepage and deformation condition of dam(with Weighting factors)
- Match Current condition score to the Annual Probability of failure
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Anchor points(DEFRA)-Current condition score
4.4.1 Indicators (in embankment, remote from structure)Maximum Guidance on condition to Score Location/ Remarks
SeepagePossible get maximum score Calc. 1 Calc. 2 (refer to Row in Sheet 1.3 where appropri
ate)
Seepage carrying fines 10 Cloud of particles 0 0 Lots of ochre, possibly some fines at toe of left abutment
Seepage increasing at same reservoir level 8 Change of 20% on previous value 0 0
Large amount of uncontrolled seepage i.e. not discharging to filtered drainage system
6 10 times Seepage Index given in Charles et al, 1996, page 7
3 0 Reduce as high natural g/w
Increased pore pressures in/ under downstream shoulder
6 Increase of 20% of reservoir head 0 0 Piezo installed in 1999
Animal burrows 1 Extensive 0 0Decaying tree roots 1 0 0 Alder cut back, but not dead
Deformation Settlement Index (Johnston et al, 1999, page 16)
4 Acceleration with increase in gradient of > 50%
0 0 Not measured
Sinkholes, depressions, local settlement 10 10 for 1m deep; 4 for 0.1m deep 2 0 Depression on d/s side of crest at mid-point of valley
Slope movement (lateral deformation, cracking)
4 Persistent crack of length> dam height
0 0
Sub-total 5 0Reduce to maximum (if necessary); maximum score is
10 5 0
Divide by factor to get component of Current Condition Score
1.0 5 035
Anchor points(DEFRA)
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-010 1 2 3 4 5 6 7 8 9 10
Annu
al p
roba
bilit
y of
failu
re, g
iven
in p
artic
ular
con
ditio
n
Current Condition Score
Graph for plotting Annual Probability of Failure vs. Current Condition Score
Internal Stability (embankment) - Calculation 1 Internal Stability (embankment) - Calculation 236
Base probability(Korea)Ø Base probability for the Worst condition damCollected historical performance data of Korean dams to get the base probability(default value) for the Worst condition dam
• Number of failure dams in Korea
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Failure mode No. Remarks
Internal Erosion(Embankment) 10
Conduit failure 4
overtopping 77
Sliding 1
Unknown 15
Total 107
Base probability(Korea)Ø Base probability for the Worst condition dam
Estimated the average probability of failure fromhistorical records
• Annual probability of failure due to internal erosion
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= Number of failures in period due to failure modeNumber of dam life years in period considered
= 10277,907= 3.6E-05/annum
Base probability(Korea)Ø Base probability for the Worst condition damKorea Results were compared with the results of previous studies
• Default value for the worst condition in U.K.(DEFRA) = 1.4E-02/annum
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Base probability(Korea)Ø Base probability for the Best condition damImplemented the Event Tree Analysis(ETA) for the dam
that assumed in excellent condition
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Node Probability14.6 18 24.6 31.1 ResEl.
5 50 1150 23000 Recurrence
Node1 1/recurrence 0.2 0.02 0.00087 0.000043
Node2 Erosion Initiates 1.E-04 1.E-03 1.E-02 1.E-01
Node3 UnfilteredExit 0.1 0.1 0.1 0.1
Node4 Roof support 0.5 0.5 0.5 0.5
Node5Crack Stopper NotPresent
0.1 0.1 0.1 0.1
Node6 Flow Unlimited 0.995 0.995 0.995 0.995
Node7InterventionUnsuccessful
0.05 0.1 0.5 0.5
Node8 Breach 0.95 0.95 0.95 0.95
1.4.E-08 2.8.E-09 2.4.E-09 1.2.E-8 5.40.E-08
Current Condition VS. POF.Ø Matching the current condition to the Probability of
Failure(POF)Can use the indicators like Seepage and Deformation
to match the probability of failure
• Important thing is to know that which indicator is how sensitively related to the failure of dams
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Summary• With the limited budget for dam safety, Risk
analysis has been developing for dam safety.-Need proper evaluating methods for the probability of failure by internal erosion
• As a preliminary study, evaluating methods for probability of failure by internal erosion are introduced and summarized.-USBR, USACE, UNSW, DEFRA
• Got the base probability for the Best and Worst condition dam by historical performance data
• Detailed study for Matching Current condition score to the Annual Probability of failure will be performed to be applicable to Korea
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Thank you !
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