Experimental study of blockage of monochromatic waves by counter currents

Blockage of waves by a current can occur if waves are propagating on a spatially varying opposing current in which the velocity is increasing in the wave propagation direction. The ongoing waves become shorter and steeper while they are propagating against the current. Blocking occurs at the location where the opposing current strength is the same as the effective wave energy transport velocity, the intrinsic wave group velocity. This implies that upstream of this location, the blocking point, there is no propagation of wave energy. A question that immediately arises is what happens with the ongoing wave energy. The theory suggests that the ongoing waves will break before reaching the blocking point due to steepening of the waves. However, other mechanisms than wave breaking may also play an important role in the wave energy dissipation such as energy dissipations due to wave interactions with the turbulence and due to viscous effects. Another possibility is that the ongoing wave energy may be partially reflected at the blocking point. Some earlier studies (e.g. Long et al [1993]) have reported some evidence of wave reflection in blocking situations. The present study aims to investigate experimentally the phenomena of wave breaking and/or wave reflection in blocking situations. For this purpose a novel experimental arrangement has been designed and implemented in a laboratory flume. Previous laboratory studies utilized a constant discharge (Q) and a varying cross section (A) to obtain a longitudinal velocity gradient. This has the disadvantage that the effects of non-uniform cross-section and non-uniform velocity are mixed. In the present study, it was decided to use a constant cross-section and a non-uniform discharge, to he obtained by withdrawal of water through a perforated false bottom. CONCLUSIONS The fitted model has shown to he capable to reproduce the wave field of monochromatic waves being blocked by a counter current reasonably well, allowing us to discriminate the reflected waves and the incoming waves. However, the model in its present form contains unknown calibration coefficients whose variation requires further study and most likely a reformulation of the dissipation model.Hydraulic EngineeringCivil Engineering and Geoscience