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Skimming flow on a flat stepped storm waterway


The oldest stepped storm waterway is probably a series of Minoan stepped culverts in Knossos, Crete. Modern applications of stepped waterways include stepped road gutters, city storm water systems, and sewers. Stepped cascades are utilized also in water treatment plants. Artificial stepped cascades and drop structures can be introduced along or beside rivers and streams to reoxygenate waters with low dissolved oxygen contents. In this study, experiments were conducted in a large, flat stepped chute (3.4°) based upon a Froude similitude. Detailed air-water flow measurements were conducted [2]. The results allow a complete characterization of the air concentration and bubble count rate distributions, as well as an accurate estimate of the rate of energy dissipation. The flow resistance, expressed in terms of a modified friction slope, was found to be about 2.5 times greater than in smooth-chute flow. A comparison between smooth- and stepped-invert flows shows that greater aeration and larger residence times take place in the latter geometry. The result confirms the air-water mass transfer potential of stepped cascades, even for flat slopes (<5°) [2]. The video shows a skimming flow above the 24 m long cascade with 0.07 m step heights. Note the aerated nature of the flow and the substantial amount of water projections above te flow. The study [2] received the 2004 American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI) award for the best practice paper in the Journal of Irrigation and Drainage Engineering "Energy Dissipation and Air Entrainment in Stepped Storm Waterway".




Hubert Chanson


1] CHANSON, H., and TOOMBES, L. (2002). "Energy Dissipation and Air Entrainment in a Stepped Storm Waterway: an Experimental Study." Jl of Irrigation and Drainage Engrg., ASCE, Vol. 128, No. 5, pp. 305-315 (ISSN 0733-9437).

[2] TOOMBES, L., and CHANSON, H. (2005). "Air-Water Mass Transfer on a Stepped Waterway." Jl of Environ. Engrg., ASCE, Vol. 131, No. 10, pp. 1377-1386 (ISSN 0733-9372).