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Study Report

Aeration Methods to Enhance Summer Dissolved Oxygen in the Wallace Dam

Tailrace Area

Wallace Dam Hydroelectric Project FERC Project Number 2413

October 2017

GK6366/GA170090_Aeration Methods Study Report i 10.11.17

TABLE OF CONTENTS

EXECUTIVE SUMMARY ............................................................................................. iii

1.0 INTRODUCTION ................................................................................................ 1 1.1 Objectives .................................................................................................... 1 1.2 Study Area ................................................................................................... 1 1.3 Background Information .............................................................................. 2

2.0 STUDY METHODS ............................................................................................. 3 2.1 Aeration Methods Assessment .................................................................... 3 2.2 Oxygen Diffuser System Site Visit ............................................................. 4

3.0 RESULTS AND DISCUSSION ........................................................................... 5 3.1 Aeration Methods Assessment .................................................................... 5 3.2 Oxygen Diffuser System Site Visit ............................................................. 9

4.0 CONCLUSIONS ................................................................................................ 11

5.0 REFERENCES ................................................................................................... 12

TABLES

Table 1 Screening of Alternative Aeration Methods for Improving Summer DO in the Wallace Dam Tailrace

FIGURES

Figure 1 Project Boundary and Surrounding Area Figure 2 Georgia Power Water Quality Stations

APPENDICES

Appendix A Aeration Assessment for Conceptual Alternatives to Enhance Dissolved Oxygen in the Wallace Dam Tailrace

GK6366/GA170090_Aeration Methods Study Report ii 10.11.17

ACRONYMS AND ABBREVIATIONS

DO dissolved oxygen FERC Federal Energy Regulatory Commission ft feet GDNR Georgia Department of Natural Resources Georgia Power Georgia Power Company GEPD Georgia Environmental Protection Division m meters mg/L milligrams per liter PD plant datum PLP Preliminary Licensing Proposal SCADA supervisory control and data acquisition TDG total dissolved gases TVA Tennessee Valley Authority USACE U.S. Army Corps of Engineers

GK6366/GA170090_Aeration Methods Study Report iii 10.11.17

EXECUTIVE SUMMARY

Georgia Power conducted a study of aeration methods that could potentially enhance summer dissolved oxygen (DO) concentrations in the tailrace of the Wallace Dam Hydroelectric Project (FERC No. 2413). The 321.3-megawatt pumped storage project operates using Lake Oconee as the upper reservoir and Lake Sinclair, located immediately downstream, as the lower reservoir. The specific study objectives were to identify and evaluate, using data collected during the first season of study, technically feasible and cost-effective aeration methods for increasing DO levels in the tailrace area. To perform the aeration methods assessment, Georgia Power contracted the support of Richard J. (Jim) Ruane, Mark H. Mobley, and Paul J. Wolff, all highly experienced water quality management specialists who have designed aeration systems for similar large reservoir systems in the southeastern U.S.

The assessment included reviewing and evaluating existing and recently collected water quality data for Lake Oconee and the tailrace and available bathymetry data and modeling the water withdrawal zone at the turbine intake. A forebay bubble plume model was developed to evaluate the conceptual design for an in-lake forebay oxygen line diffuser system to add DO to Lake Oconee, which would increase DO concentrations in the releases from the turbine units. Turbine aeration modeling was conducted to assess the potential for turbine venting and the addition of forced air to turbine releases. In all, ten aeration approaches representing a full range of alternatives were reviewed and evaluated for their technical feasibility and efficacy for improving summer DO conditions in the Wallace Dam tailrace.

Two alternatives were identified as being technically feasible, including a forebay oxygen line diffuser system and draft tube aeration using compressed air. Conceptual designs were developed for these two alternatives for further evaluation and comparison as to installation and annual operation costs, practicality of system deployment, and other potential issues associated with system operation. In addition, Georgia Power conducted site visits of two oxygen line diffuser systems operated by the U.S. Army Corps of Engineers (USACE) at J. Strom Thurmond Lake and Richard B. Russell Lake on the Savannah River, which are similar in overall design to the concept evaluated for Wallace Dam. The site visits provided an opportunity to discuss operation and maintenance of the systems with USACE personnel.

The study concluded that a forebay oxygen line diffuser system is the most technically feasible and cost-effective approach for enhancing summer DO concentrations in the Wallace Dam tailrace. Although draft tube aeration using compressed air would also be technically feasible, the installation costs would be substantially higher and important

GK6366/GA170090_Aeration Methods Study Report iv 10.11.17

limitations for such a system would include potential total dissolved gas issues, reduced generating efficiency, and turbine maintenance issues.

GK6366/GA170090_Aeration Methods Study Report 1 10.11.17

1.0 INTRODUCTION

This report presents the findings of a study of aeration methods to enhance summer dissolved oxygen (DO) concentrations in the tailrace of the Wallace Dam Hydroelectric Project (FERC No. 2413) (Wallace Dam Project, the Project). The study was conducted in 2017 to gather additional information needed by Georgia Power Company (Georgia Power) to support development of the Preliminary Licensing Proposal (PLP) for Federal Energy Regulatory Commission (FERC) relicensing of the Project. The 321.3-megawatt Wallace Dam Project is a pumped storage project consisting of a dam, a powerhouse, and Lake Oconee on the Oconee River in Hancock, Putnam, Greene, and Morgan Counties, Georgia (Figure 1). The Project operates using Lake Oconee as the upper reservoir and Lake Sinclair, located immediately downstream, as the lower reservoir. Lake Sinclair is operated by Georgia Power as part of the separately licensed Sinclair Hydroelectric Project (FERC No. 1951). Georgia Power is not proposing to add capacity or make any major modifications to the Wallace Dam Project under the new license. The current license expires May 31, 2020. Georgia Power is using FERC’s Integrated Licensing Process at 18 Code of Federal Regulations Part 5.

1.1 Objectives

The specific objectives of the Aeration Methods Study were to identify and evaluate, using existing and recently collected data, technically feasible and cost-effective aeration methods for enhancing DO concentrations in the Wallace Dam tailrace. The assessment consisted of the following tasks:

• Review and evaluate existing water quality data for the reservoir and tailrace to characterize the water withdrawal zone at the turbine intakes, and model the withdrawal zone to relate forebay water quality to tailrace water quality.

• Model turbine aeration to assess the potential for turbine venting and the addition of forced air.

• Review available bathymetry and water quality data collected in the forebay and tailrace of Wallace Dam to model in-lake aeration approaches at the conceptual level of design.

1.2 Study Area

The study area included Wallace Dam, the lower end of Lake Oconee just upstream of the dam (the forebay), and the Wallace Dam tailrace area within the FERC project boundary (Figure 1).


1.3 Background Information

The project facilities and the pumpback and generation cycles of operation are described in the Water Resources Study Report (Georgia Power, 2016) and the Wallace Dam Operations Primer, which can be found in Appendix D of the Pre-Application Document (Georgia Power, 2015). The normal full pool elevation of Lake Oconee is 435 feet (ft) plant datum (PD).1 Lake Oconee typically starts near elevation 435 ft before the Wallace Dam generation cycle, and ends near elevation 433.5 ft. During pumpback, Lake Oconee typically refills up to elevation 435 ft. The average daily fluctuation is approximately 1.5 ft.

The Georgia Department of Natural Resources (GDNR) Environmental Protection Division (GEPD) classifies the water uses of Lake Oconee and the upstream end of Lake Sinclair within the project boundary as Recreation and Drinking Water (GEPD, 2015); these classifications also support the Fishing use. In addition to general criteria applicable to all waters, specific criteria apply to these water uses, including numeric criteria for DO, temperature, and other parameters. The applicable DO numeric criteria are a daily average of 5.0 milligrams per liter (mg/L) and no less than 4.0 mg/L at all times for water supporting warm-water species of fish.

The Water Resources Study Report (Georgia Power, 2016) provides a comprehensive analysis of the relationship between project operations and water quality in the reservoir and tailrace area based on vertical profile measurements in the reservoir and continuous water quality monitoring in the tailrace at Station OCTR (Figure 2). The tailrace data were aligned with project operational data for the same periods indicating how the turbines were operating. Continuous water quality monitoring in the tailrace from July 2015 to September 2016 found that generation at Wallace Dam resulted in daily DO depressions in the tailrace to values below 4.0 mg/L during June, July, and early August periods (Georgia Power, 2016).

In its Comments on Relicensing Study Reports dated January 20, 2017, the GDNR Wildlife Resources Division expressed concern about summer DO concentrations and requested that Georgia Power examine options for improving DO concentrations at the Project. Georgia Power (2017) filed a study plan for a newly proposed “Study of Aeration Methods to Enhance Summer Dissolved Oxygen in the Wallace Dam Tailrace Area” on February 20, 2017. FERC’s Director of the Office of Energy Projects issued a determination on requests for study modifications and new studies on March 17, 2017, which approved the new study (FERC, 2017). This study develops information needed for evaluating the potential impacts of continued project operation and the feasibility of enhancing summer DO concentrations in the tailrace area in the PLP and license application.

1 Plant datum = mean sea level (NAVD88) – 0.20 ft (+/- 0.01 ft).


2.0 STUDY METHODS

The study consisted of an aeration methods assessment of conceptual alternatives following the approved study plan (Georgia Power, 2017). In addition, Georgia Power conducted a site visit to oxygen line diffuser systems in use by the U.S. Army Corps of Engineers (USACE) in two large reservoirs on the Savannah River in Georgia and South Carolina.

2.1 Aeration Methods Assessment

To assess aeration methods that could be used to enhance DO concentrations in the Wallace Dam tailrace area, Georgia Power contracted the services of Richard J. (Jim) Ruane, M.S. of Reservoir Environmental Management, Inc.; Mark H. Mobley, P.E. of Mobley Engineering, Inc.; and Paul J. Wolff, Ph.D. of WolffWare, Ltd. This team of highly experienced water quality management specialists collaborated on the review and analysis of alternative aeration methods and produced the assessment report (Ruane et al., 2017) in Appendix A. The methods used are summarized below. The detailed methods are described their report in Appendix A.

2.1.1 Review of Water Quality Data

Water quality data for the reservoir and tailrace presented in the Water Resources Study Report (Georgia Power, 2016) and available bathymetry data from Navionics were reviewed and evaluated to characterize the water withdrawal zone at the turbine intake in Lake Oconee. DO and water temperature vertical profile data for the forebay from two 24-hour, reservoir-wide sampling events conducted on July 27-28 and August 15-16, 2016, were used to develop a withdrawal zone model. The USACE Waterways Experiment Station (WES) SELECT model (as modified using Tennessee Valley Authority [TVA] ADJUST), was used to predict the vertical extent of the forebay withdrawal zone within Lake Oconee under different flow profiles to predict DO concentrations in the penstock. The DO enhancement target was then defined to be the difference between inflow DO concentration and the DO numeric criteria for the tailrace.

2.1.2 Review of Alternative Aeration Methods

Ten aeration approaches representing a full range of alternatives were reviewed and evaluated as to their technical feasibility and efficacy for improving summer DO conditions in the Wallace Dam tailrace. Two alternatives were identified as being technically feasible, including a forebay oxygen line diffuser system and draft tube aeration using compressed air. Conceptual designs were developed for these two alternatives for further evaluation, as described in Section 2.1.5 below.


2.1.3 Modeling of Forebay Oxygen Line Diffuser System

A forebay bubble plume model was developed to evaluate the conceptual design for an in-lake forebay oxygen line diffuser system to add DO to Lake Oconee which would increase DO concentrations in the releases from the turbine units. The BPI Bubble Plume Model was applied to the Wallace Dam water quality profiles and operations data. The withdrawal zone model was used to develop design inputs for an oxygen line diffuser system.

2.1.4 Modeling of Draft Tube Aeration using Compressed Air

Turbine aeration modeling was conducted to assess the potential for turbine venting and the addition of forced air to turbine releases. The Discrete Bubble Model was set up for the Wallace Dam draft tubes using turbine design parameters, including design and geometry of draft tubes, tailrace elevation and bathymetry, gate settings, power output, and head, for the generation and the pumpback units. The model was calibrated using 2015 and 2016 water quality data. Model runs were conducted to simulate DO uptake over a range of anticipated conditions of operations and water quality. The modeling was used to determine how much airflow would be needed to achieve desired DO improvements over a range of inflow DO levels and temperatures, as well as an assessment of resulting total dissolved gases (TDG).

2.1.5 Analysis of Most Feasible Aeration Alternatives

Conceptual designs developed for a forebay oxygen line diffuser system and draft tube aeration using compressed air were evaluated and compared for installation and estimated annual operation costs, using available operations data and DO levels that represent a range of Wallace Dam operations. In addition, practicality of system deployment and maintenance were considered, based in part on Georgia Power’s site visit and discussions with USACE personnel regarding oxygen line diffuser systems currently in use during summer at two nearby reservoirs (see below), in drawing conclusions about the aeration technology most suitable for the Wallace Dam Project.

2.2 Oxygen Diffuser System Site Visit

The USACE Savannah District operates two oxygen diffuser systems at J. Strom Thurmond Lake and Richard B. Russell Lake on the Savannah River in Georgia and South Carolina. Mark Mobley, co-author of the aeration methods assessment for Wallace Dam, led the design of the diffuser line systems currently in use at both USACE reservoirs. Georgia Power held a conference call with USACE personnel from Richard B. Russell Dam on May 3, 2017, to learn more about the diffuser systems and USACE’s experience with their operation and maintenance. A site visit to both systems was conducted on August 3, 2017, when both systems were operating. USACE fisheries and operations personnel who manage the systems led the site visit.


3.0 RESULTS AND DISCUSSION

3.1 Aeration Methods Assessment

This section summarizes the detailed assessment developed by Ruane et al. (2017). Their report is provided as Appendix A.

3.1.1 Summer Vertical Profiles in Forebay

Georgia Power collected hourly vertical profiles of DO and temperature during two, 24-hour, sampling events at seven locations in Lake Oconee in summer 2016 (Georgia Power, 2016). As graphed by Ruane et al. (2017; Appendix A, Figure 1), hourly vertical profiles at the forebay location (Station OC1) during the July event showed complete vertical mixing during pumpback operations at night. During the interim period following pumpback, DO concentrations and temperature in the forebay gradually stratified to a depth of about 5 meters (m). During generation, the stratification became more pronounced and extended deeper, to about 8 to 12 m. The highest DO values and warmest temperatures were near the surface. The increasing DO concentrations near the surface during generation resulted from daytime photosynthetic activity and the inflow of reservoir water from upstream of the forebay.

This pattern of complete vertical mixing in the forebay during pumpback followed by gradually increasing stratification through the interim period and generation was repeated during the August 24-hour sampling event. Vertical stratification during generation extended to a depth of about 5 to 8 m (Figure 11a in Georgia Power, 2016). The DO gradient across the upper water column during generation was about 6 mg/L during the July event and about 3 mg/L during the August event, with the highest DO concentrations near the surface (Figures 10a and 11a in Georgia Power, 2016). Monthly vertical profiles measured in the forebay in June and July of 2015 and 2016 exhibited similar degrees of DO stratification that were consistent with this observed pattern (Figure 5a in Georgia Power, 2016).

3.1.2 Generation Withdrawal Zone

Ruane et al. (2017) modeled the withdrawal zone of the powerhouse intake by incorporating available bathymetry data and hourly vertical temperature profiles measured in the forebay during 24-hour sampling events in July and August 2016 (Appendix A). The modeling showed that the intake draws generation flows primarily from the upper layers of the forebay, even though the centerline of the intake is about 70 ft deep. As generation flows increase through the turbines, the withdrawal zone generally shifts to higher in the water column. Ruane et al. (2017) attribute this upward orientation of the withdrawal zone to the angle of the powerhouse section of the dam with respect to the main-channel, the V-shaped bathymetric profile upstream from the intake, and the greater cross-sectional area of available water in the upper water column.


Empirical observations corroborate the upward orientation of the withdrawal zone relative to the intake opening. The intake opening is located approximately between elevations 386 ft and 346 ft PD (49 to 89 ft below the normal full pool elevation of 435 ft). Ruane et al. (2017) showed that a DO value of 3.3 mg/L measured in the tailrace corresponded with the same value in the forebay at an elevation of 405 ft (30 ft below the surface and 19 ft above the top of the intake). Analysis of summer DO depressions observed in the tailrace during continuous monitoring similarly found that low DO values corresponded with values in the forebay at depths of 2 to 7 m and greater (7 to 23 ft below the surface) (Georgia Power, 2016). Thus, although the withdrawal zone modeling was a simplification of the actual water currents, its accuracy was considered sufficient to conclude that most of the water drawn into the turbine intakes originates near the surface layers of the forebay.

3.1.3 Review of Alternative Aeration Methods

Based on the daily vertical mixing of the forebay during summer operations and the upward orientation of the withdrawal zone, Ruane et al. (2017) evaluated available aeration methods for their technical feasibility and efficacy for improving summer DO concentrations in the Wallace Dam tailrace (Appendix A). Table 1 summarizes their analyses and findings as to which methods would be technically feasible for achieving the tailrace DO targets (daily average value of 5.0 mg/L, minimum instantaneous value of 4.0 mg/L). Although passive turbine venting is the most common aeration approach and is currently practiced at over 50 hydropower projects, it is not a technically feasible option for Wallace Dam. The turbines sit below the tailrace elevation and this creates pressures that would prevent air from being drawn into the draft tubes. Ruane et al. (2017) identified a forebay oxygen line diffuser system and draft tube aeration using compressed air as being the most technically feasible alternatives. These two approaches were evaluated in greater detail as summarized below.

TABLE 1 Screening of Alternative Aeration Methods for Improving Summer DO in the Wallace Dam Tailrace

Method Description and Analysis Technically Feasible?

Turbine venting (most common aeration approach)

Admits air to draft tubes immediately below Francis turbine units where low pressures (vacuums) can draw air into the discharge flow; not feasible at Wallace Dam because pressures within draft tubes would prevent air from being drawn into draft tubes

No

Forebay oxygen line diffuser system (second most common aeration approach)

Oxygen bubbles diffuse into reservoir through porous hose installed in the forebay; oxygen used instead of air to avoid TDG issues and to minimize size of diffuser system

Yes

Forebay surface water pumps or mixing units

Pumps blend high-DO water near surface with low-DO water in the withdrawal zone; would not be effective because withdrawal zone already oriented toward high-

No


TABLE 1 Screening of Alternative Aeration Methods for Improving Summer DO in the Wallace Dam Tailrace

Method Description and Analysis Technically Feasible?

DO water in upper water column and pumpback operation mixes water column vertically

Draft tube aeration using compressed air

Adds air to draft tubes immediately below turbine units using compressors; pressure and time of water passage through draft tubes would allow for acceptable gas transfer efficiency but drawbacks are high cost of compressors and engineered piping systems

Yes

Forebay aeration line diffuser system

Air diffuses into reservoir through porous hose along bottom; similar to forebay oxygen line diffuser system but uses compressed air instead of liquid oxygen; unlikely to be feasible due to TDG issues and complex operational procedures, targets may not be met, and requires much larger system than liquid oxygen (air contains only 20 percent oxygen)

No

Forebay mixing system Mixes the water column by upwelling bottom waters into the upper layers; mixing induced by pumpback operation precludes need for forebay mixing system

No

Forebay skimmer devices Placement of barrier (e.g., submerged weir, curtain) along channel bottom upstream of intake to limit withdrawal zone to high-DO water near surface; could result in anoxic products upstream of barrier, requiring additional aeration measures

No

Multi-level intake structure Intake allowing selective withdrawal from water levels having acceptable water quality; costly, insufficient to meet DO objectives after pumpback, and unlikely to work with existing pumpback turbines

No

Tailwater aeration structures Aeration weirs or structures that aerate water as it passes and drops in elevation to a lower pool; not feasible due to obstruction of pumpback flows

No

Side-stream supersaturation system Pumps side-stream of flow through oxygen transfer device (e.g., Speece Cone) where gaseous oxygen is injected and dissolves due to exceptionally high pressure and large gas-water interface, then flow blended back into waterbody; determined to be too costly in previous hydropower applications

No

In addition to any aeration methods used at Wallace Dam, the planned implementation of nutrient criteria for Lake Oconee would contribute to improvements in water quality and downstream DO levels in the future. GEPD (2013) intends to propose rules for specific numeric criteria for Lake Oconee for chlorophyll-a and nutrients to reduce nutrient over-enrichment of the reservoir from human activities and natural sources in the upstream watershed. Proposed rules are expected by 2019. After criteria are established and implemented, the reduced loading of nutrients and organic matter into reservoir inflows from


the upstream watershed are expected to lower oxygen demands in the reservoir forebay, which in turn contribute to elevated DO levels in the Wallace Dam tailrace during summer.

3.1.4 Forebay Oxygen Line Diffuser System

Forebay oxygen line diffuser systems are designed to place oxygen in a reservoir in areas of low DO and/or the intake withdrawal zone to meet a target DO concentration in the dam releases. Diffuser lines consisting of porous hose weighted to sink to depth, sometimes more than a mile long, are used to spread oxygen bubbles over a large area to obtain high oxygen transfer efficiencies. The diffusers are supplied with oxygen from an on-shore liquid oxygen storage facility. As the bubbles rise in the water column, they oxygenate water in the withdrawal zone above. The system is designed so that a sufficient volume of enhanced-DO water passes through the turbines and into the tailrace to meet the DO target. Oxygen line diffuser systems are currently being successfully operated at 15 hydropower projects across the U.S., including 11 applications in the southeastern U.S. The latter include systems at nine TVA reservoirs and two USACE reservoirs on the Savannah River. In addition, a new system is currently under construction at a reservoir in Alabama.

Based on withdrawal zone modeling of the reservoir forebay and exceedance analysis of continuous temperature and DO monitoring data from the tailrace, Ruane et al. (2017) developed a conceptual design for a forebay oxygen line diffuser system at Wallace Dam (Appendix A). The analysis showed that a diffuser system with a maximum oxygen capacity of 200 tons per day would be required for worst-case conditions but that median oxygen use would be about 60 tons per day. The conceptual design includes two sets of diffuser lines to provide operational flexibility for oxygen placement in the forebay. The two sets would be installed in sequence and extend longitudinally about 0.7 mile upstream of the dam. The upstream set of diffusers would be operated to inject a low level of oxygen continuously to maintain an oxygenated forebay volume during non-generation. The set closest to the dam would be operated to boost oxygen output during generation. The diffuser lines would be placed at various levels above the bottom to efficiently aerate the withdrawal zone.

The estimated capital cost for installing a forebay oxygen line diffuser system at Wallace Dam is $4,699,000 (Appendix A; Table 1). This cost includes the diffuser lines, supply lines, and the liquid oxygen storage and supply facility. Based on the tailrace DO monitoring data for summer 2015 and 2016, annual liquid oxygen costs would range on the order of $150,000 to $240,000 to increase summer tailrace DO levels to a daily average of 5.0 mg/L and an instantaneous level of 4.0 mg/L. Bulk liquid oxygen suppliers could maintain the cryogenic components of the supply facility, monitor the levels in storage tanks, and send truck deliveries as needed to meet demand. Based on the 2015 and 2016 oxygen demand scenarios, up to eight or nine tanker trucks would need to make deliveries of liquid oxygen each week during peak oxygen demand periods.


Ruane et al. (2017) estimate that field studies and water quality/aeration modeling costing up to $750,000 would be needed to complete the final design of a forebay oxygen line diffuser system for the Wallace Dam Project.

3.1.5 Draft Tube Aeration Using Compressed Air

Draft tube aeration using compressed air forces air into the draft tube immediately below the turbine units, where relatively low pressures allow air to be added efficiently. The air-water mixture passes through the draft tubes prior to being released to the tailrace. This approach is technically feasible at Wallace Dam because the pressure and time of water passage through the draft tubes would allow for acceptable gas transfer efficiency (Ruane et al., 2017).

Ruane et al. (2017) evaluated this option using the Discrete Bubble Model to predict the airflows required to achieve a daily average DO concentration of 5.0 mg/L in the tailrace, as well as the resulting TDG (Appendix A). The modeling also was used to predict compressor operations that would enable releases to achieve the DO target over a range of unit operations. Air bubbles formed from the use of compressed air result in unit efficiency and energy losses. TDG concentrations can also become excessive. Thus, airflows were moderated in the modeling to reduce energy losses and levels of TDG.

The estimated capital cost of installing a draft tube aeration system using compressed air at Wallace Dam is $15,190,000 (Appendix A; Table 2). The estimate assumes a system capacity of 20,000 standard cubic ft per minute of air flow per turbine unit and that each unit would be equipped with two forced air blowers. In addition to reduced unit efficiency and power losses from injecting air, other potential issues for the use of this aeration method at Wallace Dam could include elevated levels of TDG in generation releases, maintenance costs, and noise of the blowers.

3.2 Oxygen Diffuser System Site Visit

The oxygen line diffuser systems operated by USACE on the Savannah River are similar in overall design to the concept evaluated by Ruane et al. (2017) for Wallace Dam. A forebay, hypolimnetic oxygen diffuser system is in use just upstream of Richard B. Russell Dam, which like Wallace Dam, operates in a pumped storage mode. The purpose of this system is to meet a target DO concentration in the dam releases. The system typically operates from July to October. Another oxygen diffuser system is located at J. Strom Thurmond Lake about 5 miles upstream of J. Strom Thurmond Dam (Mobley et al., 2012). This system places oxygen in the reservoir at the specific temperature range suitable for striped bass habitat from June to October.

During the site visit, Georgia Power observed the on-shore elements of each oxygen line diffuser system, including the equipment pads, liquid oxygen tanks, piping, vaporizers, flow control valves, and the remote SCADA operation control system at Richard B. Russell Dam.


Each oxygen line diffuser system uses two, horizontally mounted liquid oxygen storage tanks. The tanks are replenished daily during the peak oxygen-demand periods. Two tanker trucks were observed making deliveries to the Richard B. Russell system at the time of the site visit.

The J. Strom Thurmond Lake oxygen line diffuser system, which was completed in 2011, uses air vaporizers to warm the liquid oxygen to gaseous oxygen. The gaseous oxygen moves into the supply pipes and diffuser lines because of the liquid oxygen storage tank pressure. Although occupying more land space than the water-bath vaporizer used at the older Richard B. Russell system, the air vaporizers do not use water and are virtually maintenance-free. In contrast, the water-bath vaporizer at Richard B. Russell sits below ground level and relies on a small but continuous supply of water from a dedicated intake on an adjacent cove of the reservoir. Maintenance issues include occasional dredging of the cove, servicing of pipes and pumps in the reservoir, and vaporizer shut-off during power outages.

Both systems use diffuser lines consisting of porous hose in 10- to 12-foot sections which can be replaced if leaking or torn. The Richard B. Russell system uses up to ten diffuser lines, each less than a mile long and placed about 15 to 20 ft above the bottom of the forebay. DO levels are monitored by drawing water off the penstocks to a wet well for measurement. The J. Strom Thurmond system uses up to nine diffuser lines, each about 1,400 ft long and placed at four elevations to target the aeration of 18-24°C water for the enhancement of striped bass habitat in a reach of the reservoir about 5 miles upstream of the dam. Although the diffuser lines are shorter than at Richard B. Russell, they inject oxygen bubbles at higher flow rates.

The site visit was beneficial in affording Georgia Power the opportunity to view the layout and land requirements of the on-shore liquid oxygen storage and vaporizer facilities, ascertain the nature and potential extent of impacts associated with construction and operation, discuss operation, performance, and maintenance of the systems with USACE personnel, and gain important insight into the practicality of system deployment.


4.0 CONCLUSIONS

Based on the analysis developed by Ruane et al. (2017), a forebay oxygen line diffuser system is likely to be the most technically feasible approach for enhancing summer DO concentrations in the Wallace Dam tailrace. Application of this technology would involve installing multiple diffuser lines of up to a mile long in the forebay of Wallace Dam and at up to three layers of depth within the reservoir to oxygenate a sufficient volume of the withdrawal zone. The installation costs would total about $4,699,000 in capital costs. Annual liquid oxygen costs could range from $150,000 to $240,000. Draft tube aeration using compressed air would also be technically feasible, but the installation costs would total about $15,190,000, or about $10 million more than a forebay oxygen line diffuser system. In addition, important limitations for a forced air system would include potential TDG supersaturation, reduced unit generating efficiency and its impacts to power generation, and turbine maintenance issues.

Georgia Power’s site visit of two USACE applications of oxygen line diffuser systems on the Savannah River revealed that an air vaporizer system is preferable to a water-bath vaporizer because of the ease of system control, minimal maintenance, and avoidance of operations and maintenance issues associated with the use of a surface water supply source.


5.0 REFERENCES

Federal Energy Regulatory Commission (FERC). 2017. Determination on Requests for Study Modifications and New Studies – Wallace Dam Pumped Storage Project. Project No. 2413-117, Georgia Power Company. Office of Energy Projects. March 17, 2017.

Georgia Environmental Protection Division (GEPD). 2013. Georgia’s plan for the adoption of water quality standards for nutrients. Revision 2.0. Georgia Department of Natural Resources. Atlanta, Georgia.

Georgia Environmental Protection Division (GEPD). 2015. Rules and regulations for water quality control, Chapter 391-3-6. Revised May 2015. Georgia Department of Natural Resources, Atlanta, Georgia.

Georgia Power Company (Georgia Power). 2015. Pre-application Document, Wallace Dam Hydroelectric Project, FERC Project Number 2413. Prepared with Southern Company Generation Hydro Services, Geosyntec Consultants, and CH2M HILL. February 2015.

Georgia Power Company (Georgia Power). 2016. Water Resources Study Report, Wallace Dam Hydroelectric Project, FERC Project Number 2413. Prepared with Southern Company Generation Hydro Services and Geosyntec Consultants. November 2016.

Georgia Power Company (Georgia Power). 2017. Study Plan, Study of Aeration Methods to Enhance Summer Dissolved Oxygen in the Wallace Dam Tailrace Area, Wallace Dam Hydroelectric Project, FERC Project Number 2413. Prepared with Southern Company Generation Hydro Services, Reservoir Environmental Management, Inc., Mobley Engineering, Inc., and WolffWare, Ltd. February 2017.

Mobley, M. H., P. Gantzer, G. E. Hauser, R. J. Ruane, and J. A. Sykes. 2012. Oxygen diffuser system to create fish habitat and enhance hydropower water quality in J. Strom Thurmond Reservoir. HydroVision International 2012, Louisville, Kentucky.

Ruane, R. J., M. H. Mobley, and P. J. Wolff. 2017. Aeration Assessment for Conceptual Alternatives to Enhance Dissolved Oxygen in the Wallace Dam Tailrace. Reservoir Environmental Management, Inc., Mobley Engineering, Inc., and WolffWare, Ltd.

FIGURES

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Waterfront Marina

Great Waters Marina

Blue Springs Marina

Lawrence Shoals Park

Hwy 44 Public Fishing

Sugar Creek Boat Ramp

Long Shoals Boat Ramp

Dyar Pasture Hunt Camp

Greene County Boat Ramp

Boathouse at Harbor Club

Reynolds Plantation Marina

Oconee Outdoors and Marina

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Apalachee Bait Shop & Fish Camp

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Lake Sinclair

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Project Boundary and Surrounding AreaWallace Dam Project

(FERC No. 2413)

0 2 41 Miles

Interstate HighwayU.S. HighwayState HighwayMajor RoadsLocal Streets

Railroads (Local)DamRivers/CreeksLakeProject Boundary

Towns/CitiesCounty BoundaryState Managed LandsNational Park or ForestGolf Course

Recreation Access Point

Forest Service

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Georgia Power

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Putna

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The Landing Marina

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Sugar Creek Marina

North Shore Resort

Waterfront Marina

Great Waters Marina

Blue Springs Marina

Lawrence Shoals Park

Hwy 44 Public Fishing

Sugar Creek Boat Ramp

Long Shoals Boat Ramp

Dyar Pasture Hunt Camp

Greene County Boat Ramp

Boathouse at Harbor Club

Reynolds Plantation Marina

Oconee Outdoors and Marina

Redlands Access Area

Hwy 44 Public Fishing (Jerry’s)

Apalachee Bait Shop & Fish Camp

Lake Oconee

Lake Sinclair

REDLANDS WMA

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Oak W

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Rd

Greensboro Rd

Harmony Rd

Farmington Rd

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New Phoenix Rd

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Georgia Power Water Quality StationsWallace Dam Project

(FERC No. 2413)

0 2 41 Miles

Georgia Power Water Quality StationTailrace BuoyInterstate HighwayU.S. HighwayState Highway

Major RoadsLocal StreetsRailroads (Local)DamRivers/CreeksLake

Project BoundaryTowns/CitiesCounty BoundaryState Managed LandsNational Park or ForestGolf Course

Recreation Access Point

Forest Service

Public/Private

Georgia Power

OCTRTailrace Fishing Area

APPENDIX A

Aeration Assessment for Conceptual Alternatives to Enhance Dissolved Oxygen in

the Wallace Dam Tailrace

Page i

Aeration Assessment for Conceptual Alternatives to Enhance Dissolved Oxygen in the Wallace Dam

Tailrace

Prepared for Georgia Power Company

Prepared by

Richard J. Ruane, M.S.EHE, Reservoir Environmental Management, Inc.

Mark H. Mobley, P.E., Mobley Engineering, Inc.

Paul J. Wolff Ph.D., WolffWare, Ltd.

September 27, 2017

Page ii

Table of Contents

Table of Contents .......................................................................................................................................... ii

Introduction .................................................................................................................................................. 1

Review of Wallace Dam Water Quality Data ................................................................................................ 1

Review of Aeration Methods and Considerations for Selecting the Best Method for Improving DO in Wallace Dam Tailrace .................................................................................................................................... 8

DO enhancement alternatives that have been applied at other hydropower plants include the following: ...................................................................................................................................................................... 8

Analysis of the Most Feasible Aeration Alternatives for Improving DO in the Wallace Dam Tailrace ....... 10

Forebay Oxygen Line Diffuser System (Approach 2)................................................................................... 11

Application of the FB O2 LD to Lake Oconee .................................................................................... 17

Compressed Air Addition to Draft Tubes (Approach 4) .............................................................................. 23

Application of DBM Model to Predict Airflows and Resulting TDG .................................................. 24

TDG Exceedance Considerations ....................................................................................................... 29

Cost of Forced Air System at Wallace Dam ....................................................................................... 30

Cost Evaluation for Forced Air and Forebay Oxygen Line Diffuser Systems at Wallace Dam—Capital and Annual Operating Costs .............................................................................................................................. 32

Summary and Conclusions .......................................................................................................................... 33

References .................................................................................................................................................. 33

Aeration Assessment for Alternatives to Enhance Dissolved Oxygen in the Wallace Dam Tailrace

Introduction

Georgia Power Company (GPC) is assessing aeration methods that can be used to enhance dissolved oxygen (DO) in the tailrace of the Wallace Dam Hydroelectric Project (Federal Energy Regulatory Commission [FERC] Project Number 2413). DO in the Wallace Dam tailrace has been measured at levels less than the water quality criteria set by the Georgia Environmental Protection Division (GA EPD). Those criteria are 4 milligrams per liter (mg/L) instantaneous and 5 mg/L daily average. GPC is evaluating aeration approaches that may enhance summer DO concentrations in the tailrace.

This assessment includes: (1) a review of available water quality data; (2) a review of aeration methods that may improve DO; and (3) an analysis of the most feasible aeration alternatives for improving DO in the Wallace Dam Tailrace. This report is a component of the GPC study report titled “Study of Aeration Methods to Enhance Summer Dissolved Oxygen in the Wallace Dam Tailrace Area“ that will be filed with FERC by October 11, 2017.

Review of Wallace Dam Water Quality Data

GPC completed a water resources study in 2016 that included vertical water quality profiles of Lake Oconee and continuous DO monitoring in the Wallace Dam tailrace (GPC, 2016).

In that study, GPC collected hourly vertical profiles of DO and temperature for two 24-hour periods (July 27-28 and August 15-16) in the forebay and at six other locations in the reservoir. Presented here (Figure 1) are the results for the July study event at the forebay location (Station OC1) that show the following: (1) the forebay mixed vertically during pumpback operations that occurred during the hours of 12:45 a.m. to 7:15 a.m. (between hours 15 and 21 on the x axis in Figure 1); and (2) DO and temperature gradually stratified to about 5 m deep until generation with turbine releases occurred during the hours of 1:00 p.m. to 6:30 p.m. (between hours 4 and 10 on the x axis), when stratification dropped to about 8 to 12 m deep. The upper 10 meters of the forebay accumulated higher DO levels during the daytime

stratification period as a result of photosynthetic processes within the water column, as well as surface water drawn from upstream of the forebay area by generation operations.

Figure 1. Contour graphs of hourly temperature and DO over 24 hours in the forebay on July 27-28, 2016.

The withdrawal zone was modeled using 1) the U.S. Army Corps of Engineers Waterways Experiment Station (WES) SELECT model (Davis et al, 1987 and 1992) to predict the vertical extent of withdrawal and 2) the Tennessee Valley Authority (TVA) ADJUST model (Wunderlich et al, 1969) to account for forebay bathymetry. The modeling was applied to selected vertical temperature profiles measured hourly and at every 1 m of depth on July 27 and August 15, 2016, as referred to in the legend for Figure 2. Each plotted line in Figure 2 represents the modeled flow by depth for a given hourly profile measurement from one of those dates. The legend provides 1) a list of dates and times when profile data were collected and 2) the powerhouse flow levels modeled using the profiles collected at specified times.

The 24-hour sampling data were used in the withdrawal zone model to generate multiple flow profiles. These modelled profiles were then used to determine from which forebay depth the

turbines pulled water. Withdrawal zone analyses show that during generation the turbines draw most of the water from the upper layers (epilimnion) of the forebay in Lake Oconee (Figure 2) even though the centerline of the intake is about 70 feet deep. The profiles above the top of the intake opening show a high degree of flow variation that indicates water movement that is proportional to the amount of generation flow. Turbine intake openings for the Wallace units range from a bottom elevation at 346 ft (105.5 m) to the centerline at 366 ft (111.6 m) to the top elevation at 386 ft (117.7 m). In addition, peak forebay flows typically occur at elevations ranging from 405 to 430 ft (123.5 to 131.1 m) and up to the surface. Elevation levels of peak flows are proportional to peak flow magnitudes. That is, as generation flows increase through Wallace Dam, the turbines tend to withdraw an increasing proportion of flow from the upper water column.

Figure 2. Withdrawal zone vertical flow profiles showing most of the inflow to Wallace Powerhouse intakes is drawn from above 386 ft MSL, the upper elevation of the intake opening. The profiles were developed using the withdrawal zone model WES SELECT with TVA ADJUST to account for forebay profiles of temperature and DO as well as bathymetry.

The vertical withdrawal zones that predominately extend to the epilimnion of the forebay in Lake Oconee are attributed to the angle of the powerhouse with respect to the angle of the main channel in the forebay as well as the bathymetric contours upstream from the intake (Figure 3). The red line in Figure 3 indicates the direction of flow that is induced by turbine withdrawals until this flow pattern is interrupted by the natural bathymetric features upstream

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7/27/16 13:45 - 22,283 cfs7/27/16 15:00 - 22,243 cfs7/27/16 16:00 - 37,131 cfs7/27/16 17:45 - 22,435 cfs7/27/16 18:00 - 15,060 cfs7/27/16 18:00 - 37,000 cfs8/15/16 14:00 - 22,000 cfs8/15/16 14:00 - 37,000 cfs8/15/16 15:00 - 22,000 cfs8/15/16 15:00 - 37,000 cfs8/15/16 16:00 - 22,000 cfs8/15/16 16:00 - 37,000 cfs8/15/16 17:00 - 22,000 cfs8/15/16 18:00 - 22,000 cfs8/15/16 19:00 - 22,000 cfs8/15/16 20:00 - 22,000 cfs

Approx. Top of Intake Opening

Approx. Centerline of Opening

Approx. Bottom of Intake Opening

from the intakes. The green line indicates the deeper part of the channel where water is not predominately withdrawn from the forebay. An additional factor that significantly contributes to this withdrawal from the upper layers of Wallace is that the cross-sectional area of the water column is much greater in the upper layers of the forebay than near the bottom of the water column.

Figure 3. Contour graph of the forebay showing the projected red line of inflow to the Wallace Dam turbine intakes and the associated influence of bathymetric contours on withdrawal zone currents being dominated by near-surface flows. Withdrawal zone flows also travel in the direction of the green line, but the amount of this flow is much less compared to the flow along the direction of the red line.

Figure 4 shows a vertical profile of DO in the forebay of Wallace Dam during generation on July 27, 2016, at 15:57 hours (top graph) and a plot of tailrace DO measurements and generation flow from 12:00 to 19:00 hours on the same date (bottom graph). Observed DO in the tailrace at 16:00 hours was 3.3 mg/L (bottom graph), while DO in front of the intake at the same time ranged from about 0.5 to 2.7 mg/L and the centerline DO was about 1.8 mg/L (top graph). The DO value 3.3 mg/L in the tailrace corresponded with the same value at elevation about 405 feet in Lake Oconee, about 19 feet higher in the water column than the top of the intake opening. The withdrawal zone model applied here is a simplification of the actual water currents, but its accuracy is sufficient to conclude that most of the water drawn into the turbine intakes originates in the epilimnion of the forebay. Actual withdrawal zone flows can be measured in the reservoir using an acoustic Doppler current profiler (ADCP) to develop water current profiles. Further data collection and modeling would be conducted to refine the design before implementation.

Figure 4. Withdrawal zone expansion upward in the water column allows water with higher DO to enter the intakes of the units (top figure). The bottom figure presents the tailwater DO data collected over the range of generation flows—as generation increased tailwater DO gradually increased as more water was withdrawn from the upper layers of the forebay.

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Wallace Tailwater - 7/27/16

Tailwater DO

Generation Flow

The GPC Water Resources Study Report (GPC 2016) presented continuous DO and temperature data in the tailrace of Wallace Dam reservoir in 2015 and 2016. Figure 5 presents DO levels in the tailrace as a function of generation operations for July 2 through July 6. DO in the tailrace typically decreases when generation occurs and increases during off-generation and when pumpback operations occur. DO generally increases during pumpback operations.

Figure 5. Sample of continuous DO measurements taken in 2015 and 2016 to study the levels and variability of DO with operations.

Review of Aeration Methods and Considerations for Selecting the Best Method for Improving DO in Wallace Dam Tailrace

DO enhancement alternatives that have been applied at other hydropower plants include the following:

1. Turbine venting aeration (the most common aeration approach at hydropower plants)—This approach admits air to draft tubes immediately below Francis turbine units where low pressures (i.e., vacuums) can draw air into the water being released to the tailrace and tailwater. Applications of this approach are limited to site-specific characteristics at each hydropower plant, e.g., depth and length of draft tube, height of turbine wheel above tailrace, DO level of water drawn from the forebay and DO requirement in the tailrace, total dissolved gases considerations, and power losses. This approach is not feasible at Wallace Dam because pressures in the draft tubes prevent air from being drawn into the draft tubes.

2. Forebay oxygen line diffuser system (FB O2 LD) (the second most used approach at hydropower plants)—This approach is commonly applied at projects where turbine venting is not feasible. It involves installing porous hose along the bottom of the forebay so that oxygen can be released through small bubbles that diffuse into the lake bottom waters where DO levels are low and located in the withdrawal zone for hydropower operations. Oxygen is used instead of air to avoid total dissolved gas (especially nitrogen) issues and to reduce the size of the diffuser system by 80%. This approach is likely one of the most technically feasible alternatives at Wallace Dam.

3. Forebay surface water pumps or mixing units (SWPs)—This approach blends water with high DO near the surface of the forebay with water having low DO in the withdrawal zone for the turbine intakes. These units are designed to move large volumes of water and have relatively low operational costs. They are used in conjunction with forebay oxygen diffuser systems to reduce the amount of oxygen required to maintain tailrace DO objectives. They could be useful at Wallace Dam when ambient DO is high in the forebay as shown over the hours 2-10 PM in Figure 1. However, at Wallace Dam the withdrawal zone already draws a large percentage of inflow from the epilimnion so SWPs would have less benefit than what could be expected at projects that draw most of their water from the bottom of the forebay. Further, they may not be feasible considering the turbulence in the forebay induced during pumpback operations.

4. Draft tube aeration using compressed air—This approach adds air to draft tubes immediately below Francis or Kaplan turbine units where relatively low pressures allow air to be added efficiently using compressors as the air-water mixture passes through draft tubes prior to being released to the tailrace and tailwater. Applications of this approach are limited to site-specific characteristics at each hydropower plant; e.g., depth and length of draft tube, DO level of water drawn from the forebay and DO requirement in the tailrace, total dissolved gases (TDG) considerations, and power losses. This approach is technically feasible at Wallace Dam because pressure and time of water passage through the draft tubes provide opportunity for acceptable gas transfer efficiency. A drawback for compressed air systems is the high cost of the compressors combined with the engineered piping systems for supplying airflow to draft tubes for each turbine unit.

5. Forebay aeration line diffuser system—This system is similar to the FB LD O2 system (approach 2 above) except compressed air is used instead of liquid oxygen. This approach is likely not feasible due to concerns regarding TDG and more complex operational procedures that may not be sufficiently responsive to achieve desired DO improvements for Wallace Dam. Additionally, the diffuser system would be approximately five times larger than the FB LD O2 system since air contains only 20% oxygen. Implementing the system would require more data collection and modeling than would be necessary for implementing the FB O2 LD system.

6. Forebay mixing systems—These systems involve mixing the water column of the forebay by upwelling bottom water layers into the upper layers of the water column so that water quality can be improved. They can minimize accumulation of water near the sediments that could contain anoxic products with high DO demands as well as hydrogen sulfide. This approach could serve as a supplemental system if needed to enhance the primary system; however, mixing induced by Wallace pumpback is likely to eliminate the need for a forebay mixing system.

7. Forebay skimmer devices (submerged weirs, curtains, etc.)—This approach involves placing a barrier to flow along the channel bottom upstream from the turbine intakes so that water drawn into the turbines comes from the near-surface water layers that contain higher DO levels. This approach could serve as a supplemental system if needed to enhance a primary system; however, this approach could result in anoxic products (e.g., hydrogen sulfide) upstream from skimmer structure and require additional aeration measures.

8. Multi-level intake structures—This approach allows operators to select water layers in the forebay that contain acceptable water quality. These are too costly, and not sufficient to meet DO objectives at Wallace Dam after pumpback when the water

column is mixed. These also are unlikely to work with Wallace Dam’s existing pumping turbines.

9. Tailwater aeration approaches (aeration weirs, aeration structures)—This approach employs structures that aerate water as it passes and drops in elevation to a lower pool. These are not feasible due to pumpback operations since these structures block free-flowing waterways.

10. Side-stream-supersaturation systems (e.g., employing a Speece cone) that can add DO to forebays and tailraces—These systems require high pressure, engineered piping, pumping at high pressure for the side-stream flow, oxygen, and backup systems to cover “outages.” These systems have been determined to be too expensive in previous hydropower applications.

In addition to these aeration considerations, additional measures are expected to be implemented by the Georgia Department of Natural Resources Environmental Protection Division (EPD). EPD intends to propose chlorophyll and nutrient criteria for Lake Oconee by 2019 to reduce nutrient over-enrichment of the reservoir from human activities and natural sources in the upstream watershed. Reduction of excessive levels of organic matter and nutrients in reservoir inflows from the watershed would reduce DO demands that can impact DO in the reservoir forebay that in turn cause low DO in the tailrace.

Turbine venting was first practiced in the 1940s on the Fox River in Wisconsin; however, DO uptake was marginal compared to today’s systems and energy losses were significant. Alabama Power Company (APC) developed advanced technical approaches to increase DO uptake in the 1970s and developed a model to simulate aeration in the 1980s. Ruane and McGinnis in 2003/2004 advanced the APC model by adding the ability to consider air bubble size in the draft tube, and the accuracy of DO uptake predictions improved significantly such that draft tube designs and aerating wheels for new or updated units could be developed with sufficient confidence. Today, turbine venting is practiced at over 50 hydropower projects. Unfortunately, turbine venting is not feasible at Wallace Dam since the units sit below tailrace elevation. Also, total dissolved gas (TDG) is a potential concern since nitrogen gas in air bubbles is added as oxygen is added to the releases.

Analysis of the Most Feasible Aeration Alternatives for Improving DO in the Wallace Dam Tailrace

The approaches that are most technically feasible at Wallace Dam are FB O2 LD (Approach 2) and compressed air added to the draft tubes (Approach 4). These approaches are discussed in

detail below. Inflow DO levels, temperature characteristics in the forebay, and hydropower releases need to be further characterized over the full range of hydrologic conditions for designing and operating both aeration alternatives.

Forebay Oxygen Line Diffuser System (Approach 2)

Line diffusers are based on a design that was originally developed by TVA in the 1990s for hydropower reservoir release improvements. FB O2 LD systems are currently being successfully operated at 15 hydropower projects across the U.S., as well as over 21 lakes and water supply reservoirs. Of those, nine are systems that TVA has installed at reservoirs in the southeastern U.S. A new application is currently under construction in Alabama. Figure 6 illustrates the approach for a typical hydropower application.

Figure 6. Illustration of the application of a typical FB O2 LD system.

As shown in Figure 6, diffusers are placed below the areas of low DO and the turbine withdrawal zone. Since the diffusers are in the forebay and external of the hydropower plant, they cause no disruption of hydropower operations during installation or head loss during

diffuser operation. While air can be used in place of oxygen, applications at hydropower reservoirs are limited to those where only marginal increases in DO (e.g., 1-2 mg/L) are sought since air is about 80% nitrogen and can result in excessive dissolved nitrogen that can in turn cause excessive TDG levels. Therefore, because a larger DO increase is preferable at Wallace Dam, a system whereby pure O2 is injected is considered the best alternative.

The diffusers are constructed of robust materials: high density polyethylene (HDPE) piping, porous hose, concrete anchors and stainless steel connecting components, as shown in Figure 7. The porous hose is manufactured from linear low density polyethylene and rubber from recycled car tires. This hose is capable of distributing oxygen in reservoirs for up to 10 years without excessive degradation or clogging. All HDPE connections are joined by a heat fusion procedure, including all anchor and gas piping connections. Anchor tethers are constructed of nylon coated stainless steel cable. Diffuser lines are deployed and retrieved without need for divers via the use of the buoyancy pipe (shown in Figure 7) that can be filled with air or oxygen to float or water to sink.

Diffuser lines provide a uniform bubble pattern along the full length of the porous hose sections. The diffuser can deliver large quantities of oxygen with maximum oxygen transfer efficiency by providing an economical means to spread the oxygen into a large water volume in the reservoir. Diffuser lines are often more than a mile long. The buoyant oxygen bubble plume is spread over the long lines to avoid sediment disturbance.

Figure 7. Components and features of the line diffuser.

Forebay oxygen diffusers are supplied with oxygen from a liquid oxygen storage facility on shore. Liquid oxygen (LOx) is delivered by truck to insulated storage tank(s) at the facility. Ambient air vaporizers warm the oxygen to gaseous state. The expansion of oxygen from liquid to gas pressurizes the storage tank and provides flow to the diffusers at depth without the need for pumps or compressors. A flow control manifold distributes the oxygen gas flow to the desired diffusers.

In operation, the diffusers create a bubble plume in the reservoir that entrains water as the bubbles rise. Oxygen is adsorbed by the entrained water as the plume rises. In summertime stratified conditions, the water near the bottom of the reservoir is significantly cooler than near the surface. The cooler water is denser than that near the surface so cool water entrained by the plume will lose momentum and eventually detrain from the plume and fall back to an elevation of equal density, as shown in Figure 8. The bubbles will continue toward the surface and entrain and detrain additional water volumes in secondary plumes. The oxygenated water will spread laterally away from the plume as it detrains. The system is designed such that plume detrainments occur in the area of low DO in the withdrawal zone of the turbines, creating the volume of enhanced DO water that runs through the turbine units into the tailrace. The headwater DO also improves locally.

Graphic courtesy of Dr. Daniel McGinnis, Leibniz Institute of Marine Sciences Kiel, Germany

Figure 8: Plume detrainment in a stratified reservoir.

Figures 9 and 10 illustrate how oxygen can be diffused into specific layers and volumes of a reservoir where it is required for meeting design objectives and purposes. Figure 9 presents field-measured DO in fields of red (low DO) and blue (high DO) with plume model results overlaid for Tillery Hydroelectric Project. The orange bubble plume model representation has a white line for the elevation of maximum plume rise and a black line for fall back elevation (the elevation where the plume water and ambient water are of equal density). In this figure, clear layers of increased DO are visible in the reservoir at the elevations predicted by the bubble plume model, including a second plume after the first detrainment. The resulting oxygenated volume extends several thousand feet upstream of the hydropower intakes.

Figure 9: Oxygenated forebay volume and plume model predictions for Tillery Hydroelectric Project.

Oxygen plumes can be designed to add oxygen to selected layers of a reservoir when the layers have different temperatures and therefore densities within the water column. Figure 10 presents field measurements and plume model predictions for diffusers designed to place oxygen at different elevations at J. Strom Thurmond Reservoir.

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Application of the FB O2 LD to Lake Oconee

A conceptual design was prepared for application in Lake Oconee at Wallace Dam and is presented herein.

The capacity of the system was determined using the available 2015 and 2016 DO and temperature monitoring data collected in the tailrace.

Using the 2015 and 2016 operations and tailwater temperature and DO monitoring data, the oxygen requirements for a FB O2 LD system were assessed using an exceedance analysis based on hourly data. The results are plotted in Figure 11. This graph shows that an aeration system with a capacity of 200 tons/day is required for worst-case conditions based on 2015 and 2016 data (that experienced drought conditions and low DO conditions). The median oxygen use (50 percent exceedance) would be about 60 tons/day. Oxygen would be injected at a low rate at all times and supplemented with additional oxygen pulses during turbine operations. A conceptual diffuser system layout is shown in Figure 12. The use of two sets of diffuser lines as shown would provide operational flexibility for oxygen placement in the reservoir. It is likely that the layout would place the diffuser lines at different depths in the water column. The layout is subject to change upon further analysis.

The estimated capital cost for an FB O2 LD system is presented in Table 1. This cost includes the liquid oxygen storage and supply facility, but not the operations and maintenance costs of the system. The monthly and annual use and cost for oxygen is presented in Figures 13 and 14. Based on the tailrace DO monitoring conducted in 2015 and 2016, the peak monthly oxygen requirement could total up to 700 tons of oxygen or more (Figure 13) with annual oxygen costs varying on the order of $150,000 to $240,000 per year. Control system operation will realistically require some buffer above the target setting to allow for response time and increase oxygen costs by some percentage. Different hydrological and water quality conditions would also change oxygen costs. The two years used for the cost evaluation may not be worst case conditions. The evaluation does not yet take into account the full effect of pumpback with FBO2 (probably a cost decrease).

Electrical use at the facility is only for lighting and control. Maintenance requirements are minimal, requiring clearing of debris from the oxygen supply facility and replacement of the diffuser porous hose approximately every 10 years. Bulk gas suppliers under contract to provide oxygen will typically maintain the cryogenic components of the oxygen supply facility and monitor the LOx level in the storage tank(s) to send truck deliveries as needed to meet demand. Lighting and 24-hour truck access will typically be required. Under the 2015 and 2016 scenarios, it is estimated that in the maximum use month, 9 tanker trucks (750 tons per month / 23 tons per truck / 4 weeks per month = 8.2 tanker trucks) would have needed to be delivered

weekly in July 2015 and 8 (700 tons per month / 23 tons per truck / 4 weeks per month = 7.6) in July 2016.

Since the withdrawal zone for the Wallace Dam forebay extends from the surface to the bottom, diffusers would likely be placed at about three layers depending on velocity and flow measurements as well as the need for DO increases within the water column. Additional data collection would be used to guide the design of the diffuser system.

A control system would be designed to automatically place oxygen in the forebay as a function of water flow (number of generation units in operation) and required DO uptake. The majority of the oxygen addition would be during generation, with a smaller flow rate of oxygen to maintain the oxygenated forebay volume during non-generation. This control system would account for the effect of pumpback operations on water volume and release DO levels. Pumpback presents additional complexity for the FBO2 system as the majority of the water movement in the summertime is from pumpback operations that move water from the Wallace tailrace into Lake Oconee then back. If this water is already oxygenated enough to meet requirements the FBO2 system will only need to provide oxygenation for the hydro generation water movement above pumpback each day. The spread and mixing of the oxygenated water moved into the forebay and tailrace will complicate the evaluation of the oxygen needed at any one time. These mixing characteristics can be defined prior to final design and operations. The control system may have to react quickly as the oxygenated water from pumpback is removed from the forebay.

Figure 11. Oxygen flow required to achieve DO objective of 5 mg/L daily average in the releases from Wallace Dam based on 2015 and 2016 operations and DO monitoring data.

Figure 12. Conceptual diffuser layout prepared by Mobley (subject to change). Upstream diffusers would be placed and operated such that a low level of oxygen is injected all the time. Diffusers located closer to the dam are designed to be operated intermittently to provide additional oxygen boosts during turbine operation.

Table 1. Installation Costs for Line Diffuser plus Liquid Oxygen Storage Facility

In addition to the Total Cost provided in Table 1, the cost of field studies and water quality/aeration modeling of Lake Oconee and Lake Sinclair could cost up to $750,000.

MEI DIFFUSER: Labor Material TotalSubmittals $23,000 $6,000 $29,000Shop assembly $105,000 $101,000 $206,000Diffuser lines and supply lines 20,000 feet total $345,000 $647,000 $992,000Travel expenses and shipping: $26,000 $82,000 $108,000Equipment rental $0 $35,000 $35,000Installation tools: $0 $29,000 $29,000O&M As Built drawings and training $18,000 $3,000 $21,000

Diffuser System Subtotal: $517,000 $903,000 $1,420,000General Conditions 7% $99,400Overhead and Profit 20% $284,000Contingency 25% $355,000Total $2,159,000

LOx OXYGEN SUPPLY FACILITY: 200 tons O2/day Labor Material TotalFacility layout design $127,400 $19,600 $147,000Grade and drainage (gravel) $39,200 $23,520 $62,720Access road $19,600 $29,400 $49,000Concrete foundations, equipment pad and spill area $88,200 $88,200 $176,400Electrical power supply $10,000 $15,000 $25,000LOx equipment, cryogenic connection piping installation $98,000 $725,000 $823,000Supply manifold, piping, flow control valves, pipe supports $39,200 $78,400 $117,600Trench $20,000 $500 $20,500Core under road $20,000 $2,000 $22,000Lighting $29,400 $29,400 $58,800Remote operation control system $78,400 $58,800 $137,200Fence and gate $11,760 $19,600 $31,360

Oxygen Facility Subtotal: $581,160 $1,089,420 $1,670,580General Conditions 7% $116,941Overhead and Profit 20% $334,116Contingency 25% $417,645Total $2,540,000

Total $4,699,000

WallaceINSTALLATION COST BREAKDOWN

Mobley Line Diffuser SystemCryogenic Equipment Purchased

Figure 13. Estimated monthly oxygen requirements for achieving DO objectives at Wallace Dam for years 2015 and 2016.

Figure 14. Estimated annual oxygen costs at Wallace Dam for years 2015 and 2016.

Compressed Air Addition to Draft Tubes (Approach 4)

The evaluation of compressed air, or forced air, addition to the draft tubes involved the following scope of work:

1. Using the Discrete Bubble Model (DBM) setup for the Wallace Dam draft tubes, conduct model runs using operations and water quality data for 2015 and 2016 to predict airflows required to achieve desired DO improvements in the tailrace as well as resulting TDG.

2. Estimate the TDG added by the compressors considering a range of “background” dissolved gases. In addition to dissolved nitrogen at 100% (i.e., 80% of air bubbles), consider a typical range of background dissolved gases to account for other gases at a minimum of 105%, a reasonable estimate of 115%, and a high value at 125%. Apply the turbine aeration model to predict the oxygen content combined with these background dissolved gases to predict total dissolved gases (i.e., TDG).

3. Estimate the a) sizes, numbers, and cost of compressors that would reasonably be required, b) piping, and c) electrical work.

4. Estimate power losses attributed to air bubbles formed in the draft tube by the addition of air, assuming 1% power loss for each percent of gas flow required in the draft tube.

Application of DBM Model to Predict Airflows and Resulting TDG

A cross section of one of the draft tubes is shown in Figure 15. The DBM used the geometry of the draft tube as depicted in Figure 16. Modeled total airflow requirements and resulting TDG levels are presented in Figures 17 and 18. Airflows required by individual units are presented in Figure 19. Unit efficiency and energy losses attributed to air flows for Units 1 through 6 in 2015 and 2016 are presented in Figure 20. The results of these model runs were used to develop costs associated with forced air aeration operations to achieve DO objectives in the tailrace of Wallace Dam. The following section will address uncertainty associated with inflow TDG levels, i.e., background TDG levels.

Figure 15. Side view of Wallace Dam draft tube for a conventional unit. Air would be added to the draft tube at a location near the access door below the unit. The DBM simulated gas transfer from the beginning of the draft tube to the exit and then up to the surface of the tailwater.

Figure 16. Gas transfer efficiency for air added to draft tubes to increase DO in turbine discharges is affected by pressure and bubble contact time. Draft tube characteristics regarding elevation along the centerline of the draft tube (top graph) so that water pressure can be characterized as water and bubbles move along the length of the draft tube. Draft tube characteristics regarding cross-sectional area along the centerline of the draft-tube (bottom graph) so that water and bubble velocities can be characterized as water and bubbles move along the length of the draft tube.

Figure 17. 2015 results of the turbine aeration model used to predict compressor operations that would enable Wallace Dam to achieve 5 mg/L daily average in the releases through the turbines over the range of unit operations (upper graph). In these model runs, air flows are moderated using the model and tailrace DO objectives to reduce energy losses and levels of TDG. This model run was set up using minimum background dissolved gases.

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Figure 18. 2016 results of the turbine aeration model used to predict compressor operations

aeration methods to enhance summer dissolved oxygen in the ... · vertical extent of the forebay...

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