Effects of Step Roughness in Skimming Flows: An Experimental Study

On a stepped spillway at large flows, the waters skim over the pseudo-bottom formed by the step edges with very strong recirculation vortices in the step cavities. The effects of step roughness on the flow properties are little known despite the practical relevance : e.g., gabion stepped chutes, unprotected roller compacted concrete spillways, damaged concrete steps on older structures. In the present study, the effect of step roughness was investigated systematically in a new series of experiments. The work was performed in a large size laboratory facility, with step height of 0.10 m, step length of 0.25 m and chute width of 1 m, based upon an undistorted Froude similitude. Four configurations were thoroughly tested with identical flow conditions : a smooth stepped chute, and three configurations with rough step faces. The latters were achieved by placing rough screens on the step faces : i.e., on the vertical faces only (Config. B), on the horizontal step faces only (Config. C) and on both vertical and horizontal step faces (Config. A). Detailed air-water flow measurements were performed with a dual-tip phase detection probe (sensor size: 0.025 mm). Basic results included the vertical distributions of air concentration, bubble count rate, air-water velocity, turbulence level, and the air and water chord size distributions at each sampling point. The results showed some similarities between all four stepped configurations. Three basic flow regimes were observed, and the flow conditions at the change from one flow regime to another were identical for all four geometries. In skimming flows, visual observations showed that the step roughness affected the recirculation patterns in the step cavities. For the roughest configuration A, clear-water recirculation regions were observed downstream of the inception point of air entrainment. Seepage was also observed through the rough screens. At a macroscopic level, the effects of step roughness were two-fold. The location of the inception point of free-surface aeration was further downstream than for a smooth stepped chute for an identical flow rate. In turn the residual energy at the downstream end of the chute was greater on the rough stepped chute. At a microscopic level, the experimental results showed consistently several trends. The void fraction distribution results were very close for all stepped configurations, although there seemed to be slightly less entrained air in the rough stepped chute flows. Bubble count rate distributions indicated systematically lesser bubble count rates in the rough stepped chute flows. At step edges, the rough stepped chute flows were faster than smooth chute flows for a given flow rate and dimensionless location from the inception point of free-surface aeration. This was associated with lower turbulence intensities in rough stepped channel flows. A detailed analysis of air and water chord size distributions showed that, at each sampling location, the distributions of air and water chords were broad and spanned over two to three orders of magnitude. In the bubbly flow region (C < 0.3), the probability distribution functions of air chord sizes followed closely a log-normal law for all investigated configurations and flow conditions.