The Stanislaus power tunnel fish screen project, completed to meet a mandate in the Federal Energy Regulatory Commission relicensing of PG&E’s 93-MW Spring Gap-Stanislaus hydroelectric project, helps the company protect native trout in the river.
By Mark Nunnelley, Ali Yazd, Tim Buller, David Woodward and Will Scott
In preparing the Federal Energy Regulatory Commission (FERC) relicensing application for its 93-MW Spring Gap-Stanislaus hydroelectric project, Pacific Gas & Electric Co. (PG&E) performed entrainment studies and fish rescue operations to assess the potential for fish to become entrained in flow to its conveyance facilities. PG&E retained Black & Veatch as Engineer of Record for the alternatives analysis, design and construction support. The result was the successful exclusion of fish from Stanislaus power tunnel.
The Spring Gap-Stanislaus project is located on the Middle Fork Stanislaus River and South Fork Stanislaus River in Calaveras and Tuolumne counties, Calif., and occupies more than 1,000 acres within the Stanislaus National Forest, managed by the U.S. Forest Service.
Since 1909, PG&E has been diverting up to 530 cubic feet per second (cfs) from the Middle Fork Stanislaus River to provide an average annual generation of 400,000 MWh at its Stanislaus Powerhouse. TriDam Power Authority operates Sand Bar Powerhouse upstream of Sand Bar Dam.
As a condition to relicensing, FERC required that indigenous rainbow and brown trout in the Middle Fork Stanislaus River be protected from entrainment and impingement at the point of water diversion. To meet this requirement, PG&E proposed excluding fish entrainment at the intake.
The Spring Gap-Stanislaus hydro project includes four developments. These are: Relief Reservoir, which is the uppermost reservoir; Pinecrest Lake; Spring Gap Powerhouse; and Stanislaus Powerhouse.
The Spring Gap-Stanislaus diversion facilities are located just upstream of the New Melones Reservoir. The U.S. Department of Interior’s Bureau of Reclamation owns the facility and the reservoir extends easterly about 60 miles, which is impounded by a 625-foot high dam. There are resident trout in the Middle Fork Stanislaus River that are commonly captured within the associated conveyance facilities. Although this fish species is not endangered or threatened, its exclusion was one of the requirements placed by FERC during the relicensing process. Thus, PG&E agreed as part of the new operating license to include a new intake and fish screen structure, along with instream flow release and diversion control facilities and dam and intake bypass facilities.
Based on license criterion, and evaluation of the available space and needed surface area for the required flow to be screened at low velocity, PG&E determined that a cylindrical fish screen system could provide the best performance at the lowest cost. Additionally, PG&E stakeholders determined the cylindrical screen concept would offer appreciable operation and maintenance benefits, such as sediment sluicing, emergency fish screen bypassing, reliable operations, fish protection criteria compliance, and ease of maintenance.
Choosing the design
Design criteria for the new fish screen project that PG&E and regulatory agencies agreed on during preparation of the new operating license includes:
- Flow capacity of 530 cfs;
- Approach velocity of 0.33 to 0.40 feet per second (fps) at the fish screen;
- Sweeping velocity of 2 fps or greater at the fish screen; and
- Fish screen openings 1.75 mm for screen material slot width or 3/32 inch for round openings.
To arrive at a conceptual solution that would satisfy agency-required bypass criteria in the relatively short distance available at the site (given the small available foot print and very little head to accomplish the screening activity), a one-dimensional numerical hydraulic model (HEC-RAS) was constructed. The preliminary design then was evaluated by physical modeling. Northwest Hydraulic Consultants constructed the physical model on a 1:10 scale. Results of physical modeling indicated a baffled spillway channel could dissipate fluid energy so that maximum allowable velocities and water surface differentials would not be exceeded.
The positions of the slide gate openings at the intake entrance and behind each fish screen were optimized to provide balanced flow and well-distributed approach hydraulics. Ported baffles inside each screen were adjusted to uniformly balance approach velocities along the longitudinal axis of each screen.
|In this an aerial view of PG&E’s Sand Bar Dam looking upstream along Middle Fork Stanislaus River, the power tunnel intake is pictured in the lower center.|
Recessing the guide rails flush with the upstream face of the intake headwall greatly dampened near-field hydraulics at the back side of the cylindrical screens. To uniformly distribute approach velocities around the circumference of the screens, the model was used to develop cylinder partitions that subdivided each screen element into four quadrants and prevent flow-through from the upstream face of the screen to the downstream face.
Project developers, including engineering and operations teams, discovered that constricting the channel in which the fish screens were located improved sweeping velocities at the screens and enhanced sediment sluicing through the intake.
To achieve criteria compliance required by California Fish and Wildlife, including the specific criteria for flow velocities, two additional baffles were required in the bypass spillway to subdivide the overall head differential, reducing velocities and improving energy dissipation.
The hydraulic gradeline under maximum diversion conditions was readily observable in the field by noting the maximum water surface. The model affirmed the new facility would preserve a portion of the driving head (i.e., total head difference from the intake to the downstream outlet) and therefore not reduce the capacity of the diversion facilities. The model showed the total system head loss would be 1.1 feet, less than the 1.4 feet of loss observed for the original diversion.
Site condition designs/construction challenges
Several challenges were faced and overcome at the site and during construction, including varying subsurface conditions, isolation of the workspace, the need for a permanent bypass, the location within a national forest and access to power supply to complete the work.
Geologic and subsurface conditions were significant factors in how the project should be arranged and constructed. The site is interspersed with massive granitic rocks from an ancient landside, hemming in the space between the river and rapidly ascending hillside. Groundwater and soil permeability are both highly influenced by the river surface and natural water table. The combination of these factors made it imperative to understand subsurface conditions to define the scope of work and enable contractors to properly bid on the project.
PG&E utilized multiple subsurface drilling operations and test pits to gather as much intelligence as practical. In addition, piezometers and test wells were installed to assess groundwater variation throughout the year and the effect soil conductivity would have on excavation and dewatering operations.
Based on these data collection efforts, developers estimated subsurface soils were composed of roughly 50% rock and the remainder coarse grained fines with no cohesion.
Despite the need to remove more than 30,000 cubic yards of material, open excavation proved to be the most time-effective and economical.
As a fast-track solution, the team opted to install a 1-foot thick, non-structural working slab in lieu of a crushed rock base to assure better load path distribution of the future structures to foundations. In areas with low soil densities (i.e., less than 90% roller compacted) and visible yielding of more than half an inch when subjected to a vibratory drum compactor, minimal reinforcing was added. In total, the cost to address these changed conditions equated to roughly 1% of the contract price.
Cofferdamming and workspace isolation
Separating excavations from the Middle Fork Stanislaus River was an essential part of constructing the project.
Conventional cofferdamming techniques were limited. Traditional earthen berms were unattractive due to moving water, stability, and water quality requirements. Sheet piling was incompatible with the riverbed, comprised highly of cobble and large boulders. Prospective contractors would not only need a cofferdam that could isolate their work area, but also avoid constraining the river channel by more than half so that the river would not become overly constricted. This argued for completion of all adjacent below-grade and in-channel work within one construction season in the summer and fall months when river flows were least.
Using the cofferdam for only a single construction season in the summer and fall meant the cofferdam could be relatively short (less than 15 feet tall) and would preferably be of a design that could be deployed and decommissioned quickly to suit the phasing of construction. During construction, an extended California drought also helped drastically reduce river flows and lessen the hydrostatic loads at the cofferdams.
Upon reviewing site conditions and evaluating construction risks, the prospective bidders submitted a variety of gravity bypassing, pumping, and cofferdam techniques.
Syblon Reid, the site contractor, proposed a water-filled cofferdam approach. The relatively shallow water depth was well within the performance range of this style of cofferdam. Moreover, the bladder could be drained, moved and re-filled relatively quickly. Cofferdam designs were stamped, sealed and submitted to FERC as part of the license conditions.
To provide a solution for temporarily bypassing the work site, Syblon Reid proposed a single concrete culvert branching off the downstream end of the permanent bypass and rejoining the existing diversion canal just upstream of the tunnel inlet. PG&E found the concept so beneficial and practical that it elected to make the bypass a permanent project feature, terming it a supplemental bypass.
The final supplemental bypass design included several important features, including custom-hinged swing gates at the upstream and downstream ends of the supplemental bypass confluences to enable redirection of water from the permanent bypass and around the intake structure, and vice versa. In addition, a supplemental diversion control overshotgate was installed just downstream of the bypass/canal inter-tie to regulate the amount of water conveyed into the tunnel. Immediately upstream of the supplemental overshot gate, a supplemental instream flow release system was incorporated consisting of an actuated slide gate and pipeline to route water from the canal to the river.
The supplemental bypass system possessed all the key features of the new intake so that the same diversion and instream release functions could be accomplished when the permanent bypass was in use. The system performed exceptionally well between the consecutive construction seasons and proved its value at the cost of only two days of outage time to make the downstream tie-in. Diversion operations were transferred from the existing intake to the new permanent bypass and supplemental bypass once construction of these facilities was complete.
Site logistics and physical constraints
The site is located in the Stanislaus National Forest in a steep and remote canyon. PG&E collaborated with the Forest Service, Tri-Dam Power Authority and Sierra Pacific Industries to manage the limited workspace and minimize the impact to on-going operations (i.e., laydown areas, road improvements, spoils storage, batch plant and vehicular traffic).
|Looking upstream, this view shows the six cylindrical fish screens oriented towards the control building.|
PG&E secured the campground (USFS Sand Bar Flat Camp Ground located immediately upstream of the facility) for site contractor Syblon Reid. The permission to temporarily use the staging area and office space included the nearby roadside turnarounds for the contractor’s excavated material processing and concrete batch plant operations. Without these strategic areas, it would have cost more to construct the project, not to mention being more difficult to coordinate and execute.
A significant amount of new equipment and instrumentation was included in the project, requiring a considerable source of power to operate and automate.
It became clear early on that the only practical alternative was to negotiate an electrical service connection with the upstream Tri-Dam Power Authority. Developers deemed a tap off of the station service transformer at Tri-Dam’s 16-MW Jack Southern Powerhouse, located about one-half mile upstream, was the only reasonable and economical solution.
The Jack Southern Powerhouse generates electricity via a single hydro turbine-generator unit. The station service panel inside the powerhouse offered an unused capacity of more than twice the load demands estimated for the project. PG&E negotiated an arrangement with Tri-Dam to use up to roughly half of the remaining capacity available.
Transmitting needed power to the site required a one-half-mile-long distribution line PG&E installed underground.
Fish screen debris management
PG&E’s self-imposed requirement in the watershed entails the exclusive use of non-hydraulic-powered equipment considerably reduced the pool of commercially available trash rake equipment. The concern of straining water through 1/16-inch fish screens represented real challenges in maintaining a reliable and self-sustaining diversion facility.
Debris load at the site varies from large logs and branches down to pine needles and fine vegetation, including Elodea blooms that float downstream in large mats. The mats plug the intake trashracks within minutes, rapidly restricting diversion flow.
PG&E chose to use a three-stage screening process for dealing with debris accumulation and keeping the surface water diversion operational.
As a first line of defense, a coarse trashrack and traversing/telescoping mechanical rake supplied by Mecan Hydro Inc. were selected to remove heavy debris at the upstream limits of the intake.
PG&E selected 12-inch trashrack bar spacing to screen large material, such as logs, branches and heavy debris. Project personnel would manually operate the primary trash rake during high debris load periods as necessary to prevent passage of large debris that could foul the secondary trash rake system. Most of the fine debris (i.e., grasses and aquatic vegetation) will pass through the primary trashrack and be handled by the secondary trash rakes.
A telescoping, dual-boom trash rake was selected for the primary trash rake due to its all-electro-mechanical design and ability to maneuver large debris from the front face of the rack to an upstream location, where it could be manually harnessed and rigged for removal. The rake includes an overhead rail superstructure and a traversing carriage with integral boom.
When prompted, the system progressively rakes the entire trashrack over a series of individual raking stations. The carriage indexes incremental points along the trashrack, extends the boom and rake head from the top to bottom of the rack, clamps the debris, then retracts the boom and rake head to the full up position. The carriage then traverses to a designated spoil area and deposits the debris.
To address finer debris and aquatic vegetation, a gallery of nine chain-and-flight trash rakes manufactured by Duperon Corp. was incorporated in the nine intake bays downstream of the primary trash rake system. These rakes were intended to perform the bulk of the debris removal at the site. The secondary trash rake technology uses an all-electro-mechanical sprocket head drive system to drag individual flights or scrapers over a trashrack also supplied by Duperon.
The trashracks extend above the operating deck and feature galvanized steel construction with 2-inch bar spacing to screen out the majority of entrained debris able to pass through the primary rake system.
The flex-link chain system features a front-cleaning/front-return link chain system that drags multiple flights in a continuous motion upward along the face of each respective trashrack. When the flights reach the apex of the trashrack, the debris is swept over and discharged onto a Pathwinder debris belt-type conveyor supplied by Serpentix.
The conveyor is made up of individual, self-aligning belt pans, which enable it to follow a curved alignment. A drive station with electric gear motor is located at the downstream end of the conveyor, including a return station with spring tensioner at the upstream. The conveyor cantilevers over the side of the intake structure to discharge debris back to the river channel, just downstream of the dam crest.
The six fish protection screens were supplied by Intake Screens Inc. and are modular, T-type cylindrical screens to protect fish from entrainment and impingement and minimize impacts from entrained debris that made it through the primary and secondary trash rake systems.
Each screen includes a pair of submersible electric motors for rotating each pair of screen cylinders. The cylinders can be rotated at a user-adjustable speed, frequency and duration, depending on debris load, to keep the screens clean and to perform to their intended capacity. The screen units include internal baffles and partitions to uniformly distribute velocities around the screen surface area, based on the hydraulic model study results.
In addition, each fish screen has a dedicated guiderail system and overhead winch that will raise the screen if excess debris were to occlude the screen and reduce its capacity to divert.
Controls and automation
Sand Bar Dam and the Stanislaus Power tunnel intake have two primary operating functions that have several subordinate operating routines. During normal operations, equipment at Sand Bar Dam is automated to maintain minimum daily instream releases of 50 cfs to 100 cfs in the Middle Fork Stanislaus River downstream of the dam, depending on water year and calendar month per the FERC license. Additionally, the balance of inflow into Sand Bar Reservoir is diverted (up to about 530 cfs) into the Stanislaus Power Tunnel. Flow in excess of the combined instream release and diversion flow spills over Sand Bar Dam.
|The new spillway installed at the Stanislaus project safely returns fish downstream to the Sand Bar Dam toe.|
An exception to the normal operating rule is when license conditions require supplemental flows or recreational flows be provided. In these instances, the reservoir level is raised to produce spills over the dam meeting license requirements (5 cfs to 400 cfs for supplemental releases in addition to minimum daily releases, 700 cfs to 2,000 cfs for recreational flows). This is accomplished by raising the diversion control overshot gate, causing the reservoir water surface elevation to rise above the dam crest.
A significant array of equipment and instruments were provided as part of the project to perform necessary functions at the site. More than 40 electro-mechanical operating/control devices and more than 10 measurement/monitoring instruments are used to operate the intake and bypass facilities.
The project was successfully constructed and commissioned in March 2015, after one year in design, one year in bidding and contract award, and two years in construction.
Due to insufficient flows in the watershed since completion, the project has only recently been subjected to normal operating conditions and diversion flows. Although some mechanical equipment failures were experienced within the warranty period, corrections were made by manufacturers to restore function and operating capability.
In June 2016, PG&E conducted a hydraulic performance evaluation to verify the fish screen would perform in conformance with its design criteria.
Approach velocities and velocity distribution were found to be within standard industry practice, and sweeping velocities are collectively greater than approach velocities thus minimizing impingement potential and exposure at the fish screen gallery.
The fish screen bypass facility was also found to be compliant with design criteria, and will thus effectively transport fish, debris and sediment from the intake pool back to the Middle Fork Stanislaus River downstream of Sand Bar Dam. Overall, system hydraulics were found to be consistent with pre-project conditions, affirming the modified diversion will maintain historical diversion capabilities and accomplish enhanced instream flow release functions.
The project should provide decades of reliable service, fish entrainment prevention and a safe hydraulic environment for all life stages of native trout species at the point of diversion.
Editor’s Note: This article was excerpted from a technical paper presented at HydroVision International 2016 and authored by Mark Nunnelley, Ali Yazd and Scott Fee of PG&E; Tim Bueller and David Woodard of Black & Veatch; Will Scott of Syblon Reid; and Mike Donnelly of Parsons Corp.
HydroVision International 2017 next month in Denver, Colo., U.S., will feature 21 technical paper sessions and seven sessions in the Water and Environment track. To learn more or to register to attend, visit www.hydroevent.com.
Mark Nunnelley, PE, is maintenance supervisor for Pacific Gas and Electric’s hydro operation and maintenance group. Ali Yazd, PE, is a senior civil engineer with PG&E’s power generation group. Tim Buller, PE, and David Woodard, PE, are with Black & Veatch’s hydropower group. Will Scott is senior manager for Syblon Reid.