Aeration performances and air—Water mass transfer on steep stepped weirs with horizontal and inclined steps

At hydraulic structures, some strong interactions may develop between fast flowing waters and the air adjacent to the water in motion that enhance the air-water transfer of atmospheric and volatile gases in the flow. In turn, in-stream structures may contribute to the aeration and re-oxygenation during overflow. The present study aims to characterize the aeration performance of a steep stepped weir, based upon a detailed physical investigation of air-water interfacial properties across a relatively wide range of discharges. The data showed a strong fragmentation of the air-water flows, a very broad range of entrained bubbles and drops, and a large amount of particle clustering. The results implied a monotonic increase in re-aeration with increasing rate of energy dissipation, while the largest aeration efficiency was observed on the horizontal step weir chute, with the smallest on the 1V:2.33H inclined downward steps. Altogether, the study showed that a steep stepped chute can make a sizeable contribution to the re-oxygenation of the waters, although the downward inclined steps reduce the re-aeration performances.