The hydraulics of sheet flow on a smooth surface and the effect of simulated rainfall

Abstract

AbstractIf the hydraulics of a flow is to be understood, then at least a few parameters must be known accurately. When the depth and the unit discharge are measured correctly, many other parameters can be calculated, e.g. the mean velocity of the flow, the friction, the Reynolds Number, the Froude Number, and, if the study is to be connected with erosion measurements, the shear stress. A method is therefore developed for the direct measurement of the depth of disturbed water films, by weighing the entire run; the depth of the flow is thus equal to the weight of the water contained in it, one gram being the equivalent of one cm3, divided by the area of the run. Corrections to the measurement are, however, necessary because the water accelerates along the run to a steady state velocity, because the weight drops due to an acceleration in the gravity field, because the weight of the spattered drops has to be known if simulated rainfall is applied, and because the impact of the drops causes a weight. The accuracy of the method is 10 μ if the flow conditions are not to be studied on too steep slopes and if the discharge used is not too great. The main results are plotted in Figure 6 which combines the formulas for laminar, transitional and turbulent flow that are obtained. A comparison with available literature suggests mat sheet flow can either be laminar or purely turbulent, but that mixed flows prevail on low slopes, under 0.5 per cent combined with greater water depths. Since the flow equations on steep slopes obey physical laws, sin S is to be used instead of S, which stands for tan S.The experiments have shown that the effect of the rainfall impact on the Darcy‐Weisbach f does not exceed 20 per cent in the case of laminar flow on gentle slopes. This influence diminishes when the discharge increases, or when the Reynolds Number increases, as well as when the slope angle increases.