NRC PFHA Conference

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FERC’S Need for PFHA

David Lord, P.E., Dam Safety 

Risk team – Portland, Or

From Deterministic 

to Probabilistic
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FERC Regulates 2948 Dams and 

Project Structures
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834 High Hazard Potential

234 Significant Hazard Potential
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In the Beginning... for FERC

 The FERC’s modern dam safety program -

implementing regulations in 1981.

 The Division of Dam Safety and 

Inspections (D2SI) - regulatory 

responsibility for FERC-licensed 

hydroelectric dams.

 Dams are classified into low, significant, 

and high hazard potential. 
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In the Beginning... for FERC

 The high hazard classification - worst case 

dam failures might cause loss of life.

 Significant hazard dams - economic or 

sensitive environmental issues.

 Standards-based program - relied on 

conservative deterministic analyses.

 Probable Maximum Flood and Maximum 

Credible Earthquake 
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PFMA

 In 2000 - D2SI established a Potential Failure 

Modes Analysis (PFMA) program for all high and 

significant hazard (HSH) dams.

 

 In 2009 - Commission’s strategic plan - develop a 

probabilistically oriented program, titled Risk- Informed Decision Making (RIDM).

 

 In 2012-13, RIDM Engineering Guidelines - include Hydrologic Hazard Analysis (HHA) 

chapter.

 Drafts of these guidelines are to be completed by 

September 2013.
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FERC’s RIDM Program

 You will hear more extensive discussions 

of flood risks to and from dams later in 

this workshop.

 Reclamation’s program for estimating 

flood frequencies and reservoir level 

exceedance curves is well respected 

 Our new HHA chapter will rely heavily on 

the current Best Practices used by 

Reclamation.
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FERC’s RIDM Program

 However, we are a regulator and not 

a dam owner like Reclamation.

 

 FERC has hundreds of dam owners 

and dozens of consultants many with 

a limited understanding of 

probabilistic hydrologic hazard 

analyses. 
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Probable Maximum Flood 

Calculations

 FERC owner’s and consultants are good at 

Probable PMP and PMF calculations.

 Many newer studies are based on site- specific or state wide PMP analyses.

 These newer studies generally use HMR 

concepts.

 Meteorological science has progressed 

since the HMRs were developed and some 

HMR procedures are questionable
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Probable Maximum Flood

Issues

 The conservatism inherent in the 

HMR estimates of PMP have often 

been removed - up to 56 percent 

reductions in PMP.

 These reductions raises concern on 

the reasonableness of these new 

estimates.
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Probable Maximum Flood

Issues

 PMP value using the HMRs is an estimate – not 

necessarily the true upper limit

 PMF numbers are treated as having zero 

probabilities, i.e., as upper bounds that can’t be 

exceeded, rather than having a relatively low 

probability. 

 Some interior parts of the country, farther from a 

warm ocean moisture source, PMP and PMF 

estimates likely have equivalent AEPs of about 1 

X 10-7 or lower.
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Probable Maximum Flood

Issues

 In coastal regions closer to warm moisture 

sources, PMP and PMF estimates may 

have equivalent AEPs as high as between 

1 X 10-3 and 1 X 10-5. 

 Result is uneven treatment between dams

 Also these possibly relatively frequent 

AEPs means some dams might not have 

been analyzed for a flood that is 

sufficiently unlikely. 
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Probable Maximum Flood

Issues

 Dam overtopping PFMs are often from a 

combination of circumstances, i.e.,

• Trash build-up on spillway gates

• Inability to open a spillway gate during a 

relatively frequent flood.

 

 AEPs between 1 X 10-2 and 1 X 10-4 may 

be of greater concern combined with these 

events than an extreme flood event.
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FERC Dam Safety DecisionMaking

 All of our dams currently meet the standard of 

being able to pass the PMF, or are in process.

 Our owners have spent approximately $130 

million in the last 10 years for inadequate 

spillways. 

 Under current guidance, the only alternative is to 

show there is no home at risk from a dam failure 

during an extreme flood. 
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FERC Dam Safety DecisionMaking

 Two projects at the FERC have been 

evaluated using PFHA concepts.

 Panel 3 has a presentation on one of 

them.

 The Baker River Project, has another.

 The following slides will briefly show this 

process. 
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Upper Baker Dam
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West Pass Dike
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West Pass Dike
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Frequency of Maximum Reservoir 

Level
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Probable Life Loss (PLL)

PLL as follows

• Kulshan Campground 0

• Concrete 1

• Down through Hamilton

Mile 12 to 29 – 0

• Down through Sedro Woolley

Mile 29 to 42 - 0

• Down past Mt Vernon – Mile 42 to 64 *

• Total 1

*Flow less than 1 ft/s about 1 ft high
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Risk Decisions – West Pass Dike

 Flood Frequency Analysis did not include 

other alternative methods or uncertainty.

 We concluded that any repair decision 

should be delayed until the FERC’s RIDM 

program was more developed. 

 Eventually a Risk Analysis will be 

performed to further evaluate this risk.
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FERC’s Need for PFHA

 PFHA will be a critical part of our 

RIDM program to:

• Evaluate extreme flood overtopping

• Evaluate PFMs for lesser floods
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Questions about PFHA

 Should the PMP be the upper bound 

of any frequency estimates of 

extreme storms?

 Are there simple procedures for 

precipitation and flood frequency 

estimates to judge whether more 

robust estimates are needed at a 

dam?
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Questions about PFHA

 When should a probabilistic estimate 

augment or replace a deterministic 

estimate? Is it based on the risk?

 If current PMP and PMF estimates are 

inadequate at a dam, are new 

deterministic estimates needed, or 

should we rely only on new 

probabilistic calculations?
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Questions about PFHA

 How do we deal with uncertainty? 

• We can refine estimates of more frequent events using 

old historical (research of very large, non-gage recorded 

estimates) and paleoflood data 

 How are the PFHA results going to be reviewed?

 A set of expert interaction procedures similar to 

SSHAC would be helpful. Can this be developed?