Design of Pneumatic Diffuser System

During non-generation periods, leakage through the wicket gates of a hydroturbine often results in very poor quality water (low or zero dissolved oxygen) in the tailrace of the hydropower facility. Generally, the leakage rate is relatively small, usually about 5-10 cfs per turbine. A bottom-mounted diffuser system was designed based on laboratory-measured and manufacturer-supplied specifications about the gas transfer characteristics of the bubble plume generated by an 11inch flexible head diffuser. The design criteria and the overall effectiveness of the system were evaluated in field tests at Lake Eufaula, Oklahoma. The analysis of field data is reported herein. Introduction Low dissolved oxygen (DO) concentrations in the tailwaters of deep reservoirs with hydropower is not an uncommon occurrence. Even during non-generation periods, low DO can present a problem for managing the tailwater fishery. In most cases, the source of the low DO water is the hypolimnion of the reservoir. A number of alternatives exist that may prevent the development of low DO in the tailrace area. Some may be implemented in the reservoir, in the release structure, or in the tailrace area. The most appropriate technique must be selected based upon clear identification of the source and extent of the low DO water and upon the operat i ona 1 and water qual i ty object i ves of the project. Eufaula Reservoir, located on the Canadian River in Oklahoma, is a hydropower and flood control project operated by the Corps of 'Research Hydraulic Engineer and 2Research Physical Scientist, Hydraulics Laboratory, US Army Engineer Waterways Experiment Station, Vicksburg, Mississippi 1055 Published in Hydraulic Engineering: Saving a Threatened Resource—In Search of Solutions: Proceedings of the Hydraulic Engineering sessions at Water Forum ’92. Baltimore, Maryland, August 2–6, 1992. Published by American Society of Civil Engineers. 1056 HYDRAULIC ENGINEERING Engineers. During the summer months, low DO conditions have been reported in the tailrace area of the project. The cause was identified as leakage of hypol imnetic water through the wicket gates of the hydroturbines during periods of non-generation. The severity of the low-DO problem prompted supplemental releases through a spillway sluice as a mitigative measure. Tailwater aeration held several advantages over in-reservoir enhancement techniques. Specifically, in this technique, only the water being released is aerated rather than treating a significant volume of water in the hypolimnion. This paper reports on the design, installation, and pilot tests of a tailwater aeration system for Eufaula Reservoir. System Concept and Design A floating tailwater aeration system was tested at Denison Dam (Price 1990) in north Texas. Operational considerations dictated that an alternative design be developed. The concept of a bottom-mounted flexible-head diffuser system was developed. The diffuser heads would be mounted in a structural steel member and lowered to the draft tube bottom in the bulkhead slots (Figure 1). The system would remain in