Hydro Review: A Novel Approach to New Small Hydro — Constructing South Fork Powerhouse

South Fork Powerhouse

The South Fork Powerhouse Project demonstrates how power tunnels can be used when constructing small hydro plants to take advantage of new license minimum flows when placing the plant at the dam base is not feasible or economical. This article discusses several of the unique challenges faced by this U.S. Department of Energy-grant-funded project.

By Bill Collins, P.E.

The Sacramento Municipal Utility District (SMUD) has constructed a new powerhouse and boating flow release facility (BFRF) a quarter mile downstream of its existing Slab Creek Dam in California. Slab Creek Dam impounds 16,600 acre-feet of storage and has a normal full pool elevation of 1850 ft. Water is conveyed from Slab Creek Reservoir through a 4.9-mile-long power tunnel to the 224-MW White Rock Powerhouse, which discharges into Chili Bar Reservoir. A small 450-kW powerhouse at Slab Creek Dam uses the in-stream flow requirement from the original Federal Energy Regulatory Commission operating license.

The new 50-year license (FERC Project No. 2101-084), issued in July 2014, requires significantly greater instream flows immediately below the dam for aquatic preservation and recreational flows as compared to the original license. The terms of the new license allow the Slab Creek Powerhouse to use the original required dam releases, while the new South Fork Powerhouse will use the new instream flow requirements above the original license requirement. The South Fork Powerhouse routes flows back to the South Fork of the American River (SFAR) via a new penstock that is tied into the White Rock Tunnel at Adit 3. This new penstock bifurcates at the Adit 3 portal, with one leg going to the BFRF and the other to the new powerhouse. The South Fork Powerhouse contains a single turbine-generator unit with an output of 2.98 kVA. The BFRF is a valve house with an energy dissipation chamber for a 60-inch fixed cone valve capable of passing 1,300 cfs for meeting the recreational flow requirements of the license.

Developing small hydro at existing projects

Development of new small hydro projects in California is challenging. Projects that include new dams, penstocks and generation facilities face a long and arduous licensing process. However, existing hydro projects that divert water from reservoirs through long tunnel/penstock systems to a downstream powerhouse offer the potential to add/replace a powerhouse at the existing dam to generate power from minimum flows released into the bypass reach. The opportunity is particularly strong at such hydroelectric projects that have completed a relicensing process because relicensing often results in much higher reservoir releases for aquatic resource protection, recreational boating and maintenance of fluvial processes.

In the case of Slab Creek Dam, a component of SMUD’s Upper American River Project (UARP), the existing Slab Creek Powerhouse at the base of the dam generates electricity from a 36 cubic feet/second (cfs) minimum release. Under the new license, the minimum release requirement will range from 63 to 415 cfs, creating the potential to substantially increase power generation at or near the dam.

While generating electricity at existing dams after relicensing proceedings is attractive from a renewable energy and financial perspective, such projects are not without barriers, often in the areas of siting and variable flow releases. Many hydroelectric reservoirs located in deeply incised canyons are associated with concrete arch dams. The spillways of such dams, like Slab Creek, are often built into the dam design itself such that spill water falls from the spillway to the base of the dam. In the case of Slab Creek, spill water damages the powerhouse, requiring costly maintenance and repair. A new and larger powerhouse constructed at the base of the dam would be even more susceptible to the effects of spill. Another factor precluding construction at the base of the dam is access. While original dam construction occurred decades ago, when access was achieved via the riverbed below the dam site, riverbed access is not an option today. Finally, the cost of building a powerhouse at the base of the dam can be very high given that new water conveyance facilities may include penetrating the dam with a new water conduit.

A common feature of FERC relicensing proceedings in California is a substantial increase in minimum releases at project dams. However, in many cases the new minimum flow requirements mimic the natural hydrograph, which generally results in a highly variable range of releases. In the case of Slab Creek Dam, new license-required minimum releases are from 63 to 415 cfs, with the higher range required only in the spring of wet water years to mimic high snowmelt runoff. One technological challenge involved with a development of this type is designing a turbine and runner configuration that optimizes the balance between capacity, capacity factor and energy production across the highly variable, season-dependent, minimum release schedule that emerges from relicensing.

Power tunnels

As part of SMUD’s FERC relicensing process of the UARP hydroelectric facilities, SMUD was asked to assess the feasibility of performing modifications to Slab Creek Dam to increase streamflow for aquatic resources and for periodic public recreational use, including kayaking and whitewater rafting.

The potential modifications fell into two groups:

  • Flows up to 415 cfs
  • Flows up to 1,500 cfs

For flows up to 415 cfs, several technically feasible options existed with varying complexity and cost, all centering on changing out the 24-inch Howell-Bunger valve at the dam. The variation in options was largely due to whether, and to what extent, to increase the size of the Slab Creek turbine-generator to better match its flow capability with the average annual flow (in the range of 85 to 100 cfs). For flows above 415 cfs, the technical and construction challenges increase greatly. Several options did exist. However, all would have required significant structural modifications that would have been costly and whose regulatory approval was less certain.

In this view looking west toward the powerhouse, the turbine spiral case is lifted into the powerhouse. A 110-ton hydraulic crane was brought to the site for this lift.

For the 415 cfs case, the simplest solution would have to been to replace the existing 24-inch Howell-Bunger valve. If energy recovery was an objective, increasing the size of the turbine generator to handle 85 to 100 cfs and provide a capacity of 1 MW to 1.3 MW would have been an option (consistent with average annual flow and within the takeaway capacity of the Pacific Gas & Electric distribution feeder to the facility).

To achieve a flow of up to 1,500 cfs requires significantly greater modifications to the facility. Two of these options — a new penetration in the dam or an inflatable spillway — entailed some risk of not obtaining regulatory approval from a dam safety perspective.

The White Rock tunnel proceeds 25,941 feet from Slab Creek Dam along and through the south wall of the canyon to the White Rock penstock, which in turn supplies the White Rock powerhouse. Along the tunnel there are a series of three lateral adits that were used for construction and to provide tunnel access. Adit 3 is closest to Slab Creek Dam and is located about 2,200 feet downstream of the dam at an elevation of about 1,650 feet.

From a dam safety perspective, diverting water from the White Rock tunnel to the SFAR through Adit 3 was less risky than any other options for modifying Slab Creek Dam to increase streamflow for aquatic resources and for periodic public recreational use.

This option was implemented by a careful review of potential issues and scope assumptions. Some of the potential issues included FERC approval and dewatering the White Rock tunnel to construct the White Rock tunnel to Adit 3 hydraulic interface. Truck access to the White Rock tunnel via Adit 3 would no longer be possible.

This is a view in the White Rock tunnel looking downstream at the completed hydraulic reducer cone. All formworks, scaffolding and materials were removed from the tunnel by hand through the 78-inch-diameter penstock.

Planning-level design efforts for the proposed project were initiated in 2010 as SMUD awaited issuance of the new FERC License for the UARP. Upon receipt of the new FERC license in July 2014, SMUD submitted the Final Non-Capacity License Amendment Application to include the South Fork Powerhouse and BFRF Project in the license.

The project, which was completed in December 2020, includes the construction of a single unit hydroelectric generating facility and a BFRF and the installation of about 450 ft of penstock piping. The new conveyance system consists of a 6.5-ft-diameter penstock with an isolation valve (penstock valve) in the adit, a bifurcation with piping to supply the fixed cone valve in the boating flow release structure on one branch, and piping to supply the turbine-generator unit on the other branch. The turbine-generator is furnished with an upstream turbine shutoff valve and a downstream draft tube gate. The penstock is sized to meet the maximum instream flow release and recreational flow release under the FERC license. The bypass valve is sized to pass 1,300 cfs of the recreational flow release under the license, with the balance of 200 cfs to be released via the dam. The facility includes controls to maintain required instream flow releases and recreation flow amounts through the fixed cone valve as a bypass if the turbine-generator is unavailable.

The project meets all regulatory requirements. The facility has a minimum service life of 50 years and is designed for safe and reliable operation over the life of the project.

Challenges along the way

The South Fork Powerhouse Project was also noteworthy for the challenges experienced during 3.5 years of construction. The challenges faced include the White Rock tunnel dewatering, cofferdam instability, 12-kV pole line hardening, and COVID-19 impacts.

The design of a powerhouse and BFRF that could meet the new minimum streamflow requirements was substantially approved by FERC in July 2017, and the required permits were in place. Flow to the powerhouse and BFRF would be provided through a penstock connected to the White Rock Tunnel through Adit 3. The tunnel was first watered up in 1965, and there were no documented tunnel dewaterings or inspections since. To assess the tunnel condition, a full-tunnel remotely operated vehicle (ROV) inspection and 3-D sonar scan was completed. The ROV inspection and sonar scan revealed that, in general, the tunnel was in good shape, with minor rockfall found at several locations.

A tunnel dewatering procedure was developed and approved that was intended to reduce the risk of additional rockfall, by keeping the pressure change during dewatering at a conservative rate. The procedure was based on previous experience at SMUD in other tunnels and recommendations from tunnel experts. The dewatering plan included a dewatering rate of 0.5 psi/hr (~0.35 m/hr) during dewatering of the gate shaft to 1 psi/hr (~0.7 m/hr) once reaching the tunnel invert. Instrumentation was added to the tunnel to monitor pressure and transients during the dewatering period.

In August 2017, SMUD management made their decision to proceed. Reasonable assessments and contingencies had been considered and were in place to proceed. Further delay was not expected to result in any further reduction in risk. The White Rock Tunnel dewatering and penstock tie-in at Adit 3 was approved as planned. Dewatering commenced shortly after Labor Day and was completed without incident.

At the same time, the penstock connection to the White Rock Tunnel through Adit 3 was proceeding and the cofferdam at the BFRF structure was installed in preparation for starting dewatering and excavation for the structure. The cofferdam consisted of stacked polyester bags (supersacks) filled with washed sand.

With the White Rock tunnel out of service, spills over Slab Creek Dam were needed to meet minimum flow requirements. Several rainfall/runoff events that significantly exceeded forecasts increased the volume of spill at the dam, which resulted in a partial destabilization of the BFRF cofferdam. The supersack cofferdam design was thought to be well-suited for the project site conditions, and these dams rarely fail catastrophically even in the event of excessive flows. However, even before the storm events, the river continued to develop seepage paths beneath the cofferdam and a re-evaluation of the cofferdam approach was considered.

With the project site only 0.25 mile below Slab Creek Dam, SMUD decided to forego further attempts to shore up the cofferdam and instead dry up the river to start the structures excavation and concrete foundations construction. To accomplish this, SMUD would advance the schedule for two environmental measures that allowed SMUD to dry up the SFAR in the quarter-mile reach below Slab Creek Dam. These two plans had regulatory review and approval and were initially planned to be implemented after powerhouse and BFRF construction.

In April 2018, these plans were revised and resubmitted to the California State Water Resources Control Board (SWRCB). Construction activities associated with the low-flow river channel as detailed in the one of the plans would be performed simultaneously with the powerhouse/BFRF excavations and foundation constructions. During the estimated four months of construction, SMUD would dewater the quarter-mile reach between Slab Creek Dam and the new powerhouse and BFRF construction site. This would allow both the powerhouse/BFRF and environmental plan activities to be performed concurrently in the dry riverbed.

All license-required minimum flows at Slab Creek Dam were released into the SFAR below the construction site from a temporary valve fitted to the newly constructed penstock at the Adit 3 portal. Other measures included removing fish populations beneath the dam and in the quarter-mile reach by electrofishing or with buckets or dipnets as necessary during dewatering. To protect riparian vegetation during the dewatering period, SMUD installed a sprinkler irrigation system throughout the entire quarter-mile reach.

The SFAR was dewatered in July 2018, allowing the work to proceed that could not be protected by the original cofferdam approach.

Included in the scope of the project was the installation of a new 12-kV generator interconnection line between the new powerhouse and the Point of Interconnection (POI) with PG&E’s local 12-kV distribution circuit. The new 12-kV line was to consist of 13 wood poles along a 1000-ft alignment within a 32-ft easement. However, discussions with SMUD’s Line Department at the time highlighted potential changes to SMUD’s design standards for future wildfire mitigation, given the recent devastating California wildfires. It was agreed that the current construction was unacceptable in the face of changing wildfire risk and mitigation expectations, even if the current design met all published SMUD standards.

Subsequently, a decision was made to shift scope from the contractor to SMUD to modify the existing design and construct the new 12-kV interconnection line, as a pilot project for SMUD’s future standards for wildfire mitigation. The pilot project included the installation of ductile iron poles and “hardened” or insulated tree wire conductor.

In the final year of the project, COVID-19 became a pandemic just as construction levels were peaking on the project from February to March 2020. Except for several workers electing to stop working due to the virus, construction was not significantly disrupted. That was about to change as construction was completed in August 2020 and the contractor transitioned into pre-startup and pre-operational testing activities.

Manufacturers’ representatives from Austria, Poland and the Czech Republic were not allowed entry into the U.S. to support startup testing and commissioning due to COVID-19 travel restrictions. Members of SMUD’s Power Generation and IT staffs developed a virtual commissioning solution that included helmet cameras and Microsoft Teams conference calls. Remote/virtual commissioning via these methods was very successful, and having firm internet capability was key.

This view in the powerhouse shows the author standing before the horizontal turbine spiral case after installation of the Francis runner.

There were some impacts: remote/virtual commissioning requires additional engineering time to review, restructure and coordinate the commissioning plan with the manufacturer’s representatives. Also, because the manufacturer’s representatives are not able to physically be present during remote/virtual commissioning, there were inefficiencies to support remote/virtual commissioning. These inefficiencies resulted in about 30% more effort to support remote/virtual commissioning. This meant additional costs and schedule adjustments to the project.

Also, remote/virtual commissioning occurred outside the allowed contract hours. It was important to start early due to the nine-hour time difference. But in the end, remote/virtual commissioning was achievable with the creative use of the technology provided by SMUD’s IT staff.

Conclusion

Developing small hydro projects is challenging by virtue of their size. If a new small hydro project is to be sited at a natural stream or river, the costs of licensing, environmental mitigation and transmission would be cost-prohibitive in most cases. The best option for developing efficient and cost-effective new small hydro is at existing facilities, and in particular at existing hydro dams that have nearby transmission facilities.

This view, looking upstream, shows the new powerhouse with pre-fabricated control room and switchgear enclosure atop on the left and the boating flow release facility on the right at the adit portal.

The South Fork Powerhouse, which received DOE grant funding, is a small hydro project located on the SFAR that has demonstrated a novel and innovative approach to siting. Specifically, the project has demonstrated how power tunnels such as the White Rock Tunnel can be used in the construction of small hydro plants to take advantage of new minimum flow requirements when placing the plant at an existing dam is not feasible.

The South Fork Powerhouse Project was also noteworthy for the successes and challenges experienced during 3.5 years of construction. When it became impossible to stabilize a cofferdam, SMUD was granted regulatory approval to dry up the quarter-mile reach below Slab Creek Dam to complete the excavation and deep foundations for the powerhouse and BFRF. The project used the newly constructed penstock in Adit 3 to make the license-required instream flow releases during this period of construction. The project also included a pilot project for hardening infrastructure in High Wildfire Risk Areas when designing and constructing the 12-kV pole line. And lastly, the project employed remote/virtual commissioning methods to complete commissioning when manufacturer’s representatives were prohibited from entering the U.S. due to restrictions associated with COVID-19.

Bill Collins, P.E., is principal mechanical engineer with the Sacramento Municipal Utility District and was project development manager for the South Fork Powerhouse project.

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