HomeMy WebLinkAbout12.30.25 Board Correspondence_ FW_ Part 12 Consultant Safety Inspection Reports submitted in FERC P-2088-000 by Individual No Affiliation,et al.From:Clerk of the Board
To:Clerk of the Board; Connelly, Bill; Cook, Holly; Cook, Robin; Durfee, Peter; Jessee, Meegan; Kimmelshue, Tod; Kitts, Melissa; Krater, Sharleen; Lee, Lewis; Little, Melissa; Pickett, Andy;
Ritter, Tami; Stephens, Brad J.; Sweeney, Kathleen; Teeter, Doug; Zepeda, Elizabeth
Cc:Loeser, Kamie; Nuzum, Danielle
Subject:Board Correspondence: FW: Part 12 Consultant Safety Inspection Reports submitted in FERC P-2088-000 by Individual No Affiliation,et al.
Date:Wednesday, December 31, 2025 9:12:36 AM
Please see Board Correspondence
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Subject: Part 12 Consultant Safety Inspection Reports submitted in FERC P-2088-000 by Individual No Affiliation,et al.
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On 12/30/2025, the following Filing was submitted to the Federal Energy Regulatory Commission (FERC), Washington D.C.:
Filer: Individual No Affiliation
South Feather Water & Power Agency (as Agent)
Docket(s): P-2088-000
Lead Applicant: South Feather Water & Power Agency
Filing Type: Part 12 Consultant Safety Inspection Reports
Description: South Feather Water and Power Agency submits status report re corrective measures to address recommendations from Twelfth Part 12D Safety Inspection re the
Little Grass Valley Dam et al. of the South Feather Power Project under P-2088.
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December 30, 2025 Filed Electronically
Frank L. Blackett, P.E., Regional Engineer
100 First Street, Suite 2300
San Francisco, CA 94105
RE: South Feather Power Project (FERC No. 2088)
Corrective Measure Status Report in response to the 2022/Twelfth Part 12D Independent Consultant’s
Safety Inspection
Dear Mr. Blackett,
This letter provides updates to the South Feather Water and Power Agency’s (SFWPA) previously submitted plan
and schedules to address proposed recommendations outlined in the Twelfth Part 12D Independent
Consultant’s Safety Inspection Report for Little Grass Valley Dam, Sly Creek Dam, Lost Creek Dam, Ponderosa
Diversion Dam, Miners Ranch Dam transmitted to the Commission on January 27, 2023, November 21, 2024 and
March 13, 2025. Our responses below reflect on the collaborative discussions and inputs the SFWPA received
from our Chief Dam Safety Engineer. With the exception of SCD R-31, this completes the Agency’s responses to
address all 2022 Part 12D (which incorporated all outstanding 2017 Part 12D) recommendations.
Little Grass Valley Dam
Recommendation LGV R-2: It is recommended that a formal annual maintenance program be developed for the
Little Grass Valley dam spillway chute. The plan should include the identification of open joints, damage/spalling
of concrete, slab distress/displacement, and developing cracks as well as the requirement for timely repairs of
the identified issues. (PFM 13)
Response: A comprehensive spillway inspection and repair manual has been developed for the Little Grass Valley
Dam spillway and is included in Attachment 1. The manual includes descriptions of the spillway components,
general inspection plans and schedules, types of defects to identify during inspection, and general repair details
and procedures. A detailed inspection of the spillway will be performed by December 31, 2026.
Recommendation LGV R-15: It is recommended that SFWPA assess the feasibility of installing a catchment wall
or other mitigation measure to protect the spillway gate structure from future rockfalls at the steep right cutslope
above the spillway.
Response: A Geologic Assessment Report for Little Grass Valley Dam was completed on July 31, 2025, and is
included in Attachment 2. The report identified the steep right cutslope above the spillway as having the highest
potential for rockfall and recommended implementing mitigation measures to protect the spillway gates.
Mitigation measures to protect the spillway gate structures from future rockfalls are being incorporated into a
broader project addressing rockfall and slope erosion on both sides of the spillway.
In accordance with 18 CFR § 12.11(b)(2), mitigation plans will be submitted to the FERC SFO Regional Engineer
KRISTEN MCKILLOP
REGULATORY COMPLIANCE MANAGER
Document Accession #: 20251230-5330 Filed Date: 12/30/2025
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for review and acceptance at least 60 days prior to the commencement of work. At this time, we anticipate
submitting the plans for work activity on the south wall by April 30, 2026.
Recommendation LGV R-16: It is recommended that the prominent geologic shear downstream and below the
end of the spillway be added to the DSSMP for special monitoring following major spills.
Response: This feature will be added as a special monitoring area in the next update of the DSSMP. Special
monitoring will consist of capturing drone images of the area after a defined threshold spillway flow is exceeded.
The images will be compared to baseline images captured in 2024. If no adverse conditions, such as excessive
scour, are observed, the threshold flow will be set to the maximum observed flow that year. If adverse conditions
are observed or suspected, the area will be inspected by the CDSE or their designee. The initial threshold flow
will be set to 500 cfs.
Recommendation LGV R-18: It is recommended that formal standard operating procedures (SOPs) be developed
for Little Grass Valley Dam that follow Chapter 16 of the FERC Engineering Guidelines. The plan should include
descriptions of all instrumentation and response alarm levels used to monitor reservoir operations, including the
reservoir level gage and downstream flow gages.
Response: A reservoir and downstream conveyance monitoring standard operating procedure (SOP) has been
developed and is included in Attachment 3.
Recommendation LGV R-30: It is recommended that the cantilever section be evaluated for seismic loading.
Response: The spillway flip bucket structure was analyzed for the seismic load case for the anticipated MCE level
seismic event. It was analyzed for factors of safety against sliding, resultant location due to overturning, and
structural analysis of the cantilever section of the structure and construction joint within the flip bucket. The
results of the analysis found the flip bucket is also expected to meet the minimum stability criteria for the post-
seismic load case. Analysis assumptions and results are documented in a Technical Memorandum and are
included in Attachment 4.
Sly Creek Dam
Recommendation SCD R-27: It is recommended that SFWPA locate the PMF study for the modified dam and
revise Chapter 6 of the STID accordingly. If the report documenting the hydraulic analyses for the modified
condition cannot be located, an updated PMF study should be performed.
Response: The PMF study for the modified dam was not able to be located. Consequently, a study was
undertaken to review and analyze the PMF for the modified Sly Creek Dam and is included in Attachment 5. The
study included a comprehensive evaluation of inflow hydrographs and historical data from existing
documentation, confirmation of pre- and post-construction rating curves, and development of a HEC-HMS
hydrologic model. The updated HEC-HMS modeling was able to successfully validate the previous HEC-1
modeling for the original dam configuration and subsequently expanded to evaluate PMF routing for the
modified structure in the basin’s current state. The study shows that the modified spillway and dam crest
currently allows Sly Creek Reservoir to pass the PMF with 5 feet of freeboard. The study also identified
deficiencies and inconsistencies between earlier studies and reports, particularly regarding hydrologic inputs
and reservoir routing and provided recommendations to address them. These recommendations and updates
to the STID will be completed by December 31, 2026.
Recommendation SCD R-31: Based on the information provided in the 2019 report, the stability analyses for the
ogee weir, spillway slab, and cantilever discharge section are not appropriate. It is recommended that these
analyses be reviewed and revised considering the comments provided in Section 7.8 of this report. The revised
analyses should include sufficient information to support the conclusions that the structures are stable and
Document Accession #: 20251230-5330 Filed Date: 12/30/2025
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facilitate a meaningful review of the calculation steps.
Response: The spillway flip bucket structure was analyzed for the seismic load case for the anticipated MCE level
seismic event. It was analyzed for factors of safety against sliding, resultant location due to overturning, and
structural analysis of the cantilever section of the structure and construction joint within the flip bucket. The
results of the analysis found the flip bucket is also expected to meet the minimum stability criteria for the post-
seismic load case. Analysis assumptions and results are documented in a Technical Memorandum and is included
in Attachment 6.
We request an extension of time to complete the Ogee weir and spillway slab analyses, with a proposed
completion date of May 31, 2026.
Lost Creek Dam
Recommendation LCD R-9: It is recommended that the seepage water at the right side of the dam be profiled for
chemical constituents for the purpose of evaluating corrosion that may be occurring within the dam or scour slab.
Response: As stated in our letter to the FERC dated March 13, 2025, rust colored seepage is likely emanating
from ultramafic rock that contains high amounts of iron and magnesium. To reiterate, the 2018 rehabilitation
project added several feet of corrosion resistant, glass fiber reinforced polymer (GFRP) reinforced concrete to
the face of dam. The GFRP bars are corrosion resistant and not likely susceptible to corrosion in this environment.
Fundamentally, the arch dam relies on compressive load paths that are not reliant on reinforcement. If steel
reinforcement in the existing dam were to corrode, the load path would still be carried through the compressive
strength of the concrete. Further action on this recommendation is declined.
Recommendation LCD R-10: It is recommended that the voids under the scour slab on the right abutment be
repaired, removing vegetation and loose materials, then filling the voids with strong cementitious materials.
Response: As stated in our letter to the FERC dated March 13, 2025, the area is question would “…continue to
be monitored, and a closer inspection of the area will be performed in 2025 to measure the depth of the voids.
The void areas were measured and observed in June and September of 2025. Five voids were identified and
measured depths ranged from 3 to 11 inches. Based on the observations, it appears that formwork for the
downstream edge of the apron was placed against a rough bedrock surface and was not shaped to match the
contours of the rough bedrock surface. It is likely that some sort of leveling material such as sand or gravel was
used to level the surface. Construction photos of this area could not be located to confirm. This material likely
eroded during initial spillway flows. Details of the observations will be included in the next DSSMR due March
31, 2026. For the reasons outlined in the above referenced letter, this is not considered to be a dam safety issue,
and this recommendation is declined.
Recommendation LCD R-11: It is recommended that SFWPA monitor the condition of the slope above and
immediately adjacent to the MLO valves and consider implementing a plan to remediate the slope, including
improving drainage and/or implementing a retaining system to prevent future degradation.
Response: SFWPA recently installed a barrier to better capture seepage and runoff from the adjacent slope. A
staff gauge will be installed to allow for visual monitoring of any trends in seepage from the slope. Details of the
monitoring system will be included in the next update of the DSSMP to be completed by March 31, 2026. Further
measures to improve drainage and/or implement a retaining system are declined.
Recommendation LCD R-20: It is recommended that SFWPA locate the report documenting the routing of the
PMF for the modified dam and spillway and confirm the assumptions are consistent with the as-built conditions
at the Sly Creek and Lost Creek dam and spillway. The report should be summarized in sufficient detail in Chapter
6 of the STID, including inflows, outflows, peak water surface elevations, and the modified spillway capacity. A
Document Accession #: 20251230-5330 Filed Date: 12/30/2025
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discharge rating curve for the modified spillway should also be included in the STID. If the report documenting
the hydraulic analyses for the modified condition cannot be located, or the assumptions in the analyses are found
to be inconsistent, an updated PMF study should be performed.
Response: The PMF study for the modified dam could not be located. Consequently, a study was undertaken to
review and analyze the PMF for the modified Lost Creek Dam and is included in Attachment 7. The study included
a comprehensive evaluation of inflow hydrographs and historical data from existing documentation,
confirmation of pre- and post-construction rating curves, and development of a HEC-HMS hydrologic model. The
updated HEC-HMS modeling was able to successfully validate the previous HEC-1 modeling for the original dam
configuration and subsequently expanded to evaluate PMF routing for the modified structure in the basin’s
current state. The study shows that the modified spillway and dam crest currently allows Lost Creek Dam to pass
the PMF with 1.5 feet freeboard. The study also identified deficiencies and inconsistencies between earlier
studies and reports, particularly regarding hydrologic inputs and reservoir routing and provided
recommendations to address them. These recommendations and updates to the STID will be completed by
December 31, 2026.
Ponderosa Diversion Dam
Recommendation PDD R-1: It is recommended that a structural analysis be performed to assess the stability of
the left wall of the spillway approach channel under earthquake and flood loading conditions. The analysis should
include evaluating sliding, overturning, and structural failure potential. (PFM 3)
Response: The structural analysis was performed for three representative wall sections of the spillway left
approach wall for six different load cases for active and at-rest soil condition in accordance with USACE EM 1110-
2-2104. Results indicate that the approach wall generally meets the applicable factor of safety requirements for
the evaluated loading scenarios as per present design criteria. However, the analysis shows the wall is
structurally deficient at around the mid height of the wall for the unusual and extreme load cases.
Recommendations were made for continued monitoring of the wall. Those recommendations are discussed
below (Recommendation PDD R-2) and will be completed by December 31, 2026.
Recommendation PDD R-2: It is recommended that a formal monitoring plan be established to assess the long-
term performance of the left wall of the spillway approach channel. Such a plan should include installation of one
or more crack gages across the distressed joint that allow measurements in at least 2 dimensions, one of which
is perpendicular to the wall face, and regularly documented gage readings and photos during quarterly
inspections. Special inspections and gage readings should be initiated for both post-earthquake and post-flood
events. Details of the plan should be included in the DSSMP.
Response: The 1965 periodic inspection noted that the left approach wall had deflected outward by ¾ inch, likely
measured relative to the spillway bridge abutment wall, although this was not explicitly stated. To better
understand the current deflection, the offset between the walls will be measured along with the crack gauge to
quantify the total top-of-wall deflection relative to the spillway bridge abutment wall. Subsequent readings can
be made from crack gauge alone. Readings should be performed on an annual basis at a consistent low, pool
level. Additionally, readings should be taken during inspections triggered by flooding or seismic events.
Thresholds and action levels can be established once sufficient data has been collected. Procedures for collecting
the data and the basis of collecting it will be documented in the DSSMP by March 31 2026.
Recommendation PDD R-11: It is recommended that a formal SOP or O&M plan be developed for Ponderosa Dam
that follows Chapter 16 of the FERC Engineering Guidelines. The plan should include descriptions of all
instrumentation and response alarm levels used to monitor reservoir operations, including the reservoir level
gage and downstream conveyance gages.
Document Accession #: 20251230-5330 Filed Date: 12/30/2025
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Response: A reservoir and downstream conveyance monitoring standard operating procedure (SOP) has been
developed and is included in Attachment 3.
Miners Ranch Dam
Recommendation MRD R-12: It is recommended that SFWPA investigate the condition of Piezometers 1 and 2
and demonstrate that the water level measured by both piezometers does not reflect the phreatic surface within
the downstream shell. Such an investigation may include review of construction documentation and field testing.
If the piezometers are found to be unreliable, SFWPA should evaluate options for replacing or repairing the
piezometers. If the piezometers cannot be demonstrated to be unreliable, SFWPA should update the stability
analyses to reflect the higher phreatic surface within the downstream shell.
Response: A review of construction documentation, historical inspection notes, installation details, and recent
data analysis was performed to understand the reliability of the existing piezometers. Findings of the review are
documented in a report which is included in Attachment 9. The review found that the piezometers are positioned
within the downstream shell well above the drainage blanket and do not appear to measure a reservoir-driven
phreatic surface. Recent high-frequency, automated measurements supports that the instruments are
responsive to precipitation, relatively insensitive to reservoir fluctuations, and do not reflect the phreatic surface
in the embankment. A reservoir driven, developing high phreatic surface within the downstream shell would be
detected through trending changes in weir flow. Several recommendations were made to increase functionality
of piezometers. These will be addressed by December 31, 2026.
The attached documents filed under CEII designation have been reviewed by both the Chief Dam Safety Engineer
and Chief Dam Safety Coordinator. We concur with the content.
If you have any questions related to this transmittal, please feel free to contact me at (530) 534-1221 x 265, or
via email at kmckillop@southfeather.com.
Sincerely,
South Feather Water and Power Agency
cc: Rath Moseley, General Manager
Hunter Doyle, Power Division Manager
Enclosures: CUI//CEII documents submitted separately
Attachment 1 – Spillway Inspection & Repair Manual: Little Grass Valley Dam (LGVD) and Sly Creek Dam (SCD)
Attachment 2 – Geologic Assessment Report: Little Grass Valley Dam
Attachment 3 – Reservoir Monitoring Instrumentation
Attachment 4 – Technical Memorandum: Little Grass Valley Dam Spillway Flip Bucket Seismic Analysis
Attachment 5 – Probable Maximum Flood Review and Analysis: Sly Creek Dam
Attachment 6 – Technical Memorandum: Sly Creek Dam Spillway Flip Bucket Seismic Analysis
Attachment 7 – Probable Maximum Flood Review and Analysis: Lost Creek Dam
Attachment 8 – Technical Memorandum: Ponderosa Dam Spillway Left Approach Wall Structural Analysis
Attachment 9 – Miners Ranch Piezometer Evaluation Report
Document Accession #: 20251230-5330 Filed Date: 12/30/2025
Document Content(s)
to FERC_12302025_Twelfth Part 12D CSIR Corrective Measures Status
Report.pdf................................................................1
Document Accession #: 20251230-5330 Filed Date: 12/30/2025