risk analysis for irrigation dam safety gun heo...

Risk Analysis for Irrigation Dam Safety

2015. 04. 15Gun Heo

Outline

• Introduction

• Literature Review

• Research Plan

• Summary

2

3

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

5

• 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

6

< Sandae Dam > < Gyeyeon Dam >

< Naeduck Dam > < Bookgun Dam >

Background

7

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

8

Suggestions(for Prioritizing)

Phase 4 Prioritizing Risk of Dams and Rating

Where our efforts should be focused

9

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

10

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

11

Internal Erosion /Subjective probability

• Subjective probability makes difficult to maintain consistent priority for large inventory of dams

12

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

13

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

14

15

General Categories of Internal Erosion

• Internal Erosion through Embankment

• Internal Erosion through Foundation

16

General Categories of Internal Erosion

• Internal Erosion of Embankment into Foundation

• Internal Erosion into/along Structures

17

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)

19

General Types of Internal Erosion Mechanisms

• Scour Mechanism ( contact erosion )

Contact erosion process from ICOLD, 2012

20

General Types of Internal Erosion Mechanisms

• Suffusion/Suffosion Mechanism

21

• Now, How to evaluate the probability of failure of embankment dams by internal erosion?

• Event tree(USBR, USACE)• Statistical analysis(UNSW)• Anchor points(DEFRA)

22

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

23

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)

24

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.

25

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

26

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.

28

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

29

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

30

Statistical analysis(UNSW)

31

Statistical analysis(UNSW)

32

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

33

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

34

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

37

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

38

= 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

39

Base probability(Korea)Ø Base probability for the Best condition damImplemented the Event Tree Analysis(ETA) for the dam

that assumed in excellent condition

40

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

41

42

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

43

Thank you !