Choosing the Best Option for Replacing Crest Gates at Pit 3 Dam

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Faced with a trio of failing inflatable crest gates and stipulations for FERC relicensing, Pacific Gas & Electric had to evaluate how to best meet a number of needs and challenges when replacing this equipment at its Pit 3 Dam.

Pit 3 impounds water for a 70-MW hydropower plant.

Located in northern California along the Pit River near Burney, Calif., Pacific Gas & Electric’s Pit 3 Dam impounds waters for the 70-MW Pit 3 Powerhouse. Due to aging infrastructure, the Pit 3 Dam needed new spillway gates not only reservoir control, but also to meet terms of its Federal Energy Regulatory Commission (FERC) relicensing. The process of selecting the best option spanned several years, during which time the utility examined a number of alternatives and proposals.

About Pit 3

Pit 3 Dam is a 112-ft-high, reinforced-concrete gravity dam that was completed in 1925. The structure, which impounds Lake Britton, is one of five dams in PG&E’s Pit hydroelectric system and is located on the Pit River in northern California.

Pit 3’s crest features three ogee spillway bays that are roughly 84 feet wide, overlain by a concrete highway bridge. Inflatable crest gates, approximately 84-ft long by 6-ft high and manufactured by Bridgestone, were installed in 1988 to replace flash boards and they are controlled via air pipes running from a control room at the dam’s west end. These pipes run across the underside of the bridge, down through the concrete piers supporting the bridge and into each of the gates.

The gate membrane was fabricated from half-inch-thick ethylene propylene diene monomer (EPDM), which is designed to resist typical hydrostatic and impact loads. By the early 2000s, however, all three inflatable crest gates began to show signs of wear, including partial membrane tearing and a 0.8-foot sag at the gates’ mid-span that reduced the reservoir’s storage capacity. These aging signs impacted the ability to control flow, and affected air blower operation due to water penetrating the bladder’s interior.

Additionally, the FERC-mandated
relicense for Pit 3, 4 and 5 in 2007 included requirements for PG&E to measure flow over the gates, provide minimum in-stream flows, and provide down-ramping capabilities for flow release.

The aging gates at Pit 3 hindered the utility’s ability to effectively meet the license requirements. Additionally, the gates’ warranty expired in 1994 and Bridgestone had shut down its inflatable dam business in 2001. This made it unlikely PG&E could simply repair the existing structures.

Due to these factors, in 2004 PG&E knew relicensing for Pit 3, 4 and 5 — lumped together under one of four FERC licenses covering the nine Pit facilities — was imminent and began to plan accordingly. PG&E began the process of evaluating remediation alternatives to satisfy the FERC licensing requirements and maximizing storage and generation capabilities.

Due to the aging gate infrastructure, the reservoir was held at a reduced elevation which affected peaking ability of the downstream plants. The FERC license allotted for 5 feet of storage above the crest of the dam, and PG&E was only using 3 feet. The chosen alternative needed to restore the full reservoir storage capability allowed within the license.

Each of Pit 3’s ogee spillway bays is about 84 feet wide.

Design criteria

PG&E’s three primary design criteria were:

Hydraulic

The new gates needed to be 6 ft tall to maximize the storage capacity of the reservoir based on allowable water levels per the FERC license. In addition, the existing spillway discharge rating curve needed to be maintained, and PG&E needed the ability to control and measure flow for various reservoir elevations and gate heights. Finally, the new gates need to provide the ability to pass the probable maximum flood.

Structural

The new gates needed to provide adequate resistance to the site’s seismic and wind loads, as well as resistance to impact loads such as floating debris. And the gates needed to have no detrimental effects on the existing structure, piers or overhead bridge.

Operational

Accessibility and ease of operation were key, along with low maintenance, with easily-implemented procedures for inspection and repair.

Alternatives analysis

Based on the design criteria, PG&E conducted an initial alternatives analysis in 2004 that investigated Obermeyer-type gates and hydraulic flap gates as candidates. PG&E concluded the pneumatically-operated design would be preferable due to its lower cost, absence of oil and the lack of need to remove concrete at the dam’s crest.

At that time, however, FERC had not yet fully developed or implemented its relicensing requirements, although PG&E anticipated their imminent release. Due to the uncertainty of these requirements — especially specific requirements regarding flow control — PG&E elected to defer replacing the gates at that time.

When FERC published the specific relicensing requirements in 2007, PG&E decided to reevaluate the remediation project by increasing its options while still considering the inflatable recommendation from its 2004 analysis. There was concern raised that inflatable gates are susceptible to tears and “catastrophic” failure as the plate causes a wall of water to potentially be spilled rather than a “V-notch” release as with the existing bladder dams. With recreational activities downstream, PG&E wanted to evaluate other “safer” alternatives. As such, PG&E asked Black & Veatch to prepare a broader alternatives analysis in 2009.

This second analysis not only reexamined inflatable and hydraulic gates, but also considered a 1-foot flash board; 1-foot flash board in conjunction with a 6-foot flash board; a raised concrete crest and a 5-foot flash board; and an in-kind replacement using equipment as close to the original as possible, but with a new manufacturer.

Results of this second analysis showed that:

• Flashboards would be labor-intensive to maintain and adjust, causing concerns for ergonomic injuries. They would cost an estimated $1.3 million;

• Inflatable gates presented public safety concerns associated with bladder tears. They had an estimated cost of around
$4 million;

• Hydraulic gates had the potential to release hydraulic fluid into the waterway, which was not acceptable due to environmental concerns. They would cost around
$5.5 million; and

• A replacement in-kind, which had the benefits of meeting the FERC requirements and minimizing risks for PG&E, the environment and public. This option would cost around $3 million.

PG&E believed the 1-ft and 6-ft flash board option would be the preferred alternative based on heavy cost weighting, but the company began to reconsider an in-kind replacement over the ensuing three years.

The primary concern of an in-kind replacement during the 2009 analysis resulted from PG&E’s lack of familiarity with
vendor capabilities, leading to uncertainty about whether new inflatable gates could meet enough of the design criteria to warrant the cost.

Over time, however, PG&E gained more information about vendors’ capabilities, eventually becoming satisfied that the in-kind alternative, despite its higher costs would be the best design due to a number of factors — amongst those, automated and easy flow control, no hydraulic fluids, resistance to debris and low power demands.

Therefore, PG&E assembled a team including Black & Veatch as the engineer of record, Syblon Reid as general engineering contractors, and Mecan Hydro and Floecksmuhle as the inflatable crest gate subcontractor in the spring of 2012.

Design overview

The replacement crest gate design followed the same overall configuration as the previous crest gates, with one pneumatically-operated inflatable gate for each of the three
spillway bays.

These would have the ability to rise to a fully-inflated position 6 feet above the concrete crest, with the capability for seasonal pressure adjustments resulting from seasonal storage elevation variations.

The replacement gates would have several improvements over the original designs. These include:

• A second concrete anchor line, positioned a few feet downstream of the upstream anchor line, to help prevent wind-induced damage;

• Reinforced triangular concrete fillets at the end of each spillway bay to alleviate membrane stresses on the gates’ edges when fully-deflated, helping to prevent further membrane deterioration;

• New air pipes and controls for maximum efficiency and reliability;

• Stem extensions on each gate’s 2-inch condensation drain line valve, accessible from the concrete bridge above, providing the ability to open and close the valves without an operator having to access the downstream face of the dam by ropes;

• Partially independent control of each gate for operational flexibility; and

• Local, remote and automated control for improved flexibility.

Design problems

One challenge involved the coordination required amongst project team members spread across multiple continents, with PG&E, Black & Veatch, Syblon Reid and Mecan Hydro being located in North America and Floecksmuhle being in Aachen, Germany. The differences in time zones, typical nomenclature and standard units of measure necessitated extra diligence in coordinating during the gate design process. With this extra attention, including review meetings involving all stakeholders, the team ensured the design process moved as smoothly as possible despite the physical distances.

Another challenge involved the constraints of existing conditions. Mecan Hydro and Floecksmuhle needed to design their system such that all the required controls fit into the small existing control room. Furthermore, they needed to use some existing gate system components, such as pipes embedded into the concrete piers between the bays.

Meanwhile, the addition of the second, necessary anchor line was problematic. Syblon Reid performed a dimensional dam survey for use in preparing the initial designs. However, because the existing inflatable gates still covered Pit 3’s crest at the time, Syblon Reid could not feasibly obtain precise dimensions in some locations — specifically in the vicinity of the second anchor line. This created difficulties in determining the precise gate design dimensions that would achieve the 6-foot height criteria.

To exacerbate the issue, the dam has a slight curve in plan view, whereas the gates have a completely linear configuration within each bay. This discrepancy caused slight differences in the existing concrete surface elevation compared to the anchor line elevation, with an increased gap toward the middle of each bay.

Addressing the challenges

After Black & Veatch completed the project design in the spring of 2014 with assistance from Mecan Hydro and Floecksmuhle, the contractor mobilized to the site in late summer of the same year. The project team addressed two of the more significant challenges as follows:

Second anchor line elevation

Upon removing the existing gate and resurveying the exposed concrete surface, the elevation proved inconsistent and slightly lower than expected in the center of each of the gate spans. These inconsistencies were likely due to decades of spills causing minor concrete erosion. In this situation, if the gate system was installed unmodified, the gates would have extended only 5.8 ft above the dam crest instead of the intended 6 ft. To maximize Pit 3’s storage and operational capacity, PG&E opted to evaluate methods to restore the required design height. Considerations included:

1) Modifying the already-manufactured gates with an increased membrane circumference or other means;

2) Evaluating the potential for increased air pressure, causing a further gate rise without the need to modify them; or

3) Designing a slight raise to the existing dam crest via grout pad to restore the original crest profile and such that the gates could achieve the desired height unmodified.

Alternatives 1 and 2 became undesirable due to their cost and schedule impacts and uncertainty in achieving a sufficient design. Therefore, the project team came to the quick consensus that a slight raise would be the preferred alternative.

The design included a non-shrink grout pad with a layer of welded-wire fabric reinforcement, tapered as required in the left-right direction to achieve the proper height for the gate anchor bolts, as well as contoured in the upstream-downstream direction to provide a smooth transition that approximated the original ogee shape indicated in the original as-built drawings.

Because the proposed revision involved a permanent alteration to the dam, PG&E submitted the revision to the California Division of Safety of Dams and FERC for review. DSOD approved the revision in a few days and FERC approved within weeks, allowing construction to continue.

Embedded pipe replacement

Contract documents specified the need for pressure tests to determine the suitability of the existing air pipes embedded within the intermediate concrete piers that provide the required air to the gates. Tests revealed the existing pipes were leaking, unable to hold the designated air pressure, and therefore unsuitable for continued use. Therefore, removal of the existing pipes and installation of new pipes without damaging the surrounding structure quickly became the critical issue.

PG&E’s as-built drawings showed the existing pipes were installed concurrently with the existing rubber crest gates in 1988 by core drilling a 10-inch hole vertically through the bridge deck, down into the cutout area directly above the pier, and into the pier before making a short horizontal core drill for the pipes to exit the concrete and connect to the rubber crest gates. The annular space between the pipe and the core drill hole was filled with grout after pipe installation. Drawings also showed critical reinforcing steel for the bridge’s structural stability extended into the intermediate piers not far from the embedded pipes.

Therefore, the project team developed a plan to carefully core drill into the exact locations of the 1988 core drills, following the same path to every possible extent, and drilling slowly to avoid contacting any reinforcing steel. Syblon Reid cored the concrete, examining the removed cores for signs of cut reinforced steel. The cores did not damage any reinforcing steel, and the contractor proceeded with pipe installation and grout filling. Although this challenge threatened the overall project schedule, personnel worked extensive overtime hours to ensure the new piping could be installed in a timely manner. This process took about two weeks.

With quick, efficient coordination, the project team worked through these two primary construction challenges — in addition to other minor ones — to meet PG&E’s internal milestones. Both the proposed and actual construction timeframes were about four months.

The 112-foot-high Pit 3 Dam was completed in 1925.

Conclusion

PG&E began startup and testing of the new gates — including all required local, remote and automated functions — in December 2014, concluding with an official FERC return-to-service date of Jan. 22, 2015. The project resulted in crest gates that should not only meet PG&E’s storage and flow needs, but also have a design life that far exceeds inflatable gates constructed in years past with a life expectancy of over 30 years.

Overall, the project’s costs stayed reasonably near PG&E’s projections. Initial estimates for its procurement and construction were around $3 million, which was exceeded in the project’s final actual costs of about $3.36 million. The addition of $450,000 for contract engineering costs pushed its total to nearly $5 million.

John Cohn is an engineer for Pacific Gas and Electric’s Shasta watershed and was engineering team lead for the Pit 3 project. Joel Bigelow and David Woodward are civil engineers for Black & Veatch. Andy Lockwood is a mechanical engineer for Black & Veatch.

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