Gas Transfer at Gated Sill Structures In the Ohio River Valley

Abstract

The Ohio River Valley is one of the most heavily industrialized basins in the United States. As a result, the Ohio River, Monogahela, MusldngUln, Kanawha, and other tributaries often experience poor water quality. One measure of poor water quality is low dissolved oxygen concentrations during low flow periods. Most of the rivers in the Ohio River Valley have been transformed into navigable waterways through a series of locks and dams. The free flowing river is now a series of pools. Quiescent pools do not promote the transfer of oxygen into the water due to low turbulence levels. Fortunately, the hydraulic structures themselves may actually be a key componen~ in improving the water quality because of the high turbulence levels generated at these structures. The purpose of this study is to investigate the use of dam gates and hydropower installations for the addition of oxygen to the water. The amount of oxygen added to the water varies with the structural characteristics, gate opening, and hydraulic conditions. Analysis of the most recent data taken in 1998, along with data taleen in previous years, shows trends for oxygen transfer at gate sill structures. Gate tests indicate that a high tailwater submergence of the sill allows minimal oxygen transfer to occur until an unsubmerged hydraulic jUlUP forms. The transfer efficiency will increase until a maximUlU transfer is reached. Beyond this critical gate opening, the transfer efficiency will level out or may even decline. If the submergence on the sill is low, a hydraulic jUlnp can form at very low gate settings. In this case, the transfer efficiencies will increase to a maximUlU at the critical gate opening. After this gate setting is reached, the transfer efficiencies will level out or decline. Te:;ts were also perfonned on hydropower installations with the draft tube air vent opened. Analysis of all sites indicates that oxygen transfer is negligible due to insufficient suction head. The minimal retrofit scheme is not suitable for raising oxygen levels. A more detailed study of retrofit options is needed to raise oxygen levels through hydrotuI'bine venting, In conclusion, the gated sill structures show promise as a tool for improving water quality within the river system by raising oxygen levels, if operated under close-tooptimum hydraulic conditions for gate aeration.