Field experiment on thermocline destruction by aerator in dam reservoir (Visualization of water mass intrusion by measuring water temperature)

Abstract

Aerators are necessary for the conservation of water quality in dams, as they increase water circulation and oxygenate the water. To estimate aerator performance characteristics in the field, the optimal spatial range, resolution, and sampling time interval need to be determined for the targeted water mass. In this study, we conducted precise field experiments at Hiyoshi Dam in Kyoto, Japan, by employing a new type of aerator in the summer of 2012. Initially, the water temperature distribution near one aerator was observed using an automatic temperature profiler capable of horizontal movement on the water surface over a distance of 90 m at a speed of 0.1 m/s during a sampling cycle of 15 minutes. The vertical temperature distribution was determined by a series of thermocouples suspended in steps of 0.1 to 1 m on a 10-m-long cable beneath the surface. Subsequently, the water temperature distribution between two identical aerators located 275 m apart was observed using 6 pairs of vertically suspended thermocouples connected horizontally with a cable at 25-m intervals. Finally, the water flow distribution near one aerator was observed with an electromagnetic flowmeter. Analysis of the results clearly showed how an entrained water mass intruded into a thermocline that formed at a depth of 2.5 m. In addition, details regarding the speed and flow rate of the intrusion, the mixing efficiency, and the estimated number of days required for shallow layer circulation were estimated.