Discussion of “Resurrecting a Neglected Measurement Technique for Air–Water Flows”

The authors should be commended for their insightful and thought-provoking forum article that challenges the hydraulic engineering profession to implement a valuable but overlooked measurement technique for aerated flows.The approach taken by Killen (1968) was seemingly simple at the time, with the limited objective of better understanding the contorted water surface seen in high-speed photographs of developing and fully developed self-aerated chute flows.The measurements bore unexpected additional fruit when Wilhelms (1997)-concisely presented in Wilhelms and Gulliver (2005a, b)-showed that Killen's measurements of the time-varying roughened water surface supported a new view of air-water flows in which air entrapped between the wave crests at the free surface was functionally different and could be accounted for separately from the entrained air bubbles carried within the fluid mass more directly connected to the chute floor.