And Then It Sank
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.
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TVA Reservoir Aeration Diffuser System
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.
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Diffuser System Modeling and Design for Dissolved Oxygen Enhancement of Reservoirs and Releases
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.
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Surface Water Pumps to improve Dissolved Oxygen Content of Hydroposer Releases
Mark H. Mobley, Willola Tyson, Job Webb, 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.
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