Hydro Review

Partnering to Mitigate TDG through Spillway Modifications

Seattle City Light and Avista Corporation combined their knowledge to study, test and develop a method to modify the spillways at two dams to reduce total dissolved gas (TDG).

By Shari Dunlop, Guy Paul and Kimberly Pate

Elevated levels of total dissolved gas (TDG) in spill flows is a significant water quality concern at many dams in the northwestern U.S., due to the potential to induce gas bubble trauma in fish. When fish are exposed to water with elevated levels of dissolved gas, the gasses in their bloodstream equilibrate with the gasses in the water. When the fish rise in the water column, the reduction in pressure can cause bubbles to form in the fish's tissues and bloodstream; the accompanying symptoms are known as gas bubble trauma. The Cabinet Gorge and Boundary projects are located in the Clark Fork-Pend Oreille River Basin and the species of concern for both projects is salvelinus confluentus (bull trout). State water quality standards typically limit TDG to 110% of saturation. But at Boundary Dam in Washington and Cabinet Gorge Dam in Idaho, TDG can periodically exceed 130% during spill events.

The owners of these two dams, the City of Seattle (dba Seattle City Light) and Avista Corp., are developing modifications to reduce TDG generated by spill releases. Alden Research Laboratory has been assisting the two owners with the design, performance prediction, and evaluation of measures intended to reduce the level of TDG downstream of these dams, and the utilities have been sharing information to advance development. Modifying the spillways by adding roughness elements to encourage break up of the falling spillway jet appears promising.

Avista constructed a full-scale modification of one bay of the Cabinet Gorge spillway in 2012, and performance testing in 2013 confirmed that the modifications achieved the intended goal of reducing TDG.

Cabinet Gorge Dam has eight spillway bays stretched across the 395-foot crest. The dam is a 208-foot-tall variable-radius concrete arch structure with an approximate flow height of 40 feet.
Cabinet Gorge Dam has eight spillway bays stretched across the 395-foot crest. The dam is a 208-foot-tall variable-radius concrete arch structure with an approximate flow height of 40 feet.

Physical description

Cabinet Gorge Dam is on the Clark Fork River, about 10 miles upstream of Lake Pend Oreille in Idaho. Downstream of Lake Pend Oreille, the Clark Fork River becomes the Pend Oreille River. Boundary Dam is on the Pend Oreille River in Washington, about 100 miles downstream of Lake Pend Oreille. The dams impound water for hydroelectric generation. Cabinet Gorge Dam has a capacity of 263 MW and Boundary Dam has a capacity of 1,040 MW. Although the layout of the dams and spillways is quite different, the spillway configurations are similar in terms of the physics of TDG supersaturation: both have gated spillways, which pass discharge over chutes of limited length before falling freely into deep plunge pools. This allows the water jet to entrain air and plunge to depth, where increased pressure forces the air into solution, resulting in the potential to elevate the TDG concentration downstream of the projects. The following two photos provide a visual comparison of the projects.

Both dams are variable-radius concrete arch dams and have a design discharge for TDG compliance of more than 100,000 cubic feet per second (cfs); however there are notable differences between the projects with respect to TDG abatement. The height of the free fall at Boundary Dam is 177 feet compared with about 40 feet at Cabinet Gorge. Boundary Dam can release water via its powerplant (55,000 cfs), two spillway bays (108,000 cfs), or seven sluices (252,000 cfs). Releases from Cabinet Gorge are via either the powerplant (38,000 cfs) or the eight spillway bays (206,000 cfs). The length of the spillway chute is significantly shorter at Cabinet Gorge, limiting the potential location of proposed modification features. In addition, the plunge pool at Cabinet Gorge Dam has two depths, with a longitudinal splitter wall separating the shallow and deep portions. This configuration introduces a significant operational variable, in which discharge from Bays 5 through 8 have a physical constraint to the depth of plunge, whereas Bays 1 through 4 do not.

History

Avista and Seattle City Light are pursuing options to reduce TDG in response to provisions included in their Federal Regulatory Energy Commission (FERC) relicenses, which were received in 2000 and 2013, respectively. However, the requirements for implementation differ. Avista's FERC license references a TDG management plan - the gas super saturation control program (GSCP) that was developed through a collaborative process with stakeholders and was issued in 2004. The plan initially prescribed a single specific TDG abatement measure; however, following engineering studies of the prescribed measure the plan was amended to allow multiple measures to be implemented and to institute an interim compliance standard of 120%.

The issuance of the Boundary project's new FERC license initiated a 10-year TDG plan under which Seattle City Light is required to design, construct, prototype test and evaluate alternatives to achieve the water quality standard of a maximum TDG level of 110% of saturation for flows up to the maximum 7Q10 (the average peak annual flow for seven consecutive days that has a recurrence interval of 10 years). The project's 401 certification allows for up to 10 years to implement alternatives; during that period the Washington State Department of Ecology has the option of requesting alternative compliance actions or evaluating whether modifications to the standard are warranted.

Cabinet Gorge Dam

Avista began collecting TDG data in 1996. As a result of these early efforts, Avista implemented a spillway operating sequence that reduced TDG; however, it is still common for TDG to exceed the state standard downstream of Cabinet Gorge Dam during the spring freshet when spillway releases are necessary. The freshet may last up to approximately three months, with TDG levels ranging from less than 105% to almost 145%.

Engineering studies of the concept prescribed by the GSCP, which included physical and computational fluid dynamics (CFD) modeling and predictive calculations of TDG, were completed between 2004 and 2007. The studies concluded that the structural modifications identified in the final control strategy would not provide the anticipated reductions in gas supersaturation. Concepts for TDG abatement were developed in September 2007 and the amended plan was finalized in 2009.

Evaluation of five concepts commenced in 2010 and modification of the existing spillway crest through the addition of flip buckets and roughness elements was deemed feasible. Rather than attempt to optimize the modification through modeling, Avista installed a full-scale prototype to collect field data that could be used to directly evaluate the effectiveness of the concept and support future model studies if required. Detailed design and construction of the prototype modification was complete in 2012, and the first tests of TDG performance were made in early 2013.

Boundary Dam has two spillway bays, a 508-foot crest, a height of 340 feet, and an attached power plant. The approximate flow height is 177 feet.
Boundary Dam has two spillway bays, a 508-foot crest, a height of 340 feet, and an attached power plant. The approximate flow height is 177 feet.

Boundary Dam

Seattle City Light has been studying TDG production at Boundary Dam since 1999. From 1999 to 2007, Seattle City Light conducted a literature search, performed field studies and undertook a comparative analysis of potential TDG abatement alternatives. As a result of these activities, Seattle City Light modified its power plant operations to "last on, first off" for two turbine units that had been entraining air. This produced a significant reduction in TDG; however, the water quality standard is still exceeded when river flow exceeds about 70,000 cfs.There is considerable variability between years, but on average these flow conditions correspond to an occurrence of about 7.4 days per year.

Workshops were held in 2007 and 2008 to identify TDG abatement alternatives. Of the identified alternatives, three were selected for detailed study because they could be more quickly evaluated and implemented to reduce TDG: throttle sluice gates, roughen sluice flow and provide spillway flow splitters/aerators. These modifications are being evaluated and optimized through a combination of physical and CFD modeling and application of TDG predictive tools.

Process

TDG production in spillway discharge is primarily related to the depth of plunge and the residence time of bubbles at depth. The theoretical advantage of adding roughness elements to the spillway is that these elements break up the jet before it enters the plunge pool. In theory, distributed "packets" of flow would not plunge as deep as a coherent jet, thereby reducing the residence time and pressure that the bubbles in the tailrace are exposed to, and thus reducing TDG. The roughness elements are similar to the super-cavitating baffle blocks designed by the U.S. Department of Interior's Bureau of Reclamation for Folsom Dam, but have been modified for the application on a spillway rather than in a stilling basin.

Seattle City Light and Avista have taken a number of steps to develop this concept and validate the underlying theory. The key components of the evaluation have included an integrated program of physical and CFD modeling, coupled with TDG predictive analysis that was executed by Seattle City Light, followed by prototype testing and field verification of the concept, which was conducted by Avista.

Boundary physical hydraulic modeling

A 1:25 scale physical hydraulic model of Boundary Dam was constructed by Alden in late 2008. The model has been used to characterize the performance of the existing dam and to evaluate several modifications to the spillways and sluices. The spillway test program included variations on the roughness element number, size, spacing, position on the chute, number of rows and alignment.

Findings from the physical model study include:

- Roughness elements placed on the spillway chute provide significant reductions in plunge depths;

- The magnitude of the reduction in plunge depth is a function of head, unit discharge, the number of rows of elements, element size and spacing, and position on the chute;

- Minor variations in the configuration of the roughness elements result in significant changes in performance; and

- The optimized configuration for Spillway 2 was not transferable to Spillway 1.

In the visual comparison of the jet from the unmodified (right) and modified (left) bays at Cabinet Gorge Dam, the discharge is the same.
In the visual comparison of the jet from the unmodified (right) and modified (left) bays at Cabinet Gorge Dam, the discharge is the same.

Boundary CFD modeling and predictive tools

CFD models of Boundary Dam's Spillway 1, Spillway 2 and combined spillways 1 and 2 were developed in 2009, along with models of the sluiceways and downstream river channel. The CFD model was set up at 1:25 scale to allow direct comparison of CFD and physical model results. The plunge depths predicted by the two models showed reasonable agreement, and subsequent CFD studies were modeled at prototype scale, allowing the buoyant effects of entrained air to be simulated. In addition, the CFD model was used to track air bubble position with time in the tailrace, which was used as input to a TDG predictive tool.

A spreadsheet tool has been developed to estimate the potential effectiveness of reducing bubble plunge on TDG production. The overall depth/pressure time series obtained from tracking particles in the CFD models is used to estimate TDG downstream of the project. The tool algorithm has been validated against field measurements and applied to hypothetical spillway modifications at Boundary Dam. In all cases, the model predicts that TDG downstream of a spillway that has been modified with roughness elements will be less than that produced by an unmodified spillway.

Cabinet Gorge prototype testing

The Cabinet Gorge spillway chute is considerably shorter than the Boundary chute, so there are fewer options for optimizing the size and placement of roughness elements. A feasibility study of the potential for roughness elements to reduce TDG downstream of Cabinet Gorge Dam appeared promising. However, rather than follow the same progression of optimization adopted by Seattle City Light, Avista opted to construct a prototype and collect field data, recognizing that modeling may be required to optimize the configuration prior to full build-out.

Applying the knowledge gained through Seattle City Light's modeling efforts, and limited by the physical constraints of the site, Avista installed two rows of roughness elements and a flip bucket with a radius of 25 feet on Bay 2. The prototype roughness elements are 4 feet wide by 3 feet 1 inch tall at the leading edge. The height of the block was selected to maximize the block size without protruding above the spillway crest. A spillway bay with the full tailrace depth was chosen for the prototype installation to remove the potential for artificially constraining the depth of plunge, either before or after the modification. This also allowed a direct comparison between the current operating procedure, which favors use of the bays that discharge into the shallow side of the plunge pool, and the performance of a modified bay discharging into the deep side of the tailrace.

Construction of the spillway modifications took place in late 2012. Baseline data were collected prior to installation of the prototype roughness elements. The discharge was varied from 3,150 cfs to 10,600 cfs, with no corresponding power plant flow. The discharge was varied to provide a proxy for evaluating the effect of different block height to flow depth ratios, and the power plant was shut off to isolate the effect of the spillway modification. The tests were repeated in January 2013, after construction of the modifications was complete. Continuous testing of the prototype under "real-world" conditions with the power plant operating was conducted during the spring of 2013.

Results

The physical hydraulic modeling at Boundary Dam indicates that the depth of plunge may be reduced by up to 65%, and CFD and predictive analyses estimate that for the scenarios analyzed spillway modification could provide a reduction of about 5% of saturation at the downstream measurement site.

Controlled tests of the Cabinet Gorge prototype performance indicated that the spillway modifications reduce TDG by 8% of saturation for the tested scenarios.

Continuous monitoring of the prototype performance during the 2013 spill season confirmed that the roughness elements reduced TDG levels. During the 2013 spill season the modified spillway bay was opened first and was used alone for discharges up to 6,000 cfs; when the spillway discharge was greater than 6,000 cfs the additional flow was released in accordance with standard operations which prioritize spill from the shallow bays. As a result of the positive prototype performance, Avista intends to apply this concept to additional spillway bays.

Final design of roughness elements on Boundary Dam's Spillway 2 is nearing completion and construction is scheduled for the summer of 2014. Physical and CFD model testing and application of the TDG predictive tool for the proposed structural modifications associated with Spillway 1 and the sluice gates are ongoing.

Conclusions

The results of the physical and CFD model studies, application of gas transfer theory, and prototype tests indicate that the addition of roughness elements can reduce TDG at dams with free-falling jets entering a deep plunge pool. TDG produced by a modified bay is measurably less than that produced by the same bay before modification, and less than standard operations that prioritize spill into a shallower portion of the plunge pool. Site-specific conditions may have an effect on performance, but tests at Cabinet Gorge Dam have validated the theory that TDG can be reduced by breaking up the jet before it enters the plunge pool.

Cooperation and data-sharing between utilities allowed the construction of a prototype to proceed much earlier than could have occurred otherwise. Lessons learned from the physical and CFD model testing of spillway modifications proposed for Boundary Dam were applied to the development of a prototype for Cabinet Gorge Dam. The Cabinet Gorge prototype results, as well as lessons learned with respect to the final design and construction of the modifications are being shared with Seattle City Light. Ultimately, both utilities have been able to make more-informed decisions with a higher degree of confidence, and at an earlier date than if either had chosen to work alone.

Acknowledgments

The authors would like to recognize Seattle City Light and Avista Corp. for supporting the pursuit of methods to improve water quality downstream from these two projects and allowing the data sharing that enabled advancing from conceptual design to prototype testing.We also thank the project staff at Seattle City Light, Avista, Hatch, AECOM, Alden Research Laboratory and the University of Minnesota, whose collective expertise in gas transfer, modeling, project operations, engineering and design is the foundation of this work.


Shari Dunlop is a hydraulic engineer and Alden Research Laboratory's project manager for the Boundary and Cabinet Gorge total dissolved gas (TDG) abatement projects. Guy Paul is a senior engineer with Avista Utilities and is project manager for the TDG abatement project at Cabinet Gorge Dam. Kimberly Pate is the dam safety supervising engineer at Seattle City Light and manages the TDG abatement program for Boundary Dam.

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