An experimental study of effects of step roughness in skimming flows on stepped chutes

On a spillway chute, a stepped design increases the rate of energy dissipation on the chute itself and reduces the size of a downstream energy dissipater. Up to date, the effects of step roughness on the flow properties remain unknown despite the practical relevance to damaged concrete steps, rock chutes and gabions weirs. New measurements were conducted in a large-size laboratory facility with two step conditions (smooth and rough) and three types of step roughness. Detailed air-water flow properties were measured systematically for several flow rates. The results showed faster flow motion on rough step chutes. Although the finding is counter-intuitive, it is linked with the location of the inception point of free-surface aeration being located further downstream than for a smooth stepped chute for an identical flow rate. In the aerated flow region, the velocities on rough-step chutes were larger than those of smooth chute flows for a given flow rate and dimensionless location from the inception point of free-surface aeration both at step edges and between step edges. The results suggest that design guidelines for smooth (concrete) stepped spillway may not be suitable to rough stepped chutes including gabion stepped weirs, and older stepped chutes with damaged steps.

[1]  Hubert Chanson,et al.  Hydraulics of Stepped Chutes and Spillways , 2002 .

[2]  J. A. Kells,et al.  Spatially varied flow over rockfill embankments , 1993 .

[3]  Hubert Chanson,et al.  Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways , 1994 .

[4]  Subhash C. Jain,et al.  Open-Channel Flow , 2000 .

[5]  Hubert Chanson,et al.  Air–water flows down stepped chutes: turbulence and flow structure observations , 2002 .

[6]  Hubert Chanson,et al.  Experimental Investigations of Air Entrainment in Transition and Skimming Flows down a Stepped Chute: Application to Embankment Overflow Stepped Spillways , 2002 .

[7]  Paul Royet,et al.  Ecoulement et dissipation sur les déversoirs en gradins de gabions , 1991 .

[8]  Hubert Chanson,et al.  Flow Resistance in Skimming Flows and its Modelling , 2002 .

[9]  George M. Homsy.  Multi Media Fluid Mechanics , 2001 .

[10]  Larry J. Weber,et al.  The Hydraulics of Open Channel Flow: An Introduction , 2001 .

[11]  Mehmet Ali Kökpinar.  Flow over a stepped chute with and without macro-roughness elements , 2004 .

[12]  Ian R. Wood,et al.  General Method for Critical Point on Spillways , 1983 .

[13]  Hubert Chanson,et al.  Interactions between cavity flow and main stream skimming flows: an experimental study , 2004 .

[14]  Masayuki Takahashi,et al.  Flow Characteristics of Skimming Flows in Stepped Channels , 2004 .

[15]  Hubert Chanson,et al.  Air Bubble Entrainment in Free-Surface Turbulent Shear Flows , 1996 .

[16]  Hubert Chanson,et al.  Effects of Turbulence Manipulation in Skimming Flows: An Experimental Study , 2004 .

[17]  Hubert Chanson,et al.  Interactions Between Free-Surface, Free-Stream Turbulence and Cavity Recirculation in Open Channel Flows: Measurements and Turbulence Manipulation , 2004 .

[18]  Hubert Chanson,et al.  Effects of Step Roughness in Skimming Flows: An Experimental Study , 2005 .

[19]  Hubert Chanson,et al.  Strong Interactions between Free-Surface Aeration and Turbulence in an Open Channel Flow , 2003 .

[20]  Anton Schleiss,et al.  Energy dissipation and hydrodynamic forces of aerated flow over macro-roughness lining for overtopped embankment dams , 2004 .

[21]  N. Rajaratnam,et al.  Skimming Flow in Stepped Spillways , 1990 .

[22]  Carlos A. Gonzalez,et al.  An Experimental Study of Free-surface Aeration on Embankment Stepped Chutes , 2005 .

[23]  C. Wood,et al.  The Effect of Base Bleed on a Periodic Wake , 1964, Journal of the Royal Aeronautical Society.

[24]  Hubert Chanson,et al.  Comparison of energy dissipation between nappe and skimming flow regimes on stepped chutes , 1994 .

[25]  Hubert Chanson,et al.  Micro- and Macro-scopic Study of Two-Phase Flow on a Stepped Chute , 2003 .