5.3 Earthquake-Loading Potential Failure Modes
5.3.5 PFM-E4 Large Earthquake (on the Order of the 84th percentile) on the Cleveland
Crest of the Dam
Potential Failure Mode Description
The reservoir is at El. 875 feet (considered a high, normal reservoir elevation) and a large earthquake (on the order of the 84th percentile) occurs on the Cleveland Hills fault. The seismic loading causes deformation settlement of the main dam. The deformation settlement is significant and of sufficient magnitude to cause a loss of freeboard down to El. 875 feet or lower.
The loss of freeboard allows water from the reservoir to flow over the crest of the dam resulting in erosion of the embankment section. The erosion progresses downward through the embankment until the downstream shell collapses in on itself, the reservoir is lowered sufficiently to stop the erosion, or the erosion continues failing the dam.
Preliminary PFM Event Tree:
1. The reservoir is at El. 875 feet or higher.
2. Large earthquake (on the order of the 84th percentile) occurs on Cleveland Hill Fault.
3. The earthquake causes deformation of the embankment.
4. The deformation is significant enough to result in settlement of the crest down to an elevation lower than El. 875 feet at the maximum section.
5. Water flows over the crest of the dam at the location of highest degree of settlement.
6. The flow of water over the crest causes erosion and downward cutting through the embankment.
7. The erosion continues allowing significant uncontrolled release of water.
9. The downstream shell of the dam does not collapse on itself and stop the erosion process.
10. The dam fails releasing the contents of the reservoir.
The adverse and positive factors related to PFM-E4 are provided in Table 5.10.
67
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
Table 5.10 Adverse and Positive Factors for PFM-E4
PFM-E4: During normal high reservoir (El. 875 ft.) a large earthquake (on the order of the 84th percentile) occurs on the Cleveland Hill fault and leads to deformation of the main dam. The deformation settlement results in a loss of freeboard and differential settlement, and a low spot in the crest, below El. 875 ft., allowing reservoir water to flow over the crest depression and erode a section sufficient for uncontrolled release until the reservoir can be lowered to below the eroded section elevation.
Adverse Factors Positive Factors
The shear wave velocity of the embankment dam is unknown.
There is no liquefiable material in the embankment or in the foundation.
Previous analyses used only one time- history.
The embankment is very well compacted.
Three-dimensional analysis of the embankment has not been performed.
Only slump failure is considered to be possible at the embankment.
Multiple (semi-independent) investigations have shown that the dam will perform well in an
earthquake. Settlement estimates are less than 2 meters. (done with pga greater than current 50th percentile).
A moderate earthquake occurred in 1975 and the embankment performed well.
DWR performs post-earthquake inspections and crest surveys.
The actual elevation at maximum section is above El. 927 ft. and El. 925 ft. at the abutments.
The Cleveland Hill Fault has an extremely small slip rate, on the order of 0.05 - 0.1 mm/yr.
Classification: Category III (voting: 12 for Cat III; 1 for Cat IV);
Rationale: The PFMA Team would like to see new seismic deformation analyses performed based on modern criteria including three-dimensional analyses using multiple time-histories.
Possible Risk Reduction Factors:
1. Perform seismic deformation analyses based on modern criteria including three- dimensional analyses using multiple time-histories.
Other Considerations:
1. There are five accelerometers on dam.
2. FERC has question on whether 50% ground motion is appropriate.
3. Faulting and seismicity hazards for Oroville Dam should be re-visited periodically as new studies and information becomes available.
68
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
.
69
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
Table 5.11 Adverse and Positive Factors for PFM-E5
70
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
71
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
72
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
73
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
The 1979 seismic re-analysis analyzed US/DS motions
together and that they act independently. This becomes the worst case loading.
74
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
5.3.9 PFM-E8 Earthquake Loading Under Normal Reservoir Pool Causes “Debonding”
Between the Concrete at the Base of the Ogee Weir and the Rock Foundation, Dislodging an Ogee Section Monolith Resulting in Partial Release of Reservoir to Approximately El. 850 feet
Potential Failure Mode Description
During normal high reservoir (El. 875 feet) conditions, a large earthquake (greater than magnitude 5.7) occurs within 7.5 miles of the dam. The seismic forces combined with the hydrostatic and hydrodynamic forces on the concrete weir section at the emergency spillway cause uplift pressures to develop at the base of the ogee weir. The uplift forces result in
“debonding” at the contact between the concrete weir and the foundation rock. The “debonding”
allows movement of the ogee section which begins to rock in place, eventually sliding in a rotational manner (longitudinally). The sliding rotation results in a separation (opening) at the contact between the FCO and the ogee weir. The opening allows water to flow between the two structures along the outside (right side) of the FCO discharge channel training wall. Flow continues, uncontrolled until the reservoir reaches approximately El. 850 feet, the elevation of the rock foundation at the base of the ogee weir.
Preliminary PFM Event Tree:
1. The reservoir is at El. 875 feet or higher.
2. An earthquake having a magnitude greater than 5.7 occurs within 7.5 miles of the dam.
3. The seismic load causes additional hydrodynamic loads on the ogee section.
4. Uplift pressures at the base of the ogee section increase in response to the hydrodynamic loads.
5. The uplift pressures lead to “debonding” at the base of the concrete ogee spillway crest section.
6. The bond between the foundation and the concrete monolith is sheared.
7. Reduced sliding resistance at the base of the ogee section foundation interface results in sliding of the weir.
8. The monolith slides downstream and rotates, resulting in an opening between the ogee section and FCO monolith.
9. Water flows through the opening between the ogee section and right side of the FCO along the outside of the FCO downstream channel training wall.
11. Uncontrolled release through opening created by dislodged section continues until the reservoir is lowered to elevation 850 feet, the elevation of the rock foundation at the base of the ogee weir.
The adverse and positive factors related to PFM-E8 are provided in Table 5.14.
75
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
Table 5.14 Adverse and Positive Factors for PFM-E8
PFM-E8: During normal high reservoir (El. 875 ft.) conditions, earthquake increases the hydrodynamic loads on the concrete ogee weir section at the emergency spillway. The additional hydrodynamic loads cause “debonding” of the concrete foundation from the foundation rock, shearing and subsequent sliding along the “debonded” interface. Rotation of the ogee spillway section results in an opening at the contact between the FCO and Ogee sections allowing the reservoir water to flow freely to Feather River until the reservoir level reaches approximately elevation 850 feet.
Adverse Factors Positive Factors
The emergency spillway structure has not been reviewed for new seismic criteria.
The ogee section provides for a favorable geometry of the structure.
It is uncertain what uplift loads were considered in the design of the section.
The rock contact at the base of the ogee section is very rough on a macro scale which provides for favorable conditions to resist sliding.
Seepage beneath the ogee spillway weir has been observed under high reservoir.
The foundation rock is very competent and highly resistant to erosion.
Classification: Category III (voting: 8 for Cat III; 5 for Cat IV)
Rationale: The PFMA Team voted for Category III recognizing the need for review of previously performed stability analyses of the ogee spillway section and new stability analysis as deemed appropriate to address the current understanding of the seismic conditions at Oroville Dam.
Possible Risk Reduction Factors:
1. Perform seismic stability analyses on the ogee spillway.
Other Considerations:
1. Failure mode is limited to release down to a reservoir elevation of approximately 850 feet.
5.4 Operational Potential Failure Modes
Section 5.4.1 below provides a brief summary of the conditions at Oroville Dam relevant to operational potential failure modes. Operational potential failure modes that were carried forward during this PFMA are presented beginning in Section 5.4.2.
5.4.1 Conditions Relevant to Operational Potential Failure Modes 1. Powerhouse and Appurtenant Structures
Structures that were discussed under Operational PFMs include the Hyatt Power Plant, the intake structure and penstocks,
and the gates at the FCO.
76
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
The operational PFMs consider failure modes associated with operation of the plant or dam;
such as, human control of valves, gates, stoplogs, power generation, etc. Also, possible mechanical, electrical, or controls malfunctions are considered under the operational PFMs.
Recent and past case histories of failures associated with misoperation of a plant or dam or other incidents where equipment or systems used to operate the dam were discussed. Whether similar events could happen at Oroville and how to prevent them also was discussed.
2. Corrosion
Corrosion of metal structures, namely pipes and valves, was discussed in several of the Operational PFMs. Corrosion is the degradation of a metal through a chemical reaction with its environment. Commonly ferrous metals corrode by oxidation when exposed to oxygen.
Corrosion can cause material loss and a loss of strength. Chemicals carried within water as well as the pH of the water can impact the potential for corrosion of pipes and valves at a dam.
The water within Lake Oroville is not considered to be particularly corrosive.
Regular inspections are performed through areas with piping and valves, and during the inspections the inspectors observe and note signs of corrosion.
Additionally, less frequently (3 to 5-year basis), critical pipes are tested for wall thickness or coating thickness to quantify material loss and/or loss of protection at a pipe.
77
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
78
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
79
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
80
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
82
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
83
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
84
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
85
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
86
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
88
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
89
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
.
90
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
6 CANDIDATE POTENTIAL FAILURE MODES NOT CARRIED FORWARD
Several candidate potential failure modes were discussed during the 2014 PFMA workshop and were not carried forward because the Core Team did not believe the candidate was credible for one or more reasons. These candidates are summarized by category in the following sections 6.1 Flood-Loading Candidates, 6.2 Static-Loading Candidates, 6.3 Earthquake-Loading Candidates, and 6.4 Operational Candidates.
Where substantial discussion of a candidate occurred during the workshop, an effort was made to follow the normal process and develop the full PFM description, list the positive and adverse factors, and describe in some detail the rationale for not carrying the candidate forward. For some candidates, this was not done because the workshop Core Team quickly ruled out that candidate by inspection. For the latter candidates, only a brief explanation is provided for why that candidate was ruled out or not carried forward. If risk reduction measures were identified during the course of the discussion, they are presented with the candidate PFM.
6.1 Flood Loading Candidate PFMs Not Carried Forward
6.1.1 Candidate F1: PMF Event Occurring in Feather River Basin and Canyon Dam (Lake Almanor) Fails Upstream, Leading to Eventual Overtopping of Oroville Dam Candidate Description: A PMF flood event is occurring on the Feather River and as a result of the flooding Canyon Dam (retains Lake Almanor) fails. Canyon Dam has a storage capacity of 1.3 million acre-feet and is located upstream of Oroville Dam. As a result of its failure, the volume of its reservoir is released into Lake Oroville in addition to the PMF flow. Under the storm alone, the emergency spillway is releasing 10 feet of water. When the flood wave from Almanor reaches the dam, the main embankment of Oroville Dam is overtopped. The adverse and positive factors related to Candidate F1 are provided in Table 6.1.
91
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
Table 6.1 Adverse and Positive Factors for Candidate F1
Candidate F1: PMF Event Occurring in Feather River Basin and Canyon Dam (Lake Almanor) Fails Upstream, Leading to Eventual Overtopping of Oroville Dam
Adverse Factors Positive Factors
Canyon Dam has a storage volume of 1.3 million acre-feet.
From an incremental damages perspective, failure of Canyon Dam is insignificant compare to PMF flows.
Failure of Canyon Dam in combination with PMF would likely overtop Oroville Dam.
Releases from Canyon Dam are considered in Oroville PMF study.
Other dams in basin are relatively small (compared to Oroville and Almanor)
Rationale for Not Carrying PFM forward: This was considered the combination of two extreme events and is not considered credible. Additionally, the PFMA workshop Core Team understood that Canyon Dam has been modified to allow the safe passage of the PMF event. It was also stated that in the case of a PMF event, the areas downstream of Oroville would already be in a state of emergency and the overtopping of Oroville Dam would not change the level of response needed.
Potential Risk Reduction Measures:
Prompt notification to emergency management agencies is the only noted risk reduction measure.
Other Considerations:
1. There are two significant upstream dams that can impact Oroville:
a. Canyon Dam (Lake Almanor, 1.3M ac-ft.), owned by PG&E. Have one to two hours’ notice in the case of complete failure. Does not overtop Oroville Dam with reservoir at El. 900 ft. It is believed Canyon Dam can pass the PMF with adequate freeboard.
b. Bucks Lake (100,000 ac-ft.) 2. Smaller dams in watershed include:
a. Little Grass Valley (93,000 ac-ft.) b. Sly Creek (65,000 ac-ft.)
3. Could possibly overtop Oroville if all upstream dams fail during one event, but this is highly unlikely and not to be considered further.
92
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
6.1.2 Candidate F2: PMF Event Occurring at Oroville Dam and Emergency Spillway is Overtopping and Head-cutting Occurs Initiating at the Feather River
Candidate Description: A PMF flood event is occurring and over 10 feet of water is spilling over the emergency spillway at Oroville Dam. Erosion begins where the flow is entering the Feather River and progresses by head-cutting into the reservoir.
The adverse and positive factors related to Candidate F2 are provided in Table 6.2.
Table 6.2 Adverse and Positive Factors for Candidate F2 Candidate F2: PMF Event Occurring Oroville Dam and Emergency Spillway is
Overtopping and Head-cutting Occurs Initiating at the Feather River
Adverse Factors Positive Factors
Broad area below emergency spillway reduces energy of water.
Have had 50 years of flow occurring at the downstream end of the FCO spillway channel, and very little erosion has been observed. A 150,000 cfs release through the FCO has also occurred.
Distance from the Feather River to the emergency spillway is approximately 3/4 mile.
The rock between the Feather River and emergency spillway is very competent and resistant to erosion.
Rationale for Not Carrying PFM forward: The failure of Oroville Dam through head-cutting was not considered credible because of distance from the emergency spillway to the Feather River (approximately 3/4 mile), the competence of the rock, and the lack of erosion that has been observed at the end of the FCO spillway channel.
93
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
6.1.4 Candidate F4: PMF Event Occurring, the Left Spillway Chute Wall is Overtopped and Impacts the Dam Embankment
Candidate Description: A PMF flood event is occurring and during release of 150,000 cfs through the FCO spillway, the spillway chute downstream of the FCO is overtopped on the left side. The water overtopping the left side of the FCO begins to erode the embankment leading to a dam failure.
The adverse and positive factors related to Candidate F4 are provided in Table 6.4.
94
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
Table 6.4 Adverse and Positive Factors for Candidate F4
Candidate F4: PMF Event Occurring, the Left Spillway Chute Wall is Overtopped and Impacts the Dam Embankment
Adverse Factors Positive Factors
Design of the overflow section and spillway channel should prevent this.
Rationale for Not Carrying PFM forward: The spillway chute is designed to accommodate flows in excess of the operational requirements of the flood control regulation plan (150,000 cfs). The FCO has passed 150,000 cfs in the past without overtopping the wall. The emergency spillway can pass sufficient flow such that the FCO would not need to pass more than 150,000 cfs during the PMF.
6.1.5 Candidate F5: Loss of the Spillway Channel Lining Results in Erosion of the Rock Underlying the Spillway
Candidate Description: Cavitation or slabjacking results in loss of the concrete lining in the spillway chute downstream of the FCO. The rock in the spillway chute erodes and the FCO is undermined and lost.
The adverse and positive factors related to Candidate F5 are provided in Table 6.5.
Table 6.5 Adverse and Positive Factors for Candidate F5
Candidate F5: Loss of the Spillway Channel Lining Results in Erosion of the Rock Underlining the Spillway
Adverse Factors Positive Factors
The spillway channel concrete is in good condition and there is no evidence of significant erosion or stress resulting from flows experienced to date.
The rock is fresh and hard and resistant to erosion.
The duration of large flows through the FCO is not sufficient to develop significant erosion of the rock.
DWR has performed minor repairs to the spillway concrete as recently as 2009.
95
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM
CEII-Critical Energy Infrastructure Information Do Not Release
Rationale for Not Carrying PFM forward: The spillway chute is in good condition and the underlying rock is very competent. Many spillways are constructed of rock with no concrete lining. It is seen as highly unlikely that the concrete lining will fail and highly unlikely that significant erosion of the rock will occur during one spilling event.
6.1.6 Candidate F6: Scour of Soil and Debris During Flow Over the Emergency Spillway Blocks the Feather River
Candidate Description: Flow over the emergency spillway during a large flood scours soil and trees from the slope as water flows over the emergency spillway to the Feather River. This blocks the river and causes the river to backup.
The adverse and positive factors related to Candidate F6 are provided in Table 6.6.
Table 6.6 Adverse and Positive Factors for Candidate F6
Candidate F6: Scour of Soil and Debris During Flow Over the Emergency Spillway Blocks the Feather River
Adverse Factors Positive Factors
Creates adverse condition downstream. The large flows would prevent damming of the river with debris.
This would not result in a dam failure.
The slope below the emergency spillway has relatively little vegetation and surficial cover and the underlying bedrock is not subject to significant erosion.
Rationale for Not Carrying PFM forward: Damming of the Feather River is seen as highly unlikely under the heavy flows that would be occurring if the emergency spillway is activated.
Even if this did occur, and there was no scenario that would result in failure of the dam or an uncontrolled release.
6.2 Static Loading Candidate PFMs Not Carried Forward
6.2.1 Candidate S1: Clogging of Downstream Pervious Zones leading to Elevated Phreatic Surface and Slope Instability
Candidate Description: Downstream internal drain zones, Zones 5a (Blanket) or 5b (Chimney) and/or shell Zone 3 become clogged and do not adequately drain and convey seepage from the dam core Zone 1 and under-seepage from the foundation; a phreatic line (elevated pore pressures) develops in the downstream zones and a seepage face emerges on
96
20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM