Pii: S0043-1354(96)00339-9

-In storm waterways and at dam outlets, high-velocity supercritical flows are characterized by substantial air bubble entrainment. The entrainment of fine air bubbles and the strong turbulent mixing contribute both to the air-water transfer of volatile gases (e.g. oxygen, nitrogen, VOC). The paper describes new experimental data obtained in a 25 m long channel with a 4 ° slope. The analysis of the data provides new information on the air-water flow properties and on the distributions air-water interface area. Although the amount of entrained air is small (i.e. typically Cme,, < 0.12), the specific air-water interface area can reach over 100 m 2 per unit volume of air and water. The results are compared with an earlier prediction (Chanson, 1994) and confirm the significant contribution of air entrainment to air-water gas transfer in supercritical chute flows. © 1997 Elsevier Science Ltd. Key words--air-water interface area, experimental measurement, open channel flow, air bubble entrainment, gas transfer NOMENCLATURE a = specific interface area (m -~) defined as the air-water surface area per unit volume of air and water C = air concentration defined as the volume of air per unit volume of air and water; it is also called void fraction Cmean = mean air concentration defined in term of Ygo:d = Ygo(1 Croon,) ch = chord length (m) chab = air bubble chord length (m) (ch,b)m~x = maximum chord length (m) detected during the scanning time t (Ch,b)Nm = mean bubble size (m) or the number mean size defined as U.b ni(Chab)i i=1 (chab)~Ms ~ b ~ n, i=1 d = (l) flow depth (m) measured perpendicular to the flow direction (2) characteristic water flow depth (m) defined a s :