NEVADA COUNTY, Calif. April 3, 2024 – Two letters filed with the Federal Energy Regulatory Commission (FERC) reveal details of the damage at PG&E’s Lake Spaulding Powerhouse No. 1, which is out of service since early March. The failure of the powerhouse, combined with a massive rockslide over the PG&E-owned starting portion of the South Yuba Canal, have effectively cut off water supplies from the higher elevations to the Bear River and Deer Creek.
The State Historic Preservation Officer’s letter to FERC provides additional information on the damage discovered by PG&E.
During inspections of the Spaulding No. 1 Powerhouse, it was observed that two columns in the discharge pit were completely destroyed and had fallen to the ground. The columns supported the powerhouse floor level and are normally not visible. The discharge pit is normally below water from turbine and flow and requires special conditions to obtain access.
PG&E proposes to replace the failed columns in the underbelly of the Spaulding No. 1 Powerhouse. Because the columns have failed, PG&E has already installed temporary bracing in the pit to support the loads from above. The detailed design for the permanent replacement columns is still underway but new columns will be similar design under modern design standards. PG&E will include additional improvements such as adding steel cladding to the exterior of the columns to prevent erosion, as well as installing a stainless steel/concrete training wall across the area bound by the two previously existing columns. The area behind this training wall will be filled with reinforced concrete. Four steel columns will be installed that extend from the top of that concrete fill to the beam that was previously attached to the two failed concrete columns in order to keep the two new columns out of the water flow and armor the wall to prevent erosion.
SHPO letter to FERC
PG&E also dewatered the intake tunnel leading to the powerhouse, as detailed in their March 27th letter to FERC. The intake tunnel is a 963-ft-long, 104-inch-diameter rock tunnel that diverts up to 600 cfs of water from Lake Spaulding to the powerhouse.
During a monthly visual inspection on March 6, 2024, PG&E identified abnormal leakage in the area adjacent to the PRVs. On March 7, 2024, crews dewatered the bypass valve and took the PRV unit out of service to perform a more thorough inspection, using a Remotely Operated Vehicle (ROV). The ROV inspection results found significant damage to both PRV discharge horns and surrounding concrete. PG&E determined continued operation risked further damage to operating equipment and elected to keep the unit offline until the condition is repaired.
Per the 18 CFR 12.10(a) requirements, PG&E notified FERC of the incident on March 8, 2024. On March 11, 2024, PG&E began dewatering the Spaulding Power No. 1 Intake Tunnel to facilitate further inspection and repairs. … PG&E expects to be able to return the unit to service no later than June 15, 2024.
PG&E March 17 letter to FERC
PG&E spokesperson Paul Moreno confirmed the ongoing work at the powerhouse, “PG&E long ago prioritized this work to restore the Spaulding 1 powerhouse and our crews and contractors are working around the clock most days to get the powerhouse back on as quickly and safely as possible with as many people who can safely and efficiently work inside the powerhouse. June 8 is our estimated date to return the powerhouse to service.”
Additionally, the SHPO letter detailed historic aspects of the powerhouse:
The Spaulding No. 1 Powerhouse is significant under Criterion C for its unique architectural and engineering design, and it is also a contributing element of the Drum-Spaulding Hydroelectric Historic System. Although the powerhouse is similar in architectural design to powerhouses 2 and 3, it is the best example of the three. It is also unique as an adit powerhouse, carved directly into the cliff face. The old (1917) and new (1928) sections are distinctive; when standing inside the powerhouse, the construction of these two sections is obvious. The sections provide a sense of the expansion of the system through time. The character‐defining features of the powerhouse for the purposes of this project are assumed to be the functionality and integration with the system as a whole, as well as the following features: the massing (reinforced steel structure); the flat roof (with concrete gabled parapets); the fenestration (windows symmetrically placed and consisting of industrial metal sash multi‐pane lites); the floorplan; the location (inset into a cavern in a cliff); and the associated covered stairway and tramway.
SHPO letter to FERC
Compounding the issue: South Yuba Canal rockslide
During a winter storm, a rockslide occurred on the PG&E-owned portion of the South Yuba Canal.
“PG&E’s usual alternatives to provide water to the South Yuba Canal and Bear River are dependent on the South Yuba Pipe which is currently out of service due to damage from a rockslide during a recent storm. PG&E has initiated an emergency project to return the pipe to service. The location of the damaged section of pipe, snow accumulation and inability to address the stability of the slide area is inhibiting safe access and working conditions. Long lead time material have been ordered and repair plans developed to expedite repairs once safe access is established,” Moreno replied to our question.
This issue is separate from the powerhouse failure.
