Generation
`
`245 Market Street
`San Francisco, CA 94105
`
`Mailing Address:
`Mail Code N11D
`P.O. Box 770000
`San Francisco, CA 94177
`
`May 25, 2021
`
`Via Electronic Submittal (E-File)
`
`Kimberly D. Bose, Secretary
`Federal Energy Regulatory Commission
`888 First Street, N.E. Room 1A
`Washington, DC 20426
`
`Re: UPPER NORTH FORK FEATHER RIVER PROJECT, FERC NO. 2105-089
`RESPONSE TO ADDITIONAL INFORMATION REQUEST
`
`Dear Secretary Bose:
`
`Pacific Gas and Electric Company (Licensee or PG&E) submits the enclosed response to the Federal Energy
`Regulatory Commission’s (FERC or Commission) Additional Information Request (AIR) regarding the
`Licensee’s Application for New License for PG&E’s Upper North Fork Feather River Project (FERC No.
`2105), originally submitted on October 23, 2002. The existing license expired on October 31, 2004, and the
`Project has been operating under annual licenses.
`
`Response to item 10 in Schedule A of the Commission’s January 29, 2021 AIR is included.
`PG&E performed the requested analysis on the Belden Forebay Dam and determined that the
`proposed change in the normal operation range for Belden Forebay from elevation 2,965.2 to 2,915.2
`(USGS datum) will not adversely affect the safety of Belden Forebay Dam.
`The other 17 items were addressed in PG&E’s April 26, 2021 filing. This filing concl udes
`PG&E’s response to the Commi ssion’s January 29, 2021 AIR.
`Please call me at (925) 357-7120 if you have any questions.
`
`Sincerely,
`
`Tony Gigliotti
`Mail Code N11D
`P.O. Box 770000
`San Francisco, CA 94177
`
`FERC Service List
`
`List of Attachments
`Attachment 1—Item 10: Dam Safety – Updated Stability Analysis of Belden Dam
`
`

`

`Kimberly D. Bose, Secretary
`May 25, 2021
`Page 2
`
`Upper North Fork Feather River
`FERC Project No. 2105-089
`FERC SERVICE LIST
`
`DISTRIBUTION OF COVER LETTER BY U.S. MAIL
`
`Joshua Horowitz, Attorney
`Bartkiewicz, Kronick & Shanahan
`1011 22nd Street
`Sacramento, CA 95816-4907
`
`Todd Goodwalt
`Army Corps of Engineers SPK
`U.S. Army Corps of Engineers,
`Sacramento District
`1325 J Street Suite 1430
`Sacramento, CA 95814
`R2 FERC Coordinator
`California Dept. of Fish and Wildlife
`1701 Nimbus Rd.
`Rancho Cordova, CA 95670
`Richard Roos-Collins
`Director, Legal Services
`Natural Heritage Institute
`2140 Shattuck Avenue, Ste. 801
`Berkeley, CA 94704-1229
`Mr. Eric R. Klinkner
`Deputy General Manager
`City of Pasadena Dept. of Water &
`Power
`150 So. Los Robles, Suite 200
`Pasadena, CA 91101-4613
`Director, National Park Service
`333 Bush St Ste 500
`San Francisco, CA 94104-2828
`
`Kerry O'Hara, Assistant Regional
`Solicitor
`U.S. Department of Interior
`2800 Cottage Way, Rm. E-1712
`Sacramento, CA 95825
`Russell Prentice
`Pacific Gas and Electric Company
`9 MI N/W of Avila Beach
`San Luis Obispo, CA 93424-0056
`Kelly Henderson, Attorney
`Southern California Edison Company
`PO Box 800
`Rosemead, CA 91770-0800
`
`Kevin Richard Colburn
`National Stewardship Director
`American Whitewater
`1035 Van Buren Street
`Missoula, MT 59802
`Curt Aikens, General Manager
`Yuba County Water Agency
`1220 F Street
`Marysville, CA 95901
`
`Stephan Volker
`Law Offices of Stephan C. Volker
`1633 University Avenue
`Berkeley, CA 94703
`
`Thomas Berliner, Attorney
`Duane Morris LLP
`One Market Plaza, Spear Tower, Suite 2000
`San Francisco, CA 94105
`
`Michael Swiger, Partner
`Van Ness Feldman, LLP
`1050 Thomas Jefferson Street, NW
`Washington, DC 20007
`Traci Bone
`California Public Utilities
`Commission
`505 Van Ness Avenue, 5th Floor
`San Francisco, CA 94102
`Jennifer Carville, P. Advocate
`Friends of the River
`1418 20th St; Ste A
`Sacramento, CA 95811-5206
`
`Stephen M. Bowes
`U.S. Department of Interior
`333 Bush St Ste 500
`San Francisco, CA 94104-2828
`Norman Pedersen, Attorney
`Hanna and Morton LLP
`444 South Flower Street, Suite 1500
`Los Angeles, CA 90071-2916
`
`Director, U.S. Department of Interior
`1849 C St NW
`Washington, DC 20240
`
`Dan Hytrek, Attorney
`NOAA General Counsel, Southwest
`501 W. Ocean Blvd., Suite 4470
`Long Beach, CA 90802
`Mike Fitzwater, Secretary
`Fall River Wild Trout Foundation
`16862 Pasquale Rd
`Nevada City, CA 95959
`
`PG&E Law Dept. FERC Cases
`Pacific Gas and Electric Company
`77 Beale Street
`San Francisco, CA 94105
`Michael Bruce Jackson, ESQ
`Plumas County Flood Control
`178 Lee Way
`Quincy, CA 95971
`
`Jan A. Nimick, Vice President
`Pacific Gas and Electric Company
`245 Market Street
`San Francisco, CA 94105
`
`David Arthur
`Redding Electric Utility
`PO Box 496071
`Redding, CA 96049-6071
`
`

`

`UPPER NORTH FORK FEATHER RIVER PROJECT
`FERC NO. 2105
`
`ATTACHMENT 1
`ITEM 10: Updated Stability Analysis of Belden Dam
`
`Upper North Fork Feather River Project, FERC No. 2105
`
`

`

`Based on my review of the attached analysis, I confirm that adjusting the lower bound of the
`normal operation range for Belden Forebay from elevation 2,965.2 to 2,915.2 (USGS datum)
`will not adversely affect the safety of Belden Forebay Dam.
`
`Approved by: Approved by:
`
`David L. Ritzman, P.E., G. EDavid L. Ritzman, P.E., G. E
`
`Printed Name Printed Name
`
`
`
`Signature Signature
`
`5/24/2021
`5/24/2021
`
`Date Date
`
`Page 1 of 17
`
`Safety Engineer
`Safety Engineer
`
`Chief Dam
`Chief Dam
`
`

`

`GEO.21.11 R1
`
`Date:
`Subject:
`
`05/03/2021
`Belden Forebay Rapid Drawdown Analysis
`
`1
`Introduction
`The Pacific Gas and Electric (PG&E) Geosciences Department has prepared the following memorandum
`at the request of PG&E Dam Safety to provide an evaluation of the static stability of Belden Forebay
`Dam (State Dam No. 97-119) under the rapid drawdown load case. This assessment supplements the
`existing stability analysis (PG&E, 1990) for the dam, which did not consider rapid drawdown. This
`memorandum also comments on the stability of the reservoir rim during rapid drawdown of the
`reservoir. These analyses were requested by the Federal Energy Regulatory Commission (FERC) in an
`Additional Information Request for the Upper North Fork Feather River FERC project 2105. The letter,
`dated 29 January 2021, states:
`
`“The normal operating range for the Belden Forebay is from elevation 2,955 feet to
`elevation 2,975 feet (PG&E Datum). There is no rule curve or reservoir level restriction for
`the Belden Forebay. The minimum water surface elevation of 2,905 feet (PG&E Datum) is
`well below the current normal operating range of the Belden Forebay. Within 120 days
`of the date of this AIR, please provide an updated stability analysis for the revised
`drawdown condition (dam and reservoir rim). PG&E’s Chief Dam Safety Engineer should
`provide a statement confirming that the proposed change will not affect dam safety. The
`stability analysis should follow FERC Engineering Guidelines and requirements.”
`
`2 Background
`Belden Forebay Dam (also referred to as Caribou Afterbay Dam) is a compacted zoned rockfill dam
`owned and operated by PG&E. The dam is located on the North Fork of the Feather River in Plumas
`County and is part of PG&E’s Feather River hydroelectric system. The site location and adjacent project
`components are shown in Figure 1. Belden Forebay was completed in 1958 and for a period of 11 years
`immediately following completion of the project the reservoir served only as an afterbay for Caribou
`
`Page 2 of 17
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`GEO.21.11 R1
`
`Powerhouses 1 and 2. Belden Powerhouse was completed in 1969 and the dam has since served both as
`an afterbay for the Caribou Powerhouses and a forebay for Belden Powerhouse. The dam also serves as
`the forebay for Oak Flat powerhouse, a 1.3 MW mini-hydro plant constructed at the toe of the dam in
`1985. Belden Forebay is classified as a “High Hazard Potential” structure under the FERC guidelines
`(PG&E, 2015).
`
`PG&E previously performed a static stability and seismic deformation assessment for Belden Forebay
`Dam. This assessment evaluated the stability of the dam under the maximum reservoir level only and
`did not consider the stability of the dam during a rapid drawdown scenario (PG&E, 1990). The normal
`operating range for Belden Forebay is from elevation 2965.2 feet to the normal maximum water surface
`elevation of 2985.7 feet. The minimum water surface elevation of 2915.2 feet is approximately 50 feet
`below the lower bound of the normal operating range (FERC, 2021). Elevations in this memorandum are
`in USGS datum1 unless stated otherwise.
`
`2.1 Summary of Previous Analysis
`PG&E performed a static stability and seismic deformation analysis of Belden Forebay Dam in 1990
`(PG&E, 1990). This analysis evaluated both upstream and downstream failure surfaces for the static and
`pseudo-static load cases. No analysis evaluating the stability of the upstream face during the rapid
`drawdown load case was performed. The normal maximum reservoir level and the dam geometry
`associated with the maximum section were assumed for all failure surfaces and load cases. The
`computer program UTEXAS2 using the Spencer method of analysis was used. The results for the
`upstream failure surfaces are most relevant for comparison to the updated rapid drawdown assessment
`presented herein and are shown in Figure 2.
`
`Figure 1
`
` Site location plan (PG&E, 2015)
`
`1 The conversion from USGS to PG&E datum is as follows: USGS datum = PG&E datum + 10.2 feet
`
`Page 3 of 17
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`GEO.21.11 R1
`
`Figure 2
`
`1990 stability results for the upstream face of Belden Forebay Dam (PG&E,1990)
`
`
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`Page 4 of 17
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`GEO.21.11 R1
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`3 Stability Analysis Inputs
`The following sections summarize the inputs used in the updated slope stability evaluation for the rapid
`drawdown load case.
`
`3.1 Dam Geometry
`Belden Forebay Dam is a zoned rockfill dam with a central low permeability core. The dam is 152 feet
`high, 563 feet long, and has a crest elevation of 3001.2 feet at the maximum section. A 2011 survey
`indicates that the crest has settled to an elevation of 3000.8 feet at the maximum section. The as-built
`geometry at the maximum section has been modeled in this assessment. The upstream and
`downstream slopes are 2.5H:1V and 2H:1V, respectively. The upstream shell is composed of rockfill,
`faced with large basalt boulders. The downstream zone is rockfill containing smaller basalt rocks in a
`matrix of fines above approximately elevation 2900, where a transition to larger rock on the surface
`occurs. The maximum section is shown in Figure 3 and indicates that “random fill” and “special zones”
`were included in the upper half of the downstream shell which may be more fine-grained than rockfill
`(PG&E, 2015). These zones have been included in the model for this assessment.
`
`The borrow material used to construct the impervious core was classified as clayey sand (SC) to lean clay
`(CL) in the Woodward Clyde Consultants laboratory test report (WCC, 1956). The compacted core is
`protected on either side with 8-foot-thick filter-transition zones. The upstream filter section slopes at
`1/2H:1V while the downstream filter zone has a steeper slope of 1/10H:1V. Approximately 10-feet-thick
`zones of relatively fine-grained rockfill were placed between each filter zone and the adjacent rockfill.
`The 1990 analysis neglected these finer rockfill filter transition zones and they have been similarly
`neglected in this stability model.
`
`The rockfill zones are founded primarily on weathered metasiltstone, with one area of the upstream
`shell at the left abutment founded on firm talus, while the impervious core is founded on competent
`metasandstone. A grout curtain consisting of 256 grout holes was installed beneath the core zone.
`
`
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`GEO.21.11 R1
`
`Figure 3
`
`Maximum section at Belden Forebay Dam (note elevations are in PG&E datum)
`
`
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`Page 6 of 17
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`GEO.21.11 R1
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`3.2 Geotechnical Parameters
`The material properties used in the slope stability evaluation are summarized in Table 1. The unit weight
`and effective stress (drained) parameters match those developed for the PG&E (1990) analysis.
`Laboratory strength data is not available for the coarse-grained rockfill and filter materials. The PG&E
`(1990) analysis estimated the friction angle of the compacted rockfill based on the Leps (1970) database,
`which relates effective normal stress to friction angle. An average value of 45° was selected as
`representative for the anticipated stress range of the critical failure surfaces. The properties of the
`random rockfill and filter materials were chosen based on available data in similar materials published in
`the United States Bureau of Reclamation (USBR) Design Standard No. 13, Embankment Dams (USBR,
`1987).
`
`The effective shear strength parameters for the earth core were developed for the 1990 analysis based
`on site-specific laboratory test results. A suite of 3 consolidated drained triaxial shear tests were carried
`out at confining pressures of 0.5 tsf, 1 tsf, and 2 tsf. These tests were run on reconstituted samples of
`the borrow material used to construct the earth core (WCC, 1956). The effective shear strength
`parameters suggested by the laboratory test data ranged from friction angle of 30° to 38° and cohesion
`of 900 psf to 1000 psf. An effective friction angle of 32° and an effective cohesion of 900 psf were
`selected for slope stability analyses (PG&E, 2015). PG&E (1990) explains that these were selected as
`“average strength parameters” from the construction-era lab data.
`
`Analysis of the rapid drawdown load case requires undrained strength parameters be developed for
`materials with coefficients of permeability less than 10-4 cm/s. Soils with coefficients of permeability of
`10-4 cm/s or more can be assumed to be free draining during drawdown (Duncan, Wright, and Brandon,
`2014). With a laboratory measured permeability of 3x10-7 cm/s, the earth core material should be
`modeled as undrained during drawdown. The undrained strength parameters are best developed based
`on consolidated-undrained triaxial shear tests with isotropic consolidation (Duncan, Wright, and
`Brandon, 2014). This data, however, is not available for the earth core at Belden Forebay Dam.
`
`Duncan, Wright, and Brandon (2014) presents a correlation of water content and dry density to the
`intercept and slope of the total stress envelope, cr and φr. The water content and dry density data
`obtained from compaction testing during construction of the dam was used to correlate to total
`strength parameters. The estimated cohesion intercept ranges from approximately 2,000 psf to 4,000
`psf and the estimated friction angle ranges from approximately 5° to 15°. Due to the lack of site-specific
`laboratory test data, a conservative cr value of 1,000 psf and an average φr value of 10° were selected.
`
`
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`Page 7 of 17
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`GEO.21.11 R1
`
`Table 1 Summary of Material Properties used in Slope Stability Evaluation
`
`Material
`
`Effective
`Cohesion, c’ (psf)
`0
`0
`900
`900
`0
`10,000
`
`Effective Friction
`Angle, φ’ (°)
`45
`35
`32
`32
`30
`40
`
`Cohesion, cr
`(psf)
`-
`-
`1,000
`1,000
`-
`-
`
`Friction Angle, φr
`(°)
`-
`-
`10
`10
`-
`-
`
`Total Unit Weight,
`γ (pcf)
`Compacted Rockfill
`115
`Filter
`125
`Sat. Earth Core
`132
`Moist Earth Core
`130
`Random Rockfill
`125
`Foundation
`140
`3.3 Phreatic Surface
`Piezometer data to define the phreatic surface is not available at Belden Forebay Dam. The steady-state
`phreatic surface prior to drawdown has been estimated by assuming a horizontal line from the normal
`maximum reservoir elevation to the upstream face of the impervious zone, then using the methodology
`in Casagrande (1940), as recommended in FERC (2006) within the core, then linearly extending to the
`downstream corner of the downstream filter zone. The phreatic surface associated with the drawdown
`condition was developed on the same basis for the minimum reservoir elevation of 2915.2 feet (see
`Figure 6).
`
`4 Rapid Drawdown Stability Assessment
`This section summarizes the methodology used in, and the corresponding results of, the rapid
`drawdown slope stability assessment.
`
`4.1 Methodology
`Updated stability analyses to evaluate the rapid drawdown load case were performed using the
`software Slide V7.031 (Rocscience, 2018) using the Spencer method of analysis. The stability of the
`upstream face was first modeled under steady-state seepage conditions with the reservoir at the normal
`maximum water surface elevation of 2985.7 feet. This model was run to compare to the previous
`stability analysis. Both stability analyses model the dam geometry corresponding to the maximum
`section and assume the same stratigraphy and engineering soil properties, as discussed in Section 3.2.
`The estimated factor of safety values between the two analyses should therefore be similar.
`
`The stability of the upstream face of the dam was evaluated for the rapid drawdown loading scenario
`assuming a lowered reservoir from the normal maximum water surface elevation of 2985.7 feet to the
`minimum water surface elevation of 2915.2 feet. The Duncan, Wright, and Wong (1990) procedure for
`computing slope stability during rapid drawdown is the recommended approach in the U.S. Army Corps
`of Engineers (USACE) (2003) Slope Stability Manual and has been used in this evaluation. This is a multi-
`stage procedure in which the shear strength of low permeability material is computed at various stages
`during drawdown. The undrained shear strength of low permeability materials is calculated based on
`interpolation between two strength envelopes, one corresponding to isotropic consolidation (Kc =
`σ’1/ σ’3=1) and the other corresponding to anisotropic consolidation with the maximum effective
`principle stress ratio possible (Kc = Kfailure =Kf) (Duncan, Wright, and Brandon, 2014). These strength
`
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`GEO.21.11 R1
`
`envelopes are illustrated graphically in Figure 4. A final check is performed to ensure that the drained
`strength of the material is not less than the undrained strength calculated from the envelopes described
`above.
`
`The earth core was modeled as a low permeability material during rapid drawdown and the shear
`strength was calculated using the Duncan, Wright, and Wong (1990) multi-stage procedure. The rockfill
`and filter material were modeled as free draining and were therefore evaluated using the effective
`stress (drained) parameters. The Kc = 1 strength envelope is typically obtained from consolidated-
`undrained triaxial shear tests with isotropic consolidation, but was approximated for the earth core
`using the total stress parameters presented in Section 3.2 due to a lack of available laboratory test data.
`The Kc = Kf envelope is the same as the effective stress envelope and was determined based on the
`available drained triaxial shear tests (see Section 3.2).
`
`4.2 Stability Results
`The updated slope stability evaluation for the upstream face of Belden Forebay Dam under steady-state
`seepage conditions is presented in Figure 5. This analysis assumes the reservoir is at the normal
`maximum water surface elevation of 2985.7 feet. While the predicted minimum slip surface is slightly
`shallower than the minimum slip surface predicted in the previous PG&E (1990) analysis, the estimated
`factor of safety of 2.5 is similar to the previously estimated (1990) value of 2.6. The similarity in results is
`expected since the dam geometry, soil properties, and loading conditions are consistent between the
`two analyses.
`
`The rapid drawdown analysis using the Duncan, Wright, and Wong (1990) procedure is presented in
`Figure 6. The predicted minimum slip surface is deeper than that predicted for the steady-state seepage
`condition, extending to a depth approximately tangent with the bedrock contact and daylighting near
`the upstream toe of the dam. The estimated minimum factor or safety in the rapid drawdown scenario
`is 1.9, a reduction of approximately 25% from the steady-state seepage model. It should be noted that
`the intercept and slope of the total stress envelope, cr and φr, are related but not equal to the intercept
`and slope of the Kc=1 envelope proposed in Duncan, Wright, and Wong (1990), dKc=1 and ΨKc=1. The total
`stress parameters presented in Table 1 for the earth core have been converted to the corresponding
`dKc=1 and ΨKc=1 values, as shown in Figure 4.
`
`The minimum static factor of safety values computed for the upstream face of Belden Forebay Dam are
`summarized in Table 2. The computed factor of safety value of 1.9 for the rapid drawdown load case
`meets the required minimum factor of safety specified in FERC (2006) of 1.2.
`
`Table 2 Summary of Minimum Factor of Safety Values
`
`Load Case
`
`Steady-State Seepage
`Rapid Drawdown
`
`Factor of Safety
`(PG&E, 1990)
`2.6
`-
`
`Factor of Safety
`(Updated Evaluation)
`2.5
`1.9
`
`FERC Minimum
`
`1.5
`1.2
`
`
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`GEO.21.11 R1
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`Figure 4
`
`Strength envelopes used in the rapid drawdown stability analysis
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`GEO.21.11 R1
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`Figure 5
`
`Stability results for upstream face of Belden Forebay Dam under steady-state seepage conditions
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`GEO.21.11 R1
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`Figure 6
`
`Stability results for upstream face of Belden Forebay Dam under rapid drawdown conditions
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`4.3 Reservoir Rim
`A description of the geology at the reservoir is provided in chapter five of the Supporting Technical
`Information Document (STID) (PG&E, 2012). The Belden Forebay basin is entirely underlain by bedrock
`consisting of sandstone (graywacke), siltstone (phyllite), slate, and schist. The graywacke is generally
`strong, hard, and resistant to erosion while the phyllite is strongly foliated and less resistant. The dam
`was built at stubby fins along the canyon wall where the canyon constricts at a resistant band of
`graywacke. Several steep-sided tributary channels along the canyon walls exhibit geomorphic
`characteristics of debris-flow chutes and deposits from the chutes have been identified along the west
`side of the canyon, upstream of the dam. Colluvial and landslide deposits have been identified along the
`canyon walls (PG&E, 2012).
`
`A detailed geologic description, or suitable record of topography/bathymetry of the canyon walls below
`the operating level of the reservoir is not available. However, it is reasonable to assume that the
`observations made during geologic reconnaissance of the slopes above the reservoir are representative
`of the slopes below the operating level of the reservoir. Photographs of the canyon walls during
`construction of the dam are shown in Figure 7 and Figure 8. The steep canyon walls associated with the
`bedrock present above the reservoir level can be observed in these photographs, supporting the
`conclusion that a significant change in geologic conditions is not expected below the reservoir.
`
`Cotton Shires and Associates, Inc. (CSA) carried out engineering geologic reconnaissance of the reservoir
`margins in two stages. Reconnaissance along the left abutment was completed in 2010 (CSA, 2010), and
`aerial reconnaissance of the reservoir in conjunction with an engineering geologic assessment of the
`right abutment was completed in 2011 (CSA, 2011). A shallow debris slide was identified above the
`downstream left abutment that is actively raveling and allowing debris to accumulate in the drainage
`ditch along the access road. It was concluded that future small-scale slope movement is expected,
`creating a maintenance issue but posing no significant hazard to the dam (CSA, 2010). Similarly, three
`shallow landslide features were observed along the right abutment which were likely initiated by the
`grading for Caribou and Butt Valley roads. Due to the shallow nature of these slides, it was concluded
`that the potential for landslide debris to pose a safety hazard to the dam is low (CSA, 2011). Aerial
`reconnaissance identified steep tributary channels above the reservoir which have the potential to
`produce debris flows, though none appear recently active. A summary of the engineering geologic
`reconnaissance is provided in Figure 9.
`
`Based on the geologic reconnaissance, it was concluded that no large-scale landslides or active debris
`flow chutes pose a significant safety concern for the dam due to impact or overtopping (CSA, 2011). The
`potential for a large landslide, rockslide or debris avalanche falling in to the reservoir and causing a wave
`to overtop the dam was revisited as potential failure mode (PFM) 3 during the most recent potential
`failure mode analysis (PFMA) (PG&E, 2020). This failure mode was assigned Category II or judged to be
`of lesser significance and likelihood, specifically it was discussed that it was very unlikely for a landslide
`to be large enough to generate a wave that would exceed the thirteen feet of freeboard at normal
`maximum reservoir elevation. The favorable conditions for this failure mode included the fact that the
`geologic reconnaissance did not identify any potential large landslides and the dam is constructed of
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`GEO.21.11 R1
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`rockfill shells. The PFMA also noted that the hazard of a large wave developing from a landslide is
`greatest during a flood event when the reservoir is at its highest level. The risk of a large wave
`developing from a landslide is significantly reduced during the rapid drawdown condition (PG&E, 2020).
`
`Two possible dam safety hazards exist related to stability of the reservoir rim: 1) slope failure that
`impacts the dam, and 2) slope failure into the reservoir that causes an overtopping wave. The potential
`for the dam impact hazard is controlled by the high, steep slopes above the dam abutments. The
`stability of these slopes is independent of the reservoir level and should not be affected if rapid
`drawdown of the reservoir were to occur. The potential for the overtopping hazard is greatest during
`the flood conditions, when the reservoir is likely to be full and slope failure above the reservoir is most
`likely due to storm related conditions. However in these conditions slope failure below the normal
`reservoir elevation is unlikely. Conversely, during the rapid drawdown scenario the reservoir is low
`(assumed reservoir elevation of 2915.2 feet provides a freeboard of approximately 86 feet), and so the
`overtopping hazard from slope instability below the normal max water elevation is very low. For these
`reasons, the potential for instability of the reservoir rim during rapid drawdown of the reservoir to
`adversely affect dam safety is considered low.
`
`Figure 7
`
`Photograph of upstream view of outlet construction (June 04, 1957) (PG&E, 2015)
`
`
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`GEO.21.11 R1
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`Figure 8
`
`Photograph of upstream face with dam complete (December 9, 1958) (PG&E, 2015)
`
`
`
`
`
`Figure 9
`
`Engineering geologic map for Belden Forebay (CSA, 2011)
`
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`GEO.21.11 R1
`
`5 Conclusion
`In a letter dated January 29, 2021 FERC requested that the stability of Belden Forebay Dam and the
`surrounding reservoir rim be evaluated for the rapid drawdown load case. The stability of the dam at the
`maximum section was evaluated for a reservoir drawdown from the normal maximum water surface
`elevation of 2985.7 feet to a minimum water surface elevation of 2915.2 feet. A minimum slope factor
`of safety value of 1.9 was calculated for the upstream face of the dam during rapid drawdown using the
`Duncan, Wright, and Wong (1990) multi-stage procedure. The computed minimum factor of safety is
`well above FERC’s established minimum criteria of 1.2 (FERC, 2006).
`
`Detailed geologic reconnaissance of the reservoir rim below the operating level of the reservoir is not
`available. CSA concluded from detailed geologic mapping and aerial reconnaissance of the slopes above
`the reservoir that no large-scale landslides or active debris flow chutes pose a significant safety concern
`for the dam (CSA, 2011). The steep, high slopes directly above the dam present the greatest impact risk
`to the dam and are unaffected by the reservoir level. The slopes of the reservoir formed by Belden
`Forebay Dam are monitored for potential rockfall and landslides, especially during high rainfall periods
`and after significant earthquake events. Based on a recommendation by the 2015 Part 12D Independent
`Consultants, an engineering geologic assessment of the slopes will also be performed at least every five
`years. The potential for a landslide to induce a wave which overtops the dam was evaluated as part of
`the most recent PFMA workshop (PG&E, 2020). It was noted that the risk of wave overtopping is
`greatest when the reservoir is elevated during a flood; this risk is significantly reduced when the
`reservoir is at the drawn down elevation of 2915.2 feet, 86 feet below the crest. The potential for
`instability of the reservoir rim during rapid drawdown of the reservoir to adversely affect dam safety is
`considered low. Regular monitoring of slopes in the vicinity of the dam further reduces the potential
`risk.
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`Page 16 of 17
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`GEO.21.11 R1
`
`References
`1. Casagrande (1940). Seepage Through Dams. Contributing to Soil Mechanics 1925-1940, Boston
`Society of Civil Engineers, Boston, 1940.
`2. Cotton Shires and Associates, Inc. (2010). Engineering Geologic Reconnaissance – Belden
`Forebay Dam Left Abutment: letter report from Dale R. Marcum and John M. Wallace to Scott
`M. Steinberg.
`3. Cotton Shires and Associates, Inc. (2011). Engineering Geologic Reconnaissance – Belden
`Forebay Dam Right Abutment and Aerial Reconnaissance of Reservoir Margin: letter report from
`Dale R. Marcum and John M. Wallace to Scott M. Steinberg.
`4. Duncan, J.M., Wright, S.G., and Brandon, T.L. (2014). Soil Strength and Slope Stability 2nd Edition.
`John Wiley& Sons, Inc.
`5. Duncan, J.M., Wright, S.G., and Wong, K.A. (1990). Slope Stability during Rapid Drawdown.
`Proceedings of H. Bolton Seed Memorial Symposium. Vol 2.
`6. Federal Energy Regulatory Commission (2006). Engineering Guidelines for the Evaluation of
`Hydropower Projects. Chapter 4.
`7. Federal Energy Regulatory Commission (2021). Additional Information Request for the Upper
`North Fork Feather River Hydroelectric Project No. 2105-089. January 29.
`8. Leps (1970). Review of the shearing strength of rockfill. Journal of Soil Mechanics.
`9. Pacific Gas and Electric Company (1990). Belden Forebay Dam, Static Stability and Seismic
`Deformation Assessment, FERC Project 2105.
`10. Pacific Gas and Electric Company (2012). Section 5.0 Geology And Seismicity Summary, Belden
`Forebay Dam, FERC Project No. 2105-CA, December 2012.
`11. Pacific Gas and Electric Company (2015). Supporting Technical Information Document for Belden
`Forebay Dam, FERC Project No. 2105-CA.
`12. Pacific Gas and Electric Company (2020). Potential Failure Mode Analysis – Addendum 4. Belden
`Forebay Dam, FERC Project No. 2105-CA.
`13. U.S. Army Corps of Engineer (2003). Slope Stability Engineering Manual. October 31.
`14. U.S. Department of the Interior Bureau of Reclamation (1987). Design Standards No. 13
`Embankment Dams.
`15. Rocscience (2018). Slide Version 7.031.
`16. Woodward Clyde (1956). Report of Laboratory Tests on Borrow Materials for an Earth Fill Dam,
`Caribou Afterbay. September 5.
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`CERTIFICATE OF SERVICE
`
`I hereby certify that I have this day served the foregoing document upon each person
`
`
`
`designated on the official service lists compiled by the Secretary in this proceeding (P-2105-089),
`
`in accordance with Rule 2010 of the Commission’s Rules of Practice and Procedure, 18 C.F.R. §
`
`385.2010.
`
`Dated at San Francisco, CA, this 25th day of May 2021.
`
`
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`
`
`Tony Gigliotti
`Mail Code N11D
`P.O. Box 770000
`San Francisco, CA 94177
`(925) 357-7120
`Tony.Gigliotti@pge.com
`
`
`