Undular jump in open-channel flow over a sill

The paper reports experimental data on a steady flow with a free surface over a rectangular sill at the bottom of a rectangular channel. The paper focuses on the parameter range in which a qualitative change in flow pattern accompanied by formation of an undular jump is observed. It is shown that transition from the bottom state the surface state of head and tail conjugation to occurs when the second critical depth is reached over the downstream edge of the sill. Conditions of transition from a free state to a submerged state of flow over the sill are considered; conditions of validity of the assumption that the first critical depth is reached over the sill are discussed.