20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM

CEII-Critical Energy Infrastructure Information
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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.

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 20141210-5071 FERC PDF (Unofficial) 12/10/2014 11:53:13 AM

CEII-Critical Energy Infrastructure Information
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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
6.2.1

Static Loading Candidate PFMs Not Carried Forward
Candidate S1: Clogging of Downstream Pervious Zones leading to Elevated
Phreatic Surface and Slope Instability

Candidate Description: Downstream
(Chimney) and/or shell Zone 3 become
seepage from the dam core Zone 1 and
(elevated pore pressures) develops in the

internal drain zones, Zones 5a (Blanket) or 5b
clogged and do not adequately drain and convey
under-seepage from the foundation; a phreatic line
downstream zones and a seepage face emerges on

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