Gas Transfer Measurements at Hydraulic Structures on the Ohio River

Abstract

Gas transfer at hydraulic structures has been a topic of interest for many years. Navigation dams on rivers can add a large amount of atmospheric gases to the water due to the high velocities and the turbulence generated as the water passes through these structures. The increase in air-water gas transfer is due to air entrainment and the formation of bubbles in the flow. Hence, gas transfer at hydraulic structures plays an important role in the water quality of a river-reservoir system. Measurement of air-water gas transfer at hydraulic structures is a complicated process. Oxygen has, historically, been the measured gas, but concentration levels close to saturation and significant vertical stratification in oxygen concentration in the upstream pool often hinder accurate transfer measurements. The U.S. Army Corps of Engineers, The u.s. Geological Survey, and th~ University of Minnesota have been involved in this project to measure gas transfer at hydraulic structures in the Ohio River basin, using in~ situ methane as a tracer in addition to measuring dissolved oxygen. The use of the two volatile chemicals increases the possibility of worthwhile field measurements. This project is conducted in order to evaluate gas transfer characteristics at various hydraulic structures on the Ohio River so that spills through the gates can be optimized. The hydropower producers on the Ohio River may also benefit from this information, as wastage of water from the reservoir to meet water quality requirements will be minimized. The results show that gas transfer increases significantly when a hydraulic jump forms in the stilling basin at gated sill structures, which is the type most commonly seen on the Ohio River. It is also clear that gas transfer at hydraulic structures is significantly affected by the structural characteristics and the hydraulic action at each site.