Gas Transfer at Hydraulic Structures in the Ohio River Valley

The Ohio River Valley is one of the most heavily industrialized basins in the United States. As a result, the Ohio River and its 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 the key to improving the water quality because of the high turbulence levels and air entrainment generated at these structures. Field experiments were performed at several low-head structures, called gated sills, in the Ohio River Valley to determine the oxygen transfer capability. Methane was used as a tracer gas to determine oxygen transfer. The amount of oxygen transferred to the water varies with structural characteristics, gate opening, and hydraulic conditions. From the data collected at several sites, the factors that influence the transfer efficiency of a structure have been determined. The key to achieving higher transfer efficiencies lies in the ability to form an unsubmerged hydraulic jump. A submerged hydraulic jump will achieve less than 15% transfer efficiency, while an unsubmerged hydraulic jump will achieve much higher transfer. Sill submergence is also important for determining transfer efficiency. Sill submergence will govern whether an unsubmerged hydraulic jump can form, and lower submergence will result in higher transfer.