Risk-Informed Decision Making

Approach for Inflow Design Flood Selection

and Accomodation for Dams

Jason Hedien 

MWH

January 30, 2013

RAC

Engineers &Economists
RIDM for IDF Selection for Dams

• Presentation Outline

1. Background & Introduction

2. Context and Scope

3. Risk Assessment

4. Decisions Made (RIDM)
1. Background and Introduction

– Two run-of-river dams 

• Inflow = Outflow

• No flood storage in reservoirs

• Dams in series on same river

– Constructed for hydropower

– Located in rural Midwest

– Classified as high-hazard

– Regulated by FERC

– Existing dams would be 

overtopped by PMF

– Need to accommodate IDF

– RIDM approach utilized
2. Context and Scope

• Context of Risk Assessment

– Significant percentage of 

population at risk around lakes

– Flooding of dwellings around 

lakes occurs with small 

increases in lake levels

– Significant difference in winter 

and summer populations

– Significant difference in winter 

and summer flood hydrographs

– Population impacted by floods 

before overtopping of dams

– Well functioning EAP
2. Context and Scope

• Scope of Risk Assessment

– Assess baseline risks (risks with dams in existing 

condition) (BRA)

– Identify risk reduction measures

– Assess risk reduction measures (RRA)

– Evaluate risk reduction measures relative to FERC IDF 

definition
3. Risk Assessment

– Potential Failure Modes Analysis (PFMA)

• Per FERC guidelines

• Credible and Significant PFMs included in RA

– Flood-related

– Focused on overtopping and erosion of embankments at dams

– Risk Assessment Workshop

• Same participants as PFMA

• Develop event tree & system response probabilities (SRP’s)

– Including a range of uncertainty

– Spillway gate reliability and spillway plugging with debris
3. Risk Assessment

• Event Trees

.

.

.

AEPn-1 - AEPn

> AEPn

Spillway

Clear

No Failure

Erosion exposes Shallow Section

Core wall more than 15 ft deep

.

.

.

1. AnnualExceedance

Probability

AEP1 - AEP2

AEP2 - AEP3

3. Spillway

Clear / Plugged

Peak Reservoir

Elevation

4. Peak

Reservoir Elevation

2. Number of Spillway

Gates Failing to Open

No Spillway

Gate Fails

.

.

.

1 Gate Fails

to open

5. Overtopping

Erosion and 

Dam Failure

6. Core Wall Topples and

Upstream Section Erodes 

and Location of Failure

30 ft wide

Erosion exposes Maximum Section

Core wall more than 20 ft deep

Legend

Expanded Chance Node

Collapsed Chance Node

Consequences Node

60 ft wide

90 ft wide

Spillway plugging

- Case 1

Spillway plugging

- Case 2

Insufficient

Erosion

Overtopping

Erosion

7. Initial Width of

Core Wall Breach 

- Extent of Failure

8. Peak

Discharge

No Failure Peak

Discharge

Peak

Discharge

Peak

Discharge

Peak

Discharge

Breach through Right 

Abutment

No Failure
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 0.5 1 1.5 2 2.5 3 3.5 4

Peak O vertopping Depth (ft)

Cold & Warm Season Probability of Corewall Failure - WITHOUT 

REBAR

#1 #3 #4 #2 #5 Average

3. Risk Assessment

• System Response Probabilities

Individual & Group Estimates
3. Risk Assessment

• Estimating Consequences

– Breach inundation runs

– Affected structures

– Population at risk

– Life loss (LIFESim methodology by 

Aboelata and Bowles)

Season

Peak 

Discharge at 

Norway (cfs)

Width of the 

first breach 

(feet)

Depth of 

breaches (feet)

Case (new 

numbering)

No failure No failure NF1

30 20 NF2

120 20 NF3

30 15 NF4

120 15 NF5

R. Embkmt. F 8.3 NF6

No failure No failure NF7

30 20 NF8

120 20 NF9

30 15 NF10

120 15 NF11

R. Embkmt. F 8.3 NF12

No failure No failure NF13

30 20 NF14

120 20 NF15

30 15 NF16

120 15 NF17

R. Embkmt. F 8.3 NF18

No failure No failure NF19

30 20 NF20

120 20 NF21

30 15 NF22

120 15 NF23

R. Embkmt. F 8.3 NF24

No failure No failure NF25

30 20 NF26

120 20 NF27

30 15 NF28

120 15 NF29

R. Embkmt. F 8.3 NF30

No failure No failure NF31

30 20 NF32

120 20 NF33

30 15 NF34

120 15 NF35

R. Embkmt. F 8.3 NF36

No failure No failure NF37

30 20 NF38

120 20 NF39

30 15 NF40

120 15 NF41

R. Embkmt. F 8.3 NF42

No failure No failure NF43

30 20 NF44

120 20 NF45

30 15 NF46

120 15 NF47

R. Embkmt. F 8.3 NF48

39000

51000

Warm

39000

51000

Cold

70000

90000

227600

(PMF)

58200

(PMF)

Flood Zone Rate of Life Loss

(Range)

Rate of Life Loss

(Average)

Safe 0 - <1% 0.02%

Compromised 0 – 50% 12%

Chance 50 – 100% 91.45%
3. Risk Assessment

• Tolerable Risks

1. F-N Charts (ANCOLD & HSE)

2. f-N Charts (Reclamation)

3. Summary Tables

Rating Code Explanation

N-StrongL&S Strong justification for long- and short-term risk reduction

measures

N Strong justification for long-term risk reduction measures

Y-ALARP?

Diminished justification for long-term risk reduction measures, but

ALARP (as low as reasonably practicable) still needs to be

evaluated

Y Meets tolerable risk guideline and meets ALARP
3. Risk Assessment

• Risk Reduction Assessment

– Improve warning & evacuation effectiveness (not explicitly 

evaluated, but measures being implemented anyway)

– Remove dams (not feasible)

– Land acquisition (not feasible)

– Improve gate reliability (not explicitly evaluated)

– Trash/debris booms (not explicitly evaluated)

– Raise embankment dams (not considered further)

– Re-evaluate East Dam at Dam B to determine whether stability 

improvements are needed

– Add additional spillway capacity

– Armor embankments

– Do nothing (baseline case)

– Appropriate changes to event tree inputs to represent each 

alternative
Dam A Summary of Annual Probabilities of Failure

Alternative: Description:

Discharge

(cfs):

Estimated Total Annual 

Probability of Failure

Baseline 

Case:

No Blockage 

&

100% Gate

Reliability:

A Do nothing 39,000 1 in 700 1 in 1,300

B Provide functional flashboards on overflow spillway 43,700 1 in 1,400 1 in 2,800

C.0 Lower overflow spillway crest and add gates to pass 

additional flow over the overflow spillway 75,500 1 in 41,800 1 in 62,300

C.1 Lower overflow spillway crest and add gates 53,000 1 in 5,400 1 in 10,500

C.1R Same as C.1, but includes contribution of trash gate to 

overall discharge. 57,000 1 in 8,200 1 in 14,500

C.2 Lower overflow spillway crest and add gates 60,000 1 in 11,700 1 in 18,500

D Add gated spillway on left embankment and lower overflow 

spillway crest (maximize capacity through Dam A). 100,600 1 in 181,000 Not computed

E Armor left embankment and right rim 39,000 1 in 8,000,000 Not computed

3. Risk Assessment
4. Decisions Made

– Selected alternatives a result of RIDM

– Alternative C.2 at Dam A:

• 60,000 cfs capacity ~ 103% warm PMF/27% cold PMF

– Alternative B.2 at Dam B

• 65,000 cfs capacity ~ 108% warm PMF/28% cold PMF

– Meet all tolerable risk guidelines

– Spillway capacity additions accepted by FERC

– Very low cost effectiveness for reducing life loss for risk 

reduction alternatives (ALARP satisfied)

– Alternatives in design and construction phase
 END