Scale Effects in Moderate Slope Stepped Spillways Experimental Studies in Air-Water Flows

Air-water flow measurements were conducted in two large-size stepped chute facilities (θ = 3.4 & 16o) to study experimental distortion caused by scale effects and result extrapolation to prototypes. The stepped geometries corresponded to moderate slopes typical of embankment dams and storm waterways. Experimental data included distributions of air concentration, air-water flow velocity, bubble frequency, bubble chord length and turbulence intensity. For a Froude similitude, scale effects were observed in both facilities, although the geometric scaling ratio was only Lr = 2 in each case. The criterion selection for scale effects is a critical issue. In the 16o chute, major differences (i.e. scale effects) were observed in terms of bubble chord sizes and turbulence levels although little scale effects were seen in terms of void fraction and velocity distributions. The findings emphasise that physical modelling of stepped chutes based upon a Froude similitude is more sensitive to scale effects than classical smooth-invert chute studies. This is consistent with basic dimensional analysis.