Characteristics of Free Surface Flow Over Gravel Bed

Abstract

Measurements of velocity profiles of a free surface flow over a permeable gravel bed indicate that the logarithmic velocity distribution can be preserved if the reference datum is located a small distance equal to about one third of the median diameter of the bed particles below the surface. The observed value of 0.28 for the Karman constant is significantly reduced below the commonly expected value of 0.4 for impervious boundaries which indicates that the boundary resistance of the permeable bed is higher than that of the impermeable bed having identical rugosity. It is also observed that the friction factor increases with the increase in Reynolds number. A method is proposed to predict the amount of seepage flow through the permeable bed by measuring the hydraulic gradient and the velocity profile above the bed and the bed material properties such as grain size and permeability. Although the measured bed flow showed considerable deviations from the predictions for the present experiments, the proposed method can be usefully applied in practical problems.