Stability of an Overfall Founded on Permeable Soils

The criteria for the stability of an overfall founded on permeable soils are examined. The magnitude of escape velocity (exit gradient) due to both subsoil and surface flows is taken as a basis for the stability in contrast to the existing exit gradient theories. With an increase in permeable foundation depth, the magnitude of the exit gradient, the rate of percolation, and the escape velocity also increase. The net maximum escape velocity possible at any point at the downstream bed level due to the combined effect of the subsoil and wavy surface flow is considerably greater than the one given by the subsoil flow, particularly in the case of a low head overfall where the escape velocities due to subsoil flow are small. The escape velocities contributed by the wavy surface flow are found to increase with an increase in wave steepness. This indicates that immediately downstream of the structure, occurrence of a flood wave of high amplitude and sinusoidal shape would contribute to an increase in the exit hydraulic gradient.