The existence of stepped chute design dated back to more than 3,500 years ago. In modern day, applications of stepped chute design are still relevant in most waterway system. These include stepped road gutter, storm water system and sewers. Besides being recognized as an energy dissipater, stepped channel was characterized for its strong aeration flow. Artificial cascades were introduced in wastewater treatment plant and natural waterway to reoxgenate water. To date, limited studies related to the characteristics of unsteady flow down stepped storm waterway were done. The work of Lim (2002) and Tan (2002) is the only reference.
New experiments were conducted in the Hydraulics Laboratory at the University of Queensland to gain additional information on the hydrodynamic properties and aeration performance of unsteady flow down a stepped chute. Dam break waves were generated by releasing suddenly a known discharge down the dry chute. Results showed that the wave front propagate as a succession of free-falling nappe, nappe impact and horizontal runoff on each step. Visual observations highlighted a strongly aerated wave front associated with ‘white waters’ at the leading edge. The celerity data was successfully compared with a kinematic wave theory assuming a Darcy-Weisbach friction factor of about f = 0.05. Unsteady air-water measurements in term of void fraction, bubble count rates and specific interface area demonstrated quantitatively the strong aeration performance at the leading edge of the wave front. The instantaneous distributions of void fractions were successfully compared with the theoretical solutions of air bubble diffusion equation. The instantaneous velocity distributions profile in the horizontal runoff region exhibited a developing boundary layer near the step invert and a quasi-potential flow zone above, which was analogous to a start up flow. The results were successfully compared with an analytical solution of the Navier Stokes equation. High turbulence level at the wave front was also recorded.
Specific interface area data highlighted large interfacial areas in the leading edge with depth-averaged specific interface areas up to 500 m-1. Such large interfacial areas could induce significant rate of air-water mass transfer, which contributes to the oxidation of debris, pollutants and hydrocarbon trapped at the wave front. Indeed the mass transfer rate is proportionate to the air-water interfacial area. With higher reoxgenation rate associated with large interfacial area, impact of pollution level may be minimized during flash flood events, and positive effect on aquatic habitat could be induced.