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River and Reservoir Environmental Management


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Scientific Papers on TVA Tailwater Improvements


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Porous Hose Line Diffuser

Richard B. Russell Oxygen Diffuser System Replacement

(PDF 1.7MB)

Mark H. Mobley, et. al.

Upper San Leandro Hypolimnetic Oxygenation System

(PDF 0.6MB)

Mark H. Mobley, et. al.

Diffuser System Modeling and Design For Dissolved Oxygen Enhancement of Reservoirs and Releases

(PDF 1.1MB)

Mark H. Mobley, Gary E. Hauser, Dan F. McGinnis, R. Jim Ruane

In support of the Tennessee Valley Authority’s Lake Improvement Program, a line diffuser system was developed that was applied and proven effective at six TVA hydropower projects. To be effective, the placement of the diffusers and distribution of the oxygen input must be optimized for site-specific water quality and water flow conditions. Most TVA applications were relatively straightforward designs with consistent water flows, deep intakes, and the single objective of release DO enhancement. While line diffuser applications have typically been oriented longitudinally in the old river channel, they can be arranged in any configuration for special purposes. A forebay diffuser system can be designed to continuously aerate a large volume in the reservoir to handle daily volumes associated with peaking hydro turbine flows, or it can be designed with capacity to handle instantaneous peak discharges. Aeration at the proper location in a reservoir can eliminate hydrogen sulfide, iron, and manganese in water supply withdrawals or prevent release of these compounds during hydro generation. Highly intermittent hydropower applications have created a need for a base load oxygen rate combined with intermittent generation load oxygen rate. New applications often require aeration at specific locations in a reservoir to meet fish habitat or oxygen demand requirements. Such increasing complexity in diffuser designs has led to increased use of mathematical modeling to predict diffuser performance in the context of dynamic reservoir conditions. Models are now used to help optimize size, placement, and operation of the line diffuser. New pre- and post-processors are available that reduce the time and cost of using sophisticated models in the design of demanding diffuser applications. This paper describes the line diffuser design and several modeling applications. Results from operational line diffuser systems and model predictions for systems currently under design are presented.


And Then It Sank (PDF 3.5MB)

Mark H. Mobley, R. Jim Ruane, E. Dean Harshbarger

Diffuser designs for aeration of hydropower reservoirs have progressed over the past 25 years with improved operation and reduced costs.  The porous hose line diffuser design, developed for the Tennessee Valley Authority (TVA), has proven to be an efficient and economical aeration diffuser design at eleven applications.  The line diffuser design transfers oxygen efficiently, and minimizes temperature destratification and sediment disruption by spreading the gas bubbles over a very large area in the reservoir.  The development of the line diffuser was an iterative process that responded to site-specific requirements and design failures.  Each successive application described in this paper provided new challenges and design improvements.




TVA Reservoir Aeration Diffuser System (PDF 0.6MB)

Mark H. Mobley

The Tennessee Valley Authority (TVA) has developed an efficient and economical aeration diffuser design that has been installed and operated successfully at six TVA hydropower projects, one TVA nuclear plant and two non-power reservoirs.  The line diffuser transfers oxygen efficiently, and minimizes temperature destratification and sediment disruption by spreading the gas bubbles over a very large area of the reservoir, with oxygen transfer efficiencies of 90 to 95 percent.  Line diffusers are installed and maintained from the surface without the use of divers.  The diffusers can be supplied with air or oxygen, either from a bulk liquid oxygen storage tank, onsite air separation plant, or air compressors.  A line diffuser system can be designed to continuously aerate a large volume of reservoir to handle peaking hydroturbine flows.  Aeration within the reservoir can be an economical means to meet dissolved oxygen requirements for hydropower releases.


 Surface Water Pumps


Surface Water Pumps To Improve Dissolved Oxygen Content of Hydropower Releases (PDF 0.3MB)

Mark Mobley, Willola Tyson, Joe Webb, and Gary Brock

This paper describes the development, installation, and performance testing of a surface water pump system at TVA’s Douglas Dam.  Surface water pumps move a large volume of highly oxygenated surface water down to a level where it is withdrawn through the hydropower intakes to improve the water quality of hydropower releases.  TVA has tested several different arrangements and types of surface water pumps at Douglas Dam since 1986.  Operation of the current system during 1994 has demonstrated significant dissolved oxygen improvement.  Varying conditions in the forebay stratification and hydropower operations control the system effectiveness.   Under average conditions the system can increase dissolved oxygen by 1.5 to 2 mg/L in the hydro discharges.  Installation and operating costs of the pumps are presented along with a discussion of experiences with equipment, flotation, and mooring design.


 Turbine Venting


Turbine Venting For Dissolved Oxygen Improvements At Bull Shoals, Norfork, and Table Rock Dams (PDF 0.3MB)

E. Dean Harshbarger, Bethel Herrold, George Robbins, James C. Carter

The Southwestern Power Administration (SWPA) has utilized turbine venting modifications designed and installed by the Tennessee Valley Authority (TVA) to significantly improve the dissolved oxygen content (DO) in the turbine discharges from Bull Shoals, Norfork, and Table Rock Dams.  This improvement has allowed the turbine generators to be operated at higher power outputs while meeting target DO levels in the discharges.


Auto-Venting Turbines


Justifying, Specifying, and Verifying Performance of Aerating Turbines (PDF 0.1MB)

Paul Hopping, Patrick March, and Paul Wolff

Owners of hydropower facilities face increasing demands to reduce or eliminate environmental concerns arising from the impoundment and control of once-natural stream flows.  These demands are applied through regulatory criteria for mitigating the impact of hydro projects on aquatic habitat.  This paper presents a brief summary of guiding principles for justifying, specifying, and verifying performance of aerating turbines for mitigating low release dissolved oxygen concentrations.  Examples are given for Tennessee Valley Authority (TVA) hydro projects using turbine aeration.


Update on Development of Auto-Venting Turbine Technology (PDF 0.3MB)

Paul Hopping, Patrick March, Thomas Brice, Joseph Cybularz

Since the 1970s, the Tennessee Valley Authority (TVA) has been active in the development of technology for the auto-venting turbine.  Auto-venting turbines increase the concentration of dissolved oxygen in hydropower releases by aspirating and mixing air with the water passing through the runner.  Prior to 1990, methods were developed to retrofit existing units with hub baffles, headcover ventilation pipes, and vacuum breaker air passage enhancements.  Now TVA is integrating aeration into turbine design, improving hydraulic and environmental performance.  At TVA’s Norris Dam, new upgrade turbines installed in 1995 and 1996 are the first auto-venting units in the world.  This paper briefly summarizes performance of the new Norris turbines.


 Aerating Weirs


High-Performance Aerating Weirs For Dissolved Oxygen Improvement (PDF 1.0MB)

Gary E. Hauser and Doug I. Morris

New concepts in aerating weir design are being explored for improving dissolved oxygen in tailwaters downstream from hydropower projects with oxygen-deficient releases.  Reliable design procedures are being developed from experiences of the Tennessee Valley Authority (TVA) and other utilities in a project sponsored by the Electric Power Research Institute (EPRI).  The project includes laboratory and prototype field experiments tp fill gaps in existing knowledge.  This paper is intended to update water resources professionals on the project.