DWR’s unpublished studies of the Oroville emergency spillway

As explained below, laws governing the formal dam safety reviews by the Federal Energy Regulatory Commission required an assessment of the adequacy of Oroville Dam’s emergency spillway.   The safety reviews relied on two unpublished studies by the Department of Water Resources’ Division of Engineering.

As part of formal reviews in 2004 and 2009, DWR’s Division of Engineering performed studies of the impacts of erosion below the Oroville emergency spillway weir. The first study appears to have been only distributed internally.  A published agenda shows the second study was presented for 15 minutes to the Federal Energy Regulatory Commission’s dam safety review team, and was marked, “CEII,” or Critical Energy Infrastructure Information, as part of FERC’s standard practice. Although stakeholders could request the second study through FERC’s FOIA process, they would have had to (1) know about the second study, and (2) sign a nondisclosure agreement to get it.

The distribution of DWR’s 2004 and 2009 Oroville emergency spillway studies is reminiscent of the notice of demolishment of Arthur Dent’s house in Hitchhiker’s Guide to the Galaxy:

But the plans were on display…”

“On display? I eventually had to go down to the cellar to find them.”

“That’s the display department.”

“With a flashlight.”

“Ah, well, the lights had probably gone.”

“So had the stairs.”

“But look, you found the notice, didn’t you?”

“Yes,” said Arthur, “yes I did. It was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying ‘Beware of the Leopard.”

This is apparently standard for information used in FERC’s dam safety reviews.

The Federal Energy Regulatory Commission (FERC) Dam Safety Review Process

FERC is governed by the Federal Power Act. Part 12D requires a formal review of the safety of dams every five years by a panel of independent consultants.   One of the most important aspects of the Part 12D reviews is the Potential Failure Mode Analysis, or PFMA. The FERC Dam Safety Program webpage describes a PFMA as follows:

A PFMA is an informal identification and examination of “potential” failure modes for an existing dam by a team of persons. It is based on a review of all existing data and information, first hand input from field and operational personnel, site inspection, completed engineering analyses, identification of potential failure modes, failure causes and failure development and an understanding of the consequences of failure.

Section 12.35 of the FPA, Part 12D mandates that FERC’s PFMA include a review of spillways:

(b) Evaluation of spillway adequacy. The adequacy of any spillway must be evaluated by considering hazard potential which would result from failure of the project works during flood flows.

(1) If structural failure would present a hazard to human life or cause significant property damage, the independent consultant must evaluate the ability of project works to withstand the loading or overtopping which may occur from a flood up to the probable maximum flood or the capacity of spillways to prevent the reservoir from rising to an elevation that would endanger the project works.

Oroville’s engineering design includes using the emergency spillway for the probable maximum flood. So analyzing the performance of Oroville’s emergency spillway was required under Part 12D, section 12.35.

Seventh Independent Consultant’s Safety Review in 2004

FERC’s e-library records show that in 2004, FERC oversaw the Seventh Independent Consultant’s review of the safety of Oroville and Thermalito dams. FERC records show the report of the Seventh Independent Consultant recommended that an erosion study of the hillside below Oroville emergency spillway be done, because a consultant thought that sediment could cause failure of the downstream dam. Based on this recommendation, FERC’s Division of Dam Safety & Inspections requested that DWR do an erosion study of the hillside. The Department of Water Resources formally responded in 2006:

Our Division of Engineering’s Project Geology Section has reviewed the erodibility of the emergency spillway’s downstream area. There is only one to four feet of erodible top soil in the downstream area and erosion would not compromise the stability of the emergency spillway. The amount of material eroded is dependent on the volume and duration of a spill, but material transport in the Feather River is expected. Therefore, we will re-evaluate the Thermalito Diversion Dam stability analysis to account for additional lateral earth pressures on the concrete structure due to material deposited upstream of the dam prior to the Project No. 2100 Eighth Part 12D Independent Consultant Board of Safety Review in 2008…

Original Spillway plans show no boreholes on hillside below spillway

It is unclear what geotechnical data DWR’s Division of Engineering was using to analyze the erodibility of the hillside below the emergency spillway. The original plans, published in 1974 in DWR’s Bulletin 200, Volume III, p.94, show no boreholes on the hillside below the emergency spillway weir – only under the concrete weir. It is unclear if DWR did further geotechnical investigation.

A pdf with full size graphics from Bulletin 200 is posted here: Design and General Plan of Oroville Spillway.

Eighth Independent Consultant’s Safety Review in 2009

In 2009, for FERC’s Eighth Part 12D Independent Consultants’ review, DWR chose two engineers from GEI consulting, Bill Rettberg, then Vice President of GEI, and Steven Verigin, Chief Geotechnical Engineer. A GEI document on the Oroville/Thermalito safety review states that Rettberg and Verigin worked “very closely” with DWR on “clarifying and reclassifying” some of the Potential Failure Modes (PFMs) of the dams.

The team worked very closely with DWR throughout the process, working on a shortened schedule to comply with the FERC deadline. The PFMA audit session was an intense one-day activity, that the Board and the DWR/FERC/and DSOD believed was valuable to revisiting the original PFMs and clarifying and reclassifying some of the PFMs.

The agenda for the “intense one-day” PMFA audit session shows that Tim Wehling of DWR’s Division of Engineering spent 15 minutes presenting a study of the emergency spillway to the review team.

1515-1530    Study Results; Oroville Emergency Spillway  Wehling (DOE)

The Eighth Part 12D Independent Consultants’ report is not publicly available. It is also classified “CE II” – i.e., Critical Energy Infrastructure Information.  However, subsequent FERC correspondence shows no mention of erosion of the emergency spillway as a Potential Failure Mode (PFM) for Oroville Dam or Thermalito Diversion Dam.

DWR’s Division of Safety of Dams also formally reviews the safety of Oroville and Thermalito dams at least every five years. However, in practice, DSOD’s  five year review of DWR’s dams relies on the FERC five year review. Subsequent documentation of potential failure modes of Oroville by DSOD does not show erosion of the emergency spillway as a Potential Failure Mode.

Oroville’s spillway failure appears, in hindsight, to have relied on inadequate analysis of the emergency spillway as a Potential Failure Mode.  This raises real questions about DWR’s 2004 and 2009 engineering analyses.   It is currently unclear if these analyses will ever be publicly released by DWR.

This post was updated on March 16, 2017 to correct the definition of CEII classification and provide a link.


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