Propagation of a positive surge against an initially steady flow: Influence of the flow rate

A positive surge results from a sudden change in flow that increases the depth. It is the unsteady flow analogy of the stationary hydraulic jump and a geophysical application is the tidal bore. The flow properties immediately upstream and downstream of the surge front must satisfy the continuity and momentum principles. This paper presents the results of new experimental investigations conducted in a large rectangular channel. Several experiments were conducted five different initial discharges (Q=0.035, 0.045, 0.050, 0.060 and 0.070 m³/s) to investigate a positive surge propagating upstream against the initially steady flow for the same downstream gate opening after closure. In each case, a breaking (weak) bore was observed and free-surface measurements were performed using non-intrusive acoustic displacement meters. Detailed unsteady velocity measurements were further carried out with high temporal resolution using acoustic Doppler velocimetry for Q=0.035 and 0.045 m³/s. The analysis of free-surface profiles revealed the influence of the flow rate on the surge characteristics. Unsteady flow turbulence analysis highlighted some patterns in streamwise and transverse velocities of the surge due to the effect of the flow rate.