This study investigates steady, unidirectional, two-layer flow over an obstacle, with emphasis on the dynamics of mixing between the layers, and on the behavior of flows with an approach control, i.e., an internal hydraulic control in the flow as it approaches the obstacle. Experiments were conducted in a flume 12.8 m long, 60 cm high, and 38 cm wide designed to allow continuous steady flow of fresh and salt water over a fixed obstacle. This facilitated a more effective study of steady flows than in previous studies where the obstacle was towed. Basic hydraulic principles were used to develop a classification scheme that predicts the regime of flow:​ subcritical, crest-controlled, approach-controlled, or supercritical. Internal hydraulic jum ps occur in crest and approach-controlled flows. The classification scheme predicts whether these jumps will form over the lee face of the obstacle (lee jum ps) or downstream of the obstacle (free jumps).

The primary cause of mixing in flows with an internal hydraulic jump is not the jump itself, but a shear layer upstream of the jump, consisting of a series of Kelvin-Helmholtz billows, generated in the lee of the obstacle as the lower layer accelerates and the upper layer decelerates. Laser-induced fluorescence was used to visualize these billows. Velocity and conductivity measurements were used to quantify the amount of mixing. A simple method for predicting a limit on the amount of mixing, based on the theory of shear layers, was developed. This limit is only achieved if the billows grow to their maximum size;​ this is prevented if a lee jump drowns, or partially drowns, the shear layer.

To explain the behavior of approach-controlled flows the hydrostatic assumption had to be abandoned. Retention of the hydrostatic assumption has lead some investigators to postulate the existence of a stationary shock called a “hydraulic drop”. No “hydraulic drops” were observed in this study. In previous studies flows that separated downstream of the obstacle may have been misinterpreted as examples of “hydraulic drops”.

The results of this study suggest cogent explanations of phenomena observed in Canadian fjords and in the Straits of Gibraltar.